Trichotillomania

Complex MONDO:0013189 Pathograph 9 Show in embeddings browser

Hair-pulling disorder: recurrent pulling out of one's own hair producing noticeable hair loss, with repeated unsuccessful attempts to stop. Classified in DSM-5 among the obsessive-compulsive and related disorders as a body-focused repetitive behaviour. It sits across two specialties, which is part of why it is under-modelled. Patients usually present to dermatology with alopecia rather than to psychiatry with a compulsion, and the visible lesion is a consequence of a behaviour rather than of a disease of the skin or follicle. No medication is FDA-approved for it. The mechanism is genuinely unsettled. This entry curates that state rather than assembling a chain the sources decline to assert: the cited narrative review says the pathophysiology "is poorly understood and likely multifactorial". What can be supported is a corticostriatal locus, converging from two independent directions - mouse models in which deleting a single postsynaptic gene produces compulsive grooming to the point of hair loss, and a glutamate-modulating drug that works in adults.

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5
Pathophys.
4
Phenotypes
2
Gaps
9
Pathograph
3
Genes
3
Medical Actions
3
Differentials
1
Trials
2
Models
10
References
1
Deep Research
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Discussions and Knowledge Gaps

2
Why does N-acetylcysteine reduce hair pulling in adults but not in children and adolescents?
KNOWLEDGE GAP nac_age_dependent_response
Attached to
pathophysiology#Striatal Glutamate Dysregulation
Two randomised placebo-controlled trials of the same drug in the same disorder disagree, and they differ in the age of the participants. If the glutamatergic abnormality this entry curates is the drug's target, then either it is not present in the same form in children, or it is present but the behaviour is not yet maintained by it - which would mean the paediatric and adult disorders are not mechanistically the same thing despite sharing a diagnosis. The alternative reading is methodological: the paediatric trial was an add-on design in 39 participants and may simply have been underpowered. Resolving this matters for the entry's structure, not just its treatment list, because one reading implies the disorder should eventually be split by age of onset.
Proposed experiments
Age-stratified glutamatergic imaging in trichotillomania
age_stratified_striatal_mrs
Magnetic resonance spectroscopy of striatal glutamate in children, adolescents and adults with trichotillomania against matched controls, to test whether the abnormality inferred from drug response is present, and whether it varies with age.
Is rodent compulsive overgrooming a model of trichotillomania, or only of a shared corticostriatal abnormality?
KNOWLEDGE GAP rodent_overgrooming_construct_validity
Attached to
pathophysiology#Corticostriatal Synaptic Dysfunction
Both mouse models are presented by their authors as models of OCD, and the entire claim to trichotillomania relevance is that the grooming removes hair. But grooming is a species-typical rodent behaviour performed to excess, whereas human hair pulling is not a normal behaviour intensified, and much of it is automatic rather than urge-driven. Neither gene is established in human trichotillomania. The gap is worth stating explicitly because the pathophysiology section leans on these models for its strongest evidence, including the striatal rescue experiment. If the models capture only a generic corticostriatal compulsivity, then they support the circuit but not the disorder.
Proposed experiments
Behavioural dissociation of automatic and focused pulling in model animals
automatic_vs_focused_grooming_dissociation
Test whether the grooming in Sapap3 and Slitrk5 nulls is sensitive to the manipulations that distinguish automatic from focused pulling in humans, rather than assuming the topography of the behaviour implies the same mechanism.
⚙

Pathophysiology

5
Corticostriatal Synaptic Dysfunction
Impaired excitatory neurotransmission at cortico-striatal synapses, the level at which both genetic mouse models of pathological grooming converge. Deleting either SAPAP3 (a postsynaptic scaffolding protein at excitatory synapses, highly expressed in striatum) or SLITRK5 produces defective corticostriatal transmission together with a grooming phenotype severe enough to remove hair. The direction of causation is established rather than inferred in the SAPAP3 case: restoring the protein selectively in the striatum rescues both the synaptic and the behavioural defect, so the striatal synapse is not merely correlated with the behaviour.
medium spiny neuron CL:1001474 Cell Ontology (CL) Relation: this pathophysiological event involves this cell type This pathophysiological event involves medium spiny neuron (CL:1001474). CL:1001474 is a cell type from the Cell Ontology.
chemical synaptic transmission GO:0007268 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased chemical synaptic transmission (GO:0007268). GO:0007268 is a biological process from the Gene Ontology. ↓ DECREASED
Animal models
Sapap3 knockout mouse Mouse partially recapitulates Reproduces the corticostriatal synaptic lesion and a self-directed grooming behaviour that removes hair.
Slitrk5 knockout mouse Mouse partially recapitulates An independent gene converging on the same circuit, and the source of the link between this circuit and glutamate receptor composition.
Downstream
Striatal Glutamate Dysregulation Curated as indirect. The mouse models show altered glutamate receptor composition at these synapses, and the human evidence for a glutamatergic abnormality is pharmacological rather than direct, so the step between the two is inferred.
Show evidence (3 references)
PMID:17713528 SUPPORT Model Organism
"mice with genetic deletion of Sapap3 exhibit increased anxiety and compulsive grooming behaviour leading to facial hair loss and skin lesions"
Establishes that loss of a single corticostriatal synaptic protein produces grooming to the point of hair loss.
PMID:17713528 SUPPORT Model Organism
"lentiviral-mediated selective expression of Sapap3 in the striatum rescues the synaptic and behavioural defects of Sapap3-mutant mice"
A rescue experiment localising causation to the striatum rather than leaving the association correlational.
PMID:20418887 SUPPORT Model Organism
"Slitrk5(-/-) mice show selective overactivation of the orbitofrontal cortex, abnormalities in striatal anatomy and cell morphology and alterations in glutamate receptor composition, which contribute to deficient corticostriatal neurotransmission"
An independent gene converging on the same circuit, and the link to glutamate receptor composition.
Striatal Glutamate Dysregulation
Altered extracellular glutamate homeostasis in the ventral striatum, curated here principally because a treatment that acts on it works. N-acetylcysteine restores extracellular glutamate concentration in the nucleus accumbens, and in a randomised adult trial it reduced hair-pulling substantially more than placebo. This is inference from therapeutic response, which is weaker than a measurement. The node is retained because the effect size was large and the mechanism of the drug is specific - but the direction of the underlying abnormality in patients has not been measured, and the paediatric trial of the same drug was negative.
regulation of glutamate secretion GO:0014048 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves regulation of glutamate secretion (GO:0014048). GO:0014048 is a biological process from the Gene Ontology.
Downstream
Failure of Behavioural Inhibition
Show evidence (1 reference)
PMID:19581567 SUPPORT Human Clinical
"N-acetylcysteine, an amino acid, seems to restore the extracellular glutamate concentration in the nucleus accumbens and, therefore, offers promise in the reduction of compulsive behavior"
States the pharmacological rationale that this node rests on.
Failure of Behavioural Inhibition
The behavioural output: an urge to pull that is not successfully resisted, producing repeated pulling despite intact knowledge that it is damaging and repeated attempts to stop. Pulling has been described in two modes - automatic (outside awareness, often during sedentary activity) and focused (in response to an urge or an aversive state) - and treatment trials measure them separately.
regulation of behavior GO:0050795 Gene Ontology (GO) Relation: this pathophysiological event involves this biological process This pathophysiological event involves decreased regulation of behavior (GO:0050795). GO:0050795 is a biological process from the Gene Ontology. ↓ DECREASED
Downstream
Hair Loss and Cutaneous Injury
Trichophagia and Gastrointestinal Hair Accumulation Only in the subset who swallow the pulled hair. Curated as a separate branch because its consequences, its severity and its treatment are all different.
Show evidence (1 reference)
PMID:40928501 SUPPORT Human Clinical
"Trichotillomania is an impulse control disorder in which individuals fail to resist urges to pull out their own hair and is associated with significant psychiatric comorbidity and functional impairment"
Defines the disorder as a failure to resist the urge rather than as an absence of insight.
Trichophagia and Gastrointestinal Hair Accumulation
A branch of the behaviour, not of the hair loss: a subset of patients swallow the hair they pull. Ingested hair resists peristalsis and is not digested, so it accumulates in the stomach as a trichobezoar - a hair ball taking the shape of the gastric lumen. When the mass extends through the pylorus into the small intestine or beyond, it is called Rapunzel syndrome. This is where the disorder's serious morbidity and mortality come from: presentations include obstruction, cachexia and critical dehydration, and the reported masses are large enough to be mistaken for tumours on imaging. It is the one arm of trichotillomania whose definitive treatment is surgical rather than behavioural.
Show evidence (2 references)
PMID:36090734 SUPPORT Human Clinical
"The Rapunzel syndrome occurs when the trichobezoar (hair ball) extends beyond the small intestine and sometimes even into the colon, producing long, tail-like hair extensions"
Defines the syndrome and its anatomical extent.
PMID:36090734 SUPPORT Human Clinical
"trichobezoars, an extremely rare human intestinal disease caused by the ingestion of hair (trichophagia)"
Ties bezoar formation to hair ingestion as its cause.
Hair Loss and Cutaneous Injury
The dermatological consequence, and usually the presenting complaint. Because the alopecia is produced by traction rather than by follicular disease, it is mechanically patterned - irregular, incomplete, and distributed where the hands reach - which is what distinguishes it clinically from alopecia areata.
Show evidence (1 reference)
PMID:38126097 SUPPORT Human Clinical
"Trichotillomania, defined as the compulsive pulling out of one's hair, is a psychocutaneous condition associated with functional impairment and decreased quality of life"
Records the disorder's dual psychiatric-dermatological character.
⬡

Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.
Pathograph: causal mechanism network for Trichotillomania Interactive directed graph showing how pathophysiology mechanisms, phenotypes, genetic factors and variants, experimental models, environmental triggers, and treatments relate through causal and linked edges.
●

Phenotypes

4
Integument 1
Alopecia from Traction HP:0001596 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Alopecia (HP:0001596). HP:0001596 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:19581567 SUPPORT Human Clinical
"Trichotillomania is characterized by repetitive hair pulling that causes noticeable hair loss"
Ties the hair loss to the pulling rather than to follicular pathology.
Other 3
Recurrent Hair Pulling HP:0012167 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Hair-pulling (HP:0012167). HP:0012167 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:40928501 SUPPORT Human Clinical
"individuals fail to resist urges to pull out their own hair"
The core behavioural phenotype.
Psychiatric Comorbidity
Show evidence (1 reference)
PMID:40928501 SUPPORT Human Clinical
"associated with significant psychiatric comorbidity and functional impairment in affected children, adolescents, and adults"
Establishes comorbidity and impairment across the age range.
Trichophagia Pica HP:0011856 Human Phenotype Ontology (HP) Relation: this clinical feature is this phenotype This clinical feature is Trichophagia, annotated with Pica (HP:0011856). HP:0011856 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
PMID:36090734 SUPPORT Human Clinical
"trichobezoars, an extremely rare human intestinal disease caused by the ingestion of hair (trichophagia)"
Establishes hair ingestion as a distinct behaviour with its own gastrointestinal consequence.
🧬

Genetic Associations

3
SLITRK1 (MONDO asserts SLITRK1 as the causal gene for trichotillomania (MONDO:0013189 RO:0004003 HGNC:20297, verifiable with `runoak -i sqlite:obo:mondo relationships MONDO:0013189`). That assertion is recorded here rather than endorsed. The primary report of SLITRK1 variants in trichotillomania (PMID:17003809) caches with no retrievable content, so nothing from it can be quoted, and the entry cannot independently verify the strength of the finding. The first genome-wide association study of the disorder, in 2025, found no common variant reaching genome-wide significance - so a single causal gene of the kind RO:0004003 implies is not what the genome-wide data show.)
Gene: SLITRK1 hgnc:20297 HUGO Gene Nomenclature Committee (hgnc) Relation: this disease-associated gene is this gene This disease-associated gene is SLITRK1 (hgnc:20297). hgnc:20297 is a gene from the HUGO Gene Nomenclature Committee. relationship_type: SUSCEPTIBILITY
Show evidence (2 references)
PMID:39974061 SUPPORT Human Clinical
"Candidate gene studies in humans have described rare coding variants in SLITRK1"
Secondary attestation of the SLITRK1 finding from a source this entry can actually read, since the primary report caches with no retrievable content. It establishes that the finding exists and is reported in the field; it is not independent confirmation of it.
PMID:39974061 SUPPORT Human Clinical
"In the first formal genome-wide association study of TTM, we did not identify any common variants with a genome-wide significant (P < 5×10 -8 ) association level with case status"
Marked PARTIAL because it neither confirms nor refutes SLITRK1 specifically - it establishes that no common variant reaches significance, which constrains how MONDO's causal-gene assertion should be read.
Polygenic Burden and Structural Variation (The 2025 case-control study of 101 European-ancestry cases and 488 matched controls found trichotillomania cases carry a higher load of common polygenic risk for psychiatric disorders than controls, and detected copy number variants previously associated with neuropsychiatric disorders - deletions in NRXN1, CSMD1 and 15q11.2. This is the shape the genetics actually has: shared polygenic liability with psychiatric disorders generally, plus recurrent neurodevelopmental CNVs, rather than a trichotillomania-specific locus.)
relationship_type: SUSCEPTIBILITY
Show evidence (2 references)
PMID:39974061 SUPPORT Human Clinical
"We found that TTM cases carry a higher load of common polygenic risk for psychiatric disorders than unaffected controls ( P = 0.008)"
The polygenic finding, with its p-value.
PMID:39974061 SUPPORT Human Clinical
"we also detected copy number variants previously associated with neuropsychiatric disorders in TTM cases (specifically, deletions in NRXN1, CSMD1 , and 15q11.2)"
The structural-variant finding and the specific loci.
Heritable Liability (A twin study found monozygotic and dizygotic concordance rates that differed significantly, with a heritability estimate of 76.2% for noticeable non-cosmetic hair pulling. That figure should be read with its sample in view: 34 twin pairs in total, 24 monozygotic and 10 dizygotic, with a dizygotic concordance of 0% under strict DSM-IV criteria - a denominator small enough that the point estimate is fragile even though the direction is clear. No specific gene is established. SLITRK1 variants were reported in trichotillomania, but that record caches without retrievable content, so it is named here rather than cited; SLITRK5 has human genetic study only in OCD, where the rare-variant prevalence did not differ from controls.)
relationship_type: SUSCEPTIBILITY
Show evidence (1 reference)
PMID:19199280 SUPPORT Human Clinical
"Respective concordance rates for MZ and DZ twin pairs were significantly different at 38.1% and 0% for DSM-IV TTM criteria"
The concordance difference supporting a heritable contribution, quoted with the criteria it was measured under.
💊

Medical Actions

3
Habit Reversal Training
Action: Cognitive Behavior TherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Cognitive Behavior Therapy (NCIT:C64345). NCIT:C64345 is a clinical intervention from the NCI Thesaurus. NCIT:C64345
Cognitive-behavioural intervention, the best-supported treatment. Relapse in adults is described as somewhat common, so durability rather than initial response is the limiting problem.
Show evidence (1 reference)
PMID:40928501 SUPPORT Human Clinical
"Efficacious treatments have been developed and found efficacious, most notably cognitive-behavioral interventions known collectively as habit reversal training, although relapse in adults appears to be somewhat common"
Supports habit reversal training as first-line and records the relapse caveat in the same sentence.
N-Acetylcysteine
Action: PharmacotherapyNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Pharmacotherapy (NCIT:C15986). NCIT:C15986 is a clinical intervention from the NCI Thesaurus. NCIT:C15986
Agent: N-acetylcysteine CHEBI:22198 Chemical Entities of Biological Interest (CHEBI) Relation: this treatment uses this therapeutic agent This treatment uses N-acetylcysteine, annotated with acetylcysteine (CHEBI:22198). CHEBI:22198 is a therapeutic agent from Chemical Entities of Biological Interest.
Glutamate-modulating agent. In a twelve-week randomised placebo-controlled trial in adults, 56% were much or very much improved versus 16% on placebo, with improvement first apparent at nine weeks. A separate randomised trial in children and adolescents found no difference from placebo on any primary or secondary measure - so this is not a treatment that can be assumed to transfer across the age range, and both results are curated below.
Mechanism Target:
INHIBITS Striatal Glutamate Dysregulation — Acts by restoring extracellular glutamate concentration in the nucleus accumbens. The link is the drug's stated mechanism, not a measurement made in patients.
Show evidence (2 references)
PMID:19581567 SUPPORT Human Clinical
"Fifty-six percent of patients "much or very much improved" with N-acetylcysteine use compared with 16% taking placebo (P = .003)"
The adult efficacy result, with its comparator.
PMID:23452680 REFUTE Human Clinical
"No significant difference between N-acetylcysteine and placebo was found on any of the primary or secondary outcome measures"
The paediatric trial contradicts the adult result. Curated as REFUTE rather than omitted, because an age-restricted effect is information about the mechanism and not merely a caveat on the prescription.
Surgical Removal of Trichobezoar
Action: Surgical ProcedureNCI Thesaurus (NCIT) Relation: this treatment is this clinical intervention This treatment is Surgical Procedure (NCIT:C15329). NCIT:C15329 is a clinical intervention from the NCI Thesaurus. NCIT:C15329
Definitive treatment for an established trichobezoar, which does not resolve with behavioural or pharmacological therapy. In the cited series one patient required ileotomy with removal of two masses. Psychiatric treatment of the underlying pulling is the necessary follow-on - in the same series one patient's family declined it, which is precisely the circumstance in which a bezoar recurs.
Mechanism Target:
INHIBITS Trichophagia and Gastrointestinal Hair Accumulation — Removes the accumulated mass. It does not address the behaviour that produced it, which is why it is curated as treating this node rather than the disorder.
Show evidence (1 reference)
PMID:36090734 SUPPORT Human Clinical
"Ileotomy with the removal of two trichobezoars with a diameter of about 5 cm and 7 cm was performed"
Documents surgical removal as the intervention actually used.
📊

Prevalence

1
General population, pooled across 30 studies
Unknown 1140.0 per 100,000 (660.0–1960.0) >1 in 1,000
Trichotillomania 1.14% (95% CI 0.66-1.96%) across 30 studies and 38,526 participants. Recorded alongside it because the gap is the interesting part: any hair-pulling behaviour was 8.84% (95% CI 6.33-12.20%), roughly eightfold higher. The difference is what the diagnostic threshold does - requiring noticeable hair loss and distress removes seven of every eight people who pull their hair. measure_type is UNKNOWN rather than POINT_PREVALENCE: this is a random-effects pool over 30 studies whose individual designs are not uniform, so committing to a single measure type would over-specify what the meta-analysis reports.
Show evidence (2 references)
PMID:35802953 SUPPORT Human Clinical
"Meta-analyses indicated TTM had a prevalence of 1.14% (95% CI 0.66%, 1.96%), while any hair-pulling behavior had a prevalence of 8.84% (95% CI 6.33%, 12.20%)"
Both pooled estimates, quoted together because the entry's point is the ratio between them.
PMID:35802953 SUPPORT Human Clinical
"Of the 713 records identified through database searches, 30 studies involving 38,526 participants were included"
The denominator behind the pooled estimate.
🔀

Differential Diagnoses

3

Conditions with similar clinical presentations that must be differentiated from Trichotillomania:

Alopecia areata
Overlapping Features The principal dermatological differential. Distinguished by the pattern: traction alopecia from pulling is irregular and incomplete within affected patches and falls where the hands reach, whereas alopecia areata produces smooth, well-demarcated, complete patches from an autoimmune attack on the follicle.
Excoriation (skin-picking) disorder
Overlapping Features The other body-focused repetitive behaviour, sharing the DSM-5 chapter, the corticostriatal candidate mechanism and the N-acetylcysteine evidence base. Kept as a differential rather than a subtype because the target tissue and the resulting lesion differ.
Obsessive-compulsive disorder
Overlapping Features Shares the DSM-5 chapter and both animal models. Distinguished by the absence of obsessions: hair pulling is typically not performed to neutralise an intrusive thought, and a substantial proportion of pulling is automatic and outside awareness.
🔬

Clinical Trials

1
NCT00354770 PHASE_II COMPLETED
Twelve-week double-blind placebo-controlled trial of N-acetylcysteine in adults with trichotillomania, the source of the positive adult efficacy result.
Target Phenotypes: Hair-pulling HP:0012167 Human Phenotype Ontology (HP) Relation: this clinical trial targets this phenotype This clinical trial targets Hair-pulling (HP:0012167). HP:0012167 is a phenotype from the Human Phenotype Ontology.
Show evidence (1 reference)
clinicaltrials:NCT00354770 SUPPORT Human Clinical
"This is a 12-week, double-blind study of N-Acetyl Cysteine in the treatment of trichotillomania"
Registration record establishing the design and duration of the adult N-acetylcysteine trial.
🐁

Animal Models

2
Sapap3 knockout mouse
Genetic deletion of the postsynaptic scaffolding protein SAPAP3 produces compulsive grooming severe enough to cause facial hair loss and skin lesions, alleviated by an SSRI, with defects in cortico-striatal synapses and rescue by striatal re-expression.
Species
Mouse
Genotype
Sapap3 (Dlgap3) homozygous null
Publication
PMID:17713528
Pathograph links
Corticostriatal Synaptic Dysfunction partially recapitulates fidelity: moderate Reproduces the corticostriatal synaptic lesion and a self-directed grooming behaviour that removes hair.
Limitations: The authors present this as a model of OCD, not of trichotillomania, and the relevance here rests on the grooming-with-hair-loss phenotype rather than on any validation against trichotillomania itself. Rodent overgrooming is a species-typical behaviour performed to excess; human hair pulling is not grooming carried too far and is frequently reported as automatic rather than urge-driven. No SAPAP3 variant is established in human trichotillomania.
Readouts
Facial hair loss and skin lesions from compulsive grooming
Direction: INCREASED
Interpretation: Behavioural readout of the grooming phenotype.
Show evidence (1 reference)
PMID:17713528 SUPPORT Model Organism
"compulsive grooming behaviour leading to facial hair loss and skin lesions"
The measured behavioural consequence.
Show evidence (1 reference)
PMID:17713528 SUPPORT Model Organism
"These findings demonstrate a critical role for SAPAP3 at cortico-striatal synapses and emphasize the importance of cortico-striatal circuitry in OCD-like behaviours"
Supports treating this model as informative for the corticostriatal node, in the authors' own OCD framing.
Slitrk5 knockout mouse
Loss of the neuron-specific transmembrane protein SLITRK5 produces excessive self-grooming and anxiety-like behaviour, alleviated by fluoxetine, with orbitofrontal overactivation and deficient corticostriatal neurotransmission.
Species
Mouse
Genotype
Slitrk5 homozygous null
Publication
PMID:20418887
Pathograph links
Corticostriatal Synaptic Dysfunction partially recapitulates fidelity: moderate An independent gene converging on the same circuit, and the source of the link between this circuit and glutamate receptor composition.
Limitations: Also framed by its authors as an OCD model. Human SLITRK5 re-sequencing was performed in OCD rather than trichotillomania and found no significant difference in rare non-synonymous variant prevalence versus controls, so the human genetic support for this gene is weak in OCD and absent in trichotillomania.
Show evidence (1 reference)
PMID:20418887 SUPPORT Model Organism
"loss of a neuron-specific transmembrane protein, SLIT and NTRK-like protein-5 (Slitrk5), leads to OCD-like behaviors in mice, which manifests as excessive self-grooming and increased anxiety-like behaviors"
Establishes the model and its grooming phenotype.
{ }

Source YAML

click to show 
name: Trichotillomania
creation_date: "2026-08-21T00:00:00Z"
category: Complex
disease_term:
  preferred_term: trichotillomania
  term:
    id: MONDO:0013189
    label: trichotillomania
description: >-
  Hair-pulling disorder: recurrent pulling out of one's own hair producing noticeable
  hair loss, with repeated unsuccessful attempts to stop. Classified in DSM-5 among the
  obsessive-compulsive and related disorders as a body-focused repetitive behaviour.

  It sits across two specialties, which is part of why it is under-modelled. Patients
  usually present to dermatology with alopecia rather than to psychiatry with a
  compulsion, and the visible lesion is a consequence of a behaviour rather than of a
  disease of the skin or follicle. No medication is FDA-approved for it.

  The mechanism is genuinely unsettled. This entry curates that state rather than
  assembling a chain the sources decline to assert: the cited narrative review says the
  pathophysiology "is poorly understood and likely multifactorial". What can be
  supported is a corticostriatal locus, converging from two independent directions -
  mouse models in which deleting a single postsynaptic gene produces compulsive grooming
  to the point of hair loss, and a glutamate-modulating drug that works in adults.

pathophysiology:
- name: Corticostriatal Synaptic Dysfunction
  biological_scale: CELLULAR
  description: >-
    Impaired excitatory neurotransmission at cortico-striatal synapses, the level at
    which both genetic mouse models of pathological grooming converge. Deleting either
    SAPAP3 (a postsynaptic scaffolding protein at excitatory synapses, highly expressed
    in striatum) or SLITRK5 produces defective corticostriatal transmission together
    with a grooming phenotype severe enough to remove hair.

    The direction of causation is established rather than inferred in the SAPAP3 case:
    restoring the protein selectively in the striatum rescues both the synaptic and the
    behavioural defect, so the striatal synapse is not merely correlated with the
    behaviour.
  cell_types:
  - preferred_term: medium spiny neuron
    term:
      id: CL:1001474
      label: medium spiny neuron
  biological_processes:
  - preferred_term: chemical synaptic transmission
    term:
      id: GO:0007268
      label: chemical synaptic transmission
    modifier: DECREASED
  downstream:
  - target: Striatal Glutamate Dysregulation
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
    description: >-
      Curated as indirect. The mouse models show altered glutamate receptor composition
      at these synapses, and the human evidence for a glutamatergic abnormality is
      pharmacological rather than direct, so the step between the two is inferred.
  evidence:
  - reference: PMID:17713528
    reference_title: "Cortico-striatal synaptic defects and OCD-like behaviours in Sapap3-mutant mice."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "mice with genetic deletion of Sapap3 exhibit increased anxiety and compulsive
      grooming behaviour leading to facial hair loss and skin lesions"
    explanation: Establishes that loss of a single corticostriatal synaptic protein produces
      grooming to the point of hair loss.
  - reference: PMID:17713528
    reference_title: "Cortico-striatal synaptic defects and OCD-like behaviours in Sapap3-mutant mice."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "lentiviral-mediated selective expression of Sapap3 in the striatum rescues
      the synaptic and behavioural defects of Sapap3-mutant mice"
    explanation: A rescue experiment localising causation to the striatum rather than
      leaving the association correlational.
  - reference: PMID:20418887
    reference_title: "Slitrk5 deficiency impairs corticostriatal circuitry and leads to obsessive-compulsive-like behaviors in mice."
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "Slitrk5(-/-) mice show selective overactivation of the orbitofrontal cortex,
      abnormalities in striatal anatomy and cell morphology and alterations in glutamate
      receptor composition, which contribute to deficient corticostriatal neurotransmission"
    explanation: An independent gene converging on the same circuit, and the link to
      glutamate receptor composition.
- name: Striatal Glutamate Dysregulation
  biological_scale: MOLECULAR
  description: >-
    Altered extracellular glutamate homeostasis in the ventral striatum, curated here
    principally because a treatment that acts on it works. N-acetylcysteine restores
    extracellular glutamate concentration in the nucleus accumbens, and in a randomised
    adult trial it reduced hair-pulling substantially more than placebo.

    This is inference from therapeutic response, which is weaker than a measurement.
    The node is retained because the effect size was large and the mechanism of the drug
    is specific - but the direction of the underlying abnormality in patients has not
    been measured, and the paediatric trial of the same drug was negative.
  biological_processes:
  - preferred_term: regulation of glutamate secretion
    term:
      id: GO:0014048
      label: regulation of glutamate secretion
  downstream:
  - target: Failure of Behavioural Inhibition
    causal_link_type: INDIRECT_UNKNOWN_INTERMEDIATES
  evidence:
  - reference: PMID:19581567
    reference_title: "N-acetylcysteine, a glutamate modulator, in the treatment of trichotillomania: a double-blind, placebo-controlled study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "N-acetylcysteine, an amino acid, seems to restore the extracellular glutamate
      concentration in the nucleus accumbens and, therefore, offers promise in the reduction
      of compulsive behavior"
    explanation: States the pharmacological rationale that this node rests on.
- name: Failure of Behavioural Inhibition
  biological_scale: ORGANISM
  description: >-
    The behavioural output: an urge to pull that is not successfully resisted, producing
    repeated pulling despite intact knowledge that it is damaging and repeated attempts
    to stop. Pulling has been described in two modes - automatic (outside awareness,
    often during sedentary activity) and focused (in response to an urge or an
    aversive state) - and treatment trials measure them separately.
  biological_processes:
  - preferred_term: regulation of behavior
    term:
      id: GO:0050795
      label: regulation of behavior
    modifier: DECREASED
  downstream:
  - target: Hair Loss and Cutaneous Injury
    causal_link_type: DIRECT
  - target: Trichophagia and Gastrointestinal Hair Accumulation
    causal_link_type: DIRECT
    description: >-
      Only in the subset who swallow the pulled hair. Curated as a separate branch
      because its consequences, its severity and its treatment are all different.
  evidence:
  - reference: PMID:40928501
    reference_title: "Trichotillomania and its treatment: an updated review and recommendations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Trichotillomania is an impulse control disorder in which individuals fail
      to resist urges to pull out their own hair and is associated with significant
      psychiatric comorbidity and functional impairment"
    explanation: Defines the disorder as a failure to resist the urge rather than as
      an absence of insight.
- name: Trichophagia and Gastrointestinal Hair Accumulation
  biological_scale: ORGANISM
  description: >-
    A branch of the behaviour, not of the hair loss: a subset of patients swallow the
    hair they pull. Ingested hair resists peristalsis and is not digested, so it
    accumulates in the stomach as a trichobezoar - a hair ball taking the shape of the
    gastric lumen.

    When the mass extends through the pylorus into the small intestine or beyond, it is
    called Rapunzel syndrome. This is where the disorder's serious morbidity and
    mortality come from: presentations include obstruction, cachexia and critical
    dehydration, and the reported masses are large enough to be mistaken for tumours on
    imaging. It is the one arm of trichotillomania whose definitive treatment is surgical
    rather than behavioural.
  evidence:
  - reference: PMID:36090734
    reference_title: "Trichopsychodermatology: trichotillomania and trichophagia leading to Rapunzel syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The Rapunzel syndrome occurs when the trichobezoar (hair ball) extends
      beyond the small intestine and sometimes even into the colon, producing long,
      tail-like hair extensions"
    explanation: Defines the syndrome and its anatomical extent.
  - reference: PMID:36090734
    reference_title: "Trichopsychodermatology: trichotillomania and trichophagia leading to Rapunzel syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "trichobezoars, an extremely rare human intestinal disease caused by the
      ingestion of hair (trichophagia)"
    explanation: Ties bezoar formation to hair ingestion as its cause.
- name: Hair Loss and Cutaneous Injury
  biological_scale: TISSUE
  description: >-
    The dermatological consequence, and usually the presenting complaint. Because the
    alopecia is produced by traction rather than by follicular disease, it is
    mechanically patterned - irregular, incomplete, and distributed where the hands
    reach - which is what distinguishes it clinically from alopecia areata.
  evidence:
  - reference: PMID:38126097
    reference_title: "Recent advances in trichotillomania: a narrative review."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Trichotillomania, defined as the compulsive pulling out of one's hair, is
      a psychocutaneous condition associated with functional impairment and decreased
      quality of life"
    explanation: Records the disorder's dual psychiatric-dermatological character.

phenotypes:
- category: Behavioral
  name: Recurrent Hair Pulling
  description: >-
    Recurrent pulling out of one's own hair, with repeated unsuccessful attempts to
    decrease or stop.
  phenotype_term:
    preferred_term: Hair-pulling
    term:
      id: HP:0012167
      label: Hair-pulling
  evidence:
  - reference: PMID:40928501
    reference_title: "Trichotillomania and its treatment: an updated review and recommendations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "individuals fail to resist urges to pull out their own hair"
    explanation: The core behavioural phenotype.
- category: Dermatological
  name: Alopecia from Traction
  description: >-
    Noticeable hair loss produced by the pulling itself, most often scalp but also
    eyebrows and eyelashes.
  phenotype_term:
    preferred_term: Alopecia
    term:
      id: HP:0001596
      label: Alopecia
  evidence:
  - reference: PMID:19581567
    reference_title: "N-acetylcysteine, a glutamate modulator, in the treatment of trichotillomania: a double-blind, placebo-controlled study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Trichotillomania is characterized by repetitive hair pulling that causes
      noticeable hair loss"
    explanation: Ties the hair loss to the pulling rather than to follicular pathology.
- category: Psychiatric
  name: Psychiatric Comorbidity
  description: >-
    Significant psychiatric comorbidity, reported consistently across both cited reviews.
    Deliberately left unbound: HPO has no term for carrying comorbid psychiatric
    diagnoses, and the nearest candidate (HP:0000708 Atypical behavior) is near-maximally
    generic. A term that broad classifies nothing, so none is used - the same rule that
    governs the exposure bindings elsewhere in the KB. The functional-impairment half of
    this claim was split off into the Hair Loss and Cutaneous Injury node, since
    impairment is a consequence rather than a separate phenotype.
  evidence:
  - reference: PMID:40928501
    reference_title: "Trichotillomania and its treatment: an updated review and recommendations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "associated with significant psychiatric comorbidity and functional impairment
      in affected children, adolescents, and adults"
    explanation: Establishes comorbidity and impairment across the age range.

- category: Gastrointestinal
  name: Trichophagia
  description: >-
    Ingestion of the pulled hair, in a subset of patients. HPO has no term for
    trichophagia specifically, but it does have Pica - the persistent ingestion of
    non-food substances - which is exactly one hop below the abnormal-eating term this
    was first bound to, and hair-eating is a form of pica. The trichobezoar that results
    still has no HPO term.
  phenotype_term:
    preferred_term: Trichophagia
    term:
      id: HP:0011856
      label: Pica
  evidence:
  - reference: PMID:36090734
    reference_title: "Trichopsychodermatology: trichotillomania and trichophagia leading to Rapunzel syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "trichobezoars, an extremely rare human intestinal disease caused by the
      ingestion of hair (trichophagia)"
    explanation: Establishes hair ingestion as a distinct behaviour with its own
      gastrointestinal consequence.

prevalence:
- population: General population, pooled across 30 studies
  measure_type: UNKNOWN
  prevalence_class: ABOVE_1_IN_1000
  rate_per_100000: 1140.0
  rate_low: 660.0
  rate_high: 1960.0
  notes: >-
    Trichotillomania 1.14% (95% CI 0.66-1.96%) across 30 studies and 38,526 participants.
    Recorded alongside it because the gap is the interesting part: any hair-pulling
    behaviour was 8.84% (95% CI 6.33-12.20%), roughly eightfold higher. The difference is
    what the diagnostic threshold does - requiring noticeable hair loss and distress
    removes seven of every eight people who pull their hair.

    measure_type is UNKNOWN rather than POINT_PREVALENCE: this is a random-effects pool
    over 30 studies whose individual designs are not uniform, so committing to a single
    measure type would over-specify what the meta-analysis reports.
  evidence:
  - reference: PMID:35802953
    reference_title: "Prevalence and gender distribution of trichotillomania: A systematic review and meta-analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Meta-analyses indicated TTM had a prevalence of 1.14% (95% CI 0.66%, 1.96%),
      while any hair-pulling behavior had a prevalence of 8.84% (95% CI 6.33%, 12.20%)"
    explanation: Both pooled estimates, quoted together because the entry's point is the
      ratio between them.
  - reference: PMID:35802953
    reference_title: "Prevalence and gender distribution of trichotillomania: A systematic review and meta-analysis."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Of the 713 records identified through database searches, 30 studies
      involving 38,526 participants were included"
    explanation: The denominator behind the pooled estimate.

genetic:
- name: SLITRK1
  gene_term:
    preferred_term: SLITRK1
    term:
      id: hgnc:20297
      label: SLITRK1
  association: >-
    MONDO asserts SLITRK1 as the causal gene for trichotillomania
    (MONDO:0013189 RO:0004003 HGNC:20297, verifiable with
    `runoak -i sqlite:obo:mondo relationships MONDO:0013189`).

    That assertion is recorded here rather than endorsed. The primary report of SLITRK1
    variants in trichotillomania (PMID:17003809) caches with no retrievable content, so
    nothing from it can be quoted, and the entry cannot independently verify the strength
    of the finding. The first genome-wide association study of the disorder, in 2025,
    found no common variant reaching genome-wide significance - so a single causal gene
    of the kind RO:0004003 implies is not what the genome-wide data show.
  relationship_type: SUSCEPTIBILITY
  evidence:
  - reference: PMID:39974061
    reference_title: "Genomic Analysis of Trichotillomania."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Candidate gene studies in humans have described rare coding variants in
      SLITRK1"
    explanation: >-
      Secondary attestation of the SLITRK1 finding from a source this entry can actually
      read, since the primary report caches with no retrievable content. It establishes
      that the finding exists and is reported in the field; it is not independent
      confirmation of it.
  - reference: PMID:39974061
    reference_title: "Genomic Analysis of Trichotillomania."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "In the first formal genome-wide association study of TTM, we did not
      identify any common variants with a genome-wide significant (P < 5×10 -8 )
      association level with case status"
    explanation: Marked PARTIAL because it neither confirms nor refutes SLITRK1
      specifically - it establishes that no common variant reaches significance, which
      constrains how MONDO's causal-gene assertion should be read.
- name: Polygenic Burden and Structural Variation
  association: >-
    The 2025 case-control study of 101 European-ancestry cases and 488 matched controls
    found trichotillomania cases carry a higher load of common polygenic risk for
    psychiatric disorders than controls, and detected copy number variants previously
    associated with neuropsychiatric disorders - deletions in NRXN1, CSMD1 and 15q11.2.

    This is the shape the genetics actually has: shared polygenic liability with
    psychiatric disorders generally, plus recurrent neurodevelopmental CNVs, rather than
    a trichotillomania-specific locus.
  relationship_type: SUSCEPTIBILITY
  evidence:
  - reference: PMID:39974061
    reference_title: "Genomic Analysis of Trichotillomania."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We found that TTM cases carry a higher load of common polygenic risk for
      psychiatric disorders than unaffected controls ( P = 0.008)"
    explanation: The polygenic finding, with its p-value.
  - reference: PMID:39974061
    reference_title: "Genomic Analysis of Trichotillomania."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "we also detected copy number variants previously associated with
      neuropsychiatric disorders in TTM cases (specifically, deletions in NRXN1, CSMD1 ,
      and 15q11.2)"
    explanation: The structural-variant finding and the specific loci.
- name: Heritable Liability
  association: >-
    A twin study found monozygotic and dizygotic concordance rates that differed
    significantly, with a heritability estimate of 76.2% for noticeable non-cosmetic
    hair pulling. That figure should be read with its sample in view: 34 twin pairs
    in total, 24 monozygotic and 10 dizygotic, with a dizygotic concordance of 0% under
    strict DSM-IV criteria - a denominator small enough that the point estimate is
    fragile even though the direction is clear.

    No specific gene is established. SLITRK1 variants were reported in trichotillomania,
    but that record caches without retrievable content, so it is named here rather than
    cited; SLITRK5 has human genetic study only in OCD, where the rare-variant
    prevalence did not differ from controls.
  relationship_type: SUSCEPTIBILITY
  evidence:
  - reference: PMID:19199280
    reference_title: "A twin concordance study of trichotillomania."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Respective concordance rates for MZ and DZ twin pairs were significantly
      different at 38.1% and 0% for DSM-IV TTM criteria"
    explanation: The concordance difference supporting a heritable contribution, quoted
      with the criteria it was measured under.

animal_models:
- name: Sapap3 knockout mouse
  species: Mouse
  genotype: Sapap3 (Dlgap3) homozygous null
  publication: PMID:17713528
  description: >-
    Genetic deletion of the postsynaptic scaffolding protein SAPAP3 produces compulsive
    grooming severe enough to cause facial hair loss and skin lesions, alleviated by an
    SSRI, with defects in cortico-striatal synapses and rescue by striatal re-expression.
  modeled_mechanisms:
  - target: Corticostriatal Synaptic Dysfunction
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      Reproduces the corticostriatal synaptic lesion and a self-directed grooming
      behaviour that removes hair.
    limitations: >-
      The authors present this as a model of OCD, not of trichotillomania, and the
      relevance here rests on the grooming-with-hair-loss phenotype rather than on any
      validation against trichotillomania itself. Rodent overgrooming is a species-typical
      behaviour performed to excess; human hair pulling is not grooming carried too far
      and is frequently reported as automatic rather than urge-driven. No SAPAP3 variant
      is established in human trichotillomania.
    readouts:
    - name: Facial hair loss and skin lesions from compulsive grooming
      target: Corticostriatal Synaptic Dysfunction
      direction: INCREASED
      interpretation: Behavioural readout of the grooming phenotype.
      evidence:
      - reference: PMID:17713528
        reference_title: "Cortico-striatal synaptic defects and OCD-like behaviours in Sapap3-mutant mice."
        supports: SUPPORT
        evidence_source: MODEL_ORGANISM
        snippet: "compulsive grooming behaviour leading to facial hair loss and skin lesions"
        explanation: The measured behavioural consequence.
    evidence:
    - reference: PMID:17713528
      reference_title: "Cortico-striatal synaptic defects and OCD-like behaviours in Sapap3-mutant mice."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "These findings demonstrate a critical role for SAPAP3 at cortico-striatal
        synapses and emphasize the importance of cortico-striatal circuitry in OCD-like
        behaviours"
      explanation: Supports treating this model as informative for the corticostriatal node,
        in the authors' own OCD framing.
- name: Slitrk5 knockout mouse
  species: Mouse
  genotype: Slitrk5 homozygous null
  publication: PMID:20418887
  description: >-
    Loss of the neuron-specific transmembrane protein SLITRK5 produces excessive
    self-grooming and anxiety-like behaviour, alleviated by fluoxetine, with orbitofrontal
    overactivation and deficient corticostriatal neurotransmission.
  modeled_mechanisms:
  - target: Corticostriatal Synaptic Dysfunction
    relationship: PARTIALLY_RECAPITULATES
    fidelity: MODERATE
    description: >-
      An independent gene converging on the same circuit, and the source of the link
      between this circuit and glutamate receptor composition.
    limitations: >-
      Also framed by its authors as an OCD model. Human SLITRK5 re-sequencing was
      performed in OCD rather than trichotillomania and found no significant difference
      in rare non-synonymous variant prevalence versus controls, so the human genetic
      support for this gene is weak in OCD and absent in trichotillomania.
    evidence:
    - reference: PMID:20418887
      reference_title: "Slitrk5 deficiency impairs corticostriatal circuitry and leads to obsessive-compulsive-like behaviors in mice."
      supports: SUPPORT
      evidence_source: MODEL_ORGANISM
      snippet: "loss of a neuron-specific transmembrane protein, SLIT and NTRK-like
        protein-5 (Slitrk5), leads to OCD-like behaviors in mice, which manifests as
        excessive self-grooming and increased anxiety-like behaviors"
      explanation: Establishes the model and its grooming phenotype.

treatments:
- name: Habit Reversal Training
  description: >-
    Cognitive-behavioural intervention, the best-supported treatment. Relapse in adults
    is described as somewhat common, so durability rather than initial response is the
    limiting problem.
  therapeutic_modality: BEHAVIORAL
  treatment_term:
    preferred_term: Cognitive Behavior Therapy
    term:
      id: NCIT:C64345
      label: Cognitive Behavior Therapy
  evidence:
  - reference: PMID:40928501
    reference_title: "Trichotillomania and its treatment: an updated review and recommendations."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Efficacious treatments have been developed and found efficacious, most
      notably cognitive-behavioral interventions known collectively as habit reversal
      training, although relapse in adults appears to be somewhat common"
    explanation: Supports habit reversal training as first-line and records the relapse
      caveat in the same sentence.
- name: N-Acetylcysteine
  description: >-
    Glutamate-modulating agent. In a twelve-week randomised placebo-controlled trial in
    adults, 56% were much or very much improved versus 16% on placebo, with improvement
    first apparent at nine weeks. A separate randomised trial in children and adolescents
    found no difference from placebo on any primary or secondary measure - so this is not
    a treatment that can be assumed to transfer across the age range, and both results
    are curated below.
  therapeutic_modality: SMALL_MOLECULE
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: N-acetylcysteine
      term:
        id: CHEBI:22198
        label: acetylcysteine
  target_mechanisms:
  - target: Striatal Glutamate Dysregulation
    treatment_effect: INHIBITS
    description: >-
      Acts by restoring extracellular glutamate concentration in the nucleus accumbens.
      The link is the drug's stated mechanism, not a measurement made in patients.
  evidence:
  - reference: PMID:19581567
    reference_title: "N-acetylcysteine, a glutamate modulator, in the treatment of trichotillomania: a double-blind, placebo-controlled study."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Fifty-six percent of patients \"much or very much improved\" with
      N-acetylcysteine use compared with 16% taking placebo (P = .003)"
    explanation: The adult efficacy result, with its comparator.
  - reference: PMID:23452680
    reference_title: "N-Acetylcysteine in the treatment of pediatric trichotillomania: a randomized, double-blind, placebo-controlled add-on trial."
    supports: REFUTE
    evidence_source: HUMAN_CLINICAL
    snippet: "No significant difference between N-acetylcysteine and placebo was found
      on any of the primary or secondary outcome measures"
    explanation: The paediatric trial contradicts the adult result. Curated as REFUTE
      rather than omitted, because an age-restricted effect is information about the
      mechanism and not merely a caveat on the prescription.

- name: Surgical Removal of Trichobezoar
  description: >-
    Definitive treatment for an established trichobezoar, which does not resolve with
    behavioural or pharmacological therapy. In the cited series one patient required
    ileotomy with removal of two masses. Psychiatric treatment of the underlying pulling
    is the necessary follow-on - in the same series one patient's family declined it,
    which is precisely the circumstance in which a bezoar recurs.
  therapeutic_modality: SURGERY
  treatment_term:
    preferred_term: Surgical Procedure
    term:
      id: NCIT:C15329
      label: Surgical Procedure
  target_mechanisms:
  - target: Trichophagia and Gastrointestinal Hair Accumulation
    treatment_effect: INHIBITS
    description: >-
      Removes the accumulated mass. It does not address the behaviour that produced it,
      which is why it is curated as treating this node rather than the disorder.
  evidence:
  - reference: PMID:36090734
    reference_title: "Trichopsychodermatology: trichotillomania and trichophagia leading to Rapunzel syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Ileotomy with the removal of two trichobezoars with a diameter of about 5
      cm and 7 cm was performed"
    explanation: Documents surgical removal as the intervention actually used.

clinical_trials:
- name: NCT00354770
  phase: PHASE_II
  status: COMPLETED
  description: >-
    Twelve-week double-blind placebo-controlled trial of N-acetylcysteine in adults with
    trichotillomania, the source of the positive adult efficacy result.
  target_phenotypes:
  - preferred_term: Hair-pulling
    term:
      id: HP:0012167
      label: Hair-pulling
  evidence:
  - reference: clinicaltrials:NCT00354770
    reference_title: "A Double-Blind, Placebo-Controlled Study of N-Acetyl Cysteine in Trichotillomania"
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "This is a 12-week, double-blind study of N-Acetyl Cysteine in the treatment
      of trichotillomania"
    explanation: Registration record establishing the design and duration of the adult
      N-acetylcysteine trial.

differential_diagnoses:
- name: Alopecia areata
  description: >-
    The principal dermatological differential. Distinguished by the pattern: traction
    alopecia from pulling is irregular and incomplete within affected patches and falls
    where the hands reach, whereas alopecia areata produces smooth, well-demarcated,
    complete patches from an autoimmune attack on the follicle.
- name: Excoriation (skin-picking) disorder
  description: >-
    The other body-focused repetitive behaviour, sharing the DSM-5 chapter, the
    corticostriatal candidate mechanism and the N-acetylcysteine evidence base. Kept as
    a differential rather than a subtype because the target tissue and the resulting
    lesion differ.
- name: Obsessive-compulsive disorder
  description: >-
    Shares the DSM-5 chapter and both animal models. Distinguished by the absence of
    obsessions: hair pulling is typically not performed to neutralise an intrusive
    thought, and a substantial proportion of pulling is automatic and outside awareness.

discussions:
- discussion_id: nac_age_dependent_response
  kind: KNOWLEDGE_GAP
  prompt: >-
    Why does N-acetylcysteine reduce hair pulling in adults but not in children and
    adolescents?
  attaches_to:
  - pathophysiology#Striatal Glutamate Dysregulation
  rationale: >-
    Two randomised placebo-controlled trials of the same drug in the same disorder
    disagree, and they differ in the age of the participants. If the glutamatergic
    abnormality this entry curates is the drug's target, then either it is not present
    in the same form in children, or it is present but the behaviour is not yet
    maintained by it - which would mean the paediatric and adult disorders are not
    mechanistically the same thing despite sharing a diagnosis. The alternative reading
    is methodological: the paediatric trial was an add-on design in 39 participants and
    may simply have been underpowered.

    Resolving this matters for the entry's structure, not just its treatment list,
    because one reading implies the disorder should eventually be split by age of onset.
  proposed_experiments:
  - experiment_id: age_stratified_striatal_mrs
    name: Age-stratified glutamatergic imaging in trichotillomania
    description: >-
      Magnetic resonance spectroscopy of striatal glutamate in children, adolescents and
      adults with trichotillomania against matched controls, to test whether the
      abnormality inferred from drug response is present, and whether it varies with age.
- discussion_id: rodent_overgrooming_construct_validity
  kind: KNOWLEDGE_GAP
  prompt: >-
    Is rodent compulsive overgrooming a model of trichotillomania, or only of a shared
    corticostriatal abnormality?
  attaches_to:
  - pathophysiology#Corticostriatal Synaptic Dysfunction
  rationale: >-
    Both mouse models are presented by their authors as models of OCD, and the entire
    claim to trichotillomania relevance is that the grooming removes hair. But grooming
    is a species-typical rodent behaviour performed to excess, whereas human hair pulling
    is not a normal behaviour intensified, and much of it is automatic rather than
    urge-driven. Neither gene is established in human trichotillomania.

    The gap is worth stating explicitly because the pathophysiology section leans on
    these models for its strongest evidence, including the striatal rescue experiment.
    If the models capture only a generic corticostriatal compulsivity, then they support
    the circuit but not the disorder.
  proposed_experiments:
  - experiment_id: automatic_vs_focused_grooming_dissociation
    name: Behavioural dissociation of automatic and focused pulling in model animals
    description: >-
      Test whether the grooming in Sapap3 and Slitrk5 nulls is sensitive to the
      manipulations that distinguish automatic from focused pulling in humans, rather
      than assuming the topography of the behaviour implies the same mechanism.

notes: >-
  No GeneReviews chapter exists for trichotillomania. Verified by search:
  "trichotillomania GeneReviews[All Fields]" returns zero results. This is expected -
  trichotillomania is a complex behavioural disorder with no established Mendelian form.

  Correction, and a method note worth generalizing. An earlier version of this entry
  asserted that HPO has no term for hair pulling and bound the central phenotype to
  HP:0000722 Compulsive behaviors instead. That was wrong. HP:0012167 Hair-pulling
  exists, carries "Trichotillomania" as an EXACT synonym, and shares xrefs
  SCTID/SNOMEDCT_US:17155009 and UMLS:C0040953 with MONDO:0013189 - the same concept.

  The searches behind the false claim were "l~trichotillomania" and "l~hair pulling", and
  both failed for the same reason: `l~` matches LABELS ONLY. HPO's label is the
  hyphenated "Hair-pulling", so the unhyphenated query missed it, and "Trichotillomania"
  is a synonym rather than a label so that query could not reach it either.
  `search "t~trichotillomania"` returns it immediately. **Use `t~` before concluding a
  term does not exist** - a negative result from `l~` is not evidence of absence.

  The wrong binding was not merely too broad. HP:0000722 carries "OCD" and
  "Obsessive-compulsive disorder" as EXACT synonyms, so binding to it asserted the very
  identification this entry's own differential_diagnoses section argues against. HPO
  places HP:0012167 under HP:0100716 Self-injurious behavior, not under compulsive
  behaviour, which matches how the entry describes the disorder.

  HPO does still lack terms for trichophagia and for trichobezoar; both were checked with
  synonym-aware search. Trichophagia is bound to the broad HP:0100738 Abnormal eating
  behavior, and the trichobezoar consequence is carried in the pathophysiology chain
  rather than as an unbindable phenotype.

  No frequency bands are assigned. The cited reviews describe the phenotype qualitatively,
  and the one quantitative source here is a treatment trial, whose response rates are not
  phenotype frequencies.

  Two references were fetched and deliberately not cited, because both cache with
  content_type "unavailable" and nothing in them can be quoted: PMID:17003809 (SLITRK1
  mutations in trichotillomania) and PMID:35964308 (N-acetylcysteine efficacy across
  body-focused repetitive behaviours). The SLITRK1 finding is named in the genetic
  section as unavailable rather than cited on the strength of its title.

  PMID:28085938 (rare SLITRK5 mutations in OCD) was fetched and read but is not cited.
  It is an OCD study, and its own result is mixed: rare non-synonymous variant prevalence
  did not differ significantly from controls, with only the in vitro synaptogenesis assay
  separating the groups. Citing it as human genetic support for trichotillomania would
  overstate it on both counts.

  PMID:39974061 ("Genomic Analysis of Trichotillomania") is a medRxiv PREPRINT, not a
  peer-reviewed article - its cached record carries journal: medRxiv and publication type
  Preprint, and the page footer reads "which was not certified by peer review". Three
  evidence items in the genetic section rest on it. It is cited because it is the only
  genome-wide analysis of this disorder that exists, and its findings are reported here
  as what that analysis found rather than as settled; a reader weighing the polygenic and
  CNV claims should know they have not been refereed.

  Deep-research provenance: the claude_code report resolved 19/19 references with
  confabulation_rate 0.0 and no unresolved identifiers. 13 of 19 were weighed on topic
  and none was flagged off topic, though six were undecided - a record with no abstract
  cannot be assessed for relevance, so this is not a clean bill for all 19. The reference
  set curated here was assembled independently by direct PubMed search.

references:
- reference: PMID:40928501
  title: "Trichotillomania and its treatment: an updated review and recommendations."
- reference: PMID:38126097
  title: "Recent advances in trichotillomania: a narrative review."
- reference: PMID:17713528
  title: "Cortico-striatal synaptic defects and OCD-like behaviours in Sapap3-mutant mice."
- reference: PMID:20418887
  title: "Slitrk5 deficiency impairs corticostriatal circuitry and leads to obsessive-compulsive-like behaviors in mice."
- reference: PMID:19581567
  title: "N-acetylcysteine, a glutamate modulator, in the treatment of trichotillomania: a double-blind, placebo-controlled study."
- reference: PMID:23452680
  title: "N-Acetylcysteine in the treatment of pediatric trichotillomania: a randomized, double-blind, placebo-controlled add-on trial."
- reference: PMID:19199280
  title: "A twin concordance study of trichotillomania."
- reference: PMID:35802953
  title: "Prevalence and gender distribution of trichotillomania: A systematic review and meta-analysis."
- reference: PMID:36090734
  title: "Trichopsychodermatology: trichotillomania and trichophagia leading to Rapunzel syndrome."
- reference: PMID:39974061
  title: "Genomic Analysis of Trichotillomania."
📚

References & Deep Research

References

10
PMID:40928501
Trichotillomania and its treatment: an updated review and recommendations.
No top-level findings curated for this source.
PMID:38126097
Recent advances in trichotillomania: a narrative review.
No top-level findings curated for this source.
PMID:17713528
Cortico-striatal synaptic defects and OCD-like behaviours in Sapap3-mutant mice.
No top-level findings curated for this source.
PMID:20418887
Slitrk5 deficiency impairs corticostriatal circuitry and leads to obsessive-compulsive-like behaviors in mice.
No top-level findings curated for this source.
PMID:19581567
N-acetylcysteine, a glutamate modulator, in the treatment of trichotillomania: a double-blind, placebo-controlled study.
No top-level findings curated for this source.
PMID:23452680
N-Acetylcysteine in the treatment of pediatric trichotillomania: a randomized, double-blind, placebo-controlled add-on trial.
No top-level findings curated for this source.
PMID:19199280
A twin concordance study of trichotillomania.
No top-level findings curated for this source.
PMID:35802953
Prevalence and gender distribution of trichotillomania: A systematic review and meta-analysis.
No top-level findings curated for this source.
PMID:36090734
Trichopsychodermatology: trichotillomania and trichophagia leading to Rapunzel syndrome.
No top-level findings curated for this source.
PMID:39974061
Genomic Analysis of Trichotillomania.
No top-level findings curated for this source.

Deep Research

1
Claude Code ▸
Trichotillomania (Hair-Pulling Disorder): Comprehensive Research Report
claude-haiku-4-5-20251001, claude-sonnet-5 36 citations 2026-08-21T18:31:46.436288

Trichotillomania (Hair-Pulling Disorder): Comprehensive Research Report

1. Disease Information

Overview. Trichotillomania (TTM), also termed hair-pulling disorder or trichotillosis, is a psychiatric condition classified in DSM-5 under "Obsessive-Compulsive and Related Disorders" and characterized by recurrent pulling out of one's own hair, resulting in noticeable hair loss, repeated attempts to decrease or stop the behavior, and clinically significant distress or impairment not better explained by another medical or mental disorder. The HPO clinical definition captures the phenomenology precisely: "repetitive pulling out of one's hair resulting in noticeable hair loss," with "a rising subjective sense of tension before pulling out the hair and a sense of gratification or relief when pulling out the hair" (HPO term below).

Key identifiers: - OMIM: #613229 (TRICHOTILLOMANIA; TTM) — OMIM entry - MONDO: MONDO:0013189 - ICD-11: 6B25.0 (grouped under Obsessive-Compulsive or Related Disorders) - ICD-10-CM: F63.3 (Trichotillomania), historically also coded under impulse-control disorders - HPO: HP:0012167 (Trichotillomania) - MeSH: D014307 (Trichotillomania) - DSM-5 code: 312.39 (F63.3)

Synonyms: hair-pulling disorder, trichotillosis, trichomania, hair pulling disorder, alopecia due to trichotillomania (as a phenotypic consequence).

Data provenance note: Most of the literature base for TTM is aggregated disease-level (clinical trials, cross-sectional cohort/epidemiological studies, twin registries, structural neuroimaging case-control series) rather than large-scale EHR-derived data — nationwide epidemiological studies specifically targeting TTM prevalence are notably lacking, with most estimates drawn from smaller convenience or college-based samples plus a handful of population-based twin registries (e.g., TwinsUK).

2. Etiology

Disease causal model. TTM is understood as a multifactorial neuropsychiatric condition arising from an interaction of polygenic genetic vulnerability, altered cortico-striatal habit/reward circuitry, and environmental/psychological stressors, situated on the obsessive-compulsive/body-focused repetitive behavior (BFRB) spectrum alongside skin-picking (excoriation) disorder.

Genetic risk factors: - SLITRK1 (chromosome 13q31.1; OMIM 609678): Two rare variants (a frameshift and a 3′UTR SNP, var321) were identified in TTM patients and estimated to account for ~5% of TTM cases in the original family-based discovery study; mutations have also been reported in Tourette syndrome. Subsequent replication attempts in independent OCD/TTM cohorts have had mixed success. (Nature Molecular Psychiatry) - SAPAP3 (DLGAP3): A SNP (rs11583978) in SAPAP3 was associated with TTM in the OCD Collaborative Genetics Study, supporting SAPAP3 as a shared genetic risk factor across TTM and OCD-spectrum pathological grooming. (PMC) - Polygenic/GWAS evidence: The first case-control GWAS of TTM (Halvorsen et al. 2025, medRxiv/AJMG-B; 101 European-ancestry TTM cases vs. 488 matched controls) found no genome-wide-significant loci, but demonstrated that TTM cases carry a significantly elevated burden of cross-disorder psychiatric polygenic risk (using a large trans-diagnostic psychiatric GWAS as reference), plus a trend toward excess large CNV deletions impacting constrained/dosage-sensitive genes. Summary statistics were released publicly to enable future meta-analysis. (medRxiv, PubMed 40511557) - Twin heritability estimates: Two published twin studies give divergent heritability estimates — Novak et al. (24 MZ, 10 DZ pairs) estimated h²≈76%, with MZ:DZ DSM-IV concordance of 38.1% vs. 0%; Monzani et al. (TwinsUK, population-based, 5,409 female twins) estimated h²≈32%. Both support a substantial genetic component but with considerable unexplained (largely non-shared environmental) variance. (PubMed 19199280)

Environmental/psychosocial risk factors: Stress and negative affect states are the most consistently reported precipitants/triggers of pulling episodes; boredom and sedentary/passive activities (e.g., reading, watching TV) are common antecedents of "automatic" (low-awareness) pulling, while tension/anxiety precede "focused" pulling. Family history of OCD-spectrum illness is a reported correlate. Age and sex are strong demographic modifiers of course (see Sections 8–9).

Protective factors: No genetic protective variants are established. Early treatment engagement and, for very-early-onset (pre-school) pulling, spontaneous natural remission are the best-documented "protective" trajectories (see Section 8).

Gene-environment interaction: Not well characterized specifically for TTM; the field draws on the broader OCD-spectrum literature (e.g., stress reactivity interacting with polygenic risk) rather than TTM-specific G×E studies.

3. Phenotypes

Core behavioral/psychiatric phenotype

  • Hair-pulling behavior — recurrent, resulting in noticeable hair loss; may be "focused" (purposeful, preceded by rising tension and followed by relief/gratification) or "automatic" (out-of-awareness, occurring during sedentary/passive activities). Suggested term: HP:0012167 Trichotillomania.
  • Rising tension before pulling, relief/gratification after pulling — the core DSM-5 phenomenological criteria (not required for diagnosis in DSM-5, unlike DSM-IV, but frequently present).
  • Repeated unsuccessful attempts to decrease/stop pulling.
  • Trichophagia (hair-eating) — occurs in a substantial minority and is the direct precursor to trichobezoar formation.
  • Trichotemnomania (compulsive hair-cutting) — a related but distinct BFRB sometimes comorbid.

Physical/dermatologic manifestations

  • Alopecia — patchy, irregular, incomplete hair loss with hairs of varying length in affected areas (contrasted with complete, well-demarcated patches in alopecia areata). Suggested term: HP:0001596 (Alopecia) as a parent/associated term, with HP:0012167 as the more specific disease-behavior term.
  • Trichoscopic/dermoscopic findings: black dots (variable size/shape), coiled/hook hairs, flame hairs, "V-sign," tulip hairs, trichoptilosis (split ends), hair powder, follicular microhemorrhages, upright regrowing hairs; absence of exclamation-mark hairs and yellow dots helps distinguish TTM from alopecia areata. (Acta Derm Venereol, PMC3500069)
  • Trichobezoar/Rapunzel syndrome — in patients with trichophagia, ingested hair accumulates in the stomach (± extension into small bowel = Rapunzel syndrome), producing gastric outlet obstruction, ulceration, perforation, peritonitis, intussusception, pancreatitis, obstructive jaundice, protein-losing enteropathy, and iron-deficiency/megaloblastic anemia. (PMC10449238, PMC9637412)
  • Sites: scalp (most common), eyebrows, eyelashes, pubic/body hair.

Psychiatric/functional phenotypes

  • Marked distress, shame, guilt, and social avoidance related to visible hair loss.
  • Impaired occupational, academic, and social functioning; financial burden (wigs, dermatologic treatment, camouflage).

Onset, severity, progression

  • Age of onset: mean overall ~17.7 years in some series; other estimates cite mean onset ~13 years or bimodal childhood peaks around 8 years (boys) / 12 years (girls); onset before age 5 has a distinct, often self-limiting course.
  • Severity: measured clinically using the Massachusetts General Hospital Hairpulling Scale (MGH-HPS) — a 7-item, 0–4 Likert self-report scale (total range 0–28) with a two-factor structure (Severity; Resistance and Control), good internal consistency (α 0.80–0.85). (ScienceDirect)
  • Progression: chronic, waxing-and-waning course typical when onset occurs in later childhood/adolescence; later age of onset is associated with greater severity, more treatment resistance, and higher comorbidity, whereas very-early (pre-school) onset frequently resolves spontaneously ("natural recovery").
  • Frequency in affected individuals: comorbid psychiatric illness present in ~79% of TTM patients (see Section 9/comorbidity below).

Quality-of-life impact

Multiple case-control studies (e.g., Diefenbach et al.) show TTM patients report significantly worse psychological distress, self-esteem, and quality of life than non-psychiatric controls, with effects substantially mediated by comorbid depression; shame, embarrassment, social isolation, and avoidance of activities that expose hair loss (swimming, wind, intimacy, hairdressers) are consistently reported. (ScienceDirect)

4. Genetic/Molecular Information

  • Causal/risk genes: SLITRK1 (chr13q31.1, OMIM 609678) — integral membrane protein homologous to TRK neurotrophin receptors, implicated in neurite outgrowth/synaptic connectivity; a frameshift deletion and the noncoding variant var321 were identified in TTM (and Tourette) pedigrees. SAPAP3/DLGAP3 (chr1p34.3) — postsynaptic scaffolding protein at glutamatergic corticostriatal synapses, interacting with PSD-95/Shank family proteins in the striatum (caudate); rs11583978 associated with TTM/pathological grooming.
  • Variant classification: Both SLITRK1 and SAPAP3 findings are common/rare-variant association findings rather than ClinVar-curated pathogenic/likely-pathogenic single-gene Mendelian variants; TTM is not modeled as a single-gene Mendelian disorder — genetic architecture is best characterized as polygenic, overlapping substantially with general psychiatric risk (per the 2025 GWAS).
  • Structural variation: The 2025 GWAS reported a nominal excess of large, constrained-gene-impacting copy-number deletions in TTM cases vs. controls (trend-level, not genome-wide significant given small sample).
  • Functional consequence framing: SLITRK1 loss-of-function variants are hypothesized to impair neurite/synapse formation ("faulty wiring") producing the urge to pull; SAPAP3 dysfunction is hypothesized to alter glutamatergic corticostriatal synaptic structure/function, consistent with the NAC (glutamate-modulator) treatment rationale (Section 12) and with Sapap3-knockout mouse hyperglutamatergic/hypergrooming phenotypes.
  • Epigenetics/somatic: No disease-defining epigenetic or somatic-mosaicism findings are established for TTM specifically.
  • Chromosomal abnormalities: None established as causal.

Suggested annotations: HGNC gene symbols SLITRK1 (HGNC:20297) and DLGAP3/SAPAP3 (HGNC:19071); GO terms such as GO:0007416 (synapse assembly) and GO:0007268 (chemical synaptic transmission) for mechanistic framing.

5. Environmental Information

  • Psychosocial stressors: acute and chronic stress, negative affect, boredom, and sedentary cognitive load (reading, screen time) are the most consistently identified situational triggers/exacerbators, rather than toxin/occupational exposures.
  • No infectious etiology — TTM is not infection-associated; tinea capitis is a key infectious differential diagnosis to exclude (see Section 10), not a cause.
  • Lifestyle factors: sedentary/passive activity contexts specifically associated with "automatic" subtype pulling.

6. Mechanism / Pathophysiology

Causal chain (proposed): genetic vulnerability (polygenic risk overlapping general psychiatric risk; candidate SLITRK1/SAPAP3 variants) → altered cortico-striato-thalamo-cortical (CSTC) circuitry, particularly striatal (caudate/putamen), orbitofrontal/cingulate, and cerebellar nodes involved in habit formation, motor sequencing, and affect regulation → dysregulated glutamatergic neurotransmission in corticostriatal/nucleus accumbens circuits (supported by NAC's mechanism and Sapap3-knockout hyperglutamatergic grooming) → maladaptive habit-learning loop in which hair-pulling becomes negatively reinforced by tension relief (or is captured by an automatic, low-awareness habit circuit) → recurrent pulling behavior, hair-follicle trauma, and (if trichophagia present) secondary GI bezoar pathology.

  • Molecular pathways: Glutamatergic signaling in nucleus accumbens/striatum implicated via the SAPAP3/Shank–PSD95 postsynaptic density complex and via NAC's efficacy as a cystine-glutamate antiporter modulator restoring extracellular glutamate tone. Serotonergic dysregulation is also implicated (basis for SSRI trials, though evidence is weaker than for OCD proper).
  • Cellular processes: Aberrant corticostriatal synaptic structure — Sapap3-knockout mice show excess dendritic spine density and pre/postsynaptic structural defects in striatum; Hoxb8-mutant mice show corticostriatal circuit defects (excess dendritic spines, LTP and miniature postsynaptic current abnormalities) on Golgi/ultrastructural/electrophysiological study. (Nature Molecular Psychiatry)
  • Neuroimmune mechanism (Hoxb8 model): Hoxb8 lineage in brain labels bone-marrow-derived microglia exclusively; Hoxb8-mutant mice display doubled grooming time leading to hair removal and self-inflicted sores, and this pathological grooming is rescued by wild-type bone-marrow transplantation, implicating a hematopoietic/microglial origin for the corticostriatal circuit defect — a striking "neuroimmunological" mechanistic candidate for compulsive grooming disorders. (PMC2894573, Cell/ScienceDirect)
  • Structural neuroimaging in humans: Morphometric MRI shows increased gray-matter density in left striatum, left amygdalo-hippocampal formation, and cingulate/supplementary motor/frontal cortex bilaterally — regions implicated in habit learning, cognition, and affect regulation (Cambridge/BJP). A separate parcellation study found reduced cerebellar volumes in TTM (n=14) vs. controls (n=12), implicating the cerebellum's role in complex motor sequencing. White-matter volume alterations and structural brain network connectivity differences have also been reported, though the overall imaging literature is described as "limited and inconsistent," and (unlike OCD) TTM-specific CSTC circuit dysfunction has not been as extensively or consistently mapped as in OCD proper.
  • Tissue-level mechanism: repeated mechanical traction on hair follicles causes anagen hair extraction with variable catagen/telogen involvement (trichogram shows catagen and anagen hairs with ruptured root sheaths, absence of telogen hairs), producing irregular patchy alopecia without epidermal inflammation/scarring — distinguishing TTM histologically/trichoscopically from alopecia areata and cicatricial alopecias.
  • GI pathophysiology (trichobezoar): swallowed hair is resistant to digestion and peristaltic transit due to its smooth, slippery surface; it accumulates in the gastric rugae, becomes matted with mucus/food, and can extend through the pylorus into the small bowel (Rapunzel syndrome), producing obstruction, pressure necrosis/ulceration, and perforation via impaired local blood supply.

Suggested ontology terms: GO:0035640 (exploration behavior) / behavioral GO terms are limited for this domain — more precise: GO:0007268 (synaptic transmission), GO:0050890 (cognition), GO:0007612 (learning); CL terms for microglia (CL:0000129) and medium spiny neurons (CL:1001474) given the Hoxb8/microglial and striatal-neuron mechanistic threads; UBERON:0002316 (striatum), UBERON:0002037 (cerebellum), UBERON:0001870 (frontal cortex).

7. Anatomical Structures Affected

  • Organ level (primary): Skin/hair follicles of the scalp (UBERON:0002385 or more specific scalp term), eyebrows, eyelashes, and other body hair sites. Secondary: gastrointestinal tract (stomach/small intestine) via trichobezoar in patients with trichophagia.
  • Body systems: primarily the integumentary system (behaviorally driven) and central nervous system (etiological/mechanistic locus — corticostriatal circuitry); secondary gastrointestinal system involvement in trichophagic patients.
  • Tissue/cell level: hair follicle epithelium and dermal papilla (mechanical trauma); striatal medium spiny neurons and corticostriatal glutamatergic synapses (CNS mechanism); microglia (Hoxb8 mouse model).
  • Subcellular level: postsynaptic density complex (PSD-95/SAPAP3/Shank) at glutamatergic synapses; dendritic spines (structural remodeling in animal models).
  • Localization/laterality: Scalp involvement is typically patchy and can be unilateral or bilateral depending on hand dominance and pulling pattern (e.g., "Friar Tuck" pattern on the vertex/crown is a recognized presentation); eyebrow/eyelash involvement can be unilateral if one hand is dominantly used.

Suggested UBERON terms: UBERON:0002481 (skin of scalp region — or closest scalp term), UBERON:0006914 (eyebrow), UBERON:0006414 (eyelash region... check exact ID), UBERON:0000945 (stomach) for trichobezoar site, UBERON:0002108 (small intestine) for Rapunzel-syndrome extension.

8. Temporal Development

  • Onset: bimodal/variable — early childhood (mean ~8 y boys / ~12 y girls in one series, sexes roughly equally affected in childhood) versus adolescent onset (mean overall ~17.7 y; males ~19.0 y, females ~14.8 y in another series). Onset pattern is typically insidious rather than acute.
  • Progression: Childhood-onset (pre-school, <5 years) TTM frequently shows spontaneous cessation over time and is often considered a transient, self-limited habit requiring little intervention. Adolescent/later-onset TTM more often becomes a chronic, waxing-and-waning condition persisting into adulthood if untreated, and is associated with greater severity, treatment resistance, and psychiatric comorbidity than early-onset disease.
  • Course pattern: chronic, relapsing-remitting/fluctuating severity over the lifespan is the modal pattern in clinical (as opposed to community/subclinical) samples; "automatic" pulling in passive/sedentary contexts and "focused," tension-driven pulling can co-occur and fluctuate with stress.
  • Remission: Natural (untreated) recovery is documented, more common in childhood-onset disease; treatment-induced remission is achievable with behavioral therapy but relapse is common (see Section 12).
  • Critical periods: Adolescence appears to be a period of both risk (onset) and relative treatment resistance if onset occurs then, versus early childhood onset, where the window is more one of spontaneous resolution.

9. Inheritance and Population

Epidemiology: - Lifetime prevalence: estimates range 0.6–2.2%, with a pooled meta-analytic estimate of 1.14% (95% CI 0.66–1.96%) across 30 studies/38,526 participants; other single estimates cite ~1.7%. - Nationwide/population-representative incidence data are lacking; most prevalence estimates derive from college/community convenience samples.

Sex ratio: Clinical samples classically report a strong female predominance (as high as ~9:1 female:male in adult clinical populations), while population-based/meta-analytic data do not consistently confirm female preponderance, with substantial cross-study heterogeneity — sexes are reported as roughly equally affected in childhood, with the female skew emerging/persisting more strongly in adulthood (possibly reflecting differential treatment-seeking or persistence rather than true incidence difference).

Inheritance pattern: TTM is not Mendelian — best characterized as complex/multifactorial (polygenic) inheritance with candidate contributory rare variants (SLITRK1, SAPAP3) of modest individual effect. No established penetrance/expressivity figures analogous to single-gene disorders; no genetic anticipation, germline mosaicism, or founder-effect data reported. No specific carrier-frequency data (not applicable to a polygenic/complex trait in this framework).

Population demographics: No strong evidence for differential prevalence by ethnicity/geography has been robustly established in the literature reviewed; data are concentrated in North American/European clinical and web-recruited cohorts (e.g., the 2025 GWAS cohort was European-ancestry, US-recruited via the Trichotillomania Learning Center).

10. Diagnostics

Clinical criteria (DSM-5): (A) recurrent pulling out of one's hair resulting in hair loss; (B) repeated attempts to decrease/stop; (C) clinically significant distress or impairment; (D) not attributable to another medical condition (e.g., dermatologic); (E) not better explained by another mental disorder.

Physical/dermatologic exam and trichoscopy (key tool): - Trichoscopic features suggestive of TTM: black dots of variable size/shape, coiled/hook hairs, "flame hairs," "V-sign," "tulip hairs," trichoptilosis (split ends), broken hairs of varying length, hair powder, follicular microhemorrhages, upright regrowing hairs; absence of exclamation-mark hairs and yellow dots (the latter two being more typical of alopecia areata). (Acta Derm Venereol, PMC3500069) - Trichogram: catagen and anagen hairs with ruptured root sheaths; absence of telogen hairs. - Scalp biopsy (when diagnosis unclear): pigment casts, increased catagen hairs, perifollicular hemorrhage, empty follicles, trichomalacia — without significant inflammation or scarring.

Differential diagnosis: - Alopecia areata: complete hair loss within well-demarcated patches, exclamation-mark hairs present, uniform black-dot morphology, autoimmune pathogenesis. - Tinea capitis: scaling, erythema, crusting, broken hairs at the scalp surface, positive fungal culture/KOH. - Traction alopecia: history of chronic mechanical tension (braiding, tight hairstyles) rather than compulsive pulling. - Androgenetic alopecia, telogen effluvium, and other cicatricial alopecias in atypical presentations.

Standardized severity/assessment tools: - MGH-HPS (Massachusetts General Hospital Hairpulling Scale) — 7-item self-report, 0–28 total, two-factor (Severity; Resistance and Control) structure. (ScienceDirect) - Trichotillomania Impact Project scales, Milwaukee Inventory for Subtypes of Trichotillomania (MIST), NIMH-TTM Severity Scale.

Genetic testing: No clinically validated single-gene or panel-based genetic test exists for TTM diagnosis (it is a behavioral/clinical diagnosis); research-only genotyping (GWAS arrays) has been used investigationally.

Imaging: Not diagnostic/routine; structural MRI (VBM) has been used in research settings to characterize gray-matter and cerebellar volume differences (Section 6) but has no established individual diagnostic utility.

Screening: No population screening programs exist; case identification is via clinical interview, often supplemented by trichoscopy for confirmation and to exclude dermatologic mimics. GI imaging (ultrasound/CT) is indicated diagnostically when trichobezoar/Rapunzel syndrome is suspected in patients with a trichophagia history presenting with abdominal mass, obstruction, or GI bleeding.

11. Outcome / Prognosis

  • Mortality: TTM itself is not directly life-threatening, but trichobezoar-related complications (gastric/intestinal perforation, peritonitis, severe malnutrition) carry real morbidity/mortality risk in trichophagic patients if unrecognized and untreated; case-report literature documents severe outcomes including perforation peritonitis and multi-organ complications (pancreatitis, cholangitis).
  • Morbidity/functional impact: Significant impairment in psychosocial functioning, self-esteem, and quality of life, substantially mediated by comorbid depression; occupational, academic, and social impairment and financial burden are well documented.
  • Course/complications: chronic waxing-and-waning course in clinical samples; comorbidity with mood/anxiety disorders is associated with greater severity and poorer prognosis. Up to 50–67% of initial treatment responders relapse during long-term follow-up after habit reversal-based behavioral therapy, underscoring a high relapse burden even among treatment responders.
  • Prognostic factors: later age of onset, comorbid OCD/anxiety/depression, and greater experiential avoidance are associated with more severe/treatment-resistant course; very early (pre-school) onset is prognostically favorable (frequent spontaneous remission).
  • Recovery potential: Natural recovery is well documented, especially in early-onset cases; with behavioral treatment (habit reversal-based therapy), large effect sizes are achievable in the short-to-medium term (see Section 12), though durability is limited without maintenance.

12. Treatment

Behavioral/psychotherapeutic (first-line, strongest evidence base)

  • Habit Reversal Training (HRT) / broader behavior therapy — large effect vs. control conditions (SMD ≈ −1.22); superior to pharmacotherapy (clomipramine or SSRIs) in comparative meta-analyses. Suggested NCIT: closest fit is a behavioral therapy/counseling term (e.g., NCIT:C15315 Rehabilitation or a psychotherapy-specific term); no MAXO/NCIT-specific "habit reversal training" term is confirmed — use free-text preferred_term with therapeutic_modality: BEHAVIORAL.
  • Acceptance and Commitment Therapy (ACT)-enhanced HRT: RCT (Woods et al., 25 adults) found significant reductions in MGH-HPS severity, impairment, experiential avoidance, anxiety, and depression vs. waitlist; 66% response rate in the ACT/HRT arm vs. 8% in waitlist control, with gains generally maintained at 3-month follow-up. ("Strong support" designation also given in a more recent evidence review, alongside plain HRT and NAC.) (PubMed 16039603)
  • Comprehensive Behavioral (ComB) model, cognitive therapy (targeting self-control cognitions), telehealth/virtual HRT delivery — virtual-delivery real-world data show median 33.3% severity reduction (Hedges' g=1.01).
  • Limitation: 50–67% of initial responders relapse long-term.

Pharmacotherapy (adjunctive/second-line; overall low-certainty evidence per Cochrane)

  • N-acetylcysteine (NAC) — glutamate modulator (restores extracellular glutamate in nucleus accumbens via cystine-glutamate antiporter). Adult RCT (Grant, Odlaug, Kim 2009, Arch Gen Psychiatry): 50 adults, NAC 1200–2400 mg/day × 12 weeks; 56% "much/very much improved" vs. 16% placebo — significant benefit, well tolerated. (JAMA Network) Pediatric RCT (39 children/adolescents 8–17y, 12 weeks, add-on design): no significant difference from placebo on MGH-HPS. Suggests efficacy may be age-dependent (adults > children). CHEBI candidate: CHEBI:47704 (N-acetyl-L-cysteine).
  • Clomipramine (TCA with SRI action, 50→250 mg/day) — low-certainty evidence of benefit from a single small trial. NCIT/CHEBI: clomipramine has an established CHEBI ID.
  • Olanzapine (atypical antipsychotic, 2.5→10 mg/day) — low-certainty benefit from one small RCT.
  • SSRIs: commonly used clinically but evidence base for TTM specifically is weaker/inconsistent compared to OCD.
  • Overall Cochrane conclusion: evidence is low-certainty, from few, small (n often <50), short-duration (5–13 week) trials; treatment decisions should be individualized based on severity and comorbidity. (Cochrane)
  • Experimental/investigational: valbenazine (VMAT2 inhibitor) trial ongoing (NCT05207085); milk thistle pediatric/adult trial (NCT02473913).

Surgical/interventional

  • Trichobezoar/Rapunzel syndrome: surgical (open or laparoscopic) gastrotomy/enterotomy for bezoar removal is definitive management for GI complications; endoscopic removal may be attempted for smaller bezoars.

Supportive care

  • Psychoeducation, family therapy (especially pediatric cases), dermatologic camouflage/wig counseling, and treatment of comorbid mood/anxiety disorders as part of a comprehensive plan.

Treatment algorithm

Behavioral therapy (HRT ± ACT enhancement) is generally first-line given the strongest and most consistent effect sizes; pharmacotherapy (NAC in adults, or SSRIs/clomipramine/olanzapine) is used adjunctively or when behavioral therapy access is limited or comorbidity (e.g., depression, OCD) predominates.

Suggested NCIT terms: NCIT:C15986 (Pharmacotherapy) for drug treatments generally, paired with therapeutic_agent CHEBI terms for clomipramine, olanzapine, and NAC; NCIT:C15302 (Physical Therapy) is not appropriate — HRT/ACT are psychological/behavioral interventions best captured with therapeutic_modality: BEHAVIORAL.

13. Prevention

  • No established primary prevention (no vaccine, no modifiable single risk factor with proven prevention efficacy).
  • Secondary prevention/early detection: early clinical recognition in childhood-onset cases to differentiate transient (likely self-resolving) pulling from a pattern likely to become chronic, enabling earlier behavioral intervention when indicated.
  • Tertiary prevention: prevention of trichobezoar/GI complications via early identification and treatment of trichophagia in TTM patients (screening for hair-eating behavior specifically, given the outsized morbidity risk of undetected bezoar formation).
  • Genetic counseling: not a standard part of TTM management given its complex/polygenic, non-Mendelian architecture; family history of OCD-spectrum illness may be discussed as a risk correlate in counseling contexts but no formal risk-prediction tools exist.
  • Behavioral/public health interventions: stress-reduction and habit-awareness psychoeducation in at-risk youth (e.g., those with early automatic pulling behaviors or comorbid anxiety) is a plausible but not rigorously trial-validated preventive strategy.

14. Other Species / Natural Disease

TTM-like body-focused repetitive/compulsive grooming behaviors occur naturally across multiple species, supporting cross-species translational relevance:

  • Avian — feather-picking/feather-damaging behavior in parrots: proposed as a naturalistic model with phenomenological similarities to TTM (behavior pattern, proposed stress-related etiology, evoking cues, and shared responsiveness to behavioral and SSRI-based pharmacological treatment). (ScienceDirect)
  • Canine — acral lick dermatitis: dogs lick paws/flank to the point of ulceration/infection, proposed as a compulsive-grooming veterinary analog.
  • Feline and nonhuman primate — psychogenic alopecia: self-induced hair loss from excessive grooming, described as phenotypically analogous.
  • Swine: tail/ear biting behaviors described in the same conceptual family of body-focused repetitive disorders.
  • Rodent (naturalistic): inbred C57BL/6J mice display spontaneous barbering behavior with phenotypic parallels to TTM; alcohol-preferring (P) rats selectively bred for alcohol preference show elevated scratching/oral grooming predictive of subsequent dermatitis/skin lesion development, proposed as an additional naturalistic model. (PMC8875168)

These natural/spontaneous animal presentations are distinct from the induced genetic models below but reinforce that compulsive self-grooming with tissue damage is a conserved cross-species behavioral phenotype.

15. Model Organisms

Genetic/induced mouse models:

  • Sapap3 (Dlgap3) knockout mice: exhibit excessive/compulsive grooming with facial injuries and increased anxiety-like behavior, closely paralleling human TTM/OCD-spectrum pathological grooming. Quantitatively, pre-stressor (pre-spray) grooming was 31 s (KO) vs. 8 s (WT); post-spray, 167 s (KO) vs. 52 s (WT). Underlying mechanism: SAPAP3 (a PSD-95/Shank-interacting postsynaptic scaffolding protein enriched in the striatum/caudate) loss produces corticostriatal glutamatergic synaptic dysfunction. Recent extensions include study of the nucleus accumbens circuit and oxytocin as a candidate therapeutic in this model. (Scientific Reports 2025, bioRxiv)
  • Hoxb8 mutant mice: spend roughly double the grooming time of wild-type littermates, developing hair removal and self-inflicted open sores highly analogous to human TTM. Mechanistically notable for its hematopoietic/microglial origin — Hoxb8 lineage in brain exclusively marks bone-marrow-derived microglia, and pathological grooming is rescued by wild-type bone-marrow transplantation (though the associated sensory/nociceptive defect is not rescued), and corticostriatal circuit defects (excess dendritic spines, synaptic/LTP abnormalities) are demonstrable by Golgi, ultrastructural, and electrophysiological study. Disruption of Hoxb8 restricted to the hematopoietic lineage alone recapitulates pathological grooming. (PMC2894573, Molecular Psychiatry 2018)
  • iNOS (NOS2) knockout mice: proposed as an additional model of trichotillomania-like behavior (nitric oxide synthase pathway implicated in compulsive grooming; bioRxiv preprint).

Model characteristics and translational fidelity: Both Sapap3-KO and Hoxb8-mutant models recapitulate the core surface phenotype (excessive, injurious self-grooming/hair removal) and implicate convergent corticostriatal circuit and glutamatergic synaptic pathology, aligning with human structural imaging (striatal/cingulate gray-matter changes) and the clinical efficacy of the glutamate-modulator NAC. A key limitation: these are models of pathological grooming generally (shared across TTM and OCD-spectrum compulsive behavior) rather than TTM-specific models, and the Hoxb8 model's microglial/hematopoietic mechanism, while mechanistically striking, has an uncertain degree of translational specificity to human TTM (a candidate HUMAN_MODEL_MISMATCH consideration for KB curation, given no direct human confirmation of a hematopoietic/microglial causal contribution to TTM).

Research applications: These models are used to probe corticostriatal synaptic and microglial mechanisms of compulsive grooming, to screen candidate pharmacotherapies (e.g., oxytocin, NAC-related glutamate modulators), and to study circuit-level (nucleus accumbens, striatum) intervention targets.

Suggested NCBITaxon terms: NCBITaxon:10090 (Mus musculus), NCBITaxon:10116 (Rattus norvegicus, P rats), NCBITaxon:9615 (Canis lupus familiaris), and relevant Psittaciformes taxa for parrot feather-picking models.

Summary of Suggested Ontology Term Bindings

Domain Suggested term
Disease MONDO:0013189; OMIM:613229; ICD-11:6B25.0
Core phenotype HP:0012167 (Trichotillomania)
Associated phenotype HP:0001596 (Alopecia)
Genes HGNC:20297 (SLITRK1); HGNC:19071 (DLGAP3/SAPAP3)
Biological process GO:0007268 (chemical synaptic transmission); GO:0007612 (learning)
Cell types CL:1001474 (medium spiny neuron); CL:0000129 (microglial cell)
Anatomy UBERON:0002316 (striatum); UBERON:0002037 (cerebellum); UBERON:0000945 (stomach, for trichobezoar)
Chemical/drug CHEBI: N-acetyl-L-cysteine; clomipramine; olanzapine
Treatment (NCIT) NCIT:C15986 (Pharmacotherapy) + therapeutic_agent; behavioral therapy via therapeutic_modality: BEHAVIORAL (no precise NCIT HRT term identified)

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