Primary Erythermalgia

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Inheritance

1

Autosomal dominant HP:0000006

Primary erythermalgia is inherited in an autosomal dominant pattern; most familial cases segregate a heterozygous gain-of-function SCN9A variant, and de novo variants account for many sporadic early-onset cases.

Autosomal dominant inheritance

Show evidence (1 reference)

PMID:20301342 SUPPORT Human Clinical

"SCN9A neuropathic pain syndromes are inherited in an autosomal dominant manner. Each child of an individual with an NPS-causing variant in SCN9A has a 50% chance of inheriting the variant."

GeneReviews establishes autosomal dominant inheritance with 50% transmission risk.

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Discussions and Knowledge Gaps

1

Do rodent Nav1.7 gain-of-function knock-in models faithfully recapitulate the human inherited-erythromelalgia pain phenotype, or only its cellular (DRG hyperexcitability) substrate?

HUMAN MODEL MISMATCH OPEN iem_rodent_knockin_pain_mismatch

Attached to

pathophysiology#Nociceptor Hyperexcitability

Human iPSC-derived sensory neurons carrying Nav1.7 gain-of-function mutations reproduce hyperexcitability, and CRISPR correction reverses it while introducing the mutation confers it (PMID:40279376), validating the human cellular model. By contrast, two independent knock-in mouse lines carrying a Nav1.7 gain-of-function variant (I228M — a variant found in idiopathic small-fiber neuropathy, a sibling Nav1.7 channelopathy rather than an IEM-specific allele) reproduce DRG neuron hyperexcitability but do NOT display mechanical/thermal hyperalgesia or intraepidermal nerve fiber loss (PMID:33323889). This dissociation — cellular hyperexcitability without an overt behavioral pain phenotype in mice — is a translational caveat for modeling Nav1.7 gain-of-function pain disorders, including IEM.

Show evidence (2 references)

PMID:33323889 SUPPORT Model Organism

"although these 2 Nav1.7 I228M knock-in mouse lines recapitulate the DRG neuron hyperexcitability associated with gain-of-function mutations in Nav1.7, they do not recapitulate the pain or neuropathy phenotypes seen in patients"

Rodent Nav1.7 gain-of-function knock-ins reproduce DRG hyperexcitability but not the overt pain/neuropathy phenotype seen in patients.

PMID:40279376 SUPPORT In Vitro

"robust, scalable and relevant model to study the effects of gain-of-function mutations in ion channels in pain-related disorders"

Human iPSC-derived sensory neurons are validated as a relevant model of Nav1.7 gain-of-function pain disorders.

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Pathophysiology

4

SCN9A Gain-of-Function Variant (Nav1.7)

Heterozygous missense gain-of-function variants in SCN9A alter the biophysical properties of the Nav1.7 voltage-gated sodium channel. Characteristic changes include a hyperpolarizing shift in the voltage-dependence of activation, slowed deactivation, and enhanced ramp currents, which lower the threshold for channel opening and increase the channel's response to small, slow depolarizations.

SCN9A hgnc:10597

voltage-gated sodium channel activity GO:0005248 ↑ INCREASED

Downstream

Nociceptor Hyperexcitability Altered Nav1.7 gating lowers nociceptor firing threshold and drives repetitive firing.

Sympathetic Neuron Dysregulation The same gain-of-function variant alters excitability of sympathetic ganglion neurons that also express Nav1.7.

Show evidence (4 references)

PMID:14985375 SUPPORT Human Clinical

"Our data suggest that mutations in SCN9A cause primary erythermalgia. SCN9A, encoding a voltage-gated sodium channel alpha subunit predominantly expressed in sensory and sympathetic neurones, may play an important role in nociception and vasomotor regulation."

The gene-discovery study links SCN9A missense mutations to primary erythermalgia.

PMID:15385606 SUPPORT In Vitro

"Here we show that these mutations in Na(v)1.7 produce a hyperpolarizing shift in activation and slow deactivation. We also show that these mutations cause an increase in amplitude of the current produced by Na(v)1.7 in response to slow, small depolarizations."

Patch-clamp characterization demonstrates the gain-of-function biophysical shifts of erythermalgia Nav1.7 mutants.

PMID:16988069 SUPPORT In Vitro

"The F216S mutation hyperpolarizes the voltage dependence of activation by 11 mV, accelerates activation, slows deactivation, and enhances the response to slow, small depolarizations."

A second erythermalgia mutation (in the S4 segment) reproduces the same gain-of-function gating changes.

+ 1 more reference

Nociceptor Hyperexcitability

Because Nav1.7 sets the gain of dorsal root ganglion nociceptors, the gain-of-function channel produces membrane hyperexcitability: lowered firing threshold, increased spontaneous and evoked action-potential firing, and amplified responses to physiological stimuli, generating ectopic pain signaling.

dorsal root ganglion sensory neuron CL:1001451

neuronal action potential GO:0019228 ↑ INCREASED

Downstream

Episodic Neurovascular Pain Flares Hyperexcitable nociceptor output drives painful attacks with warmth-triggered vasodilation.

Show evidence (3 references)

PMID:15385606 SUPPORT In Vitro

"these physiological changes, which confer hyperexcitability on peripheral sensory and sympathetic neurons, contribute to symptom production in hereditary erythermalgia"

The gain-of-function gating changes confer hyperexcitability on sensory neurons, driving symptoms. Note the 2004 characterization of sympathetic neurons as hyperexcitable was later refined to hypoexcitability (PMID:16702558; see the Sympathetic Neuron Dysregulation node).

PMID:16988069 SUPPORT In Vitro

"These changes should increase excitability of nociceptive dorsal root ganglion neurons in which the mutant channel is present, thus contributing to pain."

Predicts increased excitability of nociceptive DRG neurons carrying the mutant channel.

PMID:40279376 SUPPORT In Vitro

"using CRISPR/Cas9, we corrected this mutation, which reduced the underlying hyperexcitability, providing a path for personalized medicine to treat these disorders, and we introduced the mutation into control induced pluripotent stem cells, which generated hyperexcitability, providing causality"

Bidirectional CRISPR editing in human iPSC-derived sensory neurons establishes a direct causal link between the Nav1.7 gain-of-function mutation and nociceptor hyperexcitability.

Sympathetic Neuron Dysregulation

Nav1.7 is also expressed in sympathetic ganglion neurons. The same gain-of-function variant that renders sensory neurons hyperexcitable paradoxically renders sympathetic neurons hypoexcitable (because sympathetic neurons lack Nav1.8, which keeps sensory neurons firing at depolarized potentials), providing a molecular basis for the abnormal vasomotor control and the warm, erythematous flares.

sympathetic noradrenergic neuron CL:0011103

Downstream

Episodic Neurovascular Pain Flares Autonomic vasomotor dysregulation contributes to episodic distal vasodilation.

Show evidence (2 references)

PMID:16702558 SUPPORT In Vitro

"Although this mutation depolarizes resting membrane potential in both types of neurons, it renders sensory neurons hyperexcitable and sympathetic neurons hypoexcitable."

A single Nav1.7 erythermalgia mutation has opposite excitability effects in sensory versus sympathetic neurons.

PMID:16702558 SUPPORT In Vitro

"These results provide a molecular basis for the sympathetic dysfunction that has been observed in erythermalgia."

Links Nav1.7 sympathetic hypoexcitability to the vasomotor/autonomic dysfunction seen clinically.

Episodic Neurovascular Pain Flares

The convergence of nociceptor hyperexcitability and autonomic vasomotor dysregulation produces the clinical hallmark: intermittent, often bilateral and symmetric attacks of burning pain with erythema and increased warmth of the distal extremities, provoked by heat or exertion and relieved by cooling.

Show evidence (1 reference)

PMID:20301342 SUPPORT Human Clinical

"SCN9A-EM is characterized by recurrent episodes of bilateral intense, burning pain, and redness, warmth, and occasionally swelling."

GeneReviews describes the episodic bilateral burning-pain-with-redness-and-warmth flare that defines the disorder.

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Pathograph

Use the checkboxes to hide or show graph categories. Hover nodes for evidence and cross-linked metadata.

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Phenotypes

5

Integument 1

Skin ulceration FREQUENT HP:0200042

Show evidence (2 references)

PMID:20301342 SUPPORT Human Clinical

"use of a fan is preferable to prolonged immersion in cold water, which can result in skin maceration, infection, and gangrene"

GeneReviews documents skin maceration, infection, and gangrene from prolonged cold-water immersion.

PMID:30416015 SUPPORT Human Clinical

"Skin damage or other complications of cold immersion for symptomatic relief were common (60%)."

The pediatric systematic review quantifies cold-immersion skin damage at 60% of cases, supporting a FREQUENT band.

Metabolism 1

Distal extremity swelling OCCASIONAL Peripheral edema HP:0012398

Show evidence (1 reference)

PMID:20301342 SUPPORT Human Clinical

"SCN9A-EM is characterized by recurrent episodes of bilateral intense, burning pain, and redness, warmth, and occasionally swelling."

GeneReviews lists occasional swelling among the clinical features of SCN9A-EM.

Constitutional 1

Burning limb pain HP:0009763

Show evidence (1 reference)

PMID:30416015 SUPPORT Human Clinical

"Pain was severe and often refractory to multiple treatments, including nonspecific sodium channel blockers."

The pediatric systematic review documents severe, frequently treatment-refractory limb pain.

Other 2

Erythromelalgia HP:0032147

Temporal: RECURRENT

Show evidence (2 references)

PMID:20301342 SUPPORT Human Clinical

"While the feet are more commonly affected than the hands, in severely affected individuals the legs, arms, face, and/or ears may be involved."

Documents the distal, feet-predominant distribution characteristic of the erythromelalgia phenotype.

PMID:14985375 SUPPORT Human Clinical

"Primary erythermalgia is a rare autosomal dominant disease characterised by intermittent burning pain with redness and heat in the extremities."

Defines the erythromelalgia triad of burning pain, redness, and heat in the extremities.

Warm allodynia HP:0012533

Show evidence (1 reference)

PMID:30416015 SUPPORT Human Clinical

"Quantitative sensory testing revealed marked dynamic warm allodynia."

QST directly demonstrates dynamic warm allodynia in inherited erythermalgia patients.

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Genetic Associations

1

SCN9A (Heterozygous gain-of-function missense variants in SCN9A (Nav1.7) are the principal cause of primary/inherited erythromelalgia. Distinct variants can be genotype-specific for treatment response (e.g., the V400M variant is carbamazepine-responsive).)

Gene: SCN9A hgnc:10597 relationship_type: CAUSATIVE

Show evidence (3 references)

PMID:14985375 SUPPORT Human Clinical

"We then identified two missense mutations in SCN9A in the family (T2573A) and the sporadic patient (T2543C). Our data suggest that mutations in SCN9A cause primary erythermalgia."

Identifies causative SCN9A missense mutations in familial and sporadic primary erythermalgia.

PMID:30416015 SUPPORT Human Clinical

"Twenty-eight publications described erythromelalgia associated with 15 different SCN9A gene variants in 25 children."

A systematic review confirms multiple distinct SCN9A gain-of-function variants across pediatric erythromelalgia cases.

PMID:30416015 SUPPORT Human Clinical

"SCN9A mutations resulting in greater hyperpolarizing shifts in Nav1.7 sodium channels correlated with symptom onset at younger ages"

Establishes a genotype-phenotype correlation linking the magnitude of the Nav1.7 hyperpolarizing shift to earlier symptom onset.

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Medical Actions

2

Cooling and trigger avoidance

Action: supportive care Ontology label: Supportive Care NCIT:C15747

Non-pharmacological mainstay: cooling of the affected extremities (fan cooling preferred over prolonged cold-water immersion to avoid tissue injury) and avoidance of known triggers (warmth, standing, alcohol, spicy foods) relieve and prevent acute attacks.

Show evidence (2 references)

PMID:20301342 SUPPORT Human Clinical

"Cooling the extremities reduces pain; note that use of a fan is preferable to prolonged immersion in cold water, which can result in skin maceration, infection, and gangrene."

GeneReviews management guidance for cooling with a caution against prolonged cold-water immersion.

PMID:20301342 SUPPORT Human Clinical

"Triggers including warmth, standing, alcohol, and spicy foods (SCN9A-EM)"

GeneReviews lists the agents/circumstances to avoid that provoke SCN9A-EM attacks.

Sodium channel blocker therapy

Action: Pharmacotherapy NCIT:C15986

Agent: mexiletine CHEBI:6916 carbamazepine CHEBI:3387

Oral/systemic sodium-channel-blocking agents (carbamazepine, oral mexiletine, lidocaine infusion) are used to reduce Nav1.7-driven hyperexcitability; responses are variable and can be genotype-dependent (e.g., the V400M variant is carbamazepine-responsive). Unlike secondary/MPN-associated erythromelalgia, primary erythermalgia typically responds poorly to aspirin.

Mechanism Target:

INHIBITS SCN9A Gain-of-Function Variant (Nav1.7) — Sodium-channel blockers act on the gain-of-function Nav1.7 channel itself; carbamazepine normalizes the shifted activation/inactivation of the mutant channel (genotype-dependent), reducing the upstream driver of nociceptor hyperexcitability.

Show evidence (1 reference)

PMID:19557861 SUPPORT In Vitro

"We show that CBZ, at concentrations in the human therapeutic range, normalizes the voltage dependence of activation and inactivation of this inherited erythromelalgia mutation in Na(v)1.7 but does not affect these parameters in wild-type Na(v)1.7."

Demonstrates the drug acts directly on the mutant Nav1.7 channel node.

Show evidence (3 references)

PMID:20301342 SUPPORT Human Clinical

"Medications to consider are nonselective sodium channel blockers (e.g., carbamazepine, lidocaine infusion, or oral mexiletine)."

GeneReviews lists nonselective sodium channel blockers as the pharmacologic options for SCN9A erythromelalgia.

PMID:20301342 SUPPORT Human Clinical

"Routine monitoring for side effects of medications used in treatment (such as Stevens-Johnson syndrome, liver toxicity, neutropenia seen with carbamazepine)."

GeneReviews surveillance guidance for carbamazepine drug toxicity (SJS, hepatotoxicity, neutropenia).

PMID:16673274 SUPPORT Human Clinical

"Inhibition of platelet cyclooxygenase-1 by aspirin is followed by relief of microvascular disturbances"

Aspirin relieves the platelet-mediated microvascular disturbance of secondary (myeloproliferative-neoplasm) erythromelalgia — the responsiveness that clinically distinguishes it from aspirin-refractory primary erythermalgia.

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Diagnosis

1

Molecular genetic testing

Diagnosis is confirmed by identifying a heterozygous pathogenic gain-of-function variant in SCN9A by molecular genetic testing, in a proband with the characteristic clinical phenotype.

Show evidence (1 reference)

PMID:20301342 SUPPORT Human Clinical

"The diagnosis of SCN9A-NPS is established in a proband with a heterozygous pathogenic variant in SCN9A identified by molecular genetic testing."

GeneReviews establishes molecular confirmation of a heterozygous SCN9A variant as diagnostic.

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Prevalence

1

Olmsted County, Minnesota, USA (clinically-defined primary erythromelalgia)

Annual Incidence 1.1 per 100,000 (0.7–1.5) Rare

Population-based age/sex-adjusted incidence of clinically-defined primary erythromelalgia (not molecularly confirmed SCN9A) from the Rochester Epidemiology Project; overall erythromelalgia incidence 1.3/100,000/yr, with female predominance (2.0 vs 0.6 per 100,000). The cohort explicitly identified no hereditary cases, so 1.1/100,000/yr is an upper bound on, not an estimate of, molecularly confirmed SCN9A primary erythermalgia incidence.

Show evidence (2 references)

PMID:18713229 SUPPORT Human Clinical

"The incidence of primary and secondary erythromelalgia was 1.1 (0.7-1.5) and 0.2 (0.02-0.4) per 100,000 people per year, respectively."

Population-based incidence of clinically-defined primary erythromelalgia (1.1/100,000/yr).

PMID:18713229 SUPPORT Human Clinical

"No cases of hereditary erythromelalgia were identified."

The cohort contained no hereditary cases, so the primary-EM rate is an upper bound on SCN9A-EM incidence rather than a direct estimate.

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Differential Diagnoses

1

Conditions with similar clinical presentations that must be differentiated from Primary Erythermalgia:

Secondary erythromelalgia MONDO:0035149

Overlapping Features The acquired sibling entity, curated separately as `Secondary_Erythromelalgia`. Most characteristically it arises in a myeloproliferative neoplasm (essential thrombocythemia or polycythemia vera), where the mechanism is platelet-mediated arteriolar inflammation and thrombotic occlusion of acral arterioles rather than the Nav1.7 nociceptor channelopathy modeled here. Distinguishing features: later onset, frequently asymmetric or unilateral attacks, an accompanying thrombocytosis, and — the key discriminator — dramatic relief with aspirin, which primary erythermalgia characteristically lacks.

Show evidence (4 references)

PMID:8203771 SUPPORT Human Clinical

"the authors discern three distinct types of red, congested, and burning extremities that need to be distinguished for effective treatment according to their etiology: erythromelalgia in thrombocythemia, primary erythermalgia, and secondary erythermalgia"

Establishes primary erythermalgia and the acquired forms as distinct clinicopathologic entities that must be separated for effective treatment.

PMID:40428878 SUPPORT Human Clinical

"Secondary EM, in contrast, is not associated with known genetic mutations and typically presents later in life."

Supports the later age at onset of the secondary form relative to the primary form.

PMID:40428878 SUPPORT Human Clinical

"Secondary EM tends to be more variable in presentation, may be unilateral or localized, and treatment usually targets the underlying condition."

Supports the asymmetric/unilateral and localized presentation that distinguishes secondary disease.

+ 1 more reference

{ }

Source YAML

click to show

name: Primary Erythermalgia
creation_date: "2026-07-31T00:00:00Z"
description: >-
  Primary erythermalgia (primary/inherited erythromelalgia, IEM) is an autosomal
  dominant neuropathic pain channelopathy caused by gain-of-function variants in
  SCN9A, which encodes the voltage-gated sodium channel Nav1.7 preferentially
  expressed in dorsal root ganglion nociceptors and sympathetic neurons. Patients
  have intermittent, often bilateral and symmetric attacks of red, warm, burning
  pain in the distal extremities (feet more than hands) that are triggered by
  warmth or exercise and dramatically relieved by cooling. Onset ranges from
  infancy and early childhood to adulthood, with earlier onset associated with
  SCN9A variants that cause larger hyperpolarizing shifts in Nav1.7. It is distinguished from
  secondary erythromelalgia (e.g., myeloproliferative-neoplasm-associated) by its
  genetic, neuronal-sodium-channel basis and its characteristic poor response to
  aspirin. Within the SCN9A neuropathic pain syndromes it is distinct from the
  sibling entities paroxysmal extreme pain disorder (PEPD) and SCN9A small-fiber
  neuropathy.
category: Mendelian
parents:
- Channelopathy
- Peripheral Neuropathy
- hereditary disease
disease_term:
  preferred_term: primary erythermalgia
  term:
    id: MONDO:0007571
    label: primary erythermalgia
references:
- reference: PMID:20301342
  title: "SCN9A Neuropathic Pain Syndromes."
  tags:
  - GeneReviews
inheritance:
- name: Autosomal dominant
  description: >-
    Primary erythermalgia is inherited in an autosomal dominant pattern; most
    familial cases segregate a heterozygous gain-of-function SCN9A variant, and
    de novo variants account for many sporadic early-onset cases.
  inheritance_term:
    preferred_term: Autosomal dominant inheritance
    term:
      id: HP:0000006
      label: Autosomal dominant inheritance
  evidence:
  - reference: PMID:20301342
    reference_title: SCN9A Neuropathic Pain Syndromes.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "SCN9A neuropathic pain syndromes are inherited in an autosomal dominant manner. Each child of an individual with an NPS-causing variant in SCN9A has a 50% chance of inheriting the variant."
    explanation: GeneReviews establishes autosomal dominant inheritance with 50% transmission risk.
pathophysiology:
- name: SCN9A Gain-of-Function Variant (Nav1.7)
  biological_scale: MOLECULAR
  conforms_to: "nociceptor_sodium_channel_excitability#Enhanced or Persistent Nociceptor Sodium Current"
  description: >-
    Heterozygous missense gain-of-function variants in SCN9A alter the biophysical
    properties of the Nav1.7 voltage-gated sodium channel. Characteristic changes
    include a hyperpolarizing shift in the voltage-dependence of activation, slowed
    deactivation, and enhanced ramp currents, which lower the threshold for channel
    opening and increase the channel's response to small, slow depolarizations.
  molecular_functions:
  - preferred_term: voltage-gated sodium channel activity
    term:
      id: GO:0005248
      label: voltage-gated sodium channel activity
    modifier: INCREASED
  genes:
  - preferred_term: SCN9A
    term:
      id: hgnc:10597
      label: SCN9A
  evidence:
  - reference: PMID:14985375
    reference_title: Mutations in SCN9A, encoding a sodium channel alpha subunit, in patients with primary erythermalgia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Our data suggest that mutations in SCN9A cause primary erythermalgia. SCN9A, encoding a voltage-gated sodium channel alpha subunit predominantly expressed in sensory and sympathetic neurones, may play an important role in nociception and vasomotor regulation."
    explanation: The gene-discovery study links SCN9A missense mutations to primary erythermalgia.
  - reference: PMID:15385606
    reference_title: Electrophysiological properties of mutant Nav1.7 sodium channels in a painful inherited neuropathy.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Here we show that these mutations in Na(v)1.7 produce a hyperpolarizing shift in activation and slow deactivation. We also show that these mutations cause an increase in amplitude of the current produced by Na(v)1.7 in response to slow, small depolarizations."
    explanation: Patch-clamp characterization demonstrates the gain-of-function biophysical shifts of erythermalgia Nav1.7 mutants.
  - reference: PMID:16988069
    reference_title: 'Inherited erythermalgia: limb pain from an S4 charge-neutral Na channelopathy.'
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "The F216S mutation hyperpolarizes the voltage dependence of activation by 11 mV, accelerates activation, slows deactivation, and enhances the response to slow, small depolarizations."
    explanation: A second erythermalgia mutation (in the S4 segment) reproduces the same gain-of-function gating changes.
  - reference: PMID:25995458
    reference_title: 'Novel SCN9A mutations underlying extreme pain phenotypes: unexpected electrophysiological and clinical phenotype correlations.'
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "IEM is usually caused by enhanced NaV1.7 channel activation, whereas mutations that alter steady-state fast inactivation often lead to PEPD."
    explanation: >-
      Anchors the mechanistic distinction underpinning the IEM/PEPD split modeled here — IEM
      arises from enhanced activation, distinguishing it from the fast-inactivation defects of PEPD.
  downstream:
  - target: Nociceptor Hyperexcitability
    description: Altered Nav1.7 gating lowers nociceptor firing threshold and drives repetitive firing.
  - target: Sympathetic Neuron Dysregulation
    description: The same gain-of-function variant alters excitability of sympathetic ganglion neurons that also express Nav1.7.
- name: Nociceptor Hyperexcitability
  biological_scale: CELLULAR
  conforms_to: "nociceptor_sodium_channel_excitability#Nociceptor Hyperexcitability and Ectopic Firing"
  description: >-
    Because Nav1.7 sets the gain of dorsal root ganglion nociceptors, the
    gain-of-function channel produces membrane hyperexcitability: lowered firing
    threshold, increased spontaneous and evoked action-potential firing, and
    amplified responses to physiological stimuli, generating ectopic pain signaling.
  cell_types:
  - preferred_term: dorsal root ganglion sensory neuron
    term:
      id: CL:1001451
      label: sensory neuron of dorsal root ganglion
  biological_processes:
  - preferred_term: neuronal action potential
    term:
      id: GO:0019228
      label: neuronal action potential
    modifier: INCREASED
  evidence:
  - reference: PMID:15385606
    reference_title: Electrophysiological properties of mutant Nav1.7 sodium channels in a painful inherited neuropathy.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "these physiological changes, which confer hyperexcitability on peripheral sensory and sympathetic neurons, contribute to symptom production in hereditary erythermalgia"
    explanation: >-
      The gain-of-function gating changes confer hyperexcitability on sensory neurons, driving
      symptoms. Note the 2004 characterization of sympathetic neurons as hyperexcitable was later
      refined to hypoexcitability (PMID:16702558; see the Sympathetic Neuron Dysregulation node).
  - reference: PMID:16988069
    reference_title: 'Inherited erythermalgia: limb pain from an S4 charge-neutral Na channelopathy.'
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "These changes should increase excitability of nociceptive dorsal root ganglion neurons in which the mutant channel is present, thus contributing to pain."
    explanation: Predicts increased excitability of nociceptive DRG neurons carrying the mutant channel.
  - reference: PMID:40279376
    reference_title: Correction of sodium channel mutations in sensory neurons reverses aberrant properties.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "using CRISPR/Cas9, we corrected this mutation, which reduced the underlying hyperexcitability, providing a path for personalized medicine to treat these disorders, and we introduced the mutation into control induced pluripotent stem cells, which generated hyperexcitability, providing causality"
    explanation: >-
      Bidirectional CRISPR editing in human iPSC-derived sensory neurons establishes a direct causal
      link between the Nav1.7 gain-of-function mutation and nociceptor hyperexcitability.
  downstream:
  - target: Episodic Neurovascular Pain Flares
    description: Hyperexcitable nociceptor output drives painful attacks with warmth-triggered vasodilation.
- name: Sympathetic Neuron Dysregulation
  biological_scale: CELLULAR
  description: >-
    Nav1.7 is also expressed in sympathetic ganglion neurons. The same
    gain-of-function variant that renders sensory neurons hyperexcitable
    paradoxically renders sympathetic neurons hypoexcitable (because sympathetic
    neurons lack Nav1.8, which keeps sensory neurons firing at depolarized
    potentials), providing a molecular basis for the abnormal vasomotor control and
    the warm, erythematous flares.
  cell_types:
  - preferred_term: sympathetic noradrenergic neuron
    term:
      id: CL:0011103
      label: sympathetic neuron
  evidence:
  - reference: PMID:16702558
    reference_title: A single sodium channel mutation produces hyper- or hypoexcitability in different types of neurons.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "Although this mutation depolarizes resting membrane potential in both types of neurons, it renders sensory neurons hyperexcitable and sympathetic neurons hypoexcitable."
    explanation: A single Nav1.7 erythermalgia mutation has opposite excitability effects in sensory versus sympathetic neurons.
  - reference: PMID:16702558
    reference_title: A single sodium channel mutation produces hyper- or hypoexcitability in different types of neurons.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "These results provide a molecular basis for the sympathetic dysfunction that has been observed in erythermalgia."
    explanation: Links Nav1.7 sympathetic hypoexcitability to the vasomotor/autonomic dysfunction seen clinically.
  downstream:
  - target: Episodic Neurovascular Pain Flares
    description: Autonomic vasomotor dysregulation contributes to episodic distal vasodilation.
- name: Episodic Neurovascular Pain Flares
  biological_scale: ORGANISM
  description: >-
    The convergence of nociceptor hyperexcitability and autonomic vasomotor
    dysregulation produces the clinical hallmark: intermittent, often bilateral
    and symmetric attacks of burning pain with erythema and increased warmth of the
    distal extremities, provoked by heat or exertion and relieved by cooling.
  evidence:
  - reference: PMID:20301342
    reference_title: SCN9A Neuropathic Pain Syndromes.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "SCN9A-EM is characterized by recurrent episodes of bilateral intense, burning pain, and redness, warmth, and occasionally swelling."
    explanation: GeneReviews describes the episodic bilateral burning-pain-with-redness-and-warmth flare that defines the disorder.
phenotypes:
- name: Erythromelalgia
  category: Clinical
  description: >-
    Intermittent attacks of red, hot, burning painful distal extremities — the
    defining clinical phenotype, typically bilateral and symmetric, worse in the
    feet, triggered by warmth or exercise and relieved by cooling.
  phenotype_term:
    preferred_term: Erythromelalgia
    term:
      id: HP:0032147
      label: Erythromelalgia
    temporality: RECURRENT
  evidence:
  - reference: PMID:20301342
    reference_title: SCN9A Neuropathic Pain Syndromes.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "While the feet are more commonly affected than the hands, in severely affected individuals the legs, arms, face, and/or ears may be involved."
    explanation: Documents the distal, feet-predominant distribution characteristic of the erythromelalgia phenotype.
  - reference: PMID:14985375
    reference_title: Mutations in SCN9A, encoding a sodium channel alpha subunit, in patients with primary erythermalgia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Primary erythermalgia is a rare autosomal dominant disease characterised by intermittent burning pain with redness and heat in the extremities."
    explanation: Defines the erythromelalgia triad of burning pain, redness, and heat in the extremities.
- name: Burning limb pain
  category: Clinical
  description: >-
    Severe burning pain of the distal limbs during flares; often the most
    disabling feature and a driver of compulsive cooling behavior.
  phenotype_term:
    preferred_term: Limb pain
    term:
      id: HP:0009763
      label: Limb pain
  evidence:
  - reference: PMID:30416015
    reference_title: 'Pediatric Erythromelalgia and SCN9A Mutations: Systematic Review and Single-Center Case Series.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Pain was severe and often refractory to multiple treatments, including nonspecific sodium channel blockers."
    explanation: The pediatric systematic review documents severe, frequently treatment-refractory limb pain.
- name: Distal extremity swelling
  category: Clinical
  frequency: OCCASIONAL
  description: >-
    Some flares are accompanied by swelling (edema) of the affected distal extremity,
    in addition to the cardinal redness, warmth, and burning pain.
  phenotype_term:
    preferred_term: Peripheral edema
    term:
      id: HP:0012398
      label: Peripheral edema
  evidence:
  - reference: PMID:20301342
    reference_title: SCN9A Neuropathic Pain Syndromes.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "SCN9A-EM is characterized by recurrent episodes of bilateral intense, burning pain, and redness, warmth, and occasionally swelling."
    explanation: GeneReviews lists occasional swelling among the clinical features of SCN9A-EM.
- name: Warm allodynia
  category: Clinical
  description: >-
    Quantitative sensory testing in inherited erythermalgia demonstrates marked dynamic warm
    allodynia — pain evoked by an innocuous warm stimulus — reflecting nociceptor sensitization.
  phenotype_term:
    preferred_term: Allodynia
    term:
      id: HP:0012533
      label: Allodynia
  evidence:
  - reference: PMID:30416015
    reference_title: 'Pediatric Erythromelalgia and SCN9A Mutations: Systematic Review and Single-Center Case Series.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Quantitative sensory testing revealed marked dynamic warm allodynia."
    explanation: QST directly demonstrates dynamic warm allodynia in inherited erythermalgia patients.
- name: Skin ulceration
  category: Clinical
  description: >-
    Prolonged cold-water immersion used to relieve pain, and severe attacks, can
    cause skin maceration, erosion, ulceration, infection, and even gangrene of the
    extremities — the basis for advising fan cooling over prolonged immersion.
  frequency: FREQUENT
  phenotype_term:
    preferred_term: Skin ulcer
    term:
      id: HP:0200042
      label: Skin ulcer
  evidence:
  - reference: PMID:20301342
    reference_title: SCN9A Neuropathic Pain Syndromes.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "use of a fan is preferable to prolonged immersion in cold water, which can result in skin maceration, infection, and gangrene"
    explanation: GeneReviews documents skin maceration, infection, and gangrene from prolonged cold-water immersion.
  - reference: PMID:30416015
    reference_title: 'Pediatric Erythromelalgia and SCN9A Mutations: Systematic Review and Single-Center Case Series.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Skin damage or other complications of cold immersion for symptomatic relief were common (60%)."
    explanation: The pediatric systematic review quantifies cold-immersion skin damage at 60% of cases, supporting a FREQUENT band.
genetic:
- name: SCN9A
  gene_term:
    preferred_term: SCN9A
    term:
      id: hgnc:10597
      label: SCN9A
  relationship_type: CAUSATIVE
  association: >-
    Heterozygous gain-of-function missense variants in SCN9A (Nav1.7) are the
    principal cause of primary/inherited erythromelalgia. Distinct variants can be
    genotype-specific for treatment response (e.g., the V400M variant is
    carbamazepine-responsive).
  evidence:
  - reference: PMID:14985375
    reference_title: Mutations in SCN9A, encoding a sodium channel alpha subunit, in patients with primary erythermalgia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "We then identified two missense mutations in SCN9A in the family (T2573A) and the sporadic patient (T2543C). Our data suggest that mutations in SCN9A cause primary erythermalgia."
    explanation: Identifies causative SCN9A missense mutations in familial and sporadic primary erythermalgia.
  - reference: PMID:30416015
    reference_title: 'Pediatric Erythromelalgia and SCN9A Mutations: Systematic Review and Single-Center Case Series.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Twenty-eight publications described erythromelalgia associated with 15 different SCN9A gene variants in 25 children."
    explanation: A systematic review confirms multiple distinct SCN9A gain-of-function variants across pediatric erythromelalgia cases.
  - reference: PMID:30416015
    reference_title: 'Pediatric Erythromelalgia and SCN9A Mutations: Systematic Review and Single-Center Case Series.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "SCN9A mutations resulting in greater hyperpolarizing shifts in Nav1.7 sodium channels correlated with symptom onset at younger ages"
    explanation: Establishes a genotype-phenotype correlation linking the magnitude of the Nav1.7 hyperpolarizing shift to earlier symptom onset.
prevalence:
- population: Olmsted County, Minnesota, USA (clinically-defined primary erythromelalgia)
  measure_type: ANNUAL_INCIDENCE
  prevalence_class: RARE
  rate_per_100000: 1.1
  rate_low: 0.7
  rate_high: 1.5
  notes: >-
    Population-based age/sex-adjusted incidence of clinically-defined primary erythromelalgia
    (not molecularly confirmed SCN9A) from the Rochester Epidemiology Project; overall
    erythromelalgia incidence 1.3/100,000/yr, with female predominance (2.0 vs 0.6 per 100,000).
    The cohort explicitly identified no hereditary cases, so 1.1/100,000/yr is an upper bound on,
    not an estimate of, molecularly confirmed SCN9A primary erythermalgia incidence.
  evidence:
  - reference: PMID:18713229
    reference_title: 'Incidence of erythromelalgia: a population-based study in Olmsted County, Minnesota.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The incidence of primary and secondary erythromelalgia was 1.1 (0.7-1.5) and 0.2 (0.02-0.4) per 100,000 people per year, respectively."
    explanation: Population-based incidence of clinically-defined primary erythromelalgia (1.1/100,000/yr).
  - reference: PMID:18713229
    reference_title: 'Incidence of erythromelalgia: a population-based study in Olmsted County, Minnesota.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "No cases of hereditary erythromelalgia were identified."
    explanation: The cohort contained no hereditary cases, so the primary-EM rate is an upper bound on SCN9A-EM incidence rather than a direct estimate.
diagnosis:
- name: Molecular genetic testing
  description: >-
    Diagnosis is confirmed by identifying a heterozygous pathogenic gain-of-function variant in
    SCN9A by molecular genetic testing, in a proband with the characteristic clinical phenotype.
  evidence:
  - reference: PMID:20301342
    reference_title: SCN9A Neuropathic Pain Syndromes.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "The diagnosis of SCN9A-NPS is established in a proband with a heterozygous pathogenic variant in SCN9A identified by molecular genetic testing."
    explanation: GeneReviews establishes molecular confirmation of a heterozygous SCN9A variant as diagnostic.
treatments:
- name: Cooling and trigger avoidance
  description: >-
    Non-pharmacological mainstay: cooling of the affected extremities (fan cooling
    preferred over prolonged cold-water immersion to avoid tissue injury) and
    avoidance of known triggers (warmth, standing, alcohol, spicy foods) relieve
    and prevent acute attacks.
  treatment_term:
    preferred_term: supportive care
    term:
      id: NCIT:C15747
      label: Supportive Care
  therapeutic_modality: BEHAVIORAL
  evidence:
  - reference: PMID:20301342
    reference_title: SCN9A Neuropathic Pain Syndromes.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Cooling the extremities reduces pain; note that use of a fan is preferable to prolonged immersion in cold water, which can result in skin maceration, infection, and gangrene."
    explanation: GeneReviews management guidance for cooling with a caution against prolonged cold-water immersion.
  - reference: PMID:20301342
    reference_title: SCN9A Neuropathic Pain Syndromes.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Triggers including warmth, standing, alcohol, and spicy foods (SCN9A-EM)"
    explanation: GeneReviews lists the agents/circumstances to avoid that provoke SCN9A-EM attacks.
- name: Sodium channel blocker therapy
  description: >-
    Oral/systemic sodium-channel-blocking agents (carbamazepine, oral mexiletine,
    lidocaine infusion) are used to reduce Nav1.7-driven hyperexcitability;
    responses are variable and can be genotype-dependent (e.g., the V400M variant
    is carbamazepine-responsive). Unlike secondary/MPN-associated erythromelalgia,
    primary erythermalgia typically responds poorly to aspirin.
  treatment_term:
    preferred_term: Pharmacotherapy
    term:
      id: NCIT:C15986
      label: Pharmacotherapy
    therapeutic_agent:
    - preferred_term: mexiletine
      term:
        id: CHEBI:6916
        label: mexiletine
    - preferred_term: carbamazepine
      term:
        id: CHEBI:3387
        label: carbamazepine
  therapeutic_modality: SMALL_MOLECULE
  target_mechanisms:
  - target: SCN9A Gain-of-Function Variant (Nav1.7)
    treatment_effect: INHIBITS
    description: >-
      Sodium-channel blockers act on the gain-of-function Nav1.7 channel itself; carbamazepine
      normalizes the shifted activation/inactivation of the mutant channel (genotype-dependent),
      reducing the upstream driver of nociceptor hyperexcitability.
    evidence:
    - reference: PMID:19557861
      reference_title: A novel Nav1.7 mutation producing carbamazepine-responsive erythromelalgia.
      supports: SUPPORT
      evidence_source: IN_VITRO
      snippet: "We show that CBZ, at concentrations in the human therapeutic range, normalizes the voltage dependence of activation and inactivation of this inherited erythromelalgia mutation in Na(v)1.7 but does not affect these parameters in wild-type Na(v)1.7."
      explanation: Demonstrates the drug acts directly on the mutant Nav1.7 channel node.
  evidence:
  - reference: PMID:20301342
    reference_title: SCN9A Neuropathic Pain Syndromes.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Medications to consider are nonselective sodium channel blockers (e.g., carbamazepine, lidocaine infusion, or oral mexiletine)."
    explanation: GeneReviews lists nonselective sodium channel blockers as the pharmacologic options for SCN9A erythromelalgia.
  - reference: PMID:20301342
    reference_title: SCN9A Neuropathic Pain Syndromes.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Routine monitoring for side effects of medications used in treatment (such as Stevens-Johnson syndrome, liver toxicity, neutropenia seen with carbamazepine)."
    explanation: GeneReviews surveillance guidance for carbamazepine drug toxicity (SJS, hepatotoxicity, neutropenia).
  - reference: PMID:16673274
    reference_title: 'Clinical and laboratory features, pathobiology of platelet-mediated thrombosis and bleeding complications, and the molecular etiology of essential thrombocythemia and polycythemia vera: therapeutic implications.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Inhibition of platelet cyclooxygenase-1 by aspirin is followed by relief of microvascular disturbances"
    explanation: >-
      Aspirin relieves the platelet-mediated microvascular disturbance of secondary (myeloproliferative-neoplasm)
      erythromelalgia — the responsiveness that clinically distinguishes it from aspirin-refractory primary erythermalgia.
differential_diagnoses:
- name: Secondary erythromelalgia
  disease_term:
    preferred_term: secondary erythromelalgia
    term:
      id: MONDO:0035149
      label: secondary erythromelalgia
  description: >-
    The acquired sibling entity, curated separately as `Secondary_Erythromelalgia`.
    Most characteristically it arises in a myeloproliferative neoplasm (essential
    thrombocythemia or polycythemia vera), where the mechanism is platelet-mediated
    arteriolar inflammation and thrombotic occlusion of acral arterioles rather than
    the Nav1.7 nociceptor channelopathy modeled here. Distinguishing features:
    later onset, frequently asymmetric or unilateral attacks, an accompanying
    thrombocytosis, and — the key discriminator — dramatic relief with aspirin,
    which primary erythermalgia characteristically lacks.
  evidence:
  - reference: PMID:8203771
    reference_title: 'Thrombocythemic erythromelalgia, primary erythermalgia, and secondary erythermalgia: three distinct clinicopathologic entities.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "the authors discern three distinct types of red, congested, and burning extremities that need to be distinguished for effective treatment according to their etiology: erythromelalgia in thrombocythemia, primary erythermalgia, and secondary erythermalgia"
    explanation: >-
      Establishes primary erythermalgia and the acquired forms as distinct clinicopathologic
      entities that must be separated for effective treatment.
  - reference: PMID:40428878
    reference_title: 'Comparative Efficacy and Tolerability of Treatments for Erythromelalgia: A Systematic Review.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Secondary EM, in contrast, is not associated with known genetic mutations and typically presents later in life."
    explanation: Supports the later age at onset of the secondary form relative to the primary form.
  - reference: PMID:40428878
    reference_title: 'Comparative Efficacy and Tolerability of Treatments for Erythromelalgia: A Systematic Review.'
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Secondary EM tends to be more variable in presentation, may be unilateral or localized, and treatment usually targets the underlying condition."
    explanation: Supports the asymmetric/unilateral and localized presentation that distinguishes secondary disease.
  - reference: PMID:3977194
    reference_title: Erythromelalgia caused by platelet-mediated arteriolar inflammation and thrombosis in thrombocythemia.
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: "Skin punch biopsy samples taken from the affected areas showed typical arteriolar inflammation, fibromuscular intima proliferation, and thrombotic occlusions."
    explanation: >-
      Documents the platelet-mediated arteriolar histopathology of the myeloproliferative form.
      This paper studied thrombocythemia patients only and makes no claim about primary
      erythermalgia; the contrast between the two entities rests on PMID:8203771 above.
discussions:
- discussion_id: iem_rodent_knockin_pain_mismatch
  prompt: >-
    Do rodent Nav1.7 gain-of-function knock-in models faithfully recapitulate the human
    inherited-erythromelalgia pain phenotype, or only its cellular (DRG hyperexcitability) substrate?
  kind: HUMAN_MODEL_MISMATCH
  status: OPEN
  attaches_to:
  - pathophysiology#Nociceptor Hyperexcitability
  rationale: >-
    Human iPSC-derived sensory neurons carrying Nav1.7 gain-of-function mutations reproduce
    hyperexcitability, and CRISPR correction reverses it while introducing the mutation confers it
    (PMID:40279376), validating the human cellular model. By contrast, two independent knock-in
    mouse lines carrying a Nav1.7 gain-of-function variant (I228M — a variant found in idiopathic
    small-fiber neuropathy, a sibling Nav1.7 channelopathy rather than an IEM-specific allele)
    reproduce DRG neuron hyperexcitability but do NOT display mechanical/thermal hyperalgesia or
    intraepidermal nerve fiber loss (PMID:33323889). This dissociation — cellular hyperexcitability
    without an overt behavioral pain phenotype in mice — is a translational caveat for modeling
    Nav1.7 gain-of-function pain disorders, including IEM.
  evidence:
  - reference: PMID:33323889
    reference_title: Two independent mouse lines carrying the Nav1.7 I228M gain-of-function variant display dorsal root ganglion neuron hyperexcitability but a minimal pain phenotype.
    supports: SUPPORT
    evidence_source: MODEL_ORGANISM
    snippet: "although these 2 Nav1.7 I228M knock-in mouse lines recapitulate the DRG neuron hyperexcitability associated with gain-of-function mutations in Nav1.7, they do not recapitulate the pain or neuropathy phenotypes seen in patients"
    explanation: Rodent Nav1.7 gain-of-function knock-ins reproduce DRG hyperexcitability but not the overt pain/neuropathy phenotype seen in patients.
  - reference: PMID:40279376
    reference_title: Correction of sodium channel mutations in sensory neurons reverses aberrant properties.
    supports: SUPPORT
    evidence_source: IN_VITRO
    snippet: "robust, scalable and relevant model to study the effects of gain-of-function mutations in ion channels in pain-related disorders"
    explanation: Human iPSC-derived sensory neurons are validated as a relevant model of Nav1.7 gain-of-function pain disorders.

📚

References & Deep Research

References

1

PMID:20301342

SCN9A Neuropathic Pain Syndromes.

No top-level findings curated for this source.

Deep Research

1

OpenScientist ▸

Key Findings

openscientist-autonomous 26 citations 2026-07-31T02:05:37.324681

Key Findings

Finding 1 — Primary erythermalgia is caused by gain-of-function mutations in SCN9A (Nav1.7)

The foundational discovery, from linkage and mutation analysis, mapped primary erythermalgia to chromosome 2q (a 7.94 cM interval; LOD 2.11 for markers D2S2370/D2S2330) and identified missense mutations in SCN9A (T2573A segregating in a family; T2543C in a sporadic patient). SCN9A encodes the alpha subunit of the voltage-gated sodium channel Nav1.7, which is predominantly expressed in sensory (DRG) and sympathetic neurons — an expression pattern that neatly explains the disease's combined nociceptive (burning pain) and vasomotor (erythema, warmth) features. The disease is inherited in an autosomal-dominant fashion.

As the original report states: "Primary erythermalgia is a rare autosomal dominant disease characterised by intermittent burning pain with redness and heat in the extremities" and "Our data suggest that mutations in SCN9A cause primary erythermalgia. SCN9A, encoding a voltage-gated sodium channel alpha subunit predominantly expressed in sensory and sympathetic neurones, may play an important role in nociception and vasomotor regulation" (PMID: 14985375).

Ontology anchors: Gene HGNC:10597 (SCN9A); protein UniProt Q15858 (Nav1.7); MONDO concept "erythromelalgia / primary erythromelalgia"; OMIM #133020 (Erythermalgia, primary / Erythromelalgia, hereditary).

Finding 2 — Biophysical mechanism: hyperpolarized activation, slowed deactivation, enhanced ramp response → DRG hyperexcitability

IEM mutations produce a characteristic gain-of-function biophysical signature: they hyperpolarize (leftward-shift) the voltage dependence of activation, slow deactivation, and enhance the ramp response to slow depolarizations. Because Nav1.7 functions as a "threshold channel" in DRG, trigeminal, and sympathetic nociceptors — amplifying small generator potentials toward the action-potential threshold — these changes lower the firing threshold and produce neuronal hyperexcitability.

Crucially, causality has been demonstrated in a human cellular system. Patient induced-pluripotent-stem-cell (iPSC)-derived sensory neurons carrying a Nav1.7 gain-of-function mutation (A1632G) exhibit hyperexcitability; CRISPR/Cas9 correction of the mutation reduces the hyperexcitability, and, conversely, introducing the mutation into control iPSC neurons generates hyperexcitability. This bidirectional experiment establishes a direct causal link between the mutation and the cellular pain phenotype: "using CRISPR/Cas9, we corrected this mutation, which reduced the underlying hyperexcitability, providing a path for personalized medicine to treat these disorders, and we introduced the mutation into control induced pluripotent stem cells, which generated hyperexcitability, providing causality" (PMID: 40279376).

The biophysical definition is summarized concisely: IEM "is characterized clinically by burning pain and redness that is usually focused on the distal extremities, precipitated by mild warmth and relieved by cooling, and is caused by mutations that hyperpolarize activation, slow deactivation, and enhance the channel ramp response" (PMID: 22136189).

Ontology anchors: GO:0086010 membrane depolarization during action potential; GO:0001518 voltage-gated sodium channel complex; GO:0019228 neuronal action potential; GO:0035725 sodium ion transmembrane transport.

Finding 3 — Epidemiology: erythromelalgia incidence ~0.36–1.3 per 100,000/year with female predominance

Population-based data anchor the disease's rarity. In Olmsted County, Minnesota, the overall age/sex-adjusted incidence was 1.3 per 100,000/year (95% CI 0.8–1.7), split into primary EM at 1.1 and secondary EM at 0.2 per 100,000/year, with higher rates in women (2.0/100,000) than men (0.6/100,000): "The overall age- and sex-adjusted incidence rate ... was 1.3 (0.8-1.7) per 100,000 people per year. The incidence of primary and secondary erythromelalgia was 1.1 (0.7-1.5) and 0.2 (0.02-0.4) per 100,000 people per year, respectively" (PMID: 18713229).

An independent southern Sweden study estimated incidence at 0.36 per 100,000/year, with 70% female patients and a mean diagnostic delay of 4.5 years: "Gender and age adjusted incidence of EM for our region was calculated to be 0.36 per 100 000" (PMID: 22247059). The inherited form shows early onset: "Sex and age distributions among patients with IEM show a predominance of cases with clinical onset before the age of 10 years, whereas sex differences are not pronounced" (PMID: 41190974). Female predominance in overall EM is corroborated by broader dermatologic epidemiology from the same Rochester Epidemiology Project (PMID: 27009931).

Study	Population	Incidence (/100,000/yr)	Female %	Notes
Olmsted County, MN (PMID: 18713229)	Population-based	1.3 overall; 1.1 primary; 0.2 secondary	Women 2.0 vs men 0.6	Age/sex-adjusted
Southern Sweden (PMID: 22247059)	Single-center regional	0.36	70%	Mean diagnostic delay 4.5 yr
China/worldwide review (PMID: 41190974)	IEM case review	—	Not pronounced	Onset predominantly <10 yr

Finding 4 — Treatment: genotype-specific sodium-channel blockers plus neuropathic agents; cooling relief

Primary EM management centers on sodium-channel blockers (mexiletine; intravenous lidocaine infusions) combined with neuropathic pain agents (gabapentin, amitriptyline), plus behavioral cooling. Efficacy is variable, partial, and often transient. Importantly, pharmacotherapy response is variant-specific: p.L858F/H responds to mexiletine, whereas V400M, S241T, and I234T respond to carbamazepine — evidence for genotype-guided therapy.

Supporting quotes: "Some patients with specific pathogenic variants respond to pharmacotherapy, such as p.L858F/H (mexiletine) and V400M, S241T, and I234T (carbamazepine), suggesting potential for personalized therapeutic approaches" (PMID: 41190974); "The treatment of primitive erythermalgia is based on sodium channel blockers such as mexiletine or lidocaine infusions, and on drugs effective on neuropathic pain, such as gabapentin or amitryptiline" (PMID: 35835622). A single-center cohort found "The most effective therapies were antihistamines, venlafaxine, and mexiletine" (PMID: 37557164).

A cautionary safety note: mexiletine has a narrow therapeutic index; overdose can be life-threatening with cardiovascular and CNS toxicity (PMID: 37661688).

MAXO/ontology anchors: MAXO:0000058 pharmacotherapy; "sodium channel blocker therapy"; CHEBI:6916 mexiletine; CHEBI:3387 carbamazepine; CHEBI:6456 lidocaine; CHEBI:42797 gabapentin; CHEBI:2666 amitriptyline; behavioral application of cold.

Finding 5 — SCN9A allelic spectrum: IEM vs PEPD vs CIP

Nav1.7 sits at the center of a spectrum of Mendelian pain disorders. Gain-of-function missense mutations cause primary erythromelalgia (IEM) and paroxysmal extreme pain disorder (PEPD), whereas nonsense/loss-of-function mutations cause channelopathy-associated congenital insensitivity to pain (CIP). A genotype–phenotype rule generally holds: IEM mutations enhance activation (hyperpolarizing shift), while PEPD mutations impair steady-state fast inactivation (depolarizing shift, increased persistent/resurgent current).

"Gain-of-function missense mutations in Na(v)1.7 have been shown to cause primary erythermalgia and paroxysmal extreme pain disorder, while nonsense mutations in Na(v)1.7 result in loss of Na(v)1.7 function and a condition known as channelopathy-associated insensitivity to pain" (PMID: 18060017); "Gain-of-function mutations are typically pain-causing and have been associated with inherited erythromelalgia (IEM) and paroxysmal extreme pain disorder (PEPD). IEM is usually caused by enhanced NaV1.7 channel activation, whereas mutations that alter steady-state fast inactivation often lead to PEPD" (PMID: 25995458).

The correlation is imperfect: A1632E is an "overlap" mutation showing both IEM and PEPD features (a continuum), and A1632T causes IEM but does so via a fast-inactivation shift rather than the classic activation shift (PMID: 24311784). Resurgent/persistent current biophysics further refine the IEM-vs-PEPD distinction (PMID: 27174182).

Disorder	Nav1.7 mutation class	Biophysical signature	Clinical picture
IEM (primary erythromelalgia)	Gain-of-function missense	Hyperpolarized activation, slow deactivation, enhanced ramp	Distal-extremity burning pain, erythema, warmth; heat-triggered
PEPD	Gain-of-function missense	Impaired fast inactivation; ↑ persistent/resurgent current	Proximal (rectal, ocular, jaw) paroxysmal pain
CIP	Nonsense / loss-of-function	Loss of channel function	Congenital insensitivity to pain, anosmia

Finding 6 — Differential diagnosis: primary (SCN9A) vs secondary erythromelalgia (myeloproliferative neoplasms, JAK2)

Secondary erythromelalgia is a recognized microvascular manifestation of myeloproliferative neoplasms (MPNs) — essential thrombocythemia (ET) and polycythemia vera (PV) — mediated by platelet-mediated occlusive thrombosis in the end-arterial circulation. Unlike primary EM, it responds dramatically to aspirin (cyclooxygenase-1 inhibition). Consequently, the workup of suspected erythromelalgia must include a complete blood count and JAK2 mutation testing to exclude an underlying MPN.

"Microvascular disturbances in essential thrombocythemia (ET) and polycythemia vera (PV), including erythromelalgia, and atypical and typical transient cerebral, ocular, and coronary ischemic attacks, are caused by platelet-mediated transient and occlusive thrombosis in the end-arterial circulation" and "Inhibition of platelet cyclooxygenase-1 by aspirin is followed by relief of microvascular disturbances" (PMID: 16673274). Contemporary guidance for thrombocytosis workup recommends that "testing for Janus kinase 2 gene sequence variations should be performed" (PMID: 42101597).

A further differential is acute monophasic pediatric erythromelalgia, which can represent post-infectious immune-mediated small-fiber neuropathy rather than inherited channelopathy — distinguished by monophasic course, autoimmune/infectious associations, skin-biopsy small-fiber loss, and response to immunotherapy (PMID: 32723684).

Finding 7 — Model systems: iPSC sensory neurons robustly model IEM; rodent knock-ins recapitulate DRG hyperexcitability but often lack overt pain

Human iPSC-derived sensory neurons carrying Nav1.7 gain-of-function mutations (e.g., A1632G, Q875E) reproduce hyperexcitability and are described as "a robust, scalable and relevant model to study the effects of gain-of-function mutations in ion channels in pain-related disorders" (PMID: 40279376). The Q875E iPSC line has been used for pharmacology (e.g., botulinum toxin studies) (PMID: 38657946).

By contrast, rodent knock-in models incompletely recapitulate the human pain phenotype. Two independent Nav1.7 I228M knock-in mouse lines showed DRG neuron hyperexcitability yet did not display mechanical/thermal hyperalgesia or intraepidermal nerve fiber loss: "Nav1.7 I228M mice do not display mechanical or thermal hyperalgesia or intraepidermal nerve fiber loss in vivo. Therefore, although these 2 Nav1.7 I228M knock-in mouse lines recapitulate the DRG neuron hyperexcitability associated with gain-of-function mutations in Nav1.7, they do not recapitulate the pain or neuropathy phenotypes seen in patients" (PMID: 33323889). A rat Nav1.7 knock-in model was generated for drug development (PMID: 31550995), and complementary optogenetic (NaV1.7-ChR2) and heterologous (HEK293, Xenopus oocyte) systems support mechanistic and pharmacological studies (PMID: 36201719).

Model	Type	Recapitulation	Key limitation
Patient iPSC sensory neurons	In vitro, human	Hyperexcitability; CRISPR-correctable	No in-vivo behavior
Nav1.7 I228M knock-in mice (×2 lines)	Mammalian, in vivo	DRG hyperexcitability	No hyperalgesia/IENF loss
Rat Nav1.7 knock-in	Mammalian, in vivo	Drug-development platform	Genotype-specific
HEK293 / Xenopus oocytes	Heterologous	Per-variant biophysics	No neuronal context

Finding 8 — Structural basis: IEM voltage-sensor mutations facilitate outward S4 gating-charge movement

Near-atomic structural work using the bacterial channel NaVAb engineered with four IEM voltage-sensor mutations recapitulated the hyperpolarizing activation shift seen in human Nav1.7. An S1-segment mutation widens the pathway for gating-charge translocation, while S4-segment mutations modify hydrophobic interactions with neighboring side chains or membrane phospholipids — both facilitating outward S4 gating-charge movement, causing channel hyperactivation, neuronal hyperexcitability, and severe pain.

"a mutation in the S1 segment of the voltage sensor facilitated the outward movement of S4 gating charges by widening the pathway for gating charge translocation. In contrast, mutations in the S4 segments modified hydrophobic interactions with surrounding amino acid side chains or membrane phospholipids that would enhance the outward movement of the gating charges" (PMID: 37903281). This provides a physical, mutation-specific basis for structure-guided therapeutic design.

Finding 9 — Prognosis/burden: chronic, lifelong, episodic, severely disabling, but not directly fatal

Primary EM greatly compromises quality of life and causes severe disability, with an episodic/paroxysmal course of burning pain, erythema, and warmth: "The symptoms greatly compromise the patients' quality of life leading to severe disability" (PMID: 37557164). It is generally not directly fatal — the Swedish cohort reported "there was no mortality directed related to EM" (PMID: 22247059). Burden data from painful small-fiber/idiopathic neuropathy — the broader category encompassing EM pain — quantify substantial comorbidity: sleep disturbance/insomnia 37%, anxiety 34%, depressive symptoms 33%, plus work impairment (~37%) and significant direct/indirect costs: "Most common comorbidities were sleep disturbance/insomnia (37.0%), anxiety (34.0%), and depressive symptoms (33.0%)" (PMID: 24673364). A 2025 systematic review evaluated comparative treatment efficacy/tolerability, reflecting the absence of a single uniformly effective therapy (PMID: 40428878). Notably, some patients resort to extreme cooling/ice immersion, causing skin maceration and secondary complications — a self-inflicted morbidity risk.

Finding 10 — Variant spectrum and molecular diagnosis: recurrent heterozygous germline SCN9A missense alleles, ultra-rare in population databases

Primary erythermalgia arises from heterozygous germline missense variants in SCN9A/Nav1.7. Recurrent/illustrative pathogenic alleles across families and cohorts include I848T, L858F/H, F1449V, V400M, S241T, I234T, Q875E, I136V, P1308L, and A1632G/T/E, plus novel alleles such as L1595R (c.4784T>G) and F1624S. Pathogenic alleles are typically absent from population databases (e.g., gnomAD) and cosegregate within families, fulfilling ACMG/AMP criteria for pathogenicity: "The variant was absent from population databases and co-segregated with the phenotype within the family, fulfilling ACMG/AMP criteria for likely pathogenicity" (PMID: 41997215).

Molecular confirmation uses single-gene SCN9A sequencing or hereditary sensory/autonomic neuropathy gene panels: "We conducted a gene-panel sequencing targeting 18 genes associated with hereditary sensory and/or autonomic neuropathy" (PMID: 37555797); "We describe a spectrum of SCN9A variants associated with IEM" (PMID: 41190974). Chromosomal microarray, karyotyping, FISH, mtDNA testing, and repeat-expansion testing are not indicated — this is a single-gene point-mutation disorder.

Mechanistic Model / Interpretation

The pathophysiology of primary erythermalgia forms a clean, well-supported causal chain from a single germline point mutation to episodic clinical symptoms:

  Germline heterozygous                Biophysical gain-of-function
  SCN9A missense variant   ──────►     • Hyperpolarized (leftward) activation
  (e.g., I848T, L858F,                 • Slowed deactivation
   V400M, A1632G)                      • Enhanced ramp/persistent current
  │                                        │
  ▼                                        ▼
  Structural change in Nav1.7          Nav1.7 = "threshold channel"
  voltage sensor (S1/S4):              amplifies subthreshold generator
  facilitated OUTWARD S4               potentials in DRG / trigeminal /
  gating-charge movement               sympathetic neurons
  │                                        │
  └───────────────┬────────────────────────┘
          ▼
DRG NOCICEPTOR + SYMPATHETIC NEURON HYPEREXCITABILITY
(spontaneous firing, lowered threshold)
          │
┌─────────────────┴───────────────────┐
▼                                      ▼
  Nociceptive output                    Vasomotor dysregulation
  → BURNING PAIN                        → ERYTHEMA + WARMTH
│                                      │
└──────────────┬───────────────────────┘
       ▼
HEAT-TRIGGERED, COOLING-RELIEVED
EPISODIC ATTACKS IN DISTAL EXTREMITIES
       │
(temperature-dependent because warmth further
 shifts channels toward the hyperactivable state)

Upstream vs downstream: The upstream trigger is the germline SCN9A variant and its structural effect on the voltage sensor (outward S4 gating-charge facilitation, PMID 37903281). The proximate downstream mechanism is altered channel gating (PMID 22136189), which drives sensory/sympathetic neuronal hyperexcitability (causally proven by CRISPR correction in iPSC neurons, PMID 40279376). The most downstream events are the clinical manifestations — burning pain (nociceptive) and erythema/warmth (vasomotor), both explicable by Nav1.7's dual expression in sensory and sympathetic neurons (PMID 14985375).

Cell types and biological processes (ontology anchors): - CL:0000101 sensory neuron; dorsal root ganglion neuron; CL:0000198 nociceptor; sympathetic (postganglionic) neuron. - GO:0019228 neuronal action potential; GO:0086010 membrane depolarization during action potential; GO:0035725 sodium ion transmembrane transport; GO:0001518 voltage-gated sodium channel complex.

Anatomical involvement (Section 7): Primary organ = skin of the distal extremities (UBERON:0002097 skin of body; UBERON:0002387 pes/foot; UBERON:0002398 manus/hand), typically bilateral and symmetric, with feet more affected than hands. Body systems: peripheral nervous system (UBERON:0000010) — specifically DRG (UBERON:0000044) and sympathetic ganglia — and the cutaneous microvasculature/integumentary system. Subcellular locus = the neuronal plasma membrane voltage-gated sodium channel (GO:0001518).

Temporal development (Section 8): Onset is usually pediatric/childhood (often <10 years) in the inherited form, insidious in onset, with a chronic, lifelong, episodic (paroxysmal) course punctuated by heat- and exercise-triggered flares and relieved by cooling. There is no established genetic anticipation, and the disorder is not self-limited (contrasting with the acute monophasic post-infectious pediatric form).

Inheritance and population (Section 9): Autosomal dominant, single-gene (SCN9A), with recurrent private missense alleles that are ultra-rare/absent in gnomAD and cosegregate in families. Both familial and de novo/sporadic cases occur. Penetrance is high but expressivity is variable, including striking intra- and interfamily phenotypic diversity for the same variant (PMID 22136189). Female predominance is seen in overall EM epidemiology, though sex differences are less pronounced in the inherited subtype.

Evidence Base

PMID	Title (abbrev.)	Evidence type	Supports
14985375	Mutations in SCN9A cause primary erythermalgia	Human genetic (linkage/mutation)	F1: causal gene, AD inheritance, Nav1.7 expression
22136189	Phenotypic diversity of a Nav1.7 GoF variant	Human clinical + biophysics	F2: IEM biophysical signature; variable expressivity
40279376	CRISPR correction in iPSC sensory neurons	In vitro human iPSC	F2, F7: causality; iPSC model validation
18713229	Incidence in Olmsted County	Population epidemiology	F3: incidence, primary vs secondary
22247059	EM in Sweden	Population/single-center	F3, F9: incidence, female %, no EM mortality
41190974	IEM review (China/worldwide)	Review	F3, F4, F10: onset <10 yr; genotype-specific therapy; variant spectrum
35835622	Paroxysmal vascular acrosyndromes	Clinical review	F4: first-line drug classes
37557164	Single-center primary EM experience	Clinical cohort	F4, F9: effective therapies; QoL/disability
37661688	Mexiletine overdose	Clinical case	F4: treatment safety/narrow index
18060017	SCN9A spectrum of pain disorders	Review	F5: IEM/PEPD/CIP allelic spectrum
25995458	Novel SCN9A mutations, phenotype correlations	Human + electrophysiology	F5: IEM vs PEPD biophysical rule
24311784	A1632T causes IEM via fast-inactivation shift	In vitro electrophysiology	F5: imperfect genotype-phenotype correlation
27174182	Slow inactivation & open-channel block	In vitro biophysics	F5: resurgent/persistent current distinctions
16673274	Platelet-mediated thrombosis in ET/PV	Clinical/mechanistic	F6: secondary EM mechanism, aspirin response
42101597	Thrombocytosis evidence review	Clinical guideline	F6: JAK2 testing
32723684	Pediatric acute monophasic EM = SFN	Clinical case series	F6: immune-mediated differential
33323889	Two Nav1.7 I228M knock-in mouse lines	Mouse model	F7: incomplete pain-phenotype recapitulation
31550995	Rat Nav1.7 knock-in	Rat model	F7: drug-development platform
36201719	NaV1.7-ChR2 optogenetic mice	Mouse model	F7: nociceptor activation tool
38657946	BoNT/A in Q875E iPSC neurons	In vitro human iPSC	F7: iPSC pharmacology model
37903281	Structural basis (NaVAb) of IEM voltage-sensor mutations	Structural biology	F8: S4 gating-charge mechanism
24673364	Burden of painful small-fiber neuropathy	Survey/chart review	F9: comorbidity/cost burden
40428878	Comparative treatment efficacy (systematic review)	Systematic review	F9: no single uniformly effective therapy
41997215	Germline SCN9A variant, unilateral erythema	Human clinical/genetic	F10: ultra-rare, cosegregating, ACMG classification
37555797	Gene-panel study of SCN9A pain disorders	Human genetic	F10: gene-panel diagnostic approach
27009931	Sex differences in skin diseases (REP)	Epidemiology	F3: female predominance in EM

Consistency and conflicts. The genetic etiology (SCN9A/Nav1.7 gain-of-function) is corroborated by convergent evidence types — human linkage/mutation studies, in-vitro electrophysiology, human iPSC causality experiments, and channel structural biology — with no contradicting evidence in the reviewed literature. The main internal tension is genotype–phenotype: while IEM generally maps to activation-enhancing mutations and PEPD to inactivation-impairing mutations (PMID 25995458), overlap variants (A1632E) and exceptions (A1632T causing IEM via an inactivation shift, PMID 24311784) show the rule is a useful heuristic rather than an absolute law. The second key tension is model fidelity: iPSC neurons faithfully model human hyperexcitability, but mouse knock-ins reproduce cellular hyperexcitability without overt pain behavior (PMID 33323889), a species/context gap important for preclinical drug development.

Section-by-Section Knowledge-Base Annotations

1. Disease information. Rare AD Mendelian sodium channelopathy; synonyms: primary/inherited/familial erythromelalgia, IEM, primary erythermalgia, Mitchell disease (historical, for EM broadly), "man on fire" syndrome. Identifiers: OMIM #133020; Orphanet ORPHA:90026 (primary erythromelalgia); MeSH "Erythromelalgia"; ICD-10 I73.81; MONDO erythromelalgia concept. Information is derived from aggregated disease-level resources and published case/family cohorts (not primarily EHR).

2. Etiology. Primary cause = heterozygous germline gain-of-function SCN9A missense variants (F1, F5, F10). Genetic risk = the causal variant itself (dominant); no established environmental risk factors cause the disease, though heat, exercise, and warm environments are potent symptom triggers. No robust protective genetic/environmental factors identified; cooling is the principal symptom-relieving intervention. Gene–environment interaction is temperature-dependent channel gating: warmth further biases already-hyperactivable channels toward opening.

3. Phenotypes (HPO). Core: burning extremity pain (HP:0012531 Pain), erythema (HP:0500252 Erythema), local skin warmth/increased temperature, episodic/paroxysmal course, heat-triggered, cooling-relieved, distal and typically bilateral. Onset childhood (HP:0011463 Childhood onset). Severity moderate–severe, variable; progression episodic/fluctuating. QoL impact severe (F9).

4. Genetic/molecular. Causal gene SCN9A (HGNC:10597; OMIM *603415), protein Nav1.7 (UniProt Q15858). Variant class: missense, heterozygous, germline, gain-of-function (F1, F5, F10). Illustrative alleles: I848T, L858F/H, F1449V, V400M, S241T, I234T, Q875E, I136V, P1308L, A1632G/T/E, L1595R, F1624S. Allele frequency: absent/ultra-rare in gnomAD; ACMG-classified pathogenic/likely pathogenic via cosegregation and functional data. No causal chromosomal abnormalities or established epigenetic drivers.

5. Environmental. No toxic/infectious cause of the primary form; heat/warmth and physical activity are triggers; a distinct acute post-infectious/immune-mediated pediatric erythromelalgia exists as a phenocopy (PMID 32723684).

6. Mechanism. See Mechanistic Model above (F2, F5, F8). Molecular pathway = voltage-gated sodium channel gating / neuronal excitability; cellular process = altered action-potential threshold and firing; protein dysfunction = gain-of-function voltage-sensor defect. No primary immune/metabolic/fibrotic mechanism.

7. Anatomy. Skin of distal extremities (feet > hands), bilateral/symmetric; peripheral sensory (DRG) and sympathetic neurons; cutaneous microvasculature. UBERON/CL/GO anchors listed above.

8. Temporal. Childhood onset, insidious, chronic lifelong, episodic/paroxysmal; heat-triggered flares; cooling-induced transient remission.

9. Inheritance/epidemiology. AD; incidence ~0.36–1.3/100,000/yr (F3); high penetrance, variable expressivity; female predominance overall.

10. Diagnostics. Clinical triad + provocation/relief pattern; CBC and JAK2 testing to exclude secondary MPN-associated EM (F6); molecular confirmation by SCN9A single-gene sequencing or HSAN gene panel (F10). Skin biopsy for small-fiber neuropathy in atypical/acquired presentations. CMA/karyotype/FISH/mtDNA/repeat-expansion testing NOT indicated.

11. Prognosis. Chronic, disabling, non-fatal (F9); high psychiatric/functional comorbidity; risk of self-inflicted cooling injury.

12. Treatment (MAXO). Sodium-channel blockers (mexiletine, carbamazepine, lidocaine), neuropathic agents (gabapentin, amitriptyline), antihistamines, venlafaxine; behavioral cooling; genotype-guided selection (F4). Emerging: selective Nav1.7 blockers, structure-guided and gene/CRISPR-based precision approaches.

13. Prevention. No primary prevention (Mendelian). Genetic counseling for AD 50% recurrence risk; cascade family testing; prenatal/preimplantation options where applicable. Tertiary prevention = trigger avoidance (heat, exertion) and avoidance of cooling-related skin damage.

14. Other species / natural disease. Human disease (NCBI Taxon 9606). Ortholog Scn9a in mouse (NCBI Gene 20274) and rat; no well-documented naturally occurring companion-animal/wildlife equivalent identified; engineered rodent orthologs used experimentally (F7).

15. Model organisms. Human iPSC-derived sensory neurons (preferred, robust; F7); mouse and rat Nav1.7 knock-in lines (cellular hyperexcitability, incomplete pain behavior); optogenetic NaV1.7-ChR2 mice; HEK293/Xenopus oocyte heterologous systems; bacterial NaVAb for structural studies. Resources: MGI, RGD, Cellosaurus/iPSC repositories.

Limitations and Knowledge Gaps

Model fidelity gap. Mouse Nav1.7 I228M knock-ins recapitulate DRG hyperexcitability but not pain behavior or nerve-fiber loss (PMID 33323889), limiting preclinical predictive validity; iPSC neurons capture excitability but lack in-vivo behavioral read-outs.
Imperfect genotype–phenotype mapping. The IEM (activation shift) vs PEPD (inactivation shift) dichotomy has documented exceptions and overlap variants (A1632E/T), complicating variant interpretation and prognostication.
Sparse population genetics. Because pathogenic alleles are private/ultra-rare, formal penetrance estimates, carrier frequencies, and founder effects are not well quantified from population databases.
Epidemiology heterogeneity. Incidence estimates (0.36 vs 1.3/100,000/yr) mix primary and secondary EM and different ascertainment methods; primary-EM-specific, molecularly confirmed incidence/prevalence is uncertain.
Treatment evidence quality. Most therapeutic data are from small single-center cohorts and case reports; the 2025 systematic review (PMID 40428878) underscores the lack of a uniformly effective, high-evidence therapy. Genotype-specific responses are promising but not yet validated in prospective randomized trials.
No omics depth. Transcriptomic/proteomic/metabolomic profiling of patient tissue is limited; QoL is documented qualitatively and via proxy small-fiber-neuropathy burden data rather than EM-specific validated instruments.
Comparative/veterinary biology for a naturally occurring animal counterpart is essentially absent.

Proposed Follow-up Experiments / Actions

Prospective, genotype-stratified pharmacology trial. Test the mexiletine-responsive (L858F/H) vs carbamazepine-responsive (V400M, S241T, I234T) hypothesis in a controlled, biophysically phenotyped patient cohort to validate genotype-guided prescribing (extends F4/F5).
iPSC pharmacology panel. Build an isogenic, CRISPR-engineered iPSC-sensory-neuron library spanning the recurrent allelic spectrum (F10) and screen selective Nav1.7 blockers and existing sodium-channel drugs against multielectrode-array excitability to prioritize variant-specific therapies (extends F7).
Structure-guided drug design. Leverage the NaVAb/Nav1.7 voltage-sensor structures (F8) to design mutation-specific state-dependent inhibitors that preferentially stabilize the resting (deactivated) voltage sensor.
Improved in-vivo model. Develop humanized-SCN9A or higher-impact knock-in/conditional models that reproduce pain behavior and intraepidermal nerve-fiber changes, closing the mouse-model gap (addresses F7 limitation).
Molecularly confirmed natural-history registry. Establish a genotyped primary-EM registry to quantify penetrance, expressivity, incidence/prevalence, progression, and EM-specific QoL with validated instruments (addresses epidemiology and prognosis gaps).
Diagnostic standardization. Formalize a diagnostic flow-chart combining clinical triad + CBC/JAK2 exclusion of secondary EM + SCN9A/HSAN panel confirmation + skin biopsy for atypical acquired cases (operationalizes F6/F10).

Evidence source types are indicated throughout: human clinical/genetic (e.g., PMIDs 14985375, 22136189, 25995458, 41190974, 41997215, 37555797), in-vitro human iPSC (40279376, 38657946), heterologous/electrophysiology (24311784, 27174182), model organism (33323889, 31550995, 36201719), structural/computational (37903281), and epidemiologic/clinical-review (18713229, 22247059, 16673274, 42101597, 24673364, 40428878, 35835622, 37557164, 27009931).

Primary Erythermalgia

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Key Findings

Finding 1 — Primary erythermalgia is caused by gain-of-function mutations in SCN9A (Nav1.7)

Finding 2 — Biophysical mechanism: hyperpolarized activation, slowed deactivation, enhanced ramp response → DRG hyperexcitability

Finding 3 — Epidemiology: erythromelalgia incidence ~0.36–1.3 per 100,000/year with female predominance

Finding 4 — Treatment: genotype-specific sodium-channel blockers plus neuropathic agents; cooling relief

Finding 5 — SCN9A allelic spectrum: IEM vs PEPD vs CIP

Finding 6 — Differential diagnosis: primary (SCN9A) vs secondary erythromelalgia (myeloproliferative neoplasms, JAK2)

Finding 7 — Model systems: iPSC sensory neurons robustly model IEM; rodent knock-ins recapitulate DRG hyperexcitability but often lack overt pain

Finding 8 — Structural basis: IEM voltage-sensor mutations facilitate outward S4 gating-charge movement

Finding 9 — Prognosis/burden: chronic, lifelong, episodic, severely disabling, but not directly fatal

Finding 10 — Variant spectrum and molecular diagnosis: recurrent heterozygous germline SCN9A missense alleles, ultra-rare in population databases

Mechanistic Model / Interpretation

Evidence Base

Section-by-Section Knowledge-Base Annotations

Limitations and Knowledge Gaps

Proposed Follow-up Experiments / Actions

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