Bulbospinal Muscular Atrophies (Bulbospinal Amyotrophies)

name: Bulbospinal Muscular Atrophies
display_name: Bulbospinal Muscular Atrophies (Bulbospinal Amyotrophies)
creation_date: "2026-08-19T00:00:00Z"
description: >-
  The bulbospinal muscular atrophies (bulbospinal amyotrophies) are the subset of
  lower-motor-neuron disorders in which degeneration involves BOTH the brainstem
  motor nuclei — producing progressive bulbar / lower-cranial-nerve palsy with
  dysarthria, dysphagia, facial and tongue wasting and fasciculation — AND the
  spinal anterior horn, producing neurogenic limb weakness, wasting and
  fasciculation. The combined bulbar-plus-spinal distribution, rather than any
  single molecular lesion, is what the name denotes: the members are
  aetiologically unrelated (an X-linked polyglutamine androgen-receptor
  proteinopathy, an autosomal recessive riboflavin-transporter deficiency, and a
  geographically restricted disorder of unknown cause), yet they converge on the
  same two vulnerable motor-neuron pools and therefore present to the clinic as
  one differential-diagnostic problem. That differential matters because it
  contains a treatable disorder: riboflavin transporter deficiency
  (Brown-Vialetto-Van Laere syndrome and its allelic form Fazio-Londe disease)
  responds to high-dose oral riboflavin, so a bulbospinal presentation is an
  indication to consider an empirical riboflavin trial rather than to settle on a
  degenerative diagnosis. The group is deliberately distinguished from the
  amyotrophic lateral sclerosis spectrum, whose bulbar-onset form (progressive
  bulbar palsy) is defined by combined UPPER and lower motor neuron degeneration
  with corticobulbar signs.
grouping_basis:
- SHARED_PHENOTYPE
- CLINICAL_CONVENTION
grouping_rationale: >-
  Grouped on a shared anatomical distribution of motor-neuron loss (brainstem
  motor nuclei plus spinal anterior horn) and on the clinical convention that
  treats these disorders as one differential-diagnostic set, NOT on a shared
  molecular mechanism — which is deliberately heterogeneous across the members
  (androgen-dependent polyglutamine androgen-receptor toxicity in Kennedy
  disease; flavocofactor depletion from defective plasma-membrane riboflavin
  transport in Brown-Vialetto-Van Laere syndrome; unknown in Madras motor neuron
  disease, where the riboflavin transporter genes and the C9orf72 expansion have
  been sequenced and are negative). The members are kept as separate Disease
  entries because they differ in causal gene and inheritance, age of onset
  (midlife vs childhood/juvenile), the presence of non-motor features (androgen
  insensitivity in Kennedy disease; sensorineural deafness and optic atrophy in
  the childhood forms), and — decisively — in treatability, since only the
  riboflavin-transporter arm has a disease-modifying therapy. The criteria are
  NECESSARY (membership entails combined bulbar and spinal lower-motor-neuron
  degeneration) rather than NECESSARY_AND_SUFFICIENT: that distribution alone
  does not make a disorder a bulbospinal muscular atrophy, because it also
  occurs in the ALS spectrum, in brainstem and cervical structural lesions, and
  in neuromuscular-junction disease that mimics bulbar palsy. This grouping sits
  below the broader Motor Neuron Disorders grouping, of which every member is
  also a member.
mappings:
  mondo_mappings:
  - term:
      id: MONDO:0016113
      label: bulbospinal muscular atrophy
    mapping_predicate: skos:narrowMatch
    mapping_source: MONDO
    mapping_justification: >-
      narrowMatch: this grouping is the curated clinical-syndrome subset of the
      MONDO bulbospinal muscular atrophy class. MONDO:0016113 is explicitly a
      grouping term — it carries the `disease_grouping`, `ordo_group_of_disorders`
      and `rare` subsets and is sourced from the Orphanet group of disorders
      Orphanet:206701 — so it is bound here to a `Grouping` rather than to a
      Disease entry. It is NOT an exact match for the existing Kennedy Disease
      entry: Kennedy disease has its own MONDO class (MONDO:0010735, bound to
      `kb/disorders/Kennedy_Disease.yaml`), and MONDO:0016113 subsumes further
      entities beyond it. Its extension therefore differs from the DisMech
      member list in both directions and its descendant set should not be treated
      as an exhaustive list of DisMech curation gaps for this grouping.
    consistency:
    - reference: MONDO
      consistent: CONSISTENT
      notes: >-
        Alignment is conceptual rather than extensional. MONDO:0016113's asserted
        subclasses are spinal atrophy-ophthalmoplegia-pyramidal syndrome
        (MONDO:0015250, whose own MONDO definition begins "is a rare, bulbospinal
        muscular atrophy") and pontocerebellar hypoplasia types 1 and 2
        (MONDO:0016396, MONDO:0016759, which reach the class through the
        anterior-horn-degeneration arm of PCH1); riboflavin transporter
        deficiency (MONDO:0008891) is linked by `disease_has_major_feature`
        rather than `is_a`. Of these, only riboflavin transporter deficiency has
        a DisMech entry whose curated phenotypes satisfy the membership criteria;
        the DisMech Pontocerebellar Hypoplasia entry is bound to the broad
        MONDO:0020135 concept and does not annotate anterior-horn or bulbar
        lower-motor-neuron involvement, so it is deliberately NOT listed as a
        member. Conversely Madras motor neuron disease (MONDO:0015307) is listed
        here on curated phenotype grounds although MONDO does not place it under
        MONDO:0016113. Spinal atrophy-ophthalmoplegia-pyramidal syndrome is an
        open curation gap.
membership_criteria:
- description: >-
    A disorder belongs to the bulbospinal muscular atrophies if its defining
    lesion is lower-motor-neuron degeneration involving BOTH the brainstem motor
    nuclei — manifesting as bulbar signs or bulbar palsy (dysarthria, dysphagia,
    facial and tongue weakness, tongue wasting and fasciculation) — AND the
    spinal anterior horn, manifesting as neurogenic limb weakness, wasting and
    fasciculation. Either abnormality in isolation is insufficient: a purely
    bulbar syndrome and a purely spinal amyotrophy are both excluded.
  criteria_semantics: NECESSARY
  logic:
    operator: AND
    operands:
    - criterion_predicate: HAS_PHENOTYPE
      description: >-
        Lower motor neuron degeneration. NOTE this conjunct does NOT encode the
        spinal half of "bulbospinal": HP:0002366 covers any lower motor neuron
        abnormality and is therefore entailed by the bulbar branch below, which
        HPO defines as due to a lower motor neuron lesion. A spinal-specific
        term (HP:0002398, HP:0006802, HP:0007277) would encode it, but no member
        entry currently annotates one, so asserting it here would report all
        three listed members as contradictions rather than police the boundary.
        The spinal-plus-bulbar requirement is therefore carried in the prose
        description of this block, in the same way the second NECESSARY block
        carries lower-motor-neuron predominance. Annotating the members with a
        specific anterior-horn term, then tightening this conjunct, is the fix.
      phenotype_term:
        preferred_term: Abnormal lower motor neuron morphology
        term:
          id: HP:0002366
          label: Abnormal lower motor neuron morphology
    - operator: OR
      description: >-
        Bulbar (brainstem motor nucleus) involvement, recorded either as the
        summary finding "Bulbar signs" or as the syndromic term "Bulbar palsy".
      operands:
      - criterion_predicate: HAS_PHENOTYPE
        description: Bulbar signs.
        phenotype_term:
          preferred_term: Bulbar signs
          term:
            id: HP:0002483
            label: Bulbar signs
      - criterion_predicate: HAS_PHENOTYPE
        description: Bulbar palsy.
        phenotype_term:
          preferred_term: Bulbar palsy
          term:
            id: HP:0001283
            label: Bulbar palsy
  evidence:
  - reference: PMID:20301508
    reference_title: "Spinal and Bulbar Muscular Atrophy."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Spinal and bulbar muscular atrophy (SBMA) is a gradually progressive
      neuromuscular disorder in which degeneration of lower motor neurons
      results in muscle weakness, muscle atrophy, and fasciculations in
      affected males
    explanation: >-
      GeneReviews states the lower-motor-neuron degeneration that this criterion
      tests for, in the archetypal member (spinal and bulbar muscular atrophy /
      Kennedy disease). Replaces an earlier citation to PMID:37360345, whose
      quoted sentence was background from the introduction of a single-patient
      gait-rehabilitation case report.
  - reference: PMID:24253200
    reference_title: "Treatable childhood neuronopathy caused by mutations in riboflavin transporter RFVT2."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      A particularly severe subgroup first described in 1894, and subsequently
      called Brown-Vialetto-Van Laere syndrome, is characterized by progressive
      pontobulbar palsy, sensorineural hearing loss and respiratory insufficiency
    explanation: >-
      Shows that the same bulbar (pontobulbar palsy) component defines a
      childhood-onset member drawn from an aetiologically unrelated disease
      family, supporting the phenotype-based rather than mechanism-based
      criterion.
- description: >-
    Lower-motor-neuron predominance. The bulbar and limb weakness must be
    neurogenic and lower-motor-neuron in origin, not a primary myopathy and not a
    neuromuscular-junction disorder (myasthenia gravis is the standard mimic of
    fatigable bulbar weakness). Coexisting pyramidal signs do NOT exclude
    membership — pyramidal dysfunction is reported in Madras motor neuron disease
    and a mixed upper/lower motor neuron picture emerges with progression in
    Brown-Vialetto-Van Laere syndrome — but a disorder whose defining lesion is
    combined upper and lower motor neuron degeneration of the amyotrophic lateral
    sclerosis spectrum is excluded. This is why the DisMech Progressive Bulbar
    Palsy entry, curated as a bulbar-onset ALS variant with corticobulbar
    (pseudobulbar) features and TDP-43 proteinopathy, is not a member. No
    structured `logic` is given for this criterion because HPO annotation records
    the presence of a finding, not its predominance, so a boolean encoding would
    over-claim what the curated annotations can decide.
  criteria_semantics: NECESSARY
members:
- member: Kennedy Disease
  member_type: DISEASE
  disease_term:
    preferred_term: Kennedy disease
    term:
      id: MONDO:0010735
      label: Kennedy disease
  differentiating_mechanisms:
  - description: >-
      The only adult-onset and the only X-linked member, and the only one whose
      mechanism is a repeat-expansion proteinopathy: a CAG trinucleotide
      expansion (>35 repeats) in exon 1 of AR encodes an elongated polyglutamine
      tract, and the mutant receptor exerts an androgen-DEPENDENT toxic gain of
      function on bulbar and spinal lower motor neurons. Two consequences are
      unique to this member within the group. First, ligand dependence makes the
      disorder essentially male-limited, with heterozygous females largely
      spared. Second, because the same receptor also mediates normal androgen
      signalling, patients show partial androgen insensitivity (gynecomastia,
      testicular atrophy, reduced male fertility) alongside the motor syndrome —
      an endocrine dimension absent from the childhood riboflavin-responsive and
      Madras forms.
    gene:
      preferred_term: AR
      term:
        id: hgnc:644
        label: AR
    evidence:
    - reference: PMID:20301508
      reference_title: "Spinal and Bulbar Muscular Atrophy."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        identification of a hemizygous expansion of a CAG trinucleotide repeat
        (>35 CAGs) in AR by molecular genetic testing
      explanation: >-
        GeneReviews establishes the AR CAG repeat expansion as the causal lesion
        that distinguishes this member from the other bulbospinal muscular
        atrophies.
  evidence:
  - reference: PMID:37360345
    reference_title: "Long-term effects of the gait treatment using a wearable cyborg hybrid assistive limb in a patient with spinal and bulbar muscular atrophy: a case report with 5 years of follow-up."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      characterized by the degeneration of lower motor neurons in the spinal
      cord and brainstem and neurogenic atrophy of the skeletal muscle
    explanation: >-
      Confirms that Kennedy disease (spinal and bulbar muscular atrophy) meets
      the combined bulbar-plus-spinal lower-motor-neuron membership criterion.
- member: Brown-Vialetto-Van Laere Syndrome
  member_type: DISEASE
  display_name: Brown-Vialetto-Van Laere syndrome / Fazio-Londe disease (riboflavin transporter deficiency)
  disease_term:
    preferred_term: riboflavin transporter deficiency
    term:
      id: MONDO:0008891
      label: riboflavin transporter deficiency
  differentiating_mechanisms:
  - description: >-
      The only TREATABLE member, and the only one whose lesion is metabolic
      rather than degenerative in origin: autosomal recessive loss of the
      plasma-membrane riboflavin transporters RFVT2 (SLC52A2) and RFVT3
      (SLC52A3) depletes the flavocofactors FAD and FMN, impairing
      flavoprotein-dependent mitochondrial electron transport and fatty-acid
      oxidation in cranial-nerve motor nuclei, spinal motor neurons and sensory
      neurons. High-dose oral riboflavin supplementation halts or reverses
      progression, so this is the diagnosis the bulbospinal differential exists
      to catch. It is also the only member that adds a sensory neuronopathy
      (sensory ataxia) and optic atrophy to the motor syndrome, and its
      sensorineural deafness is an auditory neuropathy. The allelic childhood
      form presenting as progressive bulbar palsy without prominent deafness is
      Fazio-Londe disease, now regarded as the same entity.
    gene:
      preferred_term: SLC52A2
      term:
        id: hgnc:30224
        label: SLC52A2
    evidence:
    - reference: PMID:24253200
      reference_title: "Treatable childhood neuronopathy caused by mutations in riboflavin transporter RFVT2."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        We demonstrate that SLC52A2 mutations cause reduced riboflavin uptake and
        reduced riboflavin transporter protein expression
      explanation: >-
        Establishes defective riboflavin transport as the differentiating
        molecular lesion of this member.
  - description: >-
      A second riboflavin transporter gene, SLC52A3 (RFVT3), causes the allelic
      form originally mapped in this syndrome; the two genes give overlapping
      phenotypes and the same riboflavin responsiveness, which is why the group
      is curated as one riboflavin-transporter-deficiency entity rather than as
      separate members here.
    gene:
      preferred_term: SLC52A3
      term:
        id: hgnc:16187
        label: SLC52A3
    evidence:
    - reference: PMID:22740598
      reference_title: "Exome sequencing reveals riboflavin transporter mutations as a cause of motor neuron disease."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        We believe this strongly supports the notion that defective riboflavin
        transport plays an important role in Brown-Vialetto-Van Laere syndrome
      explanation: >-
        Supports treating the SLC52A2 and SLC52A3 forms as one
        riboflavin-transport mechanism rather than two unrelated members.
  evidence:
  - reference: PMID:20206331
    reference_title: "Brown-Vialetto-Van Laere syndrome, a ponto-bulbar palsy with deafness, is caused by mutations in c20orf54."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The key features are progressive ponto-bulbar palsy and bilateral
      sensorineural deafness.
    explanation: >-
      Documents the bulbar component of the membership criterion for this member;
      the spinal lower-motor-neuron component is curated on the disease entry.
  notes: >-
    Honest caveat on the second (predominance) criterion: the same report notes
    that "A complex neurological phenotype with a mixed picture of upper and lower
    motor neuron involvement reminiscent of amyotrophic lateral sclerosis evolves
    with disease progression." Membership rests on the lower-motor-neuron
    pontobulbar-plus-spinal lesion that defines the disorder at onset, not on the
    absence of any pyramidal sign later in the course.
- member: Madras Motor Neuron Disease
  member_type: DISEASE
  disease_term:
    preferred_term: Madras motor neuron disease
    term:
      id: MONDO:0015307
      label: Madras motor neuron disease
  differentiating_mechanisms:
  - description: >-
      The only member with no identified genetic cause and the only
      predominantly sporadic one: a juvenile/young-adult-onset motor neuron
      disease reported almost exclusively from Southern India, combining
      multiple lower-cranial-nerve palsies (VII, IX-XII) with limb wasting and
      weakness and a near-obligate sensorineural hearing loss (auditory
      neuropathy); the MMND variant adds optic atrophy and cerebellar signs.
      Crucially for this grouping, it is NOT a riboflavin transporter deficiency:
      sequencing of SLC52A1, SLC52A2 and SLC52A3 and of the C9orf72 expansion has
      been negative in MMND series, so it is retained as a distinct member rather
      than lumped into the treatable riboflavin-transporter arm it clinically
      mimics. Its cause is unknown and may be a combination of genetic and
      environmental factors; management is supportive.
    evidence:
    - reference: PMID:24139842
      reference_title: "Madras motor neuron disease (MMND) is distinct from the riboflavin transporter genetic defects that cause Brown-Vialetto-Van Laere syndrome."
      supports: SUPPORT
      evidence_source: HUMAN_CLINICAL
      snippet: >-
        We sequenced the SLC52A1, SLC52A2 and SLC52A3 in affected probands and
        sporadic individuals from the MMND series as well as the C9ORF72
        expansion. No genetic defects were identified
      explanation: >-
        Establishes that Madras motor neuron disease is genetically distinct from
        the riboflavin-transporter member it phenotypically overlaps, justifying
        its separate membership.
  evidence:
  - reference: PMID:24139842
    reference_title: "Madras motor neuron disease (MMND) is distinct from the riboflavin transporter genetic defects that cause Brown-Vialetto-Van Laere syndrome."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      The characteristic features are onset in young, weakness and wasting of
      limbs, multiple lower cranial nerve palsies and sensorineural hearing loss.
    explanation: >-
      Documents the combined lower-cranial-nerve (bulbar) and limb
      lower-motor-neuron involvement required for membership.
  - reference: PMID:12686396
    reference_title: "Madras motor neuron disease variant, clinical features of seven patients."
    supports: SUPPORT
    evidence_source: HUMAN_CLINICAL
    snippet: >-
      Bulbar palsy was an invariable feature, being present in all patients
      compared to 38.3% of MMND
    explanation: >-
      Quantifies the bulbar component in the MMND variant and in the parent
      MMND series, confirming bulbar involvement is a core rather than
      incidental feature.
discussions:
- discussion_id: mondo_0016113_sbma_synonym_collision
  kind: OPEN_QUESTION
  status: OPEN
  prompt: >-
    Should MONDO:0016113 retain "SBMA", "spinal-bulbar muscular atrophy" and
    "spinal and bulbal muscular atrophy" as EXACT synonyms while being asserted
    as a grouping term?
  rationale: >-
    MONDO:0016113 is tagged `disease_grouping` and `ordo_group_of_disorders`
    (from Orphanet:206701) yet carries "SBMA" and "spinal-bulbar muscular
    atrophy" as EXACT synonyms sourced from a patient-organisation page that
    describes Kennedy disease specifically. Kennedy disease is a separate MONDO
    class (MONDO:0010735) whose own definition opens "Kennedy's disease, also
    known as bulbospinal muscular atrophy (BSMA)". A consumer resolving the
    string "SBMA" or "bulbospinal muscular atrophy" can therefore land on either
    a single X-linked disease or a heterogeneous group of disorders — precisely
    the Named Entity Confusion pattern DisMech guards against, and the reason
    this curation produced a Grouping rather than a second Kennedy disease
    entry. Worth raising upstream with MONDO.
- discussion_id: bulbospinal_curation_gaps
  kind: CURATION_TODO
  status: OPEN
  prompt: >-
    Which further bulbospinal muscular atrophies should be curated as DisMech
    Disease entries and added as members?
  rationale: >-
    Spinal atrophy-ophthalmoplegia-pyramidal syndrome (MONDO:0015250) is an
    asserted MONDO subclass of MONDO:0016113 whose definition explicitly calls it
    "a rare, bulbospinal muscular atrophy" (neonatal hypotonia, progressive
    pontobulbar and spinal palsy, pyramidal signs, deafness, external
    ophthalmoplegia) and has no DisMech entry. Pontocerebellar hypoplasia type 1
    (MONDO:0016396, EXOSC3 and related genes) is the PCH subtype that carries
    anterior-horn-cell degeneration and is the reason MONDO places PCH under this
    class; the DisMech Pontocerebellar Hypoplasia entry is bound to the broad
    MONDO:0020135 concept and does not yet annotate the anterior-horn or bulbar
    lower-motor-neuron features, so a PCH1-specific entry (or subtype-level
    annotation) would be needed before membership could be asserted without
    creating a criteria contradiction.
notes: >-
  Scoping decision for issue #8940. MONDO:0016113 is a grouping term, not a
  disease: it is bound here to a `Grouping` (validated against the `Grouping`
  class) rather than to a new `kb/disorders/` entry, because a broader Disease
  entry would have duplicated the existing `kb/disorders/Kennedy_Disease.yaml`
  (MONDO:0010735) without adding a distinct pathophysiology, and because
  MONDO:0016113 is not an exact or narrow match for Kennedy disease alone. Every
  member of this grouping is also a member of the broader `Motor_Neuron_Disorders`
  grouping; this one is the bulbar-plus-spinal lower-motor-neuron subset of it.