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Muscular Dystrophies

For further information, see CMDT Part 24-36: Myopathic Disorders

Key Features

  • Inherited myopathic disorders are subdivided by

    • Mode of inheritance

    • Age at onset

    • Clinical features (Table 24–10)

  • Duchenne muscular dystrophy

    • Due to a genetic defect on the short arm of the X chromosome

    • Affected gene codes for the protein dystrophin, which is almost absent from diseased muscles

    • Genetic defect is detectable in pregnancy

  • Becker muscular dystrophy

    • Dystrophin levels are generally normal

    • Protein is qualitatively altered

Table 24–10.Selected muscular dystrophies.1
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Table 24–10. Selected muscular dystrophies.1
Disorder Inheritance Age at Onset (years) Distribution Prognosis Genetic Association
Duchenne type X-linked recessive 1–5 Pelvic, then shoulder girdle; later, limb and respiratory muscles. Rapid progression. Death within about 15 years after onset. Xp21; Dystrophin (loss of functional expression).
Becker X-linked recessive 5–25 Pelvic, then shoulder girdle. Slow progression. May have normal life span. Xp21; Dystrophin (reduced functional expression).
Limb-girdle (Erb) Autosomal recessive, dominant, or sporadic 10–30 Pelvic or shoulder girdle initially, with later spread to the other. Variable severity and rate of progression. Possible severe disability in middle life. Multiple.
Facioscapulohumeral Autosomal dominant Any age Face and shoulder girdle initially; later, pelvic girdle and legs. Slow progression. Minor disability. Usually normal life span.

4q35.2; Double homeobox protein 4.

18p11.32; Structural maintenance of chromosome’s flexible hinge domain-containing protein 1.
Emery-Dreifuss X-linked recessive or autosomal dominant 5–10 Humeroperoneal or scapuloperoneal. Variable. Multiple.
Distal Autosomal dominant or recessive 40–60 Onset distally in extremities; proximal involvement later. Slow progression. Multiple.
Oculopharyngeal Autosomal dominant Any age Ptosis, external ophthalmoplegia, and dysphagia. Slow progression. 14q11.2–q13; Poly (A)-binding protein-2.
Myotonic dystrophy Autosomal dominant Any age (usually 20–40) Face, neck, distal limbs. Slow progression.

19q13.32; Myotonin-protein kinase.

3q21.3; Cellular nucleic acid-binding protein.

1Not all possible genetic loci are shown.

Clinical Findings

  • Muscle weakness, often in a characteristic distribution

  • Age at onset and inheritance pattern depend on specific dystrophy

  • Duchenne dystrophy

    • Pseudohypertrophy of muscles

    • Intellectual disability

    • Skeletal deformities, muscle contractures, and cardiac involvement

Diagnosis

  • Genetic testing

  • Muscle biopsy occasionally needed

Treatment

  • Antisense oligonucleotides

    • Four agents are approved by the FDA for treatment of Duchenne muscular dystrophy

      • Eteplirsen appears to benefit those patients with a dystrophin mutation amenable to exon 51 skipping

      • Golodirsen and viltolarsen benefit those with a mutation amenable to exon 53 skipping

      • Casimersen benefits those with a mutation amenable to exon 45 skipping

    • Treated patients had more functional dystrophin on muscle biopsy than controls and a slower rate of disease progression than matched historical controls

  • Prednisone (0.75 mg/kg orally daily or 10 mg/kg orally given weekly over 2 days) or deflazocort (0.9 mg/kg orally daily) compared to placebo

    • Improves muscle strength and function in boys with Duchenne dystrophy

    • However, side effects need to be monitored

    • Although both medications cause ...

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