Muscular Dystrophies

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For further information, see CMDT Part 24-35: Myopathic Disorders

Key Features

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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 (listed in alphabetical order).¹

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Table 24–10. Selected muscular dystrophies (listed in alphabetical order).¹

Disorder

Inheritance

Age at Onset (years)

Distribution

Prognosis

Genetic Association

Becker

X-linked recessive

5–25

Pelvic, then shoulder girdle.

Slow progression. May have normal life span.

Xp21; Dystrophin (reduced functional expression).

Distal

Autosomal dominant or recessive

40–60

Onset distally in extremities; proximal involvement later.

Slow progression.

Multiple.

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).

Emery-Dreifuss

X-linked recessive or autosomal dominant

5–10

Humeroperoneal or scapuloperoneal.

Variable.

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.

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.

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.

Oculopharyngeal

Autosomal dominant

Any age

Ptosis, external ophthalmoplegia, and dysphagia.

Slow progression.

14q11.2–q13; Poly (A)-binding protein-2.

¹Not all possible genetic loci are shown.

Clinical Findings

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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

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Genetic testing
Muscle biopsy needed occasionally

Treatment

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Eteplirsen
- Approved to treat Duchenne muscular dystrophy caused by specific loss-of-function mutations in the DMD gene coding for dystrophin
- 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 similar side effects, weight gain at 1 year is less with deflazacort
Important to encourage patients to lead as normal lives as possible
Prolonged bed rest must be avoided; inactivity often leads to worsening of the underlying muscle disease
Physical therapy and orthopedic procedures may help counteract deformities or contractures

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Key Features

Clinical Findings

Diagnosis

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