• Clinical science

Progressive muscular dystrophies

Summary

Muscular dystrophies are a group of progressive diseases that affect the musculoskeletal system. Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD) are X-linked recessive diseases, whereas limb-girdle muscular dystrophy (LGMD) may be either autosomal dominant or recessive, and facioscapulohumeral dystrophy (FSHD) is usually autosomal dominant. Muscular dystrophies are commonly due to mutations involving muscular genes (e.g., dystrophin-protein coding gene). Patients typically present with muscular complaints affecting specific muscle groups, particularly the pelvic girdle musculature. DMD is the most severe form of muscular dystrophy, with disease onset typically occurring at two to three years of age. BMD usually does not become evident before the age of 15. DMD progresses rapidly and typically leads to ambulatory inability by age 12. Diagnosis of DMD and BMD is established based on blood tests that show increased creatinine kinase, whereas diagnosis of LGMD and FSHD is mainly based on genetic analysis. Treatment of muscular dystrophies is usually supportive and includes physiotherapy, assistive devices (e.g., wheelchair), and psychological support. The life expectancy for patients with DMD is approx. 30 years, whereas patients with BMD and LGMD have a longer life expectancy.

Epidemiology

  • Incidence
    • DMD: 1/3500
    • BMD: 1/30,000
  • Sex: only male individuals affected in DMD and BMD
  • Age of onset
    • DMD: 2–5 years
    • BMD: adolescence or early adulthood, usually > 15 years

References:[1][2][3]

Epidemiological data refers to the US, unless otherwise specified.

Etiology

References:[2]

Pathophysiology

Duchenne is caused by Deleted Dystrophin.

Clinical features

Duchenne muscular dystrophy (DMD)

Becker muscular dystrophy (BMD)

  • Symptoms similar to those of DMD, but less severe
  • Slower progression (patients often remain ambulatory into adult life)
  • Heart involvement is more common compared to DMD.

Becker is Better.”

References:[1]

Diagnostics

  • Blood tests:
    • ↑↑ creatine kinase in serum of almost all affected individuals (also, in > 50% of female carriers)
    • ↑ serum aldolase
  • Genetic analysis (confirmatory test): detect dystrophin gene mutation
  • Muscle biopsy

References:[1]

Differential diagnoses

Facioscapulohumeral muscular dystrophy (FSHD) [4]

  • Definition: a complex genetic disorder characterized by progressive muscle weakness of the face, scapula, upper arm
  • Etiology
    • Inheritance: autosomal dominant (70–90%) or sporadic
    • Deletion of D4Z4 repeat units, each of which contains a copy of a double homeobox protein 4 gene (DUX4), in the 4q35 region
  • Epidemiology: the third most common type of muscular dystrophy
  • Clinical features
    • Progressive muscle weakness
      • Onset usually in childhood and adolescence
      • Asymmetrical involvement
      • Affects the face, scapula, and upper arms
      • Less commonly, the girdle and/or lower leg can be involved
    • Extramuscular manifestations: retinal vasculopathy, hearing loss
  • Diagnosis: genetic testing to detect the D4Z4 short repeat array

Limb-girdle muscular dystrophy (LGMD) [5]

  • Definition: a genetic disorder characterized by progressive muscle weakness that primarily affects the pelvic and shoulder girdle
  • Etiology
  • Epidemiology
  • Clinical features
    • Onset in childhood or adolescence (LGMD 1 can also present in late adulthood)
    • Paresis and subsequent atrophy of the pelvic and shoulder girdle muscles
    • Normal intellect and absent distal muscle weakness
  • Diagnosis
    • Elevated CK
    • Muscle biopsy shows a decrease in protease calpain 3
    • Genetic analysis to detect the underlying mutation

Other

The differential diagnoses listed here are not exhaustive.

Treatment

References:[6]

Prognosis

  • DMD
    • Life expectancy is approx. 30 years.
    • Cardiac or respiratory failure is usually the cause of death.
  • BMD
    • Life expectancy is 40–50 years.
    • Cardiac failure is typically responsible for the reduced life expectancy.

References:[6]