• Clinical science

Progressive muscular dystrophies

Abstract

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. 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 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
  • Prevalence: LGMD – 1/20,000
  • Sex
    • DMD and BMD: only males affected
    • LGMD: affects both genders equally
  • Age of onset
    • DMD: 2–5 years
    • BMD: usually not earlier than 15 years of age

References:[1][2][3][4]

Epidemiological data refers to the US, unless otherwise specified.

Etiology

References:[5][3][6]

Pathophysiology

  • The dystrophin protein anchors the cytoskeleton of a muscular cell to the extracellular matrix by connecting cytoskeletal actin filaments to membrane-bound dystroglycan that is, in turn, connected to extracellular laminin.
  • Dystrophin gene mutations and subsequent alterations of the protein → partial (BMD) or almost complete impairment (DMD) of the protein → disturbance of numerous cellular signaling pathways → necrosis of affected muscle cells and subsequent replacement with connective and fat tissue → muscle appears larger (“pseudohypertrophy”)

References:[5]

Clinical features

Duchenne muscular dystrophy (DMD)

Becker muscular dystrophy (BMD)

  • Symptoms identical to those of DMD
  • Slower progression (patients often remain ambulatory into adult life) and milder symptoms
  • Heart involvement is more common compared to DMD.

Limb-girdle muscular dystrophy (LGMD)

  • Paresis and atrophy of the pelvic and shoulder girdle muscles
  • Normal intellect and absent distal muscle weakness

References:[1]

Diagnostics

  • Blood tests: ↑↑ creatine kinase in serum of almost all affected individuals (also, in > 50% of female carriers)
  • Genetic analysis: detect dystrophin gene mutation
  • Electromyogram: decreased amplitude, shortened potential duration, fibrillations
  • Muscle biopsy
    • Only performed if genetic analysis is inconclusive
    • All dystrophies: muscle fibers diameter changes, later in the disease course, necrosis and replacement with adipose tissue
    • DMD: absent dystrophin
    • BMD: reduced dystrophin
    • LGMD: decrease in protease calpain 3

References:[1]

Differential diagnoses

The differential diagnoses listed here are not exhaustive.

Treatment

Currently there is no curative treatment for muscular dystrophies!

  • Medical therapy
  • Supportive therapy
    • Physiotherapy to reinforce preserved muscles (including respiratory muscles training) and to prevent contractures
    • Orthopedic assistive devices (wheelchair, walkers)
    • Psychological support
    • Ventilation support
  • If necessary, surgical treatment of the contractures, correction of scoliosis

References:[7]

Prognosis

  • DMD: Life expectancy for patients with DMD is approx. 30 years.
  • BMD: Life expectancy for patients with BMD is 40–50 years.

References:[7]