Duchenne muscular dystrophy (DMD), followed by Becker muscular dystrophy (BMD) and the relatively new phenotype DMD associated dilated cardiomyopathy (DCM) are among the most recognized of a group of X-linked muscle disorders called dystropinopathies. They also include other conditions caused by dystrophin mutations, such as X-linked cardiomyopathy and isolated quadriceps myopathy.

Duchenne Muscular Dystrophy (DMD)

  • Epidemiology: Most common muscular dystrophy of childhood, affecting 1 in 5000 boys.
  • Cause: Mutations in the dystrophin gene.
  • Age of Onset: Symptoms typically present between 3-5 years.
  • Clinical Features:
    • Common: Motor delay, gait abnormalities, frequent falls, difficulty rising from the ground.
    • Less Common: Language/global developmental delay, raised serum creatine kinase, or hepatic transaminases.
    • Typical Signs: Proximal lower limb and trunk weakness, waddling gait, prominent calves, positive Gowers’ sign, and neck flexor weakness.

    Becker Muscular Dystrophy (BMD)

    • Initial increase in motor skills until age 6, followed by progressive weakness.
    • Loss of independent ambulation by age 13.

    Key Complications in DMD

    1. Respiratory:
      • Chronic restrictive lung disease from around 12 years of age.
      • Sleep-disordered breathing progressing to nocturnal and diurnal hypercapnia.
      • Exacerbated by scoliosis.
    2. Cardiac:
      • Dilated cardiomyopathy (affects all patients over 18).
      • Asymptomatic in DMD due to low physical activity.
      • More symptomatic and severe in BMD.
    3. Cognitive:
      • Lower verbal IQ compared to performance IQ.
      • Associated neurodevelopmental conditions such as ADHD and autism spectrum disorders.
    4. Orthopedic Complications in DMD
    • Respiratory: Chronic restrictive lung disease, sleep-disordered breathing, nocturnal and diurnal hypercapnia.
    • Cardiac: Dilated cardiomyopathy, arrhythmias, with early signs such as resting tachycardia.
    • Cognitive: Lower average IQ (verbal IQ more affected), ADHD, autism spectrum disorders.

    Female Carriers of Dystrophin Mutations

    • Epidemiology: Less common and milder than DMD.
    • Cause: Mutations in the dystrophin gene, similar to DMD.
    • Clinical Features:
      • Independent ambulation typically persists beyond age 16.
      • Variable onset of weakness; some individuals remain ambulant throughout life.

      Genetic Basis of DMD

      • Higher incidence of symptomatic cardiomyopathy compared to DMD.
        • Contractures:
          • Commonly affect ankles, knees, iliotibial bands, and hips.
          • Lead to characteristic lumbar lordosis and toe-walking while ambulant.

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