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
- Respiratory:
- Chronic restrictive lung disease from around 12 years of age.
- Sleep-disordered breathing progressing to nocturnal and diurnal hypercapnia.
- Exacerbated by scoliosis.
- Cardiac:
- Dilated cardiomyopathy (affects all patients over 18).
- Asymptomatic in DMD due to low physical activity.
- More symptomatic and severe in BMD.
- Cognitive:
- Lower verbal IQ compared to performance IQ.
- Associated neurodevelopmental conditions such as ADHD and autism spectrum disorders.
- 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.