Duchenne muscular dystrophy is an X-linked disorder (Xp21) affecting approximately 1 in 3500 boys and resulting from a gene mutation of dystrophin. Becker muscular dystrophy is an allelic disorder associated with more mild symptoms; its mutations at least partially preserve the function of the resulting gene product.
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Infant boys are only rarely symptomatic in early infancy. At about 2 to 3 years of age, boys develop an awkward gait and an inability to run properly. Some have an antecedent history of mild slowness in attaining motor milestones or poor head control during infancy. Examination shows firm calf hypertrophy and mild to moderate proximal leg weakness with a hyperlordotic, waddling gait. The child typically arises from a lying position on the floor by using his arms to climb up his legs and body (Gower sign). Arm weakness is evident by age 6 years, and most boys are wheelchair dependent by age 12 years. Other manifestations include cardiomyopathy, scoliosis, respiratory decline, and, in some boys, cognitive and behavioral dysfunction. Many boys with Duchenne live into adulthood. Most die in their 20s or early 30s, usually as a result of progressive respiratory decline or cardiac dysfunction.
Serum CK levels are always markedly elevated. Diagnosis is established by genetic testing for the dystrophin gene mutation. Prenatal diagnosis is possible. Approximately one third of cases represent new mutations. Occasionally, the diagnosis is not made until a muscle biopsy shows muscle fiber degeneration and regeneration accompanied by increased intrafascicular connective tissue.
Steroid therapy is now instituted to slow the pace of the disease and delay motor disability. Supportive care includes physical therapy, bracing, proper wheelchairs, and treatment of cardiac dysfunction or pulmonary infections. A multidisciplinary approach is recommended. Through improvements in management, patients are now living significantly longer than in the pre-steroid era.
Myotonic dystrophy (DM) is the second most common muscular dystrophy and the most common form to present in adulthood. An autosomal dominant genetic disease, DM is caused by progressive expansion of a triplet repeat, CTG, in the myotonic dystrophy protein kinase gene. Presentation is roughly correlated with the number of CTG repeats, and the disease is characterized by genetic anticipation, where each generation presents with earlier and more severe symptoms.
Although DM most typically presents in adulthood, it can present at any age. Clinical features of childhood-onset “classic” DM include slowly progressive facial and distal extremity weakness as well as myotonia. Myotonia is a disorder of muscle relaxation after contraction. Patients grasp onto an object and have difficulty releasing their grasp, peeling their fingers away slowly. The facial appearance is characteristic, with hollowing of muscles around temples, jaw, and neck; ptosis; facial weakness; and drooping of the lower lip. The voice is nasal and mildly dysarthric. Not only is the striated muscle affected, but smooth muscle of the alimentary tract, uterus, and cardiac tissue are involved. Patients have variable arrhythmias, endocrinopathies, immunologic deficiencies, cataracts, and intellectual impairment.