Muscular dystrophy is actually a group of congenital disorders characterized by progressive symmetrical wasting of skeletal muscles without neural or sensory defects. Paradoxically, these wasted muscles tend to enlarge because of connective tissue and fat deposits, giving an erroneous impression of muscle strength.
Four main types of muscular dystrophy occur: Duchenne’s (pseudohypertrophic) muscular dystrophy, which accounts for 50% of all cases; Becker’s (benign pseudohypertrophic) muscular dystrophy; facioscapulohumeral (Landouzy-Dejerine) dystrophy; and limb-girdle dystrophy. Emery-Dreifuss muscular dystrophy, myotonic dystrophy, and myotonia congenita are less common.
The prognosis varies. Duchenne’s muscular dystrophy generally strikes during early childhood and usually results in death by age 20. Patients with Becker’s muscular dystrophy live into their 40s. Facioscapulohumeral and limb-girdle dystrophies usually don’t shorten life expectancy.
Muscular dystrophy is caused by various genetic mechanisms, distinguished from one another by the type of inheritance (sex-linked, dominant gene, recessive gene). Duchenne’s and Becker’s muscular dystrophies are X-linked recessive disorders. They result from defects in the gene coding for the muscle protein dystrophin. The defect can be mapped genetically to the Xp21 locus.
Duchenne’s and Becker’s muscular dystrophies affect males almost exclusively. The incidence of Duchenne’s muscular dystrophy in males is 13 to 33 per 100,000. Becker’s muscular dystrophy occurs in about 1 to 3 males per 100,000.
Facioscapulohumeral dystrophy is an autosomal dominant disorder. Limb-girdle dystrophy may be inherited in several ways, but it’s usually an autosomal recessive trait. These two types affect both sexes about equally.
Emery-Dreifuss dystrophy and myotonic dystrophy may be autosomal dominant or autosomal recessive.
Myotonia congenita (Thomsen’s disease) may be autosomal dominant or autosomal recessive. Its causative gene resides on chromosome 7. A problem in the chloride channels of the muscle cells results in accumulation of potassium and activation of sodium channels, leading to abnormal repetitive electrical discharges that cause stiffness.
Signs and symptoms
Although the four types of muscular dystrophy cause progressive muscular deterioration, the degree of severity and the age of onset vary.
Duchenne’s muscular dystrophy
Duchenne’s muscular dystrophy begins insidiously, between ages 3 and 5. It affects leg and pelvic muscles initially but eventually spreads to the involuntary muscles. Muscle weakness produces a waddling gait, toe-walking, and lordosis.
Children with this disorder have difficulty climbing stairs, fall down often, and can’t run properly; their scapulae flare out (or “wing”) when they raise their arms. Calf muscles especially become enlarged and firm. Muscle deterioration progresses rapidly, and contractures develop. Usually, these children are confined to wheelchairs by ages 9 to 12.