Myotonic Dystrophy

An autosomal dominant multisystem disorder caused by an abnormality on chromosome 19 that results in slowly progressive muscle atrophy.

Usually, myotonic dystrophy characterized by distal weakness develops in persons in their 20s and 30s; however, the elderly may present with mild forms. In the elderly, the primary symptoms may be frontal baldness and posterior capsular cataracts. Other symptoms include minimal muscle weakness with cardiac conduction defects (eg, atrioventricular block, bundle branch block), gastrointestinal hypomotility, and respiratory insufficiency caused by weakness of the respiratory muscles, primarily the diaphragm. Tendon reflexes are depressed, especially in the legs.

Diagnosis is established by DNA testing. Muscle biopsy findings and mildly to moderately elevated serum CK levels have secondary diagnostic value. The long-term prognosis depends on the severity of muscle weakness and the presence of complications.

Treatment is nonspecific. The goal is to manage complications: respiratory insufficiency; cardiac arrhythmias, particularly conduction block; and gastrointestinal hypomotility. Surgery can be problematic, because patients develop paradoxic rigidity in reaction to succinylcholine and other muscle relaxants; nondepolarizing relaxants (eg, vecuronium, atracurium) are recommended.