Muscle function may be affected by disorders elsewhere, particularly in the nervous system and connective tissue. The conditions described below affect the muscular system directly or involve the muscles and have not been described in other chapters. Any disorder of muscles is described as a myopathy.

Techniques for diagnosing muscle disorders include electrical studies of muscle in action, electromyography (EMG), and serum assay of enzymes released in increased amounts from damaged muscles, mainly creatine kinase (CK).

Muscular Dystrophy

Muscular dystrophy refers to a group of hereditary diseases involving progressive, noninflammatory degeneration of muscles. There is weakness and wasting of muscle tissue with gradual replacement by connective tissue and fat. There also may be cardiomyopathy (disease of cardiac muscle) and mental impairment.

The most common form is Duchenne muscular dystrophy, a sex-linked disease passed from mother to son. This appears at age 3 to 4, and patients are incapacitated by age 10 to 15. Death is commonly caused by respiratory failure or infection.

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