MYOPATHY

any skeletal muscle disorder that originates within the muscle fibres and does not arise secondarily from disorders of the nervous system. Myopathies are marked by muscular degeneration and weakness and may have a genetic or nongenetic basis. Heritable myopathies include the muscular dystrophies, a group of metabolic diseases in which muscle fibres, predominantly of the pelvis and shoulder areas, atrophy. (See muscular dystrophy.) The most common muscular dystrophy is Duchenne's, an X-chromosome-linked disorder involving progressive destruction of muscle tissue that usually results in death before the age of 20. Some muscle disorders, such as the mitochondrial and lipid storage myopathies, result from a genetic defect in an enzyme necessary in metabolism. Other muscle disorders can be caused by external factors such as alcohol or steroid hormones. Inflammatory myopathy, or myositis, can arise from bacteria, viruses, or parasites that specifically invade the muscle. A well-known example is trichinosis, which is caused by infection with the pork roundworm, Trichinella spiralis. In domestic animals azoturia, or blackwater, a myopathy of horses, causes hindleg paralysis; the disease may disappear in hours with rest or may progress to death within four days. The urine, voided with difficulty, may be red, brown, or blackish with pigment from destroyed muscle cells. Cording up (also called tying up), or myositis of horses, may be a mild form of azoturia. White muscle diseases of calves, lambs, and foals are severe and often fatal myopathies. The aforementioned disorders respond variably to vitamin E and selenium supplements, suggesting a deficiency mechanism. A related disorder, steatitis, or yellow fat disease, occurs in cats. It is marked by fever, inflammation of fatty tissue, and pain on touch. Food rich in fat from marine animals appears to be a contributing factor. Treatment includes vitamin E supplement and change of diet. German shepherd dogs are susceptible to a myositis of unknown cause, in which the jaw muscles and eventually the esophagus are involved, making eating and swallowing difficult. There is no cure; discomfort can be minimized by drugs. Other myopathies of mysterious origin occur in swine, cattle, horses, and rabbits.