Myopathies

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Other Types

Endocrine Myopathies
Endocrine myopathies are caused by underlying conditions caused by the over- or underproduction of hormones. These conditions can develop in children and adults and usually respond well to treatment.

• Steroid myopathy is the most common endocrine muscle disease. Steroid excess, whether caused by an adrenal gland disorder (e.g., Addison disease) or chronic administration of glucocorticoid drugs, causes muscle weakness and wasting.
• Hyperthyroid myopathy is caused by the thyroid gland producing too much thyroxine. Its symptoms include weakening and wasting of the muscles, especially in the shoulders and hips, and sometimes the eyes.
• Hypothyroid myopathy is caused by the underproduction of thyroxine and results in muscle weakening in the legs and arms. The muscles may become enlarged.
• Cushing's disease, characterized by overproduction of hormones produced by the pituitary and adrenal glands, cause myopathy.
• Excess parathyroid hormone results in hypercalcemia, which causes proximal muscle pain and weakness.
• Hormone-secreting tumors (e.g., growth-hormone secreting pituitary adenoma) can cause endocrine disorders that produce myopathy.

Inflammatory Myopathies
Inflammatory myopathies are autoimmune disorders. An autoimmune disorder is caused by the body's immune system mistakenly attacking healthy tissue. In this case, it attacks healthy muscle fibers and causes inflammation, which in turn damages the muscle. It is not known what triggers this autoimmune response.

The severity and progression of these myopathies vary considerably. Some people develop other disorders, such as abnormal heart rhythms, lung disease, gastrointestinal problems, arthritis, or cancer.

• Polymyositis (PM) can occur at any age in either sex, but is more common in children and in women between 40 and 60 years old. Most people with PM suffer muscle aches, cramping, and tenderness. The muscle weakness is, however, quite intense and may fluctuate over weeks to months. It is often worse in the neck, arms, and thighs, making it difficult to stand up from a sitting position. Many patients also experience fever, general discomfort (malaise), and loss of appetite.
• Dermatomyositis (DM) is characterized by a skin rash and all of the muscle symptoms of PM. The rash is a purple discoloration around the eyes and on the cheeks but may also appear on other parts of the body. Eventually the skin becomes thin and fragile. DM most commonly develops in children between the ages of 5 and 14 years. People who have DM have an increased risk for developing cancer.

Toxic Myopathies

Exposure to certain medications, chemicals, and excessive alcohol intake can damage skeletal muscle. Drugs and types of chemicals that can cause myopathy include the following:

• Anesthetics (e.g., lidocaine, mepivacaine, ethyl chloride)
• Cholesterol-lowering agents (e.g., clofibrate, genfibrozil, lovastatin, simivastatin, niacin)
• Glucocorticoids (e.g., triamcinolone, dexamethasone, betamethasone)
• Narcotics (e.g., cocaine, heroin, meperidine)
• Other drugs (e.g., zidovudine, D-penicillamine, procainamide, chloroquine, gallamine)
• Herbicides, insecticides, flame retardants

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