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Types of Myopathies

Types of Myopathies

There are many different types of myopathies. These can be described as;

Inherited myopathies

Inherited myopathies are caused by a genetic defect. The most common muscular dystrophies, Duchenne and Becker muscular dystrophy, result from a genetic defect on the X chromosome.

Endocrine myopathies

Endocrine myopathies are 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 (eg. Addison disease) or chronic administration of steroids, 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, causes myopathy.
  • Excess parathyroid hormone results in hypercalcemia, which causes proximal muscle pain and weakness.
  • Hormone-secreting tumors can also cause endocrine disorders that cause myopathies.

Inflammatory myopathies

Inflammatory myopathies are autoimmune disorders (ie. the body's immune system mistakenly attacks healthy tissue). In the case of myopathies, it attacks healthy muscle fibres and causes inflammation, which in turn damages the muscle.

  • Polymyositis (PM) causes muscle aches, cramping, and tenderness. The muscle weakness is severe 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 and loss of appetite.
  • Dermatomyositis (DM) is characterised by a skin rash as well as 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.

Toxic myopathies

Toxic myopathies are caused by exposure to certain medications and chemicals. Excessive alcohol intake can also damage skeletal muscle. Drugs and chemicals that can cause myopathy include the following:

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

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