Muscle twitches are brief, painless, involuntary movements of parts of muscles. Some are visible under the skin, others are just felt. Disorders affecting any part of the nervous system that controls movement, or the muscles themselves, can result in muscle twitches. While some of these are serious and even life-threatening, it is important to note that common, harmless conditions may also cause muscle twitches.

Brain and Spinal Cord Disorders

Muscle twitching accompanied by progressive weakness and muscle loss is part of amyotrophic lateral sclerosis -- also known as ALS or Lou Gehrig disease. Other ALS symptoms include difficulty swallowing and breathing. Muscle twitching is part of another disorder called spinal muscular atrophy -- the most common genetic cause of infant death -- which causes progressive weakness of the arms and legs and trouble breathing and swallowing. Muscle twitching may also occur when movement-related nerve cells in the brain and spinal cord are injured due to tumors, infections, injury or immune diseases in which the body attacks its own tissue.

Nerve Disorders

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Muscle twitching may occur when the nerves that control movement are irritated or damaged. This can happen when the body's immune system attacks its own nerves, which may occur spontaneously or when a cancer is present. Muscle twitching may also happen after nerves are damaged by infections, such as HIV, or toxins, such as lead and certain insecticides. Radiation or an abnormal buildup of metabolic byproducts, such as occurs with kidney failure, can also damage nerves and lead to twitching. Hereditary nerve diseases may also be associated with muscle twitching.

Muscle Disorders

Muscle twitches are not typical of many genetic muscle diseases, such as muscular dystrophy 1. However, they may be seen in muscle disease due to hormonal imbalances. Muscle abnormalities due to low or high thyroid hormone levels, for example, may be associated with muscle twitching. High levels of another hormone called parathyroid hormone -- which is important in regulating calcium and phosphorus levels in the blood -- is also associated with muscle twitching.

Harmless Muscle Twitching

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Some muscle twitches are harmless and unrelated to an underlying medical condition. For example, muscle twitching in the eyelid can be triggered by caffeine, fatigue or stress. Eyelid twitching is generally harmless unless associated with a change in vision, twitching elsewhere in the face or facial weakness. Another condition called benign fasciculation syndrome, occurs in healthy people who experience twitching -- especially in the calf muscles -- often triggered by exercise. People who experience this may also have muscle cramping, but not muscle weakness.

When to See a Doctor

See your doctor as soon as possible if you experience muscle twitching that is accompanied by pain, weakness, loss of sensation or tingling, changes in vision or hearing, trouble with balance, or difficulty speaking or swallowing.