Tremor is an unintentional (involuntary), rhythmical alternating movement that may affect the muscles of any part of the body. Tremor is caused by the rapid alternating contraction and relaxation of muscles and is a common symptomof diseases of the nervous system (neurologic disease). Occasional tremor isfelt by almost everyone, usually as a result of fear or excitement. However,uncontrollable tremor or shaking is a common symptom of disorders that destroy nerve tissue such as Parkinson's disease or multiple sclerosis. Tremor mayalso occur after stroke or head injury. Other tremor appears without any underlying illness.

The cause of essential tremor not linked to any other problem, is not known,although it is an inherited problem in more than half of all cases. The genetic condition has an autosomal dominant inheritance pattern, which means thatany children of an affected parent will have a 50% chance of developing the condition. Essential tremor most often appears when the hands are being used,whereas a person with Parkinson's disease will most often have a tremor whilewalking or while the hands are resting. People with essential tremor will usually have shaking head and hands, but the tremor may involve other parts ofthe body. The shaking often begins in the dominant hand and may spread to theother hand, interfering with eating and writing. Some people also develop aquavering voice. Essential tremor affects men and women equally. The shakingoften appears at about age 45, although the disorder may actually begin in adolescence or early adulthood. Essential tremor that begins very late in lifeis sometimes called "senile tremor."

Several different classes of drugs can cause tremor as a side effect. These drugs include amphetamines, classify="antidepressants drugs" project="woh" type="bookxref">antidepressant drugs, antipsychotic drugs, caffeine, and lithium. Tremor also may be a sign of withdrawal from alcohol or street drugs.

Close attention to where and how the tremor appears can help provide a correct diagnosis of the cause of the shaking. The source of the tremor can be diagnosed when the underlying condition is found. Diagnostic techniques that makeimages of the brain, such as computed tomography scan (CT scan) or magnetic resonance imaging (MRI), may help form a diagnosis of multiple sclerosis or other tremor caused by disorders of the central nervous system. Blood tests can rule out metabolic causes such as thyroid disease. A family historycan help determine whether the tremor is inherited.

Neither tremor nor most of its underlying causes can be cured. Most people with essential tremor respond to drug treatment, which may include propranolol,primidone, or a benzodiazepine. People with Parkinson's disease may respondto levodopa or other antiparkinson drugs.

Research has shown that about 70% of patients treated with botulinum toxin A(Botox) have some improvement in tremor of the head, hand, and voice. Botulinum is derived from the bacterium Clostridium botulinum. This bacteriumcauses botulism, a form of food poisoning. It is poisonous because it weakens muscles. A very weak solution of the toxin is used in cases of tremor and paralysis to force the muscles to relax. However, some patients experience unpleasant side effects with this drug and cannot tolerate effective doses. Forother patients, the drug becomes less effective over time. About half of patients don't get relief of tremor from medications at all.

Tremor control therapy is a type of treatment using mild electrical pulses tostimulate the brain. These pulses block the brain signals that trigger tremor. Some patients experience complete relief with this technique, but for others it is of no benefit at all. About 5% of patients experience complicationsfrom the surgical procedure, including bleeding in the brain. The procedure causes some discomfort, because patients must be awake while the implant is placed. Batteries must be replaced by surgical procedure every three to five years.

A patient with extremely disabling tremor may find relief with a surgical technique called thalamotomy, in which the surgeon destroys part of the thalamus. However, the procedure is complicated by numbness, balance problems, or speech problems in a significant number of cases. Pallidotomy is another type ofsurgical procedure sometimes used to decrease tremors from Parkinson's disease. In this technique, the surgeon destroys part of a small structure withinthe brain called the globus pallidus internus. The globus is part of the basal ganglia, another part of the brain that helps control movement. This surgical technique also carries the risk of disabling permanent side effects.

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