Tremor is a rhythmic, involuntary, smooth oscillation of a body part. Tremors can occur alone or as a symptom of another disorder such as Parkinson's disease. Tremors are not to be confused with tics, which are rapid, involuntary movements of the body. All true tremors are greatly reduced or disappear altogether during sleep.
Tremors are classified into three categories: intention tremors, rest tremors, and postural (or physiological) tremors. Intention tremors occur when a person makes a purposeful movement, such as reaching for a glass. The tremor subsides when the movement ceases. Rest tremors, which are characteristic of Parkinson's disease, occur while the body is at rest, and disappear when the person moves. Postural tremors occur when the limbs are outstretched.
Tremors that usually begin in early adulthood, slowly become more pronounced, and have no known cause are called essential tremors. Essential tremors are postural tremors that can occur at any age, and may become progressively more severe with time. Over half of essential tremor cases are inherited. While essential tremors usually remain mild and do not indicate serious disease, they can become a nuisance, as handwriting and utensil use can be affected. Essential tremors usually stop when the arms or legs are resting, but become obvious when the limbs are outstretched or held in uncomfortable positions. These tremors usually affect one side of the body more than the other, but may involve both sides of the body. If the vocal cords are affected, the voice will shake and quiver. Drinking alcohol reduces the severity of the tremor, but leads to rebound tremors.
Senile tremors are essential tremors that begin in old age. Familial tremors are essential tremors that occur in families.
Over 50% of essential tremors are hereditary. The remaining essential tremors have no known cause. Up to half of essential tremors are inherited as autosomal dominant disorders. This means that the gene for tremor is located on a chromosome other than the sex chromosomes, and that if a parent has the gene for tremor, there is at least a 50% chance that a child of that parent will develop a tremor as well.
A person's tremor, however, can vary in type and severity from that of an affected parent, and may affect different parts of the body.
Some tremors occur as a result of other diseases, and are called symptomatic tremors. Diseases such as Parkinson's disease, Wilson's disease, and peripheral neuropathy may cause symptomatic tremors. Multiple sclerosis, chronic alcoholism, cerebellar damage, or stroke are other conditions that can also cause tremor.
Some tremors have no known origin. Tremors with no known origin are called idiopathic tremors, and include postural tremors and task-specific tremors.
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