A tremor is an involuntary, rhythmic, oscillating movement in the muscles of part of the body; the most commonly affected muscles are those of the hands, feet, jaw, tongue, or head. Tremor is the result of rapidly alternating muscle contraction and relaxation.
Occasional tremors are experienced by the majority of people and are due to increased production of the hormone adrenaline (epinephrine). A slight, persistent tremor is common in many elderly people.
Essential tremor, which is a type that runs in families, is a fine-to-moderate tremor (about six to ten movements per second) that may be temporarily relieved by consuming a small amount of alcohol, or by taking beta-blocker drugs.
Coarse tremor (about four to five movements per second) is present at rest but reduced during movement, and is often a sign of the movement disorder Parkinson's disease. An intention tremor (worse on movement of the affected part) may be a sign of cerebellar ataxia.
Tremor may be caused by multiple sclerosis, Wilson's disease, mercury poisoning, hepatic encephalopathy, thyrotoxicosis, or drugs, such as amphetamines and caffeine, and withdrawal from drugs, including alcohol.