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Essential Tremor

Essential tremor, the most common of all movement disorders, usually begins as a postural tremor of both arms, often asymmetrically. It may progress to involve the head, voice, and, less commonly, the trunk and legs. It often occurs in families and is transmitted as an autosomal dominant trait with variation in age at onset and severity, even within a kindred. The pathology and pathophysiology are unknown, but some evidence implicates brain stem circuits involving the inferior olive. Surveys have shown prevalence rates of 0.41 to 5.6% of adults over age 40 in different populations.

The onset usually occurs in adulthood. Initially, the tremor may be apparent only during periods of emotional stress when it is felt as transient "nervousness." At first, the tremor is described best as tremulousness, with a frequency of 8 to 12 Hz and appearing in both hands. The oscillation frequency tends to decrease with increasing amplitude of the tremor and age of the person. The tremor is most prominent when the arms are held outstretched. It diminishes with the initiation of movement, but reappears to interfere with such functions as writing and feeding. The tremor is rarely present at rest, in contrast to the tremor of Parkinson disease. The tremor occasionally appears in children or adolescents; it may not be seen until advanced age (senile tremor).

The condition may remain static for many years, even decades, but usually increases gradually in severity and often spreads to involve the head in a rhythmic bobbing that may be either vertical or horizontal. It then may spread to the vocal cords or diaphragm.

Treatment.

Beta blockers and primidone, alone or in combination, are the most effective pharmacologic therapies. Propranolol has been used in doses up to 240 mg daily; primidone may be effective in doses of 50 mg or less daily. Both drugs reduce the amplitude of tremor in some patients, but do not abolish the tremor; benefit may disappear with time. Stereotactic thalamotomy can reduce essential tremor in the contralateral limbs, but the condition is rarely sufficiently disabling to warrant brain surgery. Clonazepam, methazolamide, glutethimide, and clozapine have been reported.

Differential Diagnosis.

Essential tremor is frequently misdiagnosed as parkinsonism, especially in the elderly patient. Differentiation may readily be made, however, by the absence of parkinsonian features, such as rest tremor, muscular rigidity, bradykinesia, or loss of postural control. Handwriting is large, irregular, and tremulous in striking contrast to the micrographia of parkinsonism. Head tremor rarely occurs in parkinsonism; instead, tremor of the lips, tongue, and jaw occurs. Patients with a long history of tremor of both hands occasionally develop typical signs and symptoms of Parkinson disease. It is not known whether this development is a coincidence of common condition or whether patients with essential tremor have an increased risk of parkinsonism. Absence of dysdiadochokinesis, dysmetria, exaggeration of the tremor with intention, and other cerebellar signs distinguishes essential tremor.

Hyperthyroidism, lithium or valproate intoxication, chronic alcoholism, drug addiction, and enhanced physiologic tremor are usually excluded by laboratory tests and clinical evidence. One form of tremor of the trunk and legs appears on standing and is relieved by walking; this orthostatic tremor may be a form of essential tremor. Task-specific tremors, such as tremors restricted to the act of writing, or tremor associated with a task-specific dystonic posturing, such as in writer's cramps, are diagnostic gray areas between essential tremor.