Elvis Presley might have found it exciting to be "all shook up," but for the millions of persons with essential tremor (ET), the story is just the opposite. Eighty-five percent of affected persons report that ET significantly compromises their professional and personal lives.1
Furthermore, a study of persons with hereditary ET found that 60% were unemployed; 25% changed jobs or retired early; 65% did not dine out; 30% avoided parties, participating in sports, or solo shopping; and 20% stopped driving.2
The most common movement disorder in the United States, ET is estimated to affect 0.3% to 5.6% of the general population, or roughly 5 to 10 million persons.3 Men and women appear to be equally affected.4 A 45-year population-based study of persons from Rochester, Minnesota, reported an age-adjusted annual ET incidence of 18.3 and 17.1 per 100,000 for men and women, respectively.5
While it is tempting to think of tremor as a geriatric issue, ET targets 2 age groups: those aged 15 to 20 years and those aged 50 to 70 years.4 It is not unusual, however, for ET to appear in persons aged 35 to 45 years. Rarely, newborns and infants are affected.1
Nobody knows for certain what causes ET.6-9 No evidence suggests that the earlier ET begins, the more severe and disabling it becomes,1 although its characteristics may change over time. Similarly, there is no evidence to indicate that ET has any effect on mortality.1,4
WHAT IS TREMOR?
By definition, tremor is "an involuntary, rhythmic oscillation of a body part within a fixed plane, involving alternating or simultaneous contractions of agonist and antagonist muscles entrained by a signal pattern originating from a central oscillator."10 Everyone has physiologic tremor, which is an asymptomatic, small-amplitude tremor that probably originates from spontaneous oscillatory activity within the olivocerebellar system of the brain.4 Physiologic tremor is influenced by such factors as myocardial contraction and motor neuron firing and can be enhanced under certain circumstances, including exercise, emotional stress, metabolic abnormalities, and exposure to some drugs (eg, sympathomimetic agents, lithium).10
Tremor is generally classified as being either resting or action.4 Resting tremor, which includes that seen in parkinsonism, is present when the body is fully supported in a way that requires no voluntary activation of skeletal muscles—sitting in a chair with hands resting in the lap, for instance. Resting tremor is suppressed by voluntary movement of the affected body part. If you were to evaluate someone with resting tremor, you would notice that his or her hands were shaky at rest but became steady when reaching out to pick up a book or perform some other intentional activity.
Action tremor occurs during voluntary muscle contraction and can be postural, kinetic, or isometric. ET falls into the postural and kinetic categories. Postural tremor occurs when a person voluntarily tries to maintain a position against the pull of gravity, while kinetic tremor occurs during an intended movement such as drawing or pointing. One type of kinetic tremor, intention tremor, worsens as the person is almost done with his activity or task. Another type of action tremor, task-specific tremor, occurs only during specific, skilled activities—writing or playing the piano, for instance. Isometric tremor occurs when a person voluntarily contracts a muscle against a rigid stationary object, as when squeezing a stick.4
CHARACTERISTICS OF ET
The sine qua non of ET is a bilateral postural or kinetic tremor affecting the distal upper extremities. Occasionally, patients have ET affecting the head. Tremor can involve the lips, chin, voice, palate, and tongue—which all affect speech.4 Head tremor can occur alone, but more often follows upper extremity involvement. Alexander Rajput, MD, of the University of Saskatchewan in Canada, in describing a cross section of his patients with ET, notes that about 75% of patients have tremor that originates in an upper extremity; 20% have tremor that starts in an upper extremity and the head, lips, jaw, or vocal cords; and 5% have tremor limited to the head alone.11 There is some thought that hand tremor is more common in men, while women tend to have head involvement, but this is speculative.1 Isolated leg tremor is not a presenting finding of ET and suggests that tremor is related to something else. "We have not found leg tremor to be an initial sign of ET," observes Rajput.11 However, Rajput does point out that he and others have seen that ET can extend to the lower extremities over time.11
The onset of ET is gradual. As the person ages, the frequency of tremor declines but its amplitude increases. This makes it increasingly more difficult for the patient to function normally. Elan Louis, MD, of the Gertrude H. Sergievsky Center of Columbia University in New York City, notes that as function declines, persons with ET tend to become increasingly anxious and depressed.12
Classically, ET occurs alone, unaccompanied by any other neurological disorders. However, you might occasionally encounter a patient who has subtle signs of ataxia or cognitive impairment.13 Some patients with ET also have migraine headache or report being unable to hear or smell as acutely as before the tremor started.14-16
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