Tremor is an involuntary oscillation of a group of muscles resulting in movement of an arm or a leg. In tremor, each oscillation resembles the previous one: they are rhythmical. However, the frequency and amplitude of tremors differ from disease to disease. Tremors can be classified as mild (fine and barely noticeable), mild-to-moderate (noticeable but intermit-tent), moderate (noticeable and present most of the time), or marked (high amplitude and present most of the time). Some of the Parkinson-like disorders dis-cussed in Question 24 involve tremors. Essential tremor (ET), a hyper-kinetic disorder, is actually the most common movement disorder—in fact it is 20 times more common than PD. However, fewer than 2% of people with ET have symptoms sufficiently disabling to require treatment.
The tremor of ET can usually, but not always, be distinguished from the tremor of PD. Generally, the tremor of ET starts in both hands simultaneously, while the tremor of PD usually starts in only one hand. The tremor of ET is a sustention or postural tremor, a tremor that appears when you hold your hands in front of you, contracting the muscles of your forearm while maintaining (or sustaining) a posture. ET is also an action, or kinetic, tremor; a tremor that appears when you move or use your hands. Initially ET can be bothersome or embarrassing, but not disabling.
However, after several years, ET may become disabling and impair or limit such everyday activities as dressing, feeding, shaving, and writing. Although a postural or sustention tremor is characteristic of ET, a similar tremor occurs in 20% of PD patients. In PD, the postural tremor begins in one hand. PD patients can have both a resting and a postural tremor. Such a tremor is called a complex or compound tremor. When a sustention tremor appears in one hand, with no other symptoms of PD, it may be difficult to predict whether the tremor will evolve into PD or ET. In some people ET evolves into PD, and in some families there are people with ET, people with PD, and people with ET and PD. At present, however, there is not enough data to say whether ET and PD are related.
Physiologic tremor is the tremor that is present in everyone, the tremor that is brought out with stress, a fever, or with an over-active thyroid gland. The tremor of ET and physiological tremor are thought to arise from over-activity of a “tremor circuit” in a region of the brain below the basal ganglia. Many experts believe ET is an exaggerated form of physiological tremor, one that does not go away.
Others believe ET and physiological tremor are separate and distinct. One or two drinks of alcohol may help ET, presumably by suppressing the tremor circuit. However, alcohol usually does not help the tremor of PD. A word of caution: alcohol overuse or abuse can, in time, aggravate ET. And withdrawal from alcohol after heavy drinking can itself result in a tremor: alcoholic withdrawal tremor. Whether the tremor of alcohol abuse or alcohol withdrawal is a separate tremor or an exaggeration of a physiological tremor or of ET is unknown at present.
Dystonic tremor refers to a tremor that appears in a hand, leg, or neck that is dystonic: a disorder in which a muscle or group of muscles is abnormally or over-contracted, the tone is increased, and movement is compromised. While one set of muscles is contracted, the opposing muscles are also contracted, instead of relaxing, as though your muscles are “fighting” each other. Dystonic tremor is task-specific: it appears when you attempt a specific task such as writing, playing a musical instrument, throwing a baseball, or putting in golf (where it may be called the “yips”).
It is as though the particular task has become “over-learned” and can-not be performed when you want to perform it. Dystonic tremor may occur independently of other disorders or, rarely, it may appear as part of PD. The treatment of dystonic tremor is injection of Botox into the over-contracted muscles.
Tremor may also be caused by several disorders or drugs. Such a tremor is thought to be an exaggeration of the normal physiologic tremor resulting from over-activity of the tremor circuit. The tremor is bilateral, in both hands, and is a postural, sustention, or action tremor, not a resting tremor, nor is it associated with rigidity or bradykinesia. The tremor is associated with other symptoms of the disorder, or is time-related to the start of the tremor-causing drug. The tremor dis-appears when the disorder is treated or after the offending drug is stopped. Among the more common drugs that cause tremors are:
- Amiodarone or Cordarone—used to regulate the heartbeat
- Dolbutamine—used in the hospital to maintain blood pressure and blood flow
- Ephedrine and Phenylephrine—nasal decongestants
- Haloperidol or Haldol—used to control abnormal behavior such as schizophrenia
- Lidocaine—a local anesthetic drug also used in the hospital to regulate the heartbeat
- Lithium—used to treat bipolar disorder
- Metoclopramide or Reglan—used to regulate gastric motility
- Valproic acid or Depakote—an anti-seizure drug used to treat epilepsy and bipolar disorder