This is a coarse, rhythmic tremor with a frequency of 3 to 5 Hz, characterized by bursts of activity that alternate between opposing muscle groups. The tremor is most often localized in one or both hands and forearms and less frequently in the feet, jaw, lips, or tongue (Fig. 4-7D). It occurs when the limb is in an attitude of repose and is suppressed or diminished by willed movement, at least momentarily, only to reassert itself once the limb assumes a new position. Even though it is termed a “resting” tremor, maintaining the arm in an attitude of repose requires a certain degree of muscular contraction, albeit slight. If the tremulous hand is completely relaxed, as it is when the arm is fully supported at the wrist and elbow, the tremor usually disappears. It is difficult, however, for the parkinsonian patient to relax and instead it is typical to maintain a state of slight tonic contraction of the trunk and proximal muscles.
Parkinsonian tremor is “alternating” in the sense that it takes the form of flexion–extension or abduction–adduction of the fingers or the hand; pronation–supination of the hand and forearm is also a common presentation. Flexion–extension of the fingers in combination with adduction–abduction of the thumb yields the characteristic “pill-rolling” tremor of Parkinson disease. The tremor continues and may worsen while the patient walks, unlike essential tremor; indeed, it may first become apparent to the patient during walking. When the legs are affected, the tremor takes the form of a flexion–extension movement of the foot, sometimes the knee. In the jaw and lips, it is seen as up-and-down and pursing movements, respectively. The eyelids, if they are closed lightly, tend to flutter rhythmically (blepharoclonus), and the tongue, when protruded, may move in and out of the mouth at about the same tempo as the tremor elsewhere.
The cogwheel effect is a ratchet-like interruption perceived by the examiner on passive movement of an extremity (the Negro sign) as mentioned earlier. It is said by many authors to be no more than a palpable tremor superimposed on rigidity and as such, is not specific for Parkinson disease although it is most often recognized in that condition. This explanation is called into question by the numerous cases in which Parkinson patients display minimal or no resting tremor but nonetheless have the cogwheel phenomenon. Cogwheeling can be brought out by having the patient engage the opposite limb, such as tracing circles in the air; called the Froment sign, this finding was originally described in essential tremor.
The parkinsonian tremor frequency is surprisingly constant over long periods, but the amplitude is variable. Emotional stress augments the amplitude and may add to the effects of an enhanced physiologic or essential tremor. With advance of the disease, increasing rigidity of the limbs obscures or reduces it. It is curious how little the tremor interferes with voluntary movement; for example, it is possible for a tremulous patient to raise a full glass of water to his lips and drain its contents without spilling a drop; this is not always the case with “benign” essential tremor, as already emphasized.