The subject of tremor is considered at this point because of its association with diseases of the basal ganglia and cerebellum. In addition, a group of miscellaneous movement disorders—myoclonus, facial and cervical dyskinesias, focal limb dystonias, and tics—is described in this chapter. These disorders are largely involuntary in nature and can be quite disabling but they have an uncertain pathologic basis, as alluded to in Chap. 4, and an indefinite relationship to the extrapyramidal motor disorders or to other standard categories of neurologic disease. They are brought together here mainly for convenience of exposition.
Tremor may be defined as involuntary rhythmic oscillatory movement produced by alternating or irregularly synchronous contractions of reciprocally innervated muscles. Its rhythmic quality distinguishes tremor from other involuntary movements, and its oscillatory nature distinguishes it from myoclonus and asterixis.
A normal, or physiologic, tremor is embedded in the motor system. It is present in all contracting muscle groups and persists throughout the waking state and even in certain phases of sleep. The movement is so fine that it can barely be seen by the naked eye, and then only if the fingers are firmly outstretched; in most instances special instruments are required for its detection though asking the patient to aim a laser pointer at a distant target will often expose it. It ranges in frequency between 8 and 13 Hz, the dominant rate being 10 Hz in adulthood and somewhat less in childhood and old age. Several hypotheses have been proposed to explain physiologic tremor, a traditional one being that it reflects the passive vibration of body tissues produced by mechanical activity of cardiac origin, but this cannot be the whole explanation. As Marsden has pointed out, several additional factors—such as spindle input, the unfused grouped firing rates of motor neurons, and the natural resonating frequencies and inertia of the muscles and other structures—are probably of greater importance. Certain abnormal tremors, namely, the metabolic varieties of postural or action tremor and at least one type of familial tremor, are considered by some to be variants or exaggerations of physiologic tremor—i.e., "enhanced physiologic tremor," as discussed further on.
The following types of tremors, the clinical features of which are summarized in Fig. 6-1 and Table 6-1, are encountered most frequently in clinical practice. In clinical analysis they are usually distinguishable on the basis of (1) relation to movement and posture, (2) frequency, (3) the pattern of activity of opposing (agonist-antagonist pairs) muscles, i.e., synchronous or alternating, and (4) affected body parts. Such a classification also differentiates tremors from a large array of nontremorous movements, such as fasciculations, sensory ataxia, myoclonus, asterixis, epilepsia partialis continua, clonus, and rigor (shivering).
Table 6–1 Main Types of Tremor