A third type, formerly thought to be a variant of the Mount-Reback type mentioned above, is precipitated by prolonged exercise. In addition to a response to benzodiazepines, it has the unique characteristic of improving with acetazolamide.
More common than these familial dyskinesias are sporadic cases secondary to focal brain lesions such as stroke, trauma, encephalitis, perinatal anoxia, multiple sclerosis, HIV encephalitis or as a result of associated toxoplasmosis, lymphoma; and also generalized metabolic disorders such as hypoparathyroidism, thyrotoxicosis, and particularly, nonketotic hyperosmolarity. Demirkirian and Jankovic classified the acquired paroxysmal dyskinesias according to the duration of each attack and the event or activity that precipitates the abnormal movements (kinesigenic, nonkinesigenic, exertional, or hypnagogic). As with the familial cases, the acquired kinesigenically induced movements often improve with antiepileptic drugs; some cases respond particularly to clonazepam.
The most severe instances of paroxysmal dyskinesia in our experience have been related to the previously mentioned multiple sclerosis (“tetanoid spasms”), and from the secondary brain lesions of HIV. These patients were relatively unresponsive to medications. Also, it should be recalled that oculogyric crises and other nonepileptic spasms have occurred episodically in patients with postencephalitic parkinsonism; these phenomena are now rarely seen with acute and chronic phenothiazine intoxication and with Niemann-Pick disease (type C).
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 the other involuntary movements described earlier, and its oscillatory nature distinguishes it from myoclonus and asterixis. The many varieties of tremor can be considered in terms of their frequency, amplitude, location, and positional activation, and the enhancement or attenuation of the tremor by certain drugs. In some processes, such as Parkinson disease, more than one tremor may be displayed and tremor may be a component of other movement disorders such as dystonia and cerebellar ataxia. The characteristics of the main tremors seen in practice are summarized in Table 4-6.
A normal, or physiologic, tremor is embedded in the motor system. The movement is so fine that it can barely be seen by the naked eye, and then only if the fingers are firmly outstretched; asking the patient to aim a laser pointer at a distant target will often expose the tremor. It is present in all contracting muscle groups and persists throughout the waking state and even in certain phases of sleep. 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—that is, “enhanced physiologic tremor,” as discussed further on.