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 a more or less involuntary
and rhythmic oscillatory movement produced by alternating or irregularly
synchronous contractions of reciprocally innervated muscles. Its
rhythmic quality distinguishes tremor from other involuntary movements,
and the involvement of agonist and antagonistic muscles distinguishes
it from clonus.
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. 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. Assuredly this is not
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 workers to be variants or
exaggerations of physiologic tremor—i.e., “enhanced
physiologic tremor,” as discussed further on.
Abnormal or pathologic tremor, which is what one means when the
term tremor is used clinically, preferentially
affects certain muscle groups—the distal parts of the limbs
(especially the fingers and hands); less often the proximal parts;
the head, tongue, jaw, or vocal cords; and rarely the trunk—and
is present only in the waking state. The rate in most forms is from
4 to 7 Hz, or about half that of physiologic tremor. In any one
case, the rate is fairly constant in all affected parts.
The following types of tremors, the features of which are summarized
in Table 6-1, are encountered most frequently
in clinical practice. In clinical analysis they are usually distinguishable
on the basis of (1) rhythmicity, (2) frequency, (3) relation to
movement and posture, and (4) the pattern of activity of opposing
muscles, i.e., synchronous or alternating. ...