As will be discussed in Chap. 5, the word intention is ambiguous in this context because the tremor itself is not intentional and occurs not when the patient intends to make a movement but only during the most demanding phases of active performance. In this sense it is a kinetic or action tremor, but the latter term has connotations of the essential tremor to neurologists, as described earlier. The term ataxic is a suitable substitute for intention, because this tremor is always combined with cerebellar ataxia and adds to it. Its salient feature is that it requires for its full expression the performance of an exacting, precise, projected movement. The tremor is absent both when the limbs are inactive and during the first part of a voluntary movement, but as the action continues and fine adjustments of the movement are demanded (e.g., in touching the tip of the nose or the examiner’s finger), an irregular interruption of forward progression appears. These side-to-side oscillations are more or less rhythmic and may continue for several beats after the target has been reached. Unlike essential and parkinsonian tremors, the oscillations occur in more than one plane but are mainly horizontal and perpendicular to the trajectory of movement. The tremor and ataxia may seriously interfere with the patient’s performance of skilled acts. In some patients there is a rhythmic oscillation of the head on the trunk (titubation), or of the trunk itself, at approximately the same rate. As already indicated, this type of tremor points to disease of the cerebellum or its outflow connections, but certain peripheral nerve diseases may simulate it.
Ataxic tremor has been produced in monkeys by inactivating the deep cerebellar nuclei or by sectioning the superior cerebellar peduncle or the brachium conjunctivum below its decussation. A lesion of the nucleus interpositus or dentate nucleus causes an ipsilateral tremor of ataxic type, as one might expect, associated with other manifestations of cerebellar ataxia. In addition, such a lesion gives rise to a “simple tremor,” which is the term that Carpenter applied to a “resting” or parkinsonian tremor. He found that the latter was most prominent during the early postoperative period and was less enduring than ataxic tremor. Nevertheless, the concurrence of the two types of tremor and the fact that both can be abolished by ablation of the contralateral ventrolateral thalamic nucleus suggest that they have related neural mechanisms, at least in monkeys.
There is another, higher amplitude tremor associated with cerebellar ataxia, in which every movement, even lifting the arm slightly or maintaining a static posture with the arms held out, results in a wide-ranging, rhythmic 2to 5-Hz “wing-beating” movement. This tremor can be of sufficient force to throw the patient off balance. In such cases, the lesion is usually in the midbrain, involving the rostral projections of the dentatorubrothalamic fibers and the medial part of the ventral tegmental reticular nucleus. Because of the location of the lesion in the region of the red nucleus, Holmes originally called this a rubral tremor. However, experimental evidence in monkeys indicates that the tremor is produced not by a lesion of the red nucleus per se but by interruption of fibers that traverse this nucleus—that is, the cerebellar efferent fibers that form the superior cerebellar peduncle (Carpenter). This type of tremor is seen most often in patients with multiple sclerosis or Wilson disease, occasionally with vascular and other lesions of the tegmentum of the midbrain and subthalamus, and rarely as an effect of antipsychosis medications. Beta-adrenergic blocking agents, anticholinergic drugs, and L-dopa have little therapeutic effect. It is abolished by a surgical lesion in the opposite ventrolateral nucleus of the thalamus. Thalamic stimulation may be particularly helpful in severe cases that are the result of demyelinating lesions in the cerebellar peduncles.