Patients with these various abnormalities of tone may show little or no impairment of motor power (see below), indicating that the maintenance of posture involves more than the voluntary contraction of muscles. It is noteworthy that the signs of cerebellar dysfunction (dysmetria, clumsiness, tremor) are absent in the hypotonic muscles of peripheral nerve disease—indicating that the cerebellum exerts a unique modulating effect on movement that is beyond its control of muscle tone.
Other Symptoms of Cerebellar Disease
It has been stated by some authors that there is a slight loss of muscular power and fatigability of muscle (asthenia) with acute cerebellar lesions. Insofar as these symptoms cannot be explained by other disturbances of motor function, they may be regarded as primary manifestations of cerebellar disease, but they are never severe or persistent and are of little clinical importance; anything approaching a hemiparesis in distribution or severity is not attributable to cerebellar disease.
Myoclonic movements—i.e., brief (50to 100-ms), random contractions of muscles or groups of muscles—are, in some disease processes, combined with cerebellar ataxia. When multiple myoclonic jerks mar a volitional movement, they may be mistaken for an ataxic tremor. Action myoclonus may be the principal residual sign of postanoxic encephalopathy, referred to as the Lance-Adams syndrome and discussed further in Chap. 39. It has been proposed that this condition has a cerebellar origin. Myoclonus is described more fully in Chap. 4, where it is pointed out that it more often has its origin in diseases of the cerebral cortex.
In addition to its motor functions, it has been established that the cerebellum participates in certain aspects of cognitive function and behavior (see the reviews by Schmahmann and Sherman and by Leiner et al). These authors have described a wide range of subtle alterations of memory and cognition, language function, and behavior in patients with disease apparently limited to the cerebellum (as determined by CT and MRI). However, it is not entirely clear if there is a uniform clinical pathologic syndrome in which a distinctive group of cognitive–behavioral deficits are related to a cerebellar lesions. These recent investigations into the cerebral influences of the cerebellum are accurate and novel contributions to neurology, but at the same time, the changes referred to are subtle in the bedside neurologic examination. Rarely, as in a patient under our care, a recovered aphasia from a prior cerebral infarct was unmasked by an acute cerebellar stroke. Slowly developing cerebellar disorders, such as tumors, do not appear to demonstrate this phenomenon.
Noncerebellar Sources of Ataxia
Ataxia has a distinct appearance to the examiner and is even identifiable by lay persons as wildly erratic movements. However, several clinical abnormalities simulate the incoordination of ataxia, among them myoclonus, spasticity, convulsions, tics, tremors of various types, and psychogenic movement disorders.