Unlike the animal experiments mentioned earlier, it is uncertain if disequilibrium can result from lesions of the flocculonodular lobe alone. Attribution of gait instability to damage in this area rests on the observation that medulloblastomas may cause unsteadiness but no tremor or incoordination of the limbs. Insofar as these tumors are thought to originate from cell rests in the posterior medullary velum, at the base of the nodulus, it has been inferred that the disturbance of equilibrium results from involvement of this portion of the cerebellum. By the time such tumors are imaged or viewed at operation or autopsy, however, they have spread beyond the confines of the nodulus, and strict clinicopathologic correlations are not possible.
The point to be made is that midline anterior cerebellar lesions may produce solely a disorder of stance and gait, i.e., nystagmus, dysarthria, and limb ataxia are absent. Moreover, the entire problem may be missed if the examination does not include an assessment of the patient’s ability to stand and walk.
This refers to a decrease in the normal resistance that is offered by muscles to passive manipulation. It is the least conspicuous of the cerebellar signs of disease but may explain certain clinical features of the ataxic syndrome. As mentioned earlier, Holmes believed that hypotonia was a fundamental defect in cerebellar disease, accounting not only for the defects in postural fixation (see later) but also for certain elements of ataxia and the tremor.
Hypotonia is related to a depression of gamma and alpha motor neuron activity, as discussed in Chap. 3. Experimentally in cats and monkeys, acute cerebellar lesions and hypotonia are associated with a depression of fusimotor efferent and spindle afferent activity. With the passage of time, fusimotor activity is restored and hypotonia disappears (Gilman et al).
Hypotonia is much more apparent with acute than with chronic lesions and may be demonstrated in a number of ways. A conventional test for hypotonia is to tap the wrists of the outstretched arms and to detect a displacement through a wider range than normal in the affected limb (or both limbs in diffuse cerebellar disease), sometimes causing the limb to oscillate; this is presumably the result of a failure of the hypotonic muscles to fixate the arm at the shoulder. Another test for hypotonia is to shake the affected limb and demonstrate that the flapping movements of the hand are of wider excursion than normal. If the patient places his elbows on the table with the arms flexed and the hands are allowed to hang limply, the hand of the hypotonic limb will sag. If the standing patient is rotated to and fro at the shoulders, the hypotonic arm will be seen to continue to swing after the other has come to rest. Holmes also attributed pendular knee jerks to hypotonia but he described a more complex phenomenon in which the normal contraction of the antagonist hamstring fails to occur. This deficiency may be palpated by the examiner during reflex testing. Babinski also was impressed with gross alterations of posture related to hypotonia. These take the form of extension of the neck and involuntary bending of the knees, which are apparent when the patient is lifted from a bed or chair or upon first standing, or slumping of the shoulder on the affected side.
Failure to check a movement is a closely related phenomenon. Thus, after strongly flexing one arm against a resistance that is suddenly released, the patient may be unable to check the flexion movement, to the point where the arm may strike the face. This is the result of a delay in contraction of the triceps muscle, which ordinarily would arrest overflexion of the arm. Stewart and Holmes, who first described this test, made the point that when resistance to flexion is suddenly removed, the normal limb moves only a short distance in flexion and then rebounds very briefly in the opposite direction. Thus, although the two are often conflated, there is a subtle difference between the signs of deficient checking and an excessive rebound of the limb that follows.