In these patients, difficulty in standing and walking cannot be attributed to spasticity or paralysis. Hypotonia and poverty of movement are the initial motor abnormalities—as they are in athetoid cerebral palsy. The cerebellar deficit becomes manifest only later when the patient begins to sit, stand, and walk. There may or may not be a delay in reaching the normal motor milestones. Attempts to attain sitting balance early on reveal an unsteadiness that is not soon overcome, even with practice. Reaching for a proffered toy is accomplished by jerky, incoordinated movements. The first steps are unsteady, as would be expected, with many tumbles, but the gait remains clumsy. Instability of the trunk may be accompanied by similar, more or less rhythmic bobbing movements of the head—titubation. Despite the severity of the ataxia, the muscles are of normal size, and voluntary movements, although weak in some patients, are possible in all the limbs. The tendon reflexes are present, and the plantar reflexes are either flexor or extensor. In some cases, the ataxia is later associated with spasticity rather than hypotonia (spastic-ataxic diplegia). Relative improvement may occur in later years. In the older child, a cerebellar gait, ataxia of limb movements, nystagmus, and uneven articulation of words are readily distinguished from myoclonus, chorea, athetosis, dystonia, and tremor.
In only a few cases have the pathologic changes been studied. Aplasia or hypoplasia of the cerebellum has been observed, but sclerotic lesions of the cerebellum are more common. The CT or MRI verifies the cerebellar atrophy. However, in a few cases of a cerebellar-like tremor in adults under our care that had been attributed to neonatal injury the MRI did not show cerebellar atrophy. A cerebral and cerebellar lesion may coexist in patients with congenital ataxia, which is the reason for the term cerebrocerebellar diplegia.
Several risk factors have been identified in the congenital ataxias. Most importantly, cerebellar ataxia may be the most prominent or sole effect of neonatal ischemia-hypoxia. A genetic factor is operative in some cases (Hagberg and Hagberg). Mercury poisoning in utero is another cause of congenital ataxia. The many cases that are not the result of a degenerative condition, some of which are described just below, remain unexplained in our experience.
Aside from the congenital ataxia described above, there are several rare familial forms in which a failure of cerebellar development is associated with developmental delay. What has now come to be called Joubert syndrome was reported in a family in which the central feature is dysgenesis of the vermis; developmental delay; episodic hyperpnea; irregular, jerky eye movements; and unsteady gait in 4 of 6 siblings. In other reports, choroidal-retinal colobomas, polydactyly, cryptorchidism, and prognathism have been mentioned. Detailed examination of the cerebra of such individuals has been lacking but the MRI has a characteristic configuration of a “molar tooth sign” that reflects a deep invagination caused by vermian hypoplasia with a narrow cleft separating the cerebellar hemispheres and thickening of the superior cerebellar peduncles. Several genetic loci have been implicated, most acting as recessive traits.