Autosomal dominant cerebellar ataxia type I. Nerve conduction and evoked potential studies in families with SCA1, SCA2 and SCA3.

Forty-one patients suffering from autosomal dominant cerebellar ataxia type I (ADCA-I) were subjected to a genotype-phenotype correlation analysis using molecular genetic assignment to the spinocerebellar ataxia type 1, 2 or 3 (SCA1, -2 or -3) genetic locus, clinical examination and nerve conduction as well as evoked potential studies. Pyramidal tract signs, pale discs, and dysphagia were more frequent in SCA1 compared with SCA2 and SCA3 patients, while double vision occurred less frequently. Visual evoked potentials and motor evoked potentials following transcranial magnetic stimulation were abnormal in almost all SCA1 patients, but only in a minority of SCA2 and SCA3 patients. In contrast, somatosensory evoked potentials were delayed or absent in the majority of patients with no significant differences between the mutations. Abnormalities of brainstem auditory evoked potentials were found in about half of the patients irrespective of the underlying mutation. In addition, reduced sensory nerve action potentials, suggesting sensory axonal neuropathy were found in all three mutations. These findings provide electrophysiological evidence that pyramidal and visual pathways are differentially affected in SCA1, SCA2 and SCA3 patients.