Spinocerebellar ataxia type 6. Frequency of the mutation and genotype-phenotype correlations.

Spinocerebellar ataxia type 6 (SCA6) is the most recently identified mutation causing autosomal-dominant cerebellar ataxia without retinal degeneration (ADCA). The SCA6 mutation is allelic with episodic ataxia type 2 (EA-2), but the two differ clinically because of the presence of progressive, rather than episodic, ataxia in SCA6. SCA6 accounts for 12% of families with ADCA in an ethnically heterogeneous population of patients. Clinical examination, quantitative eye movement testing, and imaging data show that the brainstem is normal in most patients with SCA6, especially within the first 10 years of symptoms. Most patients show progressive ataxia from the onset, but several patients show an episodic course resembling EA-2. Thus, SCA6 mutations not only account for patients with ADCA I and ADCA III phenotypes but also for some patients presenting with episodic features that are typical for EA-2. Interestingly, a compound heterozygote for the SCA6 expansion manifested an earlier onset and more rapid course than family members with the same larger expanded allele.