The autosomal dominant cerebellar degenerative disorders are generally referred to as ‘spinocerebellar ataxias,’ (SCAs, even though ‘spinocerebellar’ is a hybrid term, referring to both clinical signs and neuroanatomical regions (Margolis, 2003). Neuropathologists have defined SCAs as cerebellar ataxias with variable involvement of the brainstem and spinal cord, and the clinical features of the disorders are caused by degeneration of the cerebellum and its afferent and efferent connections, which involve the brainstem and spinal cord (Schols et al., 2004Taroni and DiDonato, 2004).

Historically, Harding (1982) proposed a clinical classification for autosomal dominant cerebellar ataxias (ADCAs). ADCA I was characterized by cerebellar ataxia in combination with various associated neurologic features, such as ophthalmoplegia, pyramidal and extrapyramidal signs, peripheral neuropathy, and dementia, among others. ADCA II was characterized by the cerebellar ataxia, associated neurologic features, and the additional findings of macular and retinal degeneration. ADCA III was a pure form of late-onset cerebellar ataxia without additional features. SCA1, SCA2 (183090), and SCA3, or Machado-Joseph disease (109150), are considered to be forms of ADCA I. These 3 disorders are characterized at the molecular level by CAG repeat expansions on 6p24-p23, 12q24.1, and 14q32.1, respectively. SCA7 (607640), caused by a CAG repeat expansion in the ATXN7 gene (607640) on chromosome 3p13-p12, is a form of ADCA II. SCA5 (600224), SCA31 (117210), SCA6 (183086), and SCA11 (600432) are associated with phenotypes most suggestive of ADCA III. However, Schelhaas et al. (2000) noted that there is significant phenotypic overlap between different forms of SCA as well as significant phenotypic variability within each subtype.

Classic reviews of olivopontocerebellar atrophies and of inherited ataxias in general include those of Konigsmark and Weiner (1970), who identified 5 types of olivopontocerebellar atrophy, Berciano (1982)Harding (1993)Schelhaas et al. (2000), and Margolis (2003).