Cognitive Changes in the Spinocerebellar Ataxias Due to Expanded Polyglutamine Tracts: A Survey of the Literature

The dominantly-inherited ataxias characterised by expanded polyglutamine tracts-spinocere bellar ataxias (SCAs) 1, 2, 3, 6, 7, 17, dentatorubral pallidoluysian atrophy (DRPLA) and, in part, SCA 8-have all been shown to result in various degrees of cognitive impairment. We survey the literature on the cognitive consequences of each disorder, attempting correlation with their published neuropathological, magnetic resonance imaging (MRI) and clinical features. We suggest several psychometric instruments for assessment of executive function, whose results are unlikely to be confounded by visual, articulatory or upper limb motor difficulties. Finally, and with acknowledgement of the inadequacies of the literature to date, we advance a tentative classification of these disorders into three groups, based on the reported severity of their cognitive impairments, and correlated with their neuropathological topography and MRI findings: group 1-SCAs 6 and 8-mild dysexecutive syndrome based on disruption of cerebello-cortical circuitry; group 2-SCAs 1, 2, 3, and 7-more extensive deficits based largely on disruption of striatocortical in addition to cerebello-cerebral circuitry; and group 3-SCA 17 and DRPLA-in which cognitive impairment severe enough to cause a dementia syndrome is a frequent feature.