Unraveling movement disorders in spinocerebellar ataxia

Spinocerebellar ataxia (SCA) is a clinically heterogeneous group of neurodegenerative disorders characterized by progressive degeneration of the cerebellum and its associated connections. Genetic defects causing SCA include trinucleotide repeat expansions in the coding and non-coding regions of the genes, gene rearrangements, and conventional mutations. Various non-ataxic manifestations, such as dementia, peripheral neuropathy, and movement disorders (MDs) are described in SCA. MDs are the most common non-ataxic manifestations of SCA, and their prevalence and type vary according to the underlying genetic defects as well as the geographical and ethnic differences. In addition to the size of the repeat expansions, genetic modifiers contribute to the phenotypic pleiotropy of SCA. When present in association with ataxia, MDs may provide an important diagnostic clue for genotyping. However, patients with SCA presenting with MDs can be a diagnostic challenge when cerebellar ataxia is subtle or absent. Certain MDs may be more frequent in particular SCA subtypes compared to others. Similarly, MD may be an infrequent but pertinent manifestation in specific subtypes of SCA. Knowledge about MDs in SCA can help clinicians choose the genetic tests appropriately. Our paper comprehensively reviews the spectrum of MDs in SCA, and attempt to guide clinicians in choosing appropriate genetic tests for SCA in patients presenting with isolated or prominent MDs.