The spinocerebellar ataxias (SCA) are a clinically and genetically heterogeneous group of neurodegenerative disorders characterized by limb and gait ataxia. More than 25 different types of SCA have been described, although the causative genes have not yet been identified for all types. SCA type 8 (SCA8) is a predominantly cerebellar syndrome characterized by gait, limb, and speech difficulties. Dysphagia (difficulty swallowing), hyperreflexia, spasticity, oculomotor disorders, sensory disturbance, abnormal movements, and tremor are additional features that may be seen in SCA8 patients. The onset of SCA8 symptoms is usually in the third or fourth decade of life, although the patient age at disease onset is highly variable and onset in early childhood has been reported. The condition is slowly progressive and is not expected to shorten lifespan. SCA8 is
caused by the expansion of a CTG repeat in the ATXN8OS gene, which is located on chromosome 13 at band q21. A stable but polymorphic CTA repeat is located immediately upstream of the CTG repeat tract and is included in the repeat count when testing patients. Compared with other types of SCA, SCA8 is remarkable in that the causative repeat is highly unstable and intergenerational changes can be extreme, the repeat tends to expand when maternally inherited but contracts upon paternal transmission, and it is not associated with genetic anticipation (earlier onset and increased severity with successive generations).