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. The features of SCA type 7 (SCA7) include the characteristic signs of ataxia (such as gait and balance problems), along with visual disturbance. A variety of ocular features are possible, such as tritanopia (blue-yellow color blindness), a slowing of rapid eye movements, and ophthalmoplegia (paralysis of 1 or more of the eye muscles), but the condition is characterized by progressive retinal degeneration and eventual blindness. Other common features of SCA7 include dysarthria (slow distorted speech), dysphagia (difficulty swallowing), spasticity, brisk reflexes, and hearing impairment. The onset of symptoms in individuals with SCA7 is typically in the second to fourth decade, although infantile- or juvenile-onset is not uncommon. SCA7 is caused by expansion of a
CAG repeat in the ataxin-7 (ATXN7) gene located on chromosome 3 at p12 to p13. Like other trinucleotide repeat disorders, SCA7 exhibits genetic anticipation, in which the severity of the condition worsens and the patient age at onset decreases with successive generations. However, because of the exceptional instability of the ATXN7 CAG repeat in transmission from father to child, extreme expansions may occur and may result in the diagnosis of SCA7 in a child before symptoms of the disease are seen in the parent.