Spinocerebellar Ataxia Type 7

SCA7 is a rare form of SCA in all populations. The proportion of SCA patients with SCA7 ranges from 0.5% in Germany and Russia (0% in Portugal, Korea, India and China) to 16% in some regions in Spain, and is found in about 4.5% of US SCA patients. The age of onset of the symptoms ranges widely, from infancy, to the fifth or sixth decade. SCA7 is characterized by abnormalities in color vision and central visual acuity, due to a macular pigmentary degeneration, and in adults visual symptoms often precede the cerebellar symptoms, or are detectable upon development of imbalance. Typical cerebellar signs of gait unsteadiness, incoordination and dysarthria develop soon afterward and progress to a bedridden state. Rate of progression depends on age of onset. In the infantile-onset form there is rapidly progressive cerebellar and brainstem degeneration and visual loss may be difficult to ascertain before death. Otherwise retinal degeneration is progressive and leads to blindness. When visual symptoms appear at or before adolescence, blindness can occur within a few years. Ataxia progresses to wheelchair confinement in 5-10 years. The retinal degenerative component distinguishes this hereditary ataxia from other forms of SCA. The diagnosis of SCA7 is established by the demonstration of an expansion in one CAG repeat alleles in the SCA7 locus greater than 36 repeats in an ataxia patient.