Spinocerebellar ataxia type 1 is a progressive form of cerebellar ataxia with features that include lack of coordination and balance and dysarthria. Additional symptoms include vision problems such as nystagmus, dysphagia, hypotonia, peripheral neuropathy, and dysmetria. Bulbar signs such as facial muscle atrophy, difficulty chewing and swallowing, and perioral fasciculations may develop in the latter stages. Cognitive impairment may occur over time. Average age of onset is in the 20s-30s although childhood and late-adult onset have been reported.
SCA1 is inherited in an autosomal dominant pattern, and it is caused by a CAG trinucleotide repeat expansion in the ATXN1 gene. One to three CAT interruptions are normally found within the CAG repeat stretch, and the presence of CAT interruptions helps to stabilize the repeat region. In the absence of CAT interruptions, the repeat region is less stable and susceptible to expansion. Repeat ranges are as follows: Normal (CAT absent 6-35; CAT present 6-44), Intermediate (CAT absent 36-38), Pathogenic (CAT absent ≥ 39; CAT present ≥ 45). Anticipation is more likely to occur with paternal transmission.