Spinocerebellar Ataxia Type 6

The proportion of SCA patients with SCA6 ranges from 2% in Italy (0% in China, Portugal, and Brazil) to 31% in Japan, and is found in about 15% of US SCA patients. The age of onset ranges from 19 to 71 years, with a mean age of onset in the fifth decade. Age of onset may vary between siblings with the same mutation by as much as 12 years. Initial symptoms are gait unsteadiness, stumbling, and imbalance in about 90% of cases. The remainder of patients present with dysarthria. Symptoms progress slowly, and eventually all patients have gait ataxia, upper limb incoordination, intention tremor, and dysarthria. Patients with SCA6 occasionally manifest episodic ataxia or episodic vertigo. Diplopia occurs in about 50% of patients. Others experience visual disturbances related to difficulty fixating on moving objects. Dysphagia and choking are common. Lifespan is not shortened.

SCA6 is caused by an expansion in a CAG repeat in the gene CACNA1A, on 19p13.1 previously recognized to encode the alpha1A subunit, the main pore-forming subunit for the P/Q-type voltage-gated calcium channel. The diagnosis of SCA6 is established by the demonstration of an expansion in one CAG repeat alleles in the SCA6 locus greater than 19 repeats in an ataxia patient.