The olivopontocerebellar degenerations have their onset from adolescence to 65 years of age. They are divided into three genotypes. Sininocerebellar atrophy type 1, type 2, and ty 3. SCA 1 is the only type that may have onset before age 20. The genotype is a trinucleotide repeat on chromosome 6.
Paternal transmission is associated with larger repeat size and earlier onset of symptoms.
SCA 2 and SCA3 have adult onset and the abnormal gene sites are on chromosome 12 and 14, respectively. Type 1 is invariably present and is usually the initial feature. Patients with childhood onset often have mental retardation and seizures, tendon reflexes are exaggerated and the plantar response is usually extensor. Optic nerve atrophy is constant finding in some families. Other variable features include mystagmus, dysarthria, decreased pupillary response, ptosis, and impaired position sense.