Spinocerebellar Ataxia Type 20

Excerpt

Clinical characteristics: Spinocerebellar ataxia type 20 (SCA20) is characterized by a slowly progressive ataxia and dysarthria. Approximately two thirds of those affected also display palatal tremor ("myoclonus") and/or abnormal phonation clinically resembling spasmodic adductor dysphonia. Dysarthria, which may be abrupt in onset, precedes the onset of ataxia in about two thirds of affected individuals, sometimes by a number of years. Hypermetric horizontal saccades (without nystagmus or disturbance of vestibulo-ocular reflex gain) are seen in about half of affected persons. Although minor pyramidal signs (brisk knee jerks, crossed adductor spread) may be seen, spasticity and extensor plantar responses are not. Cognition is normal. Clinical information is based on the findings in 16 personally examined affected members of a single Australian family of Anglo-Celtic descent.

Diagnosis/testing: The diagnosis of SCA20 is based on clinical findings and neuroimaging. Within five years of disease onset CT scan shows pronounced dentate calcification, typically without concomitant pallidal calcification. In addition to evidence of dentate calcification, MRI shows mild-to-moderate pan cerebellar atrophy and normal cerebrum and brain stem (except for increased inferior olivary T₂ signal in those with palatal tremor). The locus for SCA20 lies within the pericentromeric region of chromosome 11; the gene is unknown. A 260-kb duplication of 11q12.2-11q12.3 has been proposed as the probable cause of SCA20 in the index family.

Management: Treatment of manifestations: Physical and occupational therapy; guidance from a speech pathologist expert in the management of neurogenic dysphagia.

Prevention of secondary complications: Prevention of falls by using appropriate gait aids and home modifications; personal alarm system.

Genetic counseling: SCA20 is inherited in an autosomal dominant manner. Each child of an affected individual has a 50% chance of inheriting the pathogenic variant.