The electrophysiology of spinocerebellar ataxias

Spinocerebellar Ataxias (SCAs) are a group of autosomal dominantly inherited neurodegenerative diseases, involving the cerebellum and the brainstem. Genetic testing is the most important method of diagnosis. Nowadays, nearly 40 types of SCAs have been identified by genetic analysis. Peripheral nerve impairment is common in SCAs: electrophysiological examination of SCA1, SCA2 and SCA3 usually shows sensorimotor and sensory neuropathy, while pure motor neuropathy is more rare, being seen only in SCA2. The abnormal VEP of SCA1, SCA2 and SCA3 include prolonged P100 latencies and reduced P100 amplitudes. Abnormal BAEP involves prolonged interpeak latency of I-III and III-V. Abnormal SEP usually show absent P40 wave and prolonged P40 latency. The abnormal MEP usually shows prolonged central motor conduction time or absent responses. SCA2 is not associated with gaze-evoked nystagmus and dysmetric saccades. SCA3 usually presents as saccadic intrusions and oscillations. Whether peripheral nerves are involved in SCA6 is uncertain; although abnormal electrophysiology has been reported, neuropathological examinations have not found degenerative changes or reductions in the number of neurons in the anterior horns and/or dorsal root ganglia in SCA6. It is therefore hypothesized that this might be a displayed feature of axonopathy. The clinical presentation of most cases of SCA6 includes spontaneous and positional downbeat nystagmus, and perverted head-shaking nystagmus. Opinion about peripheral nerve involvement in SCA7 varies between authors. Losing P100 is a predominant feature of SCA7, while III and IV/V wave absence is common in SCA17. Electrophysiological study of other types is currently limited, requiring large-scale studies for confirmation. Similar and overlapping clinical features make it difficult to differentiate each type. Electrophysiological testing can therefore play an important role in helping to identify the common phenotypes of SCAs, and determine the extent and progression of disease.