Electrophysiology in spinocerebellar ataxias: Spread of disease and characteristic findings
Spinocerebellar ataxias (SCAs) comprise a clinically and genetically heterogeneous group of autosomal dominantly inherited neurodegenerative disorders affecting the cerebellum and to variable degrees further parts of the nervous system. Electrophysiology is a potent tool to prove impairment of multiple neuronal systems and fibre tracts and even to decipher subclinical affection. Electrooculography, evoked potentials, nerve conduction studies and polysomnography are especially helpful in the setting of SCAs. Severely slowed saccades are a hallmark of SCA2. Vertical nystagmus occurs most frequently in SCA3 and SCA6. Visual potentials recede especially in SCA7. Substantially prolonged central motor conduction times in motor-evoked potentials occur frequently in SCA1 even in patients without clinical signs of pyramidal affection. Thus, electrophysiological analyses may help to predict the SCA genotype and direct molecular genetic diagnostics. Polymsomnography is a helpful tool in the analysis of sleep disorders and frequently helps to decipher treatable causes like periodic leg movement in sleep and REM sleep behaviour disorder in SCAs. Nerve conduction studies reveal sensory neuropathy in all common SCA subtypes, but to variable degrees. Age rather than CAG repeat length appears to be the most important determinant for neuropathy and makes sensory nerve action potentials a potential progression marker in SCA.