a progressive ataxic syndrome that develops in older patients who manifest cerebellar or pontocerebellar atrophy on MRI scans, and yet who have no family history of ataxia, no identifiable gene mutations, and no known causes of acquired ataxia. Some of these patients subsequently develop signs such as parkinsonian features and autonomic failure, thereby qualifying them for the diagnosis of multiple system atrophy. SYN: late onset sporadic ataxia.