Sporadic adult-onset ataxia of unknown etiology.

Sporadic adult-onset ataxia of unknown etiology (SAOA) denotes the non-hereditary degenerative adult-onset ataxia disorders that are distinct from multiple system atrophy (MSA). Rather than being a defined disease entity, SAOA has to be regarded as a group of disorders of unknown etiology that are defined by a common clinical syndrome and the exclusion of known disease causes. Epidemiological studies have revealed prevalence rates ranging from 2.2 to 8.4 per 100000, which are higher than those of hereditary ataxias. Clinically, SAOA is characterized by a slowly progressive cerebellar syndrome starting around the age of 50 years. About one-third of SAOA patients have either polyneuropathy or pyramidal tract involvement accompanying cerebellar ataxia. Cognitive impairment is not the rule, and, if present, is only mild. More than half of SAOA patients have signs of mild autonomic dysfunction that do not meet the criteria of severe autonomic failure required for a diagnosis of MSA. Neuropathological and imaging studies show an isolated cerebellar cortical degeneration with no or only mild brainstem involvement. There is no established therapy for SAOA.