Sphincter EMG as a diagnostic tool in autonomic disorders.

Multiple system atrophy (MSA) is a neurodegenerative disease presenting with a combination of parkinsonian, cerebellar, and autonomic (including cardiovascular, urinary, and anorectal) dysfunction. It is pathologically defined, but at present lacks a definitive clinical diagnostic test. The majority of patients with probable MSA have an abnormal sphincter EMG. Patients with idiopathic Parkinson's disease do not show marked sphincter EMG abnormalities. Therefore, these abnormalities can be used to distinguish MSA from idiopathic Parkinson's disease in the first 5 years after disease onset. In contrast, similar sphincter EMG abnormalities are found in some, though not many, patients with dementia with Lewy bodies, pure autonomic failure, progressive supranuclear palsy, and spinocerebellar ataxia type 3. Thus, the limitations of the sphincter EMG test should also be kept in mind. Sphincter EMG and relevant sacral autonomic tests are diagnostic tools for autonomic disorders, reflecting the common and significant involvement of the sacral cord in MSA.