Sphincter EMG and differential diagnosis of multiple system atrophy

Multiple system atrophy (MSA) is a degenerative disease manifesting a combination of parkinsonism, cerebellar, pyramidal, and autonomic (including urinary, sexual, and anorectal) dysfunction. It is pathomorphologically defined, but lacks a definitive clinical diagnostic test. Sphincter electromyography (EMG), reflecting Onuf's nucleus degeneration, has been proposed as a helpful test; its value has been reevaluated by a critical review of the literature. In patients with probable MSA, abnormal sphincter EMG, as compared to control subjects, has been found in the majority of patients in all the different forms of the disease in most studies, including patients who, as yet, have no urological or anorectal problems. The prevalence of abnormalities in the early stages of MSA is as yet unclear. Patients with Parkinson's disease (PD) as a rule do not show severe sphincter EMG abnormalities in the early stage of the disease. Anal sphincter EMG abnormalities (abnormal spontaneous activity or motor unit potential changes three standard deviations above valid control data) distinguish MSA from PD in the first 5 years after the onset of symptoms and signs, and from pure autonomic failure, as well as from cerebellar ataxias, if other causes for sphincter denervation have been ruled out. With such criteria, the sensitivity of the method is, however, low. EMG does not distinguish MSA from progressive supranuclear palsy. Future studies should use standardized anal sphincter EMG to better compare results from different centers and precisely define the sensitivity and specificity of the method.