Sphincter electromyography and multiple system atrophy.

Electromyographic studies of the sphincter in patients with multiple system atrophy have shown increased duration and polyphasia of motor unit potentials. These electrophysiological markers have been used to argue for the selective degeneration of sacral motor neurons in Onuf's nucleus in patients with multiple system atrophy. Studies comparing sphincter electromyographic changes in patients with multiple system atrophy and Parkinson's disease have shown significant differences between these two patient populations. Despite the controversy surrounding this claim, recent studies using quantitative electromyographic techniques support the view that reinnervation of the anal sphincter muscles may be a useful diagnostic marker for distinguishing multiple system atrophy from Parkinson's disease. A critical review of these data is needed to assess the validity and reliability of electromyographic changes in multiple system atrophy.