Quantitative electromyography of the external anal sphincter in Parkinson's disease and multiple system atrophy.

The distinction of multiple system atrophy (MSA) from Parkinson's disease (PD) can be difficult, especially early in the disease. In MSA degeneration of sacral anterior horn cells (Onuf's nucleus) results in denervation-reinnervation of anal and urethral sphincter muscles, which can be recognized as neurogenic electromyographic (EMG) changes of motor unit potentials. Sphincter EMG has therefore been recommended as a test for distinguishing MSA from PD. Our results confirm the presence of marked neurogenic EMG changes of the external anal sphincter muscle in patients with probable MSA compared to healthy controls. However, in patients with probable PD, our quantitative EMG data show a scatter from normal to marked neurogenic changes and the degree of EMG abnormality is correlated to the duration of the disease. Thus an abnormal sphincter EMG cannot be taken as a strong indicator of MSA rather than PD in the individual patient, especially in long-standing cases.