Multiple system atrophy (MSA) is an idiopathic neuro-degenerative disorder involving many neuronal structures including substantia nigra, putamen, cerebellum, pontine nuclei, olives, intermediolateral cell column of the spinal cord. Anterior horn
cells
innervating external urethral and external anal sphincter muscles in the sacral cord, so called Onuf's nucleus, are also affected. Neuronal loss of the Onuf's nucleus may cause denervation and reinnervation in the external urethral and the
external
anal
sphincter muscles. As a result, in the patients with MSA electromyographic study of the external urethral and the external anal sphincter muscles may show prolonged duration of motor unit potentials (MUSPs). Such abnormal MUPs of the external
urethral
sphincter muscles has been known to be a useful indicator distinguishing patients with MSA from those with idiopathic Parkinson's disease (IPD) in which minimal or no abnormal MUPs can be found. However. the results of external anal sphincter
MUPs
analysis has not been reported yet. In the patients with IPD and probable MSA, we measured the duration, the number of phases and the proportion of pathological MUPs from the external anal sphincter muscles. Cut off value of more than 60% of
abnormal
MUPs could differentiate patients with MSA and IPD with sensitivity of 70.3% and specificity of 90.9%. External anal sphincter MUP analysis seems to be a helpful laboratory method differentiating patients with MSA and IPD.
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