Sphincter EMG for Diagnosing Multiple System Atrophy and Related Disorders

One of the hallmarks in the pathology of multiple system atrophy (MSA) is neuronal loss in the sacral Onuf’s nucleus11,33,37. Onuf’s nucleus plays a key role in urinary and fecal continence12. Neurons in this nucleus receive not only cortical inputs, but also noradrenergic and serotonergic facilitatory inputs via interneurons from various brainstem structures, including the pontine urine-storage center57,68. External anal sphincter (EAS)electromyography (EMG) is an established method to detect neurogenic change in motor unit potentials (MUP), which mostly reflects denervation and reinnervation of the sphincter muscle30. The significance of the EAS-EMG in MSA has been well known30,69,74. Physiologically, external urethral sphincter (EUS) and EAS share sacral pudendal innervation from Onuf’s nucleus20. In this article, we review the normal physiology and pathophysiology of the lower urinary tract and the lower gastrointestinal tract briefly, the current methods and interpretations of EAS or EUS-EMG, and sphincter EMG in autonomic disorders.