The value of external anal sphincter electromyography for the diagnosis ofmultiple system atrophy

OBJECTIVE: To assess the value of external anal sphincter electromyography (ASEMG) for the diagnosis of multiple system atrophy (MSA) among various ca uses of parkinsonism. ASEMG denervation profiles have previously been propo sed as a diagnosis test for MSA, but their specificity is disputed. METHODS: ASEMG variables of 52 parkinsonian patients were analyzed accordin g to the clinical diagnosis: MSA (n = 31) or no MSA (n = 21). Mean motor un it potential duration, percentage of polyphasicity, and the electromyograph er's interpretation were analyzed according to clinical diagnosis, disease duration, genitourinary symptoms, gender, parity, and history of pelvic sur gery. RESULTS: All patients with MSA showed ASEMG denervation. Mean motor unit po tential duration was the most discriminant variable. No patient with MSA ha d a mean duration less than 12 ms and no patient without MSA had one greate r than 16 ms. ASEMG discriminates between patients with MSA and Parkinson's disease. Using a threshold of 13 ms, the sensitivity was 80% and specifici ty was almost 70% (positive predictive value, 80%) for the diagnosis of MSA . Age, history of pelvic surgery, and to a lesser extent, female gender, pa rity, disease duration, and presence of urinary symptoms increased the like lihood of abnormal ASEMG. CONCLUSION: ASEMG was highly sensitive and rather specific for the diagnosi s of MSA.