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

OBJECTIVE: To assess the value of external anal sphincter electromyography (ASEMG) for the diagnosis of multiple system atrophy (MSA) among various causes of parkinsonism. ASEMG denervation profiles have previously been proposed as a diagnosis test for MSA, but their specificity is disputed. METHODS: ASEMG variables of 52 parkinsonian patients were analyzed according to the clinical diagnosis: MSA (n = 31) or no MSA (n = 21). Mean motor unit potential duration, percentage of polyphasicity, and the electromyographer's interpretation were analyzed according to clinical diagnosis, disease duration, genitourinary symptoms, gender, parity, and history of pelvic surgery. RESULTS: All patients with MSA showed ASEMG denervation. Mean motor unit potential duration was the most discriminant variable. No patient with MSA had a mean duration less than 12 ms and no patient without MSA had one greater than 16 ms. ASEMG discriminates between patients with MSA and Parkinson's disease. Using a threshold of 13 ms, the sensitivity was 80% and specificity was almost 70% (positive predictive value, 80%) for the diagnosis of MSA. Age, history of pelvic surgery, and to a lesser extent, female gender, parity, disease duration, and presence of urinary symptoms increased the likelihood of abnormal ASEMG. CONCLUSION: ASEMG was highly sensitive and rather specific for the diagnosis of MSA.