Measuring the cortical silent period can increase diagnostic confidence for amyotrophic lateral sclerosis.

We evaluated a modified measurement of the cortical silent period (CSP) as a simple procedure to add further confidence in the diagnostic work-up for ALS. Thirty-seven consecutive patients with a suspicion of having ALS were included together with 25 healthy volunteers, and followed until a final diagnosis (ALS versus 'ALS mimic') was reached. Using a CSP cut-off value of 200 ms for males and 150 ms for females, the following test characteristics were obtained for ALS versus ALS mimics together with controls: sensitivity for excluding ALS 0.83, specificity 0.56 (males) and sensitivity 0.81, specificity 0.82 (females). A CSP longer than the mentioned cut-off values should alarm the clinician for the presence of a disorder other than ALS.