Ipsilateral silent period: A marker of callosal conduction abnormality in early relapsing–remitting multiple sclerosis?

ObjectiveThe corpus callosum (CC) is commonly affected in multiple sclerosis (MS). The ipsilateral silent period (iSP) is a putative electrophysiological marker of callosal demyelination. The purpose of this study was to re-assess, under recently established optimised protocol conditions [Jung P., Ziemann U. Differences of the ipsilateral silent period in small hand muscles. Muscle Nerve in press.], its diagnostic sensitivity in MS, about which conflicting results were reported in previous studies.MethodsISP measurements (onset, duration, and depth) were obtained in the abductor pollicis brevis (APB) muscle of either hand in 49 patients with early relapsing–remitting MS (RRMS) (mean EDSS, 1.3). Standard central motor conduction times to the APB (CMCTAPB) and tibial anterior muscles (CMCTTA), and magnetic resonance images (MRI) were also obtained.ResultsISP measurements showed a similar diagnostic sensitivity (28.6%) as CMCTAPB (24.5%), while diagnostic sensitivities of CMCTTA (69.4%) and MRI of the CC (78.6%) were much higher. Prolongation of iSP duration was the most sensitive single iSP measure. ISP prolongation occurred more frequently when CMCTAPB to the same hand was also prolonged (40.0% vs. 8.4%, p < 0.0001). The correlation between iSP duration and CMCTAPB was significant (Pearson's r = 0.24, p < 0.02), suggesting that iSP duration can be contaminated by demyelination of the contralateral corticospinal tract. ISP duration did not correlate with MRI abnormalities of the CC.ConclusionsISP measures are neither a sensitive nor a specific marker of callosal conduction abnormality in early RRMS.