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

Abstract

Objective The 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. Methods ISP 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 (CMCT APB) and tibial anterior muscles (CMCT TA), and magnetic resonance images (MRI) were also obtained. Results ISP measurements showed a similar diagnostic sensitivity (28.6%) as CMCT APB (24.5%), while diagnostic sensitivities of CMCT TA (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 CMCT APB to the same hand was also prolonged (40.0% vs. 8.4%, p < 0.0001). The correlation between iSP duration and CMCT APB 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. Conclusions ISP measures are neither a sensitive nor a specific marker of callosal conduction abnormality in early RRMS.