Motor-evoked potentials from multiple target muscles in multiple sclerosis and cervical myelopathy

Motor-evoked potentials (MEP) from abductor digiti minimi (ADM), biceps brachii (BB) and tibialis anterior (TA) muscle were analysed in 101 patients with multiple sclerosis (MS) and in 72 patients with compressive cervical myelopathy (CM). In definite MS the results were abnormal in 82% and in probable or possible MS in 64% of patients. In CM results were abnormal in 56% when the compression site was at the C4/C5 interspace or higher (CM_C1-C4-), and in 100% when the compression fell between the C5 and C7 level (CM_C5-C7). The pattern of normal responses from BB with abnormal responses from ADM was found in 37% of patients with CM_C5-C7, in 10% of patients with MS (p = 0.01) and in 19% of patients with CM_C1-C4+. Abnormal MEP from both ADM and BB were significantly more frequent in MS than in CM (p = 0.002). Although “specific patterns” of MEP abnormality could be significantly correlated with patient groups and anatomical compression sites, they cannot be taken as a reliable diagnostic clue in the individual case. In particular, abnormal responses from BB were frequently also found in CM_C5-C7. These discrepancies cannot be explained on the basis of a localized mechanical mechanism and raise the question of the pathogenesis in CM.