P21 TMS correlates of pyramidal signs and clinical motor status: Insights from a spinal perspective

Introduction: Motor evoked potentials (MEPs) to transcranial magnetic stimulation (TMS) have showed a good clinical and radiological correlation in stroke patients. However, MEPs are known to be susceptible to several sources of variability, such as age, sex, and body height, that may challenge the reliability of the results previously reported. Moreover, few and non-recent evidences on the TMS correlates of pyramidal signs and clinical motor status have been reported . Objectives: To assess the diagnostic value of MEPs in patients with pyramidal signs and motor deficit compared to those with pyramidal signs but without any clinical weakness. Materials & methods: A total of 62 participants with clinical and radiological evidence of cervical spondylotic myelopathy were dichotomized in 29 patients with pyramidal signs associated with mild motor deficit (Group 1) and 33 patients with pyramidal signs with normal muscle strength (Group 2). Both groups were compared with 33 healthy controls (Group 0), all matched for age, sex, and body height. All participants were right-handed, without any contraindication to Magnetic Resonance Imaging or TMS execution. MEPs were recorded through a circular coil applied over the ‘‘hot spot” of the First Dorsal Interosseous (FDI) and Tibialis Anterior (TA) muscle, bilaterally. Central motor conduction time (CMCT) was estimated as the difference between MEP cortical latency and the peripheral motor latency (PML) by magnetic stimulation of the motor nerve root. Peak-to-peak MEP amplitude and right-to-left differences were also measured. Results: As showed in Fig. 1, MEP latency at four limbs, CMCT at lower limbs, and PML at lower limbs were significantly more prolonged in Group 1 with respect to the other two. Compared to the same groups, MEP amplitude from left FDI and TA bilaterally was significantly decreased in Group 1. The other measures, including the inter-side differences, did not differ among the groups; in particular, no statistically significant difference was observed between Group 0 and Group 2. At the backward linear regression analysis (Fig. 2), pyramidal signs were not associated with any of the TMS measure, also when age, sex, and height were considered as possible confounding factors. Conclusion: MEPs represent a highly accurate diagnostic test in patients with even mild motor deficit; conversely, isolated pyramidal signs should be interpreted as neurophysiologically ‘‘mute” unless associated with other clinical or instrumental findings. However, these ‘‘irritative signs” might probably indicate a preclinical involvement of the pyramidal tract, the lesion of which would become evident when clear ”deficitary signs” of cortico-spinal bundle (i.e. latent or overt motor deficit) appear. Future TMS studies in this and other clinical settings with longitudinal exams and more extensive measurements are needed for a further understanding of the diagnostic and prognostic role of MEPs in neurological practice.