Long-latency reflexes, somatosensory evoked potentials and transcranial magnetic stimulation: relation of the three...

Afferent and efferent central pathways were tested by electrically elicited long-latency reflexes (LLRs), somatosensory evoked potentials (SEPs), and motor potentials (MEPs) evoked by transcranial magnetic stimulation (TMS) in 37 patients with multiple sclerosis. Of these 27 (72.9%) had abnormal results, with bilateral abnormalities in 17 (45.9%). MEPs were abnormal in 21 (56.8%), LLRs in 17 (45.9%) and SEPs in 16 (43.2%). Compared with TMS (MEPs) alone, additional testing of LLRs and SEPs revealed abnormalities in 6 additional patients (2 LLRs, 3 SEPs, 1 LLR and SEP). When the hands were analysed separately 32 (47.3%) showed abnormal results with TMS, 29 (39.2%) with LLRs and 22 (29.7%) with SEPs. All hands with absent or delayed N20 components of the SEP also had abnormal LLRs, supporting the hypothesis that LLRs and SEPs share the same afferent pathways. In contrast to this, 10 hands had normal LLRs but slightly delayed central motor conduction times. Calculating the sum of the latency of the N20 component of the SEP and the latency of the MEP, we found a mean cortical relay time for the LLR of 8.5 +/- 4.7 msec, which is compatible with a polysynaptic transcortical pathway. This is supported by our finding of a linear correlation between the LLR latency and the sum of N20 and MEP latencies.