Monitoring spinal cord motor and somatosensory evoked potentials in anesthetized primates

Monitoring Motor Evoked Potential (MEP) to Transcranial Stimulation (TMS) m onitoring (MEP) is a growing technique to assess motor function under anest hesia. The following primate study was conducted to analyze the non-myogeni c spinal motor and sensory volleys and to examine their reproducibility und er nitrous oxide-methohexidone anesthesia. The traveling periodic spinal de scending MEP to TMS and ascending somatosensory (SEP) to posterior tibial n erve stimulation across the thoracic cord were recorded in 12 cynomolgus mo nkeys. Through a small T-11-T-12 laminotomy, an insulated stainless steel e lectrode was inserted into the epidural thoracic space. The potentials were analyzed under 50 vol% NO in O-2 with methohexital (0.1-0.2 mg kg(-1) min( -1)). A well-defined periodic TMS-MEPs and PTN-SEPs were recorded with high reproducibility and consistency in repeated trials under N2O-methohexital anesthesia. MEP tracing consisted of an initial peak (direct (D) wave), occ urring at 2.43 (+/- 0.28) msec followed by subsequent five positive (indire ct (I) waves). Spinal SEPs-MEPs were clearly defined, morphologically stabl e, and consistent over time under N2O-methohexitone anesthesia. The present primate study may set a model to monitor both modalities in anesthetized n eurosurgical patients.