Optimum interpulse interval for transcranial electrical train stimulation to elicit motor evoked potentials of maximal amplitude in both upper and lower extremity target muscles

Abstract

ObjectiveThe aim of this study was to determine the optimum interpulse interval (OIPI) for transcranial electrical train stimulation to elicit muscle motor evoked potentials (TES-MEP) with maximal amplitude in upper and lower extremities during intra-operative spinal cord monitoring. MethodsIntraoperative spinal cord monitoring with TES-MEP was performed in 26 patients who had (corrective) spine surgery. Optimum interpulse interval (OIPI) were determined for the abductor pollicis brevis muscle (APB) representing the upper extremity and the anterior tibialis muscle (TA) representing the lower extremity. The IPI was varied between 0.5 and 4.0ms, where the OIPI was defined as the IPI with the highest muscle MEP amplitude for each muscle group. Differences between upper and lower extremity OIPIs were analyzed. Furthermore, the MEP amplitudes difference between the upper and lower extremity OIPIs and between the OIPI and IPI 2ms was determined. ResultsThe mean OIPIAPB representing the upper extremity was 1.78±1.09ms on the left side and 1.82±0.93ms on the right side. The lower extremity showed a mean OIPITA of 2.26±1.16ms on the left and 2.73±0.88ms on the right side. The mean differences between the OIPIAPB and OIPITA were significant for p=0.019 (Student’s T-test). No within patient differences in OIPIs between the left and the right side were found.The mean MEP amplitude reduction, the APB amplitude at OIPITA compared to the APB at OIPIAPB, was 32.5±27.9%. For the TA a mean amplitude reduction of 33.4±27.4% was found. The mean amplitude reduction for the OIPI amplitudes compared to the amplitudes at IPI 2ms was 53.6±25.5% for the APB and 45.8±28.3% for the TA. ConclusionLarge intra- and interindividual differences were found between the mean OIPIs of the TA and APB muscles (range 1.78–2.73ms) representing the upper and lower extremity. SignificanceBased on the results of this study, it is advisable to perform a set-up procedure for each individual patient undergoing TES-MEP to determine the optimal parameter settings when using supramaximal intensity of TES.