Motor threshold-tracking transcranial electrical stimulation with double and single train stimulation under sevoflurane anesthesia.

Introduction and goals: Intraoperative motor evoked potentials (MEP) are commonly used in spinal surgery, including scoliosis correction. Inhaled anesthetics cause significant depression of the MEP, especially in the presence of an initial neurological deficit, as a result of that the search for optimal stimulation parameters to reduce this problem is an important issue. The aim of the study was to compare the effectiveness and the threshold of the transcranial electrical MEP obtained by automatic tracking with single and double train stimulation. Materials and methods: In 25 patients (21 women, 4 men; median age 26 years) undergoing anterior idiopathic scoliosis correction, we compared MEP threshold with automatic tracking (7 consecutive stimulations for each muscle) induced by stimulation C1-C2/С2-С1 with single and double trains of 5 pulses, 0,2 ms duration, 3 ms interpulse interval. Electrodes for MEPs were placed in the muscles abductor hallucis (AH), abductor pollicis brevis (APB), bilaterally. The maximum stimulation level was 500 V, the minimum response amplitude was considered as 50 μv. Inhalation anesthesia with sevoflurane was used, without muscle relaxants at the main stage of the operation. Results: Successful MEP monitoring with double train stimulation performed in all patients, only 1 patient had no reliable unilateral MEP APB response. MEPs were absent bilaterally with a single train stimulation in 4 (AH) and 10 (APB) patients, unilaterally in 4 and 3 additionally at maximum intensity of stimulation. The median threshold MEP AH was 216 and 259 μv (p<0.05) for a single and double train, respectively, for APB - 183 and 224 μv (p<0.05). Discussions and Conclusions: The use of double train stimulation which has greater efficiency and a lower MEP threshold is recommended when performing intraoperative monitoring of MEP under inhalation anesthesia.