The influence of anesthesia on intraoperative neurophysiological monitoring during spinal surgeries

BACKGROUND: Intraoperative neuromonitoring using tc-MEPs satisfactorily detects motor tract integrity changes during spinal surgery. However, tc-MEP is affected by anesthesia and other factors, in which the stimulation threshold increases because the waveform amplitude decreases over time with the accumulation and boluses of anesthetics. METHODS: We conducted a retrospective study of 139 patients. The average age was 30 years. Tc-MEPs were recorded bilaterally from the tibialis anterior muscle and the abductor hallucis muscle. Statistical tests were used to investigate the changes to evaluate anesthetic effects. RESULTS: There were no significant differences in tc-MEP amplitude change (%) between the groups of propofol (13 %), remifentanil (22 %) and sufentanil (26 %, p < 0.01). Significant differences were found between the groups of propofol, remifentanil, and sufentanil (20 %) and bolus sufentanil (-30 %), and bolus ketamine (730 %, p < 0.008). Major differences were observed between bolus sufentanil (-30 %) and bolus ketamine (730 %, p < 0.001). When comparing tc-MEPs with no amplitude, no significant difference was found between the groups of propofol (26 %), remifentanil (24 %), and sufentanil (28 %, p < 0.007). Substantial difference was found between the groups of propofol, remifentanil, and sufentanil (mean 26 %) and the group where ketamine boluses were administered. We didn't observe any loss of amplitude (0 %, p < 0.0002). CONCLUSION: IONM may be useless in patients where boluses of sufentanil are administered and also with Medical Research Council grades 3 and below. Consider applying IONM in patients with severe spinal deformity along with a higher age of over 50 and neurological deficit. Increasing stimulus intensity or facilitation techniques may be considered to improve the usefulness of tc-MEP. Our concept of findings supports the neurophysiological monitoring findings in other studies (Tab. 10, Ref. 45).