Why Does Threshold Level Change in Transcranial Motor-evoked Potentials During Surgery for Supratentorial Lesions?

Background: Rising threshold level during monitoring of motor-evoked potentials (MEP) using transcranial electrical stimulation (TES) has been described without damage to the motor pathway in the cranial surgery, suggesting the need for monitoring of affected and unaffected hemisphere. We aimed to determine the factors that lead to a change in threshold level and to establish reliable criteria for adjusting stimulation intensity during surgery for supratentorial lesions. Materials and Methods: Between October 2014 and October 2015, TES-MEP were performed in 143 patients during surgery for unilateral supratentorial lesions in motor-eloquent brain areas. All procedures were performed under general anesthesia using a strict protocol to maintain stable blood pressure. MEP were evaluated bilaterally to assess the percentage increase in threshold level, which was considered significant if it exceeded 20% on the contralateral side beyond the percentage increase on the ipsilateral side. Patients who developed a postoperative motor deficit were excluded. Volume of subdural air was measured on postoperative magnetic resonance imaging. Logistic regression was performed to identify factors associated with the intraoperative recorded changes in threshold level. Results: A total of 123 patients were included in the study. On the affected side, 82 patients (66.7%) showed an increase in threshold level, which ranged from 2% to 48% and 41 patients (33.3%) did not show any change. The difference to the unaffected side was under 20% in all patients. The recorded range of changes in the systolic and mean pressure did not exceed 20 mm Hg in any of the patients. Pneumocephalus was detected on postoperative magnetic resonance imaging scans in 87 patients (70.7%) and 81 of them (93.1%) had an intraoperative increase in threshold level on either sides. Pneumocephalus was the only factor associated with an increase in threshold level on the affected side (P < 0.001), while each of pneumocephalus and length of the procedure correlated with a change in threshold level on the unaffected side (P < 0.001 and 0.032, respectively). Conclusions: Pneumocephalus was the only factor associated with increase in threshold level during MEP monitoring without damaging motor pathway. Threshold level on the affected side can rise up to 48% without being predictive of postoperative paresis, as long as the difference between the increased threshold of the affected and unaffected side is within 20%. Changes in systolic or mean blood pressure within a range of 20 mm Hg do not seem to influence intraoperative MEP.