gms | German Medical Science

Objective: Different warning criteria have been described to prevent and predict postoperative neurological deficits during monitoring of motor evoked potentials (MEP) using transcranial electrical stimulation (TES) in cranial surgery. This study was undertaken prospectively to compare sensitivity and specificity between threshold criterion and amplitude criterion in the same patient cohort.

Methods: Between 11/2015 and 10/2016, TES-MEP were performed in 125 patients during surgery for unilateral supratentorial lesions in motor-eloquent brain areas. MEP were recorded from abductor pollicis brevis (APB), and from orbicularis oris and/or tibialis anterior and 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. MEP amplitude was measured with a stimulus intensity of 150% of the threshold level, at the baseline and at closure. We studied the relationship between recorded changes in threshold level and amplitude and postoperative motor function.

Results: 16 of 125 patients showed a significant change in threshold level as well as an amplitude reduction over 50% in the MEP recorded from APB and 12 of them had a postoperative arm paresis, while one patient with no MEP changes had a postoperative arm paresis. Recording from tibialis anterior was performed in 64 patients and 3 of them developed a postoperative leg paresis, all showed a significant change in threshold level while an amplitude reduction over 50% took place in one of them. Amplitude reduction over 50% occurred in further 7 patients who had no significant change in threshold level and did not develop a postoperative leg paresis. Recording from orbicularis oris was performed in 57 patients, one of them developed a postoperative facial paresis and had a significant change in threshold level as well as an amplitude reduction over 50 %. Another 6 patients had an amplitude reduction over 50% but had no significant change in threshold level and did not develop a postoperative facial paresis. Sensitivity and specificity of threshold criterion and of amplitude criterion were 92 % and 97%, respectively, when MEP were recorded from APB. Recording from the leg and face muscles, threshold criterion had a 100% sensitivity and specificity, while amplitude criterion had a specificity of 89 % (leg and face) and a sensitivity of 33% from the leg and 100% from the face.

Conclusion: Sensitivity and specificity of threshold criterion was comparable to the amplitude criterion with a stimulus intensity set at 150% of threshold level, when recording MEP from APB and superior to it when recording MEP from the leg or the face.