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Why does threshold-level change in transcranial motor evoked potentials during surgery for supratentorial lesions?
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Veröffentlicht: | 8. Juni 2016 |
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Objective: Rising threshold-levels during monitoring of motor evoked potentials (MEP) using transcranial electrical stimulation (TES) have been described without damage to motor pathway in cranial surgery, suggesting the need for monitoring of affected and unaffected hemisphere. We aimed to determine the influence of blood pressure and pneumocephalus on changes in threshold-level and to establish reliable criteria for adjusting stimulation intensity during surgery for supratentorial lesions.
Method: Between 09/2014 and 09/2015, TES-MEP were performed in 143 patients during surgery for supratentorial lesions in motor-eloquent brain areas, under general anesthesia using a strict protocol. 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 deterioration were excluded. The volume of subdural air was measured on postoperative MRI.
Results: 123 patients were included. On the affected side, 82 patients (66.7%) showed an increase in threshold-level (range 2 to 48%) and 41 patients (33.3%) didn’t show any change in threshold-level. The difference to the unaffected side was less than 20 % in all patients. The recorded range of changes in the systolic pressure as well as in the mean pressure didn’t exceed 20 mm Hg in any of the patients. Subdural air was detected on postoperative MRI scans in 87 patients (70.7%). Of these patients, 81 (93.1%) had an intraoperative increase in threshold-level on either sides. While in 36 patients (29.3%) no subdural air was found and 10 of them (27.8%) developed an increase in threshold-level on either sides. Subdural air was the only factor associated with an increase in threshold-level on the affected and unaffected side (p= 0.001 and p= 0.003, respectively), while the measured volume of subdural air didn’t correlate with extent of increase in threshold-level (p= 0.074).
Conclusions: Subdural air was the only factor associated with an increase in threshold-level during MEP monitoring without damaging motor pathway. Threshold levels on the affected side can rise up to 48% without being predictive of a postoperative paresis, as long as the difference to the unaffected side is less than 20 %. Changes in systolic or mean blood pressure within a range of 20 mm Hg doesn’t seem to influence intraoperative MEP.