Motor evoked potentials (MEP) during brainstem surgery as predictor of postoperative corticospinal function

Background: Brainstem surgery bears a risk of damage to the corticospinal tract (CST). Motor Evoked Potentials (MEP) are used intraoperatively to monitor CST function in order to detect CST damage at a reversible stage and thus impede permanent neurological deficits. However, warning criteria in this context are not generally agreed on.

Methods: We analysed 104 consecutive patients who underwent microsurgical resection of lesions affecting the brainstem (58 intraaxial, 44 extraparenchymal). Motor grade was documented prior to surgery, early postoperatively and at discharge. MEP was recorded after transcranial electrical stimulation (TES). To obtain the initial TES intensity threshold, we started with TES at 30 mA and then increased in 5 mA steps until the target muscle responded reliably, or until a self-imposed limit of up to 220 mA was reached. Testing was repeated when brainstem function was assumed to be at greatest risk. MEP were considered unstable and the surgical team was notified whenever the threshold increased by 20 mA or MEP response fell under 50% at the TES limit.

Results: On the first postoperative day, 16 patients experienced a new motor deficit that resolved until discharge in 10 individuals. In 9/16 patients, TES intensity had to be increased by 20 mA or more; in this group the paresis persisted until discharge in 3/9 patients. In 5/16 patients MEP amplitude was reduced by 50% at the TES limit, three of them showing persisting paresis. In the remaining 2/16 patients, intraoperative MEP were stable although new paresis appeared. Detailed analysis of postoperative MR revealed that these patients had suffered from postoperative bleeding. In one of these patients the new motor deficit persisted until discharge. In 10/15 children MEP were unstable, but only one of them showed a new motor deficit at discharge. Of all 104 patients, 7 have deteriorated in motor grade at discharge, 92 remained unchanged and 5 patients have improved.

Conclusions: In this study of MEP monitoring during brainstem surgery, TES intensity increase by 20 mA or 50% response reduction was established as a reliable criterion to identify unstable MEP. While usually response reduction is used as warning criterion, even small intensity increase may predict new motor deficits in brainstem surgery.

Significance: Meticulous MEP monitoring may help to prevent permanent motor deficits during demanding neurosurgical procedures on the brainstem.