Intraoperative preservation of corticospinal function in the brainstem

Background: The corticospinal tract features a largely exposed course through the brainstem, and is therefore at risk in many brainstem-related procedures. No large case series on motor-evoked potential (MEP) monitoring during brainstem surgery have been reported as yet. Objective: To understand intraoperative MEP changes during brainstem-related surgery, and to explore the value of MEP monitoring for preventing permanent new paresis. Methods: Myogenic MEPs after transcranial electrical train stimulation were monitored in 70 cases of intraparenchymal (n=39) and extraparenchymal (n=31) brainstem-related tumours and vascular lesions. MEP recordings failed in another five cases. Motor outcome and intraoperative MEP results were documented prospectively and correlated for this study. Results: Significant MEP changes occurred in 46% of cases. Stable and only reversibly deteriorated MEPs warranted unimpaired motor outcome (n=50, 71% of all cases). Irreversible deterioration and reversible loss (n=19, 27%) indicated a 37% risk for transient deficit. Irreversible loss (one case, 1.5%) predicted permanent paresis. MEPs and motor outcome correlated equally well in intra-and extraparenchymal lesions. Somatosensory-evoked potentials (SEPs) did not reliably reflect motor outcome. Permanent motor deficit occurred in one out five cases (20%) with failed MEP recordings. Conclusions: MEP monitoring-as opposed to SEPs-is a valid indicator of corticospinal function in brainstem-related surgery, independent from the type of lesion operated on. New deficit occurs only after more pronounced MEP changes than in supratentorial surgery, but complete loss as in spinal surgery is not required. MEPs may help to prevent permanent new paresis.