Pure intraoperative MEP losses: Assessment of neuro-logical and neuroradiological postoperative outcome

Introduction: In supratentorial surgery, the cortical depth of excitation of the pyramidal neurons by transcranial electric stimulation (TES) is critical to detect even most superficial cortical lesions affecting the motor cortex. An excitation caudally to a lesion consequences false negative results. Exclusive (i.e. no changes in other modalities as SEPs) mMEP changes were correlated to clinical and neuroradiological outcome to evaluate their significance.

Methods: MMEPs (all elicited with TES; in 14 patients also direct cortical stimulation) were elicited with a train of 5 anodal rectangular pulses (0.5ms pulse width, interstimulus interval of 2–4ms; repetition rate of up to 1Hz; stimulation intensity 10% above motor threshold). Out of a prospective neuromonitoring database (1900 patients), patients were analyzed for the type of exclusive mMEP deterioration (loss; >50% amplitude decrement; increment of motor threshold; duration), for lesions affecting the motor cortex or motor pathways in postoperative MRI (T2, T2*, DWI) and postoperative motor status (<24 hr postoperatively, 6 months).

Results: In 40/965 patients (4.1%) undergoing intracerebral procedures only mMEPs were affected. Those changes were permanent in 18 (14 postoperative MRI (poMRI)) and transient in 22 patients (19 poMRI). All 8/14 (57%) patients with permanent losses suffered a permanent motor deficit. In all those patients, poMRI either revealed an edema (4) or infarct (4) affecting the motor pathways at the internal capsule (IC; 3), corona radiata (CR; 5) and/or precentral gyrus (PreCG; 2). Of the 6/14 (43%) patients with permanent deterioration in mMEPs 4 patients developed a transient motor deficit. The poMRI was normal in 1 patient and showed an edema in 5 patients affecting the PreCG (2), postcentral gyrus (1) and IC (1). 8/19 (42%) transient changes were losses. 3 patients suffered a postoperative motor deficit. But, the poMRI demonstrated an infarct of the frontal gyrus in 1 patient and edema in 5 patients (IC 3; Pons 1; PreCG). In 11/19 (58%) transient mMEP deterioration occurred, which resulted in a transient slight hemiparesis in one patient. In 5 of those 19 patients, the poMRI revealed an edema affecting the PreCG (4), thalamus (1); and a small hemorrhage (not requiring surgical intervention) close to the IC in 2 patients. Losses of mMEPs are accompanied by lesions affecting the IC or CR in 50% (8/16) compared to only 12% (2/17) following deterioration of mMEPs, which is significantly less (p=0.026).

Conclusion: In intracerebral surgery, losses of MEPs result in subcortical poMRI alterations affecting the motor pathways – mostly at the level of the IC or CR and lead to permanent motor deficit. Deterioration of MEPs is indicative for cortical poMRI alterations affecting the cortical pre- or postcentral gyrus. Even deterioration of mMEPs should be taken as a warning sign.