During intraoperative monitoring of motor evoked potentials (MEP), heterogeneity across studies in terms of study populations, intraoperative settings, applied warning criteria, and outcome reporting exists. A scoping review of MEP warning criteria in supratentorial surgery was conducted in accordance with the Preferred Reporting Items for Systematic reviews and Meta-Analyses extension for Scoping Reviews (PRISMA-ScR). Sixty-eight studies fulfilled the eligibility criteria. The most commonly used alarm criteria were MEP signal loss, which was always a major warning sign, followed by amplitude reduction and threshold elevation. Irreversible MEP alterations were associated with a higher number of transient and persisting motor deficits compared with the reversible changes. In almost all studies, specificity and Negative Predictive Value (NPV) were high, while in most of them, sensitivity and Positive Predictive Value (PPV) were rather low or modest. Thus, the absence of an irreversible alteration may reassure the neurosurgeon that the patient will not suffer a motor deficit in the short-term and long-term follow-up. Further, MEPs perform well as surrogate markers, and reversible MEP deteriorations after successful intervention indicate motor function preservation postoperatively. However, in future studies, a consensus regarding the definitions of MEP alteration, critical duration of alterations, and outcome reporting should be determined.

Objective We recently developed a new subcortical mapping technique based on the concept of stimulating the tissue at the site of and synchronously with resection. Our hypothesis was that instead of performing resection and mapping sequentially, a synchronized resection and mapping could potentially improve deficit rates. Methods We report our 5-year series of patients who prospectively underwent tumor surgery adjacent to the corticospinal tract (CST) (defined as < 1 cm using diffusion tension imaging and fiber tracking) with simultaneous subcortical short train cathodal monopolar mapping, equipped with a new acoustic motor evoked potential (MEP) alarm. Continuous (temporal coverage) and dynamic (spatial coverage) mapping was realized technically by integrating the mapping probe at the tip of a new suction device. Motor function was assessed using the Medical Research Council scale (from M1 to M5) 1 day after surgery, at discharge, and at 3 months. Results Technically, the method was successful in all 182 cases. The lowest individual motor thresholds reached during resection were > 10 mA, n = 56; 6–10 mA, n = 31; 4–5 mA, n = 37; and 1–3 mA, n = 58. At 3 months, six patients (3%) had a persisting postoperative motor deficit that was caused by direct mechanical injury in three of these patients (1.7%). Conclusions Continuous dynamic mapping was found to be a feasible and ergonomic technique for localizing the exact site of the CST and distance to the motor fibers. This new technique may improve the safety of motor eloquent tumor surgery.

We investigated the feasibility of recording cortico-cortical evoked potentials (CCEPs) in patients with low- and high-grade glioma. We compared CCEPs during awake and asleep surgery, as well as those stimulated from the functional Broca area and recorded from the functional Wernicke area (BtW), and vice versa (WtB). We also analyzed CCEP properties according to tumor location, histopathology, and aphasia. We included 20 patients who underwent minimally invasive surgery in an asleep-awake-asleep setting. Strip electrode placement was guided by classical Penfield stimulation of positive language sites and fiber tracking of the arcuate fascicle. CCEPs were elicited with alternating monophasic single pulses of 1.1 Hz frequency and recorded as averaged signals. Intraoperatively, there was no post-processing of the signal. Ninety-seven CCEPs from 19 patients were analyzed. There was no significant difference in CCEP properties when comparing awake versus asleep, nor BtW versus WtB. CCEP amplitude and latency were affected by tumor location and histopathology. CCEP features after tumor resection correlated with short- and long-term postoperative aphasia. CCEP recordings are feasible during minimally invasive surgery. CCEPs might be surrogate markers for altered connectivity of the language tracts. This study may guide the incorporation of CCEPs into intraoperative neurophysiological monitoring.