Intraoperative Motor Evoked Potential Alteration in Intracranial Tumor Surgery and Its Relation to Signal Alteration in Postoperative Magnetic Resonance Imaging

OBJECTIVE: To determine the degree to which the pattern of intraoperative isolated, unilateral alteration of motor evoked potential (MEP) in intracranial surgery was related to motor outcome and location of new postoperative signal alterations on magnetic resonance imaging (MRI). METHODS: In 29 patients (age, 42.8 +/- 18.2 years; 15 female patients; 25 supratentorial, 4 infratentorial procedures), intraoperative MEP alterations in isolation (without significant alteration in other evoked potential modalities) were classified as deterioration (> 50% amplitude decrease and/or motor threshold increase) or loss, respectively, or reversible and irreversible. Postoperative MRI was described for the location and type of new signal alteration. RESULTS: New motor deficit was present in all 5 patients with irreversible MEP loss, in 7 of 10 patients with irreversible MEP deterioration, in 1 of 6 patients with reversible MEP loss, and in 0 of 8 patients with reversible MEP deterioration. Irreversible compared with reversible MEP alteration was significantly more often correlated with postoperative motor deficit (P < .0001). In 20 patients, 22 new signal alterations affected 29 various locations (precentral gyrus, n = 5; corticospinal tract, n = 19). Irreversible MEP alteration was more often associated with postoperative new signal alteration in MRI compared with reversible MEP alteration (P = .02). MEP loss was significantly more often associated with subcortically located new signal alteration (P = .006). MEP deterioration was significantly more often followed by new signal alterations located in the precentral gyrus (P = .04). CONCLUSION: MEP loss bears a higher risk than MEP deterioration for postoperative motor deficit resulting from subcortical postoperative MR changes in the corticospinal tract. In contrast, MEP deterioration points to motor cortex lesion. Thus, even MEP deterioration should be considered a warning sign if surgery close to the motor cortex is performed.