Motor evoked potential monitoring and continuous dynamic mapping: Warning criteria during surgery on motor eloquent intra-axial brain tumors.

Objective We investigated the relationship between intraoperative changes of direct cortical stimulation (DCS) motor evoked potentials (MEP), lowest motor threshold (MT) reached during subcortical dynamic mapping, and the patient's motor outcome.Methods We included data from 473 intra-axial brain tumor surgeries, 432 of which had motor outcome reported directly after surgery, at discharge and the 3-month follow-up visit. The MEP changes and MT groups were correlated with clinical parameters. Ordinal logistic regression was used to model the motor outcome with respect to the severity of the deficits based on MEP changes, MT groups and clinical parameters.Results The lower the MT, the higher was the chance of significant MEP changes. MEP changes, MT group and histopathology were predictors of deficit. The more significant the MEP changes and the lower the MT, the higher the likelihood of more severe deficits. MEP loss was associated with 184 times higher odds of causing a deficit than no MEP changes. The lowest dynamic mapping MT group (1-3 mA) was associated with 20 % of significant MEP changes and 20 % permanent deficits.Conclusion Irreversible MEP alterations, MEP loss and lower MT increase the risk of postoperative motor deficits.Significance This study validates IONM warning criteria.