Abstracts

Rationale: Resective brain surgeries are inevitable to treat brain tumors or intractable epilepsy in some patients. Such an intervention causes the risk of cognitive or motor deficits if functional cortical regions are damaged. Cortical regions related to movement, sensation or language function are often called the “eloquent cortex”. It is the important role of functional mapping to preserve the eloquent cortex and minimize post-surgical functional deficits. The current gold standard mapping method is electrical cortical stimulation (ECS), where pulses of electrical current are injected into the patient’s brain, provoking or suppressing body functions. The symptoms can be aggregated into a functional brain map based on the stimulated sites. However, ECS comes with substantial drawbacks as it is a tedious process with the risk of pain or seizures. Passive brain mapping with electrocorticography (ECoG) is an alternative method to ECS that rapidly detects task-related functions on multiple cortical locations. Thus, the patient performs certain tasks (movement, listening, speaking) while cortical broadband γ activity (60-170Hz) is extracted simultaneously for all recording electrodes. This yields a functional map within minutes without any risk for the patient. Methods: This retrospective study assesses the performance of a precedent ECoG mapping to assist ECS mapping to reduce stimulation time and risk. Four epilepsy patients underwent clinical ECS and ECoG mappings. The latter required the patients to move their hand and tongue, and to listen to words. Based on both mapping outcomes, the potential stimulation reduction of ECoG guided ECS was calculated and compared to random guidance. Results: From 208 electrode pairs that were stimulated, 52 pairs (or 75 electrodes) revealed symptoms related to motor, sensory, or language functions. On the other hand, ECoG mapping identified 112 out of 494 electrodes potentially covering locations of the eloquent cortex. If the ECoG mapping results had been used as a guidance for ECS, 69 out of the 75 sites that were originally revealed by ECS would have been stimulated and the required stimulation pairs would have been reduced by 39.8%. The ECoG guidance is significantly better than chance (p < 0.05) and could have improved the ECS protocol for all four subjects. Conclusions: The six electrodes that were missed by the ECoG mapping can be explained by differences between ECS and ECoG categories. Electrodes classified as ”mouth motor” in ECS elicited lips movement symptoms and were not detected by ECoG, whose mapping protocol included only movement of the tongue. We argue that proper and diligent design of the tasks and comparison thereof can overcome this problem. The considerable reduction of necessary stimulations might become highly relevant in the near future, since there is a growing tendency to use electrode grids with higher density and increased channel count. Although the study population was small, we are confident that ECoG mapping tremendously facilitates functional mapping with ECS in clinical practice. Funding: This work was supported by ENIAC Joint Undertaking Project DeNeCoR (No. 324257) and the Eurostars RAPIDMAPS 2020 (ID 9273).