Abstracts

Rationale: Perioperative intracranial electrical stimulation (ES) mapping is the golden standard for phase II surgical epilepsy workup. No expert would advocate resection of a cortical area without the knowledge of what function it may subserve. Sensorimotor, language, and visual modalities are usually tested, depending on implant location. Occasionally, mapping fails to elicit expected responses in anatomically congruent areas. In 1998 and in 2007 we reported a noninvasive adjuvant testing method after Pfurtscheller and Crone, based on event-related desynchronization (ERD) of intracranial EEG. Among our test subjects there were two cases of "failed ES mapping", where ES was negative, but ERD identified hand and face areas in plausible locations. At the time, we had no explanation for ES failure. Now we do. Methods: A new-generation mapping hardware was used to map a complicated customized 8x8 temporo-occipital grid implant, which was cut and bended to cover the lateral occipital convexity. On this platform, there is a closed-loop reporting of both stimulus applied and of the current delivered. An observant fellow operator noted that in multiple instances "current delivered" did not correspond to "current programmed to apply" ?" for example, 5 milliamperes were applied, but only 0.2 milliamperes were delivered, as reproduced at the same electrode pair, while other electrode pair stimulations were successful. Anatomically appropriate visual association cortex mapping was achieved partially. Results: Analysis revealed that the electrode pairs that failed stimulation were at the corners or/and folds of the grid. Normal-appearing signal was recorded from those electrodes prior to ES testing, which were, save for two, not around the epileptogenic zone. Yet, stimulating them failed to deliver the programmed current, possibly due to increased capacitance of corner and bend-based contacts. Other contacts, outside of fold/cut and corner zones were mapped successfully to reveal a nice visual processing organization, with more primitive monochrome shapes posteriorly and inferiorly, and colored bright flashes anteriorly and superiorly. Conclusions: Penfield mapping, or ES perioperative functional mapping, a golden standard of perisurgical workup ?" is gilded, after all. A failure of defining a functional area upon ES should not be interpreted as a true negative ?" perhaps, corner or bent grid effect is preventing current from entering the cortex efficiently. This calls for further development and utilization of adjuvant functional testing methods. Funding: None