Abstracts

Rationale: Focal cortical dysplasia (FCD) are a common cause of drug resistant focal epilepsy and epilepsy surgery might frequently be the best therapeutic option. In addition to the determination of the minimum amount of brain tissue that should be resected to render the patient seizure-free, evaluation of the risk of cognitive deficit following epilepsy surgery is mandatory. Few data about the functionality of FCD are available. Some authors reported fMRI activation of the dysplastic cortex during cognitive tasks, whereas studies using direct electrical stimulations suggested that FCD might be considered as a non-eloquent cortex. The occurrence of high frequency oscillations (HFO) is correlated with the performance of a cognitive task with exquisite anatomical, functional and temporal specificity and mapping of task-related HFO has been proposed as an additional tool to assess cognitive functioning during presurgical evaluation. However, whether or not task-related increase of HFO might provide information about the functionality of FCD has never been investigated. Methods: We reviewed the data of 90 patients investigated with intracranial recordings (SEEG) in whom a set of standardized cognitive tasks (i.e motor, audition, reading, visual perception, attention, verbal working memory, visuo-spatial working memory), specifically designed to map task-related increase of HFO, had systematically been performed during SEEG. The ten patients whom MRI data, pathological data and/or SEEG recordings were consistent with the diagnosis of type II FCD were included in the study. Their SEEG recordings and MRI data were systematically reviewed to determine the precise localization of the FCD. For each cognitive task, a time-frequency map, which compared the EEG power following the cognitive task with the baseline using a 50-150 Hz band-pass filter, was generated. The duration of the time window varied across tasks. The time-frequency maps were then analyzed to determine if a task-related increase of HFO was observed within the FCD Results: All patients demonstrated a pattern of subcontinuous spiking activity within the FCD. A task-related increase of HFO was elicited within the non-dysplastic cortex by at least one cognitive task in all patients. In contrast, only one patient showed a marked (>30%) task-related increase of HFO within the FCD. This increase was associated with a motor task and the FCD was localized within the primary motor cortex. In three other patients, we observed a marked (>30%) task-related increase of HFO on the electrode contacts immediately adjacent to the FCD whereas no task-related variation of HFO was observed on the contacts localized within the FCD Conclusions: Although, our data confirm that task-related increase of HFO can rarely be elicited in type II FCD, most FCD might be HFO silent. These results might have implications when considering the presurgical assessment of functionality of FCD. The absence of task-related increase of HFO within the large majority of FCD might be related to severe perturbations of neural networks triggered by either the cortical malformation, the subcontinuous epileptic activity or both Funding: This work was supported by IHU CESAME, within the program "Investissements d'Avenir" (ANR-10-IBHU-0003)