26. Functional organization of the irritative zone in neocortical epilepsy

The irritative zone is an area of the brain generating interictal epileptiform discharges (IEDs) that is used together with other results when planning epilepsy surgery. This zone possesses relatively low diagnostic value due to lack of a specific marker that would identify IEDs generated within the epileptogenic tissue. To increase diagnostic yield of irritative zone, we examined its functional organization. Intracranial EEG recordings from 14 patients with refractory neocortical epilepsy were analysed using an algorithm that separates IEDs according to their spatial distribution into clusters. Analysis of cluster properties enabled to determine: (1) activity – percentage contribution to all IEDs in the given recording; (2) origins – the contacts where cluster initiates and (3) active region – the area of the most common propagation. On average 16,868 ± 16,813 IEDs per patient were analysed. The results demonstrated that in all patients the irritative zone was composed of multiple clusters with an average number of 12.5 ± 4.7 clusters per patient. The strongest cluster generated 43.5 ± 18.9% of all IEDs Average size of active region was 4.5 ± 4.5 contacts and contained 2.2 ± 1.7 origins. Evaluation of the cluster resection with the outcome suggested that the topology of the irritative zone may determine the success of the resection. This study demonstrates the complex organization and modular nature of the neocortical irritative zone in epilepsy patients. It can be stratified into functional components, each with distinct pathophysiological and clinical significance. The ability to identify the key components of this network and its topology has potential to improve the results of epilepsy surgery. Supported by grants from IGA NT11460, NT13357, NT14489, GACR 14-02634S and Neuron Fund (NFKJ 10/2012).