Cortical abnormalities in epilepsy revealed by local EEG synchrony

Abstract

Abnormally strong functional linkage between cortical areas has been postulated to play a role in the pathogenesis of partial epilepsy. We explore the possibility that such linkages may be manifest in the interictal EEG apart from epileptiform disturbances or visually evident focal abnormalities. We analyzed samples of interictal intracranial EEG (ICEEG) recorded from subdural grids in nine patients with medically intractable partial epilepsy, measuring interelectrode synchrony using the mean phase coherence algorithm. This analysis revealed areas of elevated local synchrony, or "hypersynchrony" which had persistent spatiotemporal characteristics that were unique to each patient. Measuring local synchrony in a subdural grid results in a map of the cortical surface that provides information not visually apparent on either EEG or structural imaging. We explore the relationship of hypersynchronous areas to the clinical evidence of seizure localization in each case, and speculate that local hypersynchrony may be a marker of epileptogenic cortex, and may prove to be a valuable aid to clinical ICEEG interpretation.

Detecting and localizing the foci in human epileptic seizures

“…In most of these works the increase in the synchronization was found only during the seizure, while synchronization clusters, identified in [13], remain synchronized also during interictal periods. Similar stability of local hyper-synchronous regions was found also in [14]. In spite of demonstrated success, formulation of these methods involve some assumptions, that we would like to relax in our proposed approach, as it will be explained in the next Section.…”

“…Previous observations involving different linear and non-linear measures [13,14,21,22,23] indicate that the values of the corresponding measures are altered over tumors and regions thought to be part of the epileptogenic zone. The regions of such a locally enhanced or suppressed activity are stable and have well defined edges.…”

“…Recently, concepts from synchronization analysis were brought to the study of ECoG signals [9,10,11,12,13,14,15], leading to a series of promising results on localization and predictability of epileptic seizures. In most of these works the increase in the synchronization was found only during the seizure, while synchronization clusters, identified in [13], remain synchronized also during interictal periods.…”

Remission of benign epilepsy with rolandic spikes: An EEG-based connectivity study at the onset of the disease and at remission

“…In epilepsy, abnormally synchronization rapidly spreads across a lot of cortical columns and even greater cortical areas via the dense network of short intracortical connections (Chagnac-Amitai and Connors, 1989) of about 1 to 5 millimeters length (Braitenberg, 1978). Abnormally synchronized EEG background activity does not respect anatomical borders and shows smooth gradients of the electric field as demonstrated by intracranial EEG (Schevon et al, 2007). Cross-modal validations and simulation studies showed that LORETA localization shows good correspondence with the reference imaging methods in these circumstances (Pascual-Marqui, 1999Oakes et al, 2004).…”

“…An alternative way of thinking and research may start with realizing that all sorts of epilepsy are dynamic disorders. The epileptic brain may be characterized by abnormal network activities at large spatial scales (Bragin et al, 2000;Schevon et al, 2007;Spencer, 2002;Ponten et al, 2007;Horstmann et al, 2009). However, epilepsy is neither steady nor irreversible.…”