Epileptogenicity of brain structures in human temporal lobe epilepsy: a quantified study from intracerebral EEG

Abstract

The identification of brain regions generating seizures ('epileptogenic zone', EZ) in patients with refractory partial epilepsy is crucial prior to surgery. During pre-surgical evaluation, this identification can be performed from the analysis of intracerebral EEG. In particular, the presence of high-frequency oscillations, often referred to as 'rapid discharges', has long been recognized as a characteristic electrophysiological pattern of the EZ. However, to date, there has been no attempt to make use of this specific pattern to quantitatively evaluate the degree of epileptogenicity in recorded structures. A novel quantitative measure that characterizes the epileptogenicity of brain structures recorded with depth electrodes is presented. This measure, called 'Epileptogenicity Index' (EI), is based on both spectral (appearance of fast oscillations replacing the background activity) and temporal (delay of appearance with respect to seizure onset) properties of intracerebral EEG signals. EI values were computed in mesial and lateral structures of the temporal lobe in a group of 17 patients with mesial temporal lobe epilepsy (MTLE). Statistically high EI values corresponded to structures involved early in the ictal process and producing rapid discharges at seizure onset. In all patients, these high values were obtained in more than one structure of the temporal lobe region. In the majority of patients, highest EI values were computed from signals recorded in mesial structures. In addition, when averaged over patients, EI values gradually decreased from structure to structure. For lateral neocortex, higher EI values were found in patients with normal MRI, in contrast with patients with hippocampal sclerosis. In this former sub-group of patients, a greater number of epileptogenic structures was also found. A statistically significant correlation was found between the duration of epilepsy and the number of structures disclosing high epileptogenicity suggesting that MTLE is a gradually evolving process in which the epileptogenicity of the temporal lobe tends to increase with time.

Glutamatergic pre-ictal discharges emerge at the transition to seizure in human epilepsy

“…6a left). A subset of pyramidal cells (7/39) increased firing in the 25 ms before the onset of PID field potentials (19±26 Hz, 95% CI: [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28], n=30 events from 3 cells; Fig. 6a right).…”

“…Changes in excitatory synaptic mechanisms may also be involved in the transition to seizure-like activity 12,13 . Several studies suggest that the initiation, properties and propagation of population activities generated by epileptic networks are modified before ictal-like activity is initiated [14][15][16] .…”

Advances of Intracranial Electroencephalography in Localizing the Epileptogenic Zone

“…Their definition of the EZ is different from the North American concept of ''minimal-cortex-to-remove area'' [9]. Bartolomei et al from the Marseilles group coined the term ''epileptogenicity index'' to quantify the degree of epileptogenicity in each brain region [10]. This measurement is based on both the spectral (appearance of fast oscillations in the beta and gamma bands) and temporal (delay of appearance with respect to seizure onset) properties of ictal iEEG signals.…”

High frequency oscillations and seizure frequency in patients with focal epilepsy

“…Whether the same can be assumed for humans is unknown. In humans, the extent of higher frequencies during seizure onset is correlated to the duration of disease (Bartolomei et al, 2008). If HFOs are more widespread in patients with frequent seizures or long disease duration, they could offer a measure for disease severity, enabling a better estimation of potential outcome after treatment.…”