Brain Advance Access originally published online on June 13, 2008
Brain 2008 131(7):1818-1830; doi:10.1093/brain/awn111
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Epileptogenicity of brain structures in human temporal lobe epilepsy: a quantified study from intracerebral EEG
1INSERM, U751, Marseille, F-13000, 2Aix Marseille Université, Faculté de Médecine, Marseille, F-13000, 3Assistance Publique – Hôpitaux de Marseille, Hôpital de la Timone, Service de Neurophysiologie Clinique, Marseille, F-13000, 4INSERM, U642, Rennes, F-35000 and 5Université de Rennes 1, LTSI, Rennes, F-35000, France
Correspondence to: Fabrice Bartolomei, Hôpital de la Timone, Service de Neurophysiologie Clinique, 264 Rue Saint-Pierre, Marseille 13005, France E-mail: fabrice.bartolomei{at}ap-hm.fr
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.
Key Words: partial epilepsy; high-frequency oscillations; epileptogenic zone; seizure onset
Abbreviations: EC, entorhinal cortex; EI, epileptogenicity index; EZ, epileptogenic zone; HS, hippocampal sclerosis; MRI, magnetic resonance imaging; MTLE, mesial temporal lobe epilepsy; STG, superior temporal gyrus
Received January 20, 2008. Revised April 3, 2008. Accepted May 12, 2008.
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