Seizures of temporal lobe epilepsy: identification of subtypes by coherence analysis using stereo-electro-encephalography

Objectives: Two subtypes of temporal lobe epilepsy (TLE) according to the s tructures initially involved during seizures are currently recognized: medi al TLE (MTLE) and lateral (or neocortical) TLE (LTLE). A few reports have s uggested that the classification of TLE subtypes might be larger according to variations in the interactions between medial structures and the neocort ex. In this study, we analyzed these interactions using coherence analysis of stereo-encephalographic (SEEG) signals during spontaneous seizures. Methods: Twenty-seven patients with drug-resistant TLE, diagnosed from icta l SEEG recordings obtained during pre-surgical evaluation, were studied. Or thogonally implanted depth electrodes with multiple leads according to Tala irach's method were used to sample medial and neocortical structures. Coher ence analysis of ictal discharges was performed between two SEEG bipolar si gnals from adjacent leads located either in medial structures (amygdala and hippocampus) or in neocortical regions of the temporal lobe. A new algorit hm, which was designed to reduce the bias inherent in coherence estimation, was used to compute the coherence. Results: We were able to classify TLE seizures (TLES) into 4 distinct categ ories: (1) 'medial' TLES, characterized by medial onset with later involvem ent of the neocortex in the form of a 'phasic' discharge. High ictal cohere nce values were observed between medial structures; (2) 'medial-lateral' TL ES which started in medial structures with a fast low-voltage discharge (FL VD) which rapidly affects the neocortex ( less than or equal to 3 s). High coherence values were observed between medial and lateral structures; (3) ' lateral-medial' TLES, which are different from medial-lateral TLES in that the FLVD starts in the lateral neocortex and involves the amygdala and/or h ippocampus almost immediately after; (4) 'lateral' TLES: characterized by a neocortical onset, a delayed involvement of medial structures (when presen t), and high coherence values between neocortical structures. Conclusions: These results demonstrate the existence of numerous interactio ns between medial limbic structures and the neocortex during TLE seizures. Such findings could have implications for surgical strategies and the progn osis of epilepsy surgery, particularly when limited resection is indicated. (C) 1999 Elsevier Science Ireland Ltd. All rights reserved.