Temporal and spatial determination of EEG-seizure onset in the frequency domain

Objective: A quantitative analysis of scalp electric fields in patients suf fering from pharmacoresistant temporal lobe epilepsy was performed in order to study the development of rhythmic ictal activities over time. Methods: A method that calculates phase-corrected voltage maps in the frequ ency domain (FFT-approximation) was applied to ictal multichannel recording s in 10 epileptic patients. The onset of the ictally dominant frequency was determined and its temporal evolution over a time period of 46 s around th e ictal EEG onset was studied. The analysis was completed by a linear inver se solution that estimated the sources of the dominant frequency. Results: This method permitted the identification of an ictally dominant fr equency which started on the average prior to the onset of initial EEG sign s as determined by visual inspection. The frequency incremented during the evolution of the seizure in all patients. The linear inverse solution algor ithm localized the source of this frequency to the brain region which was c linically determined as the site of seizure onset and whose resection rende red all patients seizure-free. Conclusions: Our data suggest that the constant increase of the ictally dom inant frequency is related to the amount of temporal lobe tissue generating the ictal discharges. Frequential analysis of ictal electric fields can be reliably used to detect focal pathological activity early during seizure o nset arising in deep structures such as the mesial temporal lobe. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.