Source localisation of epileptiform activity in epilepsy of temporal lobe origin

University dissertation from Clinical Neurophysiology

Abstract: In patients with drug resistant partial epilepsy, who are under consideration for epilepsy surgery, interictal and ictal EEG recorded with extracranial electrodes plays an important role in the localisation of the seizure onset area. During the past years, methods have been developed, which can be used to estimate the location of the source of epileptiform activity in the brain from extracranial EEG data. In the present thesis, the location of the source of interictal epileptiform activity is determined by use of two different concepts, dipole analysis and LORETA (Low Resolution Electromagnetic Tomography). Localisation of the source of ictal epileptiform activity is made by analysing the frequency content of the seizure activity, with subsequent dipole localisation of the dominant frequency. Dipole reconstruction of the source of interictal spikes, shown to be emanating from the mesiotemporal area, gives quite specific results, with an anterior temporal dipole location, and an oblique posterior, slightly elevated dipole orientation. Dipole reconstruction of frequency analysed ictal activity, in seizures of mesiotemporal origin, gives dipole results, which are quite similar to the results obtained for mesiotemporal interictal epileptiform activity. The characteristic dipole locations and orientations, whether obtained from interictal analysis in the time domain, or from ictal analysis in the frequency domain, thus give an indication of epilepsy of mesolimbic origin. Dipole results deviating markedly from this characteristic pattern, indicate an epileptogenic region outside mesolimbic structures. LORETA analysis of the extracranial fields of temporal lobe interictal spikes identified with intracranial electrodes, consistently shows activation of the temporal area corresponding to the intracranial spikes, even when the extracranial signal to noise-ratio is very unfavourable. Dipole and LORETA analysis of extracranially recorded interictal data, as well as dipole analysis of frequency analysed extracranially recorded ictal data, are useful supplementary methods for localising the epileptogenic region in patients under consideration for epilepsy surgery

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