Abstracts

RATIONALE: The purpose of this study is to evaluate, using reviewers who are blinded to all clinical and imaging data, the ability of Low Resolution Electromagnetic Tomography to localize the epileptic focus in patients with intractable lesional partial epilepsy.
METHODS: : Patients with medically intractable symptomatic partial epilepsy and well defined symptomatic MRI lesions were studied using Phase Encoded Frequency Spectral Analysis (PEFSA) combined with Low Resolution Electromagnetic Tomography (LORETA). Twenty-five patients admitted to the epilepsy monitoring unit with MRI lesions and intractable partial epilepsy who had seizure free outcomes after epilepsy surgery were studied using 31-electrode scalp EEG. The scalp electrodes and patients[ssquote] head shape were electronically digitized, and then manually co-registered with their MRI. Each ictal scalp EEG was reviewed by one of the authors (T.D.L and F.W.S) who were blinded to all clinical and imaging data. Compressed Spectral Array (CSA) displays were used to identify the dominant ictal frequencies and the onset of the seizure. PEFSA was used to obtain a phase encoded scalp map for the dominant ictal frequencies. The electromagnetic inverse solution for the ictal generators was obtained from the scalp map using LORETA. In addition to the ictal generators, the generators of interictal epileptogenic spikes were determined using LORETA in the time domain. Co-registration of the LORETA ictal and interictal generators with the patient[ssquote]s MRI was used to determine the accuracy of source localization.
RESULTS: The LORETA ictal generators at the onset of temporal lobe seizures were correctly lateralized in all patients. The LORETA generators were found to be rostral to the MRI lesion in all but one of the patients with temporal lobe lesions. In the cases of temporal lobe epilepsy investigated, the LORETA derived generators of the interictal spikes that were ipsilateral to the seizure onset correlated well with the ictal generators. In the patients with frontal lobe epilepsy the ictal generators determined when the spectral power of the ictal discharge was maximal gave the most reliable localization to the MRI lesions.
CONCLUSIONS: : LORETA combined with PEFSA of the ictal discharge can localize ictal EEG discharges, and improves correlation with brain anatomy by allowing co-registration of the ictal generator with the MRI. LORETA of interictal spikes was less useful than analysis of the ictal discharge in patients with more than one spike focus. The rostral displacement of the generator in temporal lobe seizures probably reflects the inadequate volume conductor model (3-sphere model).