Abstracts

Mapping of interictal epileptiform discharges is typically based on analysis of the highest amplitude to a non-active reference. Particulalry when electrodes other than the typical 10-20 system are maximal it is often difficult to grasp the location of the maximal EEG activity. Dipole modeling assumes that a scalp EEG field is generated by one or a small number of dipolar sources. This technique can be applied for the analysis of interictal epileptiform discharges. The goal of this study was to retrospectively evaluate whether dipole source analysis using 32 electrode during surface video-EEG could be useful in the presurgical work up of focal epilepsy. We analysed retrospectively the interictal sharp waves of 10 patients that underwent epilepsy surgery. EEG was aquired digitally with 32 channels with the 10-20 EEG system and with additinal temporal electrodes. Sphenoidal recordings and EKG were disregarded in the analysis. The interictal sharp waves were analysed using BESA 5 software. Homogenous samples of typical sharp waves were averaged to improve signal-to-noise ratio and decrease artifacts. A semi-automated dipole modeling technique was then then applied to these waves to solve inverse problem, yielding the most likely intracranial source of the observed surface activity. A blinded reader classified the resultant dipole location into one of the following regions: anterior, middle, posterior, mesial vs.non-mesial temporal, mesial, basal frontal or other frontal areas, and parietal regions. The region of the dipole was correlated with data obtained from conventional EEG, MRI, and invasive EEG. Four patients had mesial temopral sclerosis, 2 patients had other lesional temporal epilepsy, 1 patients had non-lesional temporal epilepsy, and 3 patients had lesional extra-temporal epilepsy. Surgeries consisted of temporal lobectomy in 5 cases, lesionectomy with intra-operative electrocorticography in 2 patients, and subdural grids in 3 patients. Dipole Source Analysis (DSA) evaluation revealed that in the 7 patients with temporal lobe epilepsy, the independent analysis was accurate in localizing the epileptogenic region. In 3 out of 7 cases, the DSA was limited in differentiating mesial vs neocortical epileptogenic foci. The DSA findings correlated accurately with the MRI and invasive EEG data in the 3 extra-temporal lesional patients. Our study revealed that the use of EEG source analysis of interictal sharp waves can provide useful data in the presurgical work up for patiens with focal epilepsy. In our patients, DSA allowed accurate localization of the interictal discharges to specific affected brain regions. The DSA visual display allows also recognition of brain regions, for professionals not familiar with EEG electrode nomenclature.