Mapping Epileptic Foci with the Dipole Tracing Method: A Comparison with Intraoperative Electrocorticograms

Abstract

Purpose: To show the high accuracy of the dipole tracing method (DTM; Homma et al.Electroencephalogr Clin Neurophysiol 1994;91: 374–82), we estimated the position of the epileptic foci in two patients with epileptic seizures by using the DTM and compared the results with electrocorticograms (ECoGs) recorded during surgical resection. Case Report: Patient 1 was a 28‐year‐old man. At age 23 years, he began to experience epileptic attacks, which lasted for 1 min 7–8 times a day. During the first stage of the attacks, the patient felt delightful emotions, followed by impairment and finally loss of consciousness. The electroencephalograms (EEGs) showed frequent focal spikes in the right temporal area. Magnetic resonance imaging (MRI) of the head revealed an atrophy of the right hippocampus. Patient 2 was a 24–year‐old man. At age 18 years, the patient began to experience “bad odors” lasting 20–30 s, 2–3 times a day. At age 20 years, he began to experience attacks of unconsciousness lasting ‐1 min, 1–2 times a month. The EEGs showed frequent focal spikes in the right temporal area. An MRI of the head revealed a tumor in the right hippocampal region. Methods: The EEG was recorded from 21 electrodes arranged according to the international 10–20 system. The reference electrode was placed on the left ear lobe. All of the EEGs were recorded by using a data recorder for later analysis. One‐dipole analysis was performed on the spike‐peaks by using an EEG analyzer (CDT1000; Chuo Electronics Co. Ltd., Tokyo, Japan). We used a realistic three‐shell head model for the analysis. Dipole estimations of the same spikes were performed at five different consecutive time points at an interval of 1 ms. The equivalent current dipole (ECD) was computed as a vector on the coordinates of a three‐dimensional model of the head, and the calculated position was marked on a MRI. The EEG recording time was ‐1 h, and representative spikes were chosen by inspection as the subject for the analysis. As a result of the analysis, only ECDs in which the dipolarity was $98% were adopted. In both patients, the ECoGs were recorded from two places, one in the right hippocampus and the other in the right temporal lobe. Results: In patient I, the locations of the two independent foci were analyzed. One was thought to be in the tail of the right hippocampus, and the other, in the right basal temporal cortex. The ECoG recorded spikes in the right hippocampal tail and the right basal temporal cortex. These places matched the positions mapped by the DTM. In patient 2, the locations of the two independent foci were analyzed. One was thought to be in the right hippocampus, and the other, in the right superior temporal gyrus. The ECoG recorded spikes in the right hippocampus and the right superior temporal gyrus. These places matched the positions mapped by the DTM. Conclusions: The results of the DTM in the two patients were in close agreement with those observed by using the ECoGs. Therefore we conclude that the DTM is a highly accurate method for the estimation of foci in patients with epileptic seizures.