Abstracts

Rationale: Scalp recorded interictal epileptiform discharges (ied) typically appear as negative polarity sharp transients. They are generated on superficial layers of the cortex by the summation of extracellular EPSPs. A passive return current generates a positive pole equivalent in the deeper layers of the cortex: this can be also recorded by scalp electrodes at some distance from the negative maxima as a lower amplitude positive sharp transient. The identification of the positive maximum of an ied may define the orientation of the dipole and help in localizing it. Some scalp recorded ieds exhibit a prevalent positive polarity without evident negative maxima; other ieds show an initial low amplitude positive component preceding a subsequent prevalent negative sharp transient. These positive scalp ieds may originate from a cortical source with orientation rotated by ~180 degrees relative to that of the hemispheric gyral surfaces, i.e. for example, the mesial inter-hemispheric, mesial temporal and the internal opercular cortices. We have identified EEG recordings presenting these positive waves and looked for evidence indicating a source in any of these regions. Methods: A retrospective chart review of the Epilepsy Program of our Hospital identified at least 6 patients (pts) with positive scalp temporal discharges. On all pts seizure history, brain MRI and video-EEGs were obtained: 2 pts were implanted intracranial electrodes. 3 pts had a Magneto encephalogram (MEG). Results: Seizure semiology of all pts indicated mesial temporal lobe seizures with staring and orofacial/manual automatisms. MRI showed temporal lobe abnormalities in 4 pts (with mesial temporal sclerosis (mts) in 3 pts (see MRI image)and cavernoma in 1 pt). In the remaining 2 pts no temporal lobe lesions were seen. EEGs showed positive sharp transients over the temporal and parietal electrodes in 4 pts (see EEG traces), more pronounced and persistent in the 3 pts with hippocampal atrophy. In 2 pts typical negative ieds in the temporal electrodes were preceded by an initial low voltage positive component. Intracranial recordings in 2 patients confirmed the presence of parahippocampal discharges. MEG was performed in 3 pts and showed basal or mesial temporal epileptiform discharges. Conclusions: On a theoretical basis, ieds originating from the mesial temporal cortex may be recorded by ipsilateral scalp electrodes as low voltage sharp transients of positive polarity. We have provided experimental data of such low voltage positive polarity discharges in the ipsilateral lateral chain of electrodes in patients with evidence of mesial temporal lobe epileptiform activity. These positive discharges are more pronounced in pts with ipsilateral hippocampal atrophy. Some pts exhibit scalp temporal ieds with an initial low voltage positive component and this may correspond to an initial source over the mesial temporal cortex.