Abstracts

Rationale: Seizure freedom, the ultimate goal of epilepsy surgery, is achieved by successful resection of the epileptogenic zone. Delineation of ictal onset zone (IOZ) is considered the most accurate approximation of the epileptogenic zone and the gold standard in surgical evaluation of partial onset epilepsies. This concept of focal ictogenesis is the basis of the clinical practice of epilepsy surgery. By contrast, the network concept of ictogenesis posits that widely distributed interconnected networks of neurons produce the electro-clinical phenomenon of a seizure. There is little known about single neuron activity during ictogenesis in humans. We examined ictal dynamics of 3 seizures captured in one patient at the neuronal level, in an area outside the IOZ, and compare this with regional EEG changes.Methods: A 23 year old right-handed male with suspected left temporal lobe epilepsy underwent invasive intracranial monitoring. An 8x8 grid of electrodes covered the left lateral fronto-temporal areas and two 1x4 electrode strips were placed subtemporally. A 96 tine Utah penetrating microelectrode array (UEA) was implanted in the left middle temporal gyrus (MTG), later determined to be the non-ictal onset zone. Digital intracranial EEG was recoded with XLTEK (EMU 128) with a sampling rate (SR) of 1 KHz, LFF 1 Hz and HFF of 70 Hz. UEA data was recorded with a SR of 30 KHz. For analyzing action potential (AP) firing the LFF was set to 300 Hz and HFF to 7.5 KHz. APs were identified using a PCA based spike sorting algorithm. For analyzing local field potentials (LFPs) the LFF was set to 0.3 Hz and HFF to 500 Hz.Results: Three stereotypical seizures were captured (40-51 seconds in duration). All seizures started with 1-5 seconds of irregular spiking in the mesial contacts of the subtemporal electrodes followed by 7-10 seconds of focal ictal alpha activity (10-13 Hz) followed by spread to lateral temporal neocortical areas. The patient underwent a left anterior-temporal lobectomy with Engel Ia outcome at 30 months. The UEA-LFP signal closely resembled the signal recorded from physically adjacent EEG contacts. Notably, analysis of the UEA-AP data revealed changes at ictal onset. The seizure onset in the mesial temporal lobe was accompanied by a cessation of AP firing in the MTG. Conclusions: In this case of temporal lobe epilepsy (with no mesial temporal sclerosis) successful surgery was performed based on the focal concept of ictogenesis as guided by intracranial EEG. The network concept of ictogenesis would suggest that seizures be accompanied by measurable changes in neuronal activity, even in non-IOZ. Seizure onset in the mesial temporal lobe was not accompanied by clear changes in LFP or EEG activity in the MTG in this patient. However, significant changes in the action potential firing rates in the temporal neocortex reflected seizure onset in the mesial area. The network and focal concepts of ictogenesis reflect differing attributes regarding human epilepsy. Simultaneous analysis of neuronal activity in differing frequency ranges (AP, LFP) might provide complimentary insights into local and distant cortical activity.