Appendix e-1
“Congruence and Discrepancyof Interictal and Ictal EEG with MRI Lesions in Focal Epilepsies”
Rémi J, Vollmar C, de Marinis A,Heinlin J,Peraud A and Noachtar S
Assignment of lesional regions
We divided the patients into four groupsaccording to the localization of their MRI lesion: temporal, frontal, parieto-occipital and central.The “central region” is not one of the four telencephalic lobes but this term has been used from the early days of epilepsy surgery to describe the region encompassing the pre- and postcentral gyri because epilepsy surgery of these structures is especially challenging and the functional anatomy differs (Penfield W, Jasper H. Epilepsy and the functional anatomy of the human brain. Boston: Brown Little & Co, 1954). We used the term “region”, because our groups are not confined to the four lobes. Please see figure e-1 for an illustration of these regions.For the purpose of this study, patients with lesions extending over more than one region were not included.
Localization of EEG pathologies
In our preoperative epilepsy patients, the 10-20 electrode set was used which was extended with electrodes from the interspaced 10-10 system in the region(s) of interest. Thus, in temporal lobe epilepsy for instance, the electrodes FT7/FT8, FT9/FT10 or additional electrodes like sphenoidal electrodes were used.
For this study, we assigned EEG electrodes to the regions according to Fig. e-1 of this appendix. It is standard practice at our lab to note the electrode of maximum negativity for all IEDs (there were no positive spikes in this preoperative patient sample) and to draw localization maps of the electrode(s) within 70% of the highest negativity. This localizing information is important for electrodes where their assignment to a single lobe or region is not obvious. For example: since the electrodes F7/F8are anatomically located over the border between anterior temporal lobe and lateral frontal lobe, we grouped the epileptiform discharge as“temporal” when the next highest negativity was temporal (for example SP2) and as “frontal” when the next highest negativity was frontal (for example F4). The same localizing procedure is used for ESPs. In ESPs, the first discernable seizure pattern was used for its localization. Flattening of the EEG, developing into a rhythmic ESP, was only used as the ESP onset in case it was regional. When the onset of seizure patterns was obscured by EMG artifact, the first discernable EEG seizure pattern was used to classify the region. When the entire seizure was artifact obscured it was grouped to the “no seizure pattern” group for the purpose of this study, because no localizing information could be derived.
Figure e-1: Illustration of the regions as used in this study
The 10-10 electrode system and a head outline are overlaid on a reconstruction of a T1-volume acquisition MRI scan. Green: frontal lobe/region; Red: temporal lobe/region; Yellow: parieto-occipital region; Blue: central region.