Sensitivity of recordings at sphenoidal electrode site for detecting seizure onset: evidence from scalp, superficial and deep foramen ovale recordings

Objectives: Some authors have recently stressed that the position of the ti p of sphenoidal electrodes plays a crucial role in their efficacy in detect ing ictal onset. An opportunity to test this hypothesis is provided by reco rdings from the most superficial contacts of foramen ovale (FO) electrode b undles because these contacts are located at the FO, in a position equivale nt to that of optimally located sphenoidal electrodes. To simplify wording, recordings obtained by superficial FO electrodes will hereafter be called sphenoidal recordings, although they have not been obtained with standard s phenoidal electrodes. The sensitivities of simultaneous scalp and sphenoida l recordings for detecting ictal onset have been compared with each other, and with a 'gold standard' provided by simultaneous deep intracranial FO re cordings from the mesial aspect of the temporal lobe. Methods: Three hundred and fourteen seizures obtained from 110 patients und er telemetric presurgical assessment for temporal lobe epilepsy have been s tudied. Scalp electrodes included anterior temporal placements. All scalp e lectrodes were considered when identifying seizure onset but the anterior t emporal electrodes were most frequently involved. Results: Ictal onset time at sphenoidal and scalp recordings: initial ictal changes appeared simultaneously in scalp and sphenoidal recordings in 123 seizures (39.2%). Initial changes occurred earlier in sphenoidal recordings in 63 seizures (20.1%), whereas they were seen earlier on the scalp in 76 seizures (24.2%). Artefacts prevented the comparison between sphenoidal and scalp recordings in 16 seizures (5.1%) and no ictal changes were seen on t he scalp and/or sphenoidal recordings in 36 seizures (11.5%). In most of th e 63 seizures where ictal changes appeared earlier in sphenoidal recordings , a delayed ipsilateral scalp onset was seen as the signal amplitude increa sed or scalp changes could be identified retrospectively on the scalp with an onset which appeared simultaneous and ipsilateral to the initial sphenoi dal changes. Sphenoidal recordings supplied additional information when com pared to scalp recordings in only 22 seizures (7%): in 5 seizures with arte facts on the scalp, in 6 seizures with no changes on the scalp and in 11 se izures with discrepant laterality at onset. Congruence in laterality with r espect to deep intracraneal FO recordings: of the 61 seizures with unilater al onset on the scalp, onsets at sphenoidal recordings and deep FO electrod es were ipsilateral in most cases. In only 3 of these 61 seizures (4.9%), s phenoidal recordings lateralized ipsilateral to the deep FO electrodes in t he presence of a contralateral onset on the scalp. In 14 among the 122 seiz ures (11.5%) with bilateral asymmetrical onset on the scalp, sphenoidal rec ordings lateralized seizure onset ipsilateral to the deep FO electrodes in the presence of a contralateral scalp onset. Thus, when compared with scalp EEG, sphenoidal recordings increased laterality congruence with respect to deep FO electrodes in 17 seizures (5.4%). Conclusions: Extracranial electrodes located next to the FO at the sphenoid al electrode site yield an improvement over suitable surface electrodes in the identification of ictal onset in only 5.4-7% of seizures. Such improvem ent derives from the fact that the low amplitude signals often seen at seiz ure onset may show higher amplitude on sphenoidal than on scalp recordings. (C) 2001 Elsevier Science ireland Ltd. All rights reserved.