Lateralizing and Localizing Values of Ictal Onset Recorded on the Scalp: Evidence from Simultaneous Recordings with Intracranial Foramen Ovale Electrodes

Abstract

Summary:Purpose: The value of scalp recordings to localize and lateralize seizure onset in temporal lobe epilepsy has been assessed by comparing simultaneous scalp and intracranial foramen ovale (FO) recordings during presurgical assessment. The sensitivity of scalp recordings for detecting mesial temporal ictal onset has been compared with a "gold standard" provided by simultaneous deep intracranial FO recordings from the mesial aspect of the temporal lobe. As FO electrodes are introduced via anatomic holes, they provide a unique opportunity to record simultaneously from scalp and mesial temporal structures without disrupting the conducting properties of the brain coverings by burr holes and wounds, which can otherwise make simultaneous scalp and intracranial recordings unrepresentative of the habitual EEG.Methods: Simultaneous FO and scalp recordings from 314 seizures have been studied in 110 patients under telemetric presurgical assessment for temporal lobe epilepsy. Seizure onset was identified on scalp records while blind to recordings from FO electrodes and vice versa.Results: Bilateral onset (symmetric or asymmetric) was more commonly found in scalp than in FO recordings. The contrary was true for unilateral seizure onset. In seizures with bilateral asymmetric onset on the scalp, the topography of largestamplitude scalp changes at onset does not have localizing or lateralizing value. However, 75-76% of seizures showing unilateral scalp onset with largest amplitude at T1/T2 or T3/T4 had mesial temporal onset. This proportion dropped to 42% among all seizures with a unilateral scalp onset at other locations. Of those seizures with unilateral onset on the scalp at T1/T2, 65.2% showed an ipsilateral mesial temporal onset, and 10.9% had scalp onset incorrectly lateralized with respect to the mesial temporal onset seen on FO recordings. In seizures with a unilateral onset on the scalp at electrodes other than T1/T2, the proportions of seizures with correctly and incorrectly lateralized mesial temporal onset were 37.5 and 4.2%, respectively. Thus the ratio between incorrectly and correctly lateralized mesial temporal onsets is largely similar for seizures with unilateral scalp onset at T1/T2 (16.7%) and for seizures with unilateral scalp onset at electrodes other than T1/T2 (11.2%). The onset of scalp changes before the onset of clinical manifestations is not associated with a lower proportion of seizures with bilateral onset on the scalp, or with a higher percentage of mesial temporal seizures or of mesial temporal seizures starting ipsilateral to the side of scalp onset. In contrast, the majority (78.4%) of mesial temporal seizures showed clinical manifestations starting after ictal onset on FO recordings.Conclusions: A bilateral scalp onset (symmetric or asymmetric) is compatible with a mesial temporal onset, and should not deter further surgical assessment. Although a unilateral scalp onset at T1/T2 or T3/T4 is associated with a higher probability of mesial temporal onset, a unilateral onset at other scalp e...

Role of Electroencephalography in Presurgical Evaluation of Temporal Lobe Epilepsy

“…FO electrodes offer a unique opportunity for simultaneous intracranial and surface EEG recording without breach of the skull. They may lateralize seizures in adults and children with mesial TLE when scalp ictal EEG fails [20–22]. It serves as intermediary between surface and invasive recordings.…”

Cortical Substrates of Scalp EEG Epileptiform Discharges

“…Simultaneous scalp and intracranial EEG recording techniques have been performed historically for the similar purposes of this study (Alarcon et al, 2001;Delucchi et al, 1962;Risinger et al, 1989). For the most part, however, those studies were compromised either by limited intracranial and/or by scalp electrode coverage.…”

Prognostic value of the second ictal intracranial pattern for the outcome of epilepsy surgery

“…The type, number and location of the electrodes were determined by the suspected location of the ictal onset region, according to non-invasive evaluation: clinical history, scalp EEG recordings obtained with the Maudsley system (Fernandez Torre et al, 1999b;Alarcon et al, 2001;Kissani et al, 2001), neuropsychology (Akanuma et al, 2003) and neuroimaging. The selection criteria and implantation procedures have been described in detail elsewhere (Alarcon et al, 2006;Alarcon, 2012).…”

“…Approximately 50% of patients undergoing ressective surgery for the treatment of epilepsy continue suffering from seizures after surgery (Kumar et al, 2013) available to identify the epileptogenic zone preoperatively, including interictal and ictal scalp electroencephalography (Adachi et al, 1998;Alarcon et al, 2001), neuropsychology (Akanuma et al, 2003) and neuroimaging (Koutroumanidis et al, 2004;Duncan, 2010).…”