Lateralization of mesial temporal lobe epilepsy with chronic ambulatory electrocorticography

Abstract

Objective: Patients with suspected mesial temporal lobe (MTL) epilepsy typically undergo inpatient video-electroencephalography (EEG) monitoring with scalp and/or intracranial electrodes for 1 to 2 weeks to localize and lateralize the seizure focus or foci. Chronic ambulatory electrocorticography (ECoG) in patients with MTL epilepsy may provide additional information about seizure lateralization. This analysis describes data obtained from chronic ambulatory ECoG in patients with suspected bilateral MTL epilepsy in order to assess the time required to determine the seizure lateralization and whether this information could influence treatment decisions.

Methods: Ambulatory ECoG was reviewed in patients with suspected bilateral MTL epilepsy who were among a larger cohort with intractable epilepsy participating in a randomized controlled trial of responsive neurostimulation. Subjects were implanted with bilateral MTL leads and a cranially implanted neurostimulator programmed to detect abnormal interictal and ictal ECoG activity. ECoG data stored by the neurostimulator were reviewed to determine the lateralization of electrographic seizures and the interval of time until independent bilateral MTL electrographic seizures were recorded.

Results: Eighty-two subjects were implanted with bilateral MTL leads and followed for 4.7 years on average (median 4.9 years). Independent bilateral MTL electrographic seizures were recorded in 84%. The average time to record bilateral electrographic seizures in the ambulatory setting was 41.6 days (median 13 days, range 0-376 days). Sixteen percent had only unilateral electrographic seizures after an average of 4.6 years of recording.

Significance: About one third of the subjects implanted with bilateral MTL electrodes required >1 month of chronic ambulatory ECoG before the first contralateral MTL electrographic seizure was recorded. Some patients with suspected bilateral MTL seizures had only unilateral electrographic seizures. Chronic ambulatory ECoG in patients with suspected bilateral MTL seizures provides data in a naturalistic setting, may complement data from inpatient video-EEG monitoring, and can contribute to treatment decisions.

Keywords: Ambulatory EEG; EEG monitoring; Electrocorticography; Intracranial EEG; Localization; Responsive stimulation.

Figure 1

Examples of bilateral seizure onsets recorded in one subject. Panels A and B show left- and right- sided seizure onsets (respectively) recorded in the same subject. In Panel A, the onset in the left hippocampus begins with spiking followed by high amplitude fast activity on channel 1. The flag labeled “B1″ on the first channel at 89.8 s denotes detection of abnormal electrographic activity by the neurostimulator based on the programmed detection settings. The flags labeled “Tr” at 90 s indicate delivery of responsive stimulation. There is an artifact in the recording when responsive stimulation is delivered. In Panel B, the onset in the right hippocampus begins with rhythmic beta activity on channel 3. The flag labeled “B2” on the third channel at 105.3 s denotes detection of abnormal electrographic activity by the neurostimulator based on the programmed detection settings. The flags labeled “Tr” at 105.5 s indicate delivery of responsive stimulation.

Figure 2

CT/MRI co-registered images of hippocampal lead implants. Panels A, B, and D show pre-implant MRI images co-registered with post-implant CT images. Panel A is an axial slice along the axis of the hippocampus showing the depth leads implanted bilaterally in the hippocampi. Panel B shows a sagittal image of the same implant, where the cross-hairs identify the second electrode of the depth lead implanted in the left hippocampus. Panel C shows a CT image of the neurostimulator (implanted in the parietal skull) connected to bilateral sub-temporal cortical strip leads. Panel D shows a coronal image of a depth lead implanted in the left hippocampus after a left temporal resection.

Figure 3

Time to record bilateral temporal onsets.