Combined Bilateral Depth and Subdural Electrode Investigation in Temporal Lobe Epilepsy

Ma, Hyeo Il; Kang, Joong Koo; Kwak, Kyu Hwan; Lee, Jung Kyo; Lee, Sang Ahm

Abstract

BACKGROUND: Depth and subdural electroencephalographic (EEG) recordings are often required to identify an area of the brain for epileptic surgery. We simultaneously compared bilaterally placed depth and subdural electrode EEGs to determine the site of seizure origins from the temporal lobes.
METHODS
We included nine consecutive patients with medically refractory temporal lobe epilepsy, whose noninvasive evaluations such as magnetic resonance imaging, scalp and sphenoidal EEG, and other tests had not proved consistent lateralization. All patients had bilateral temporal depth electrodes, anterior and lateral temporal subdural strip electrodes. Thirty-eight clinical seizures and 3 subclinical seizures were evaluated.
RESULTS
Seven out of 9 patients (78%) had unitemporal seizures, one patient had bilateral seizures, and the other had lateral temporal seizures in an invasive study. Ictal onset was localized by depth electrodes in 8 patients, and subdural strip electrodes in one. In ictal recordings, the ictal rhythms never spread to the contralateral neocortex before the ipsilateral neocortex. Most of the ictal rhythms began focally with periodic spikes or fast activities in the depth electrode, then spread to the ipsilateral strip electrode after 14 to 90 seconds (mean : 35.2 seconds). If ictal rhythms propagated to the contralateral side, it took 14 to 140 seconds (mean : 64.2 seconds). Subdural strip electrodes were less sensitive than depth electrodes in the detection of seizure onset and subclinical seizures, but were accurate when lateralized.
CONCLUSIONS
We conclude that EEG recordings with depth and subdural strip electrodes correctly identify and lateralize temporal lobe seizures more often than subdural electrodes alone.