In patients with mesial temporal lobe epilepsy (MTLE) associated with hippocampal sclerosis, foramen ovale electrodes may be a reliable method for lateralization of seizures that are not clearly recorded by surface EEG monitoring.

Tonicarlo R. Velasco, MD, from the University of São Paulo, Brazil, and colleagues studied 65 patients with bitemporal independent seizures or seizures with nonlateralized ictal onset or ictal onset initiating contralaterally to the side of hippocampal sclerosis. Participants were recruited between January 1996 and June 2004; 35 (53.8%) were women; mean age at recruitment was 37.8. Mean age at epilepsy onset was 12.2, mean epilepsy duration was 25.5 years, and mean seizure frequency before surgery was 6.8 episodes per month. Results were reported in the August Epilepsia.

In 16 patients with nonlateralized ictal onset on surface EEG, 14 (87.5%) had lateralized EEG during evaluation using foramen ovale electrodes, while among 23 patients with contralateral onset on surface EEG, 12 (52.2%) had lateralized EEG. The likelihood of lateralization with foramen ovale electrodes in these two patient subgroups was increased compared with patients with bilateral independent seizures on surface EEG, of whom nine out of 36 (34.6%) demonstrated lateralization. Foramen ovale electrode EEG onsets were unilateral but contralateral to the side of hippocampal sclerosis in eight patients.

Dr. Velasco’s team pointed out that foramen ovale electrode implantation was necessitated by independent bilateral seizures revealed by surface EEG in 26 patients (40%), by surface EEG onset contralateral to the side of hippocampal atrophy in 23 patients (35.4%), and by nonlateralized ictal discharge onset in 16 patients (24.6%). Transitory complications that correlated with foramen ovale electrode implantation included temporary facial pain in 12 patients, hypoesthesia in trigeminal territory in three patients, and temporomandibular joint dysfunction and recurrence of labial herpes in two patients each. In addition, there was one case of retromandibular hematoma and one case of transient complete atrioventricular block despite premedication with atropine.

Foramen ovale electrode evaluation had a substantial effect on presurgical evaluation of the MTLE patients in the study cohort, the researchers noted, despite the high rate of patients with bilateral hippocampal sclerosis and the potentially poor postsurgery outcome suggested by bilateral surface EEG. "It is important to emphasize that this proportion [66%] was similar in patients with bilateral and unilateral hippocampal sclerosis revealed by MRI," they pointed out.

While studies suggest that bilateral hippocampal damage is common among patients with temporal lobe epilepsy, hippocampal sclerosis tends to be asymmetrical, noted the investigators. In addition, given the agreement of evaluation data with the side of hippocampal atrophy, the ability to indicate temporal lobectomy may be present, which is typically followed by a good postsurgical outcome.

"Clinical decision making is more complicated when neurophysiologic and neuroimaging data indicate bilateral involvement of mesial structures," the group said. "Although data from the present series indicate frequent bilateral involvement of mesial structures, we found evidence that foramen ovale electrode evaluation provides accurate neurophysiologic data about lateralization of seizures that were not clearly lateralized by surface EEGs."

The team concluded that "in properly selected patients, foramen ovale electrodes are a reliable method for lateralization of seizures that are not clearly recorded by surface EEGs, usually providing sufficient information to indicate epilepsy surgery."

NR