Abstracts

Rationale: To compare spontaneous and photic induced epileptiform EEG abnormalities in scalp EEG recordings of a well documented, pedigreed colony of baboons at the Southwest Foundation for Biomedical Research (SFBR; San Antonio, Texas) with those of photosensitive epilepsy patients, for future extrapolation of pathophysiological studies. Methods: Scalp EEG studies utilizing intermittent photic stimulation (IPS) were performed in 676 baboons. These studies lasted 60 minutes, and the baboons were lightly sedated (ketamine 5-6 mg/kg) from electrode placement and transfer between their cages and the primate chairs. Scalp EEG studies were performed in a Dutch epilepsy patient population of 2313 patients: 100 (4.3 %; 61 F: Caucasians) were found to be repeatedly photosensitive and were studied in detail in comparison to 100 non-photosensitive (age- and sex- matched) patients from the same population. Detailed data analysis with comparison of spontaneous and IPS evoked epileptiform discharges with special emphasis on onset and spreading of the discharges was performed. If no onset could be found due to fast spreading, the maximum amplitude was considered. Results: The prevalence of seizures in the baboon colony is 20%, with an incidence of 1-2%. Witnessed seizures generally lasted 15-60 seconds, and were either myoclonic or generalized tonic-clonic. The EEGs demonstrated interictal epileptiform discharges (IEDs) in 332 (49%) of EEG studies. Most of the discharges were 4-6 Hz frequency, rarely 2-3 or 6-7 Hz. The IEDs were maximal over the fronto-central regions. Seizures were recorded in 221 (33%) baboons, and included eyelid or facial myoclonus, truncal myoclonus, rarely GTCS or absence seizures. Photosensitivity, defined by evidence of time-locked or increased IED rates as well as myoclonic seizures associated with IPS, was found in 157 (23%) of baboons. Photosensitivity was identified in 40% of the baboons with spontaneous IEDs. While the photic driving response is posteriorly predominant, the photoparoxysmal responses (PPRs) were expressed frontocentrally. Sixty-five percent of photosensitive epilepsy patients had a history of myoclonic seizures or absences besides usually a GTCS. In those epilepsy patients with spontaneous IEDs (66%), 50% were photosensitive. The IEDs were more temporo-parieto-occipitally ( 170/259 foci, 66%) than fronto-centrally. The PPRs had an occipital onset or maximum in 64%, a temporo-parietal in 29% and were frontocentrally in only 7%. Conclusions: While earlier electrophysiological data in the baboon Papio papio of Senegal presented by Naquet and again our scalp EEG data suggest a frontal predominance of spontaneous and IPS-induced IEDs or seizures compared to the temporo-parieto-occipital predominance in humans, further functional neuro-imaging and EEG studies might elucidate a common role of the parietal lobes in the epileptic networks with differences in spreading pattern between the species.