Abstracts

Rationale: Interictal epileptiform discharges (IEDs) on electroencephalography (EEG) are highly specific for epilepsy, but are traditionally seen as having no clinical impact on cortical function. Recent research has shown that IEDs may in fact have a measurable effect on cortical function. We therefore studied whether IEDs that occur before a visual stimulus change the visual cortical fMRI-BOLD-signal using visual stimulation during periods of IEDs and periods without IEDs in patients with occipital lobe epilepsy.Methods: All MRI and EEG-fMRI data were acquired on a 3T MRI scanner with a 12 channel head coil. After anatomical T1-MPRAGE acquisition, simultaneous EEG and fMRI was performed. After preprocessing, fMRI data was analyzed in BrainVoyager QX 2.3. The EEG data was cleaned of the fMRI artifact with the acquisition station software and the cardioballistogram was removed with an independent component analysis (ICA). The spikes of interictal activity were identified by an epileptologist. For the purpose of this study, we compared the BOLD responses to visual checkerboard stimuli that were preceded by IEDs or not. Since IEDs create a BOLD-change themselves, IEDs had to precede the stimulus by at least 4 seconds, but not more than 10 seconds.Results: We studied 5 patients with occipital lobe epilepsy. In all patients, a robust BOLD signal resulted from the visual stimulation. In 2 of the 5 patients, the EEG-IEDs were not recorded over the occipital lobe, but rather over the temporal lobes. In these patients there was no difference in the BOLD responses to the visual stimuli with and without preceding IEDs. In two patients, too few IEDs were recorded to allow analysis. In one patient, though, we could demonstrate a reduction of up to 40% in the BOLD response, when the stimuli were preceded by IEDs. When comparing the single time points with a rank-sum-test, the differences were significant (p<0.01, corrected).Conclusions: Occipital IEDs impair the physiological occipital BOLD response to visual stimulation. These results demonstrate that although IEDs occur interictally , they have an impact on cortical functioning. That impact may not be as clinically evident as a seizure, but may be enough to impair the functional performance of our epilepsy patients.