Abstracts

Rationale: In simultaneous EEG and functional MRI (EEG/fMRI) the BOLD (blood oxygenation level-dependent) responses related to interictal epileptic discharges (IEDs) can be positive (activation) and negative (deactivation). Activation has been assumed to reflect increased neuronal activity and energy demand, but the significance of a deactivation in the spike field is uncertain, although it has been associated in a few cases with an earlier activation. The aim of this study was to better characterize this phenomenon, investigating the correlation between BOLD deactivation and both location of the irritative zone and morphology of IEDs.Methods: Among patients with focal epilepsy who underwent EEG/fMRI from April 2006 to May 2011, those with a negative BOLD response with a maximum t-value in the EEG spike field were selected. As a control group the same number of subjects with maximum activation in the spike field was selected. Patients underwent a 120min recording session (3T scanner, anatomical acquisition and BOLD fMRI data collected with the patient resting). For each patient IEDs similar to those recorded outside the scanner were marked in the filtered EEG. We established the relationship between the type of response (activation or deactivation) and: 1) Presence of slow wave in the IEDs. A slow wave was defined as a waveform lasting at least 300ms and having the same polarity as the preceding spike. 2) Location of the epileptic focus at the sublobar level. 3) Occurrence as short (<500ms) or long (>500ms) event. 4) Extent of the EEG. The EEG was considered diffuse when the discharge involved bilateral homologous regions or almost all channels of one hemisphere. Results: 13 patients with deactivation and 13 with activation were included. In the deactivation group, IEDs were accompanied by a slow wave in all cases. In 7/13 (54%) the focus was in the occipital or posterior quadrant region, frontal in 4 (31%) and temporal in 2 (15%). In 10/13 (77%), IEDs were of long duration. Finally, 5 (38%) patients had a diffuse discharge (in 2 the discharge involved bilateral homologous regions, in 3 they were bilateral and diffuse). In the activation group, IEDs were accompanied by a slow wave in 5/13 cases (38%). In 2/13 (15%) the focus was in the posterior quadrant, frontal in 1 (8%), temporal in 9 (69%) and parietal in 1 (8%). In 5/13 (38%), IEDs were of long duration. Only one patient of this group had bilateral and diffuse discharges.Conclusions: The IEDs were accompanied by a slow wave in all patients whose primary BOLD was a deactivation and only in 38% of patients with activation. In the deactivation group, the epileptic focus was more frequently in the posterior quadrant and involving larger cortical areas, whereas in the activation group it was more frequently temporal. IEDs were more frequently of long duration in the deactivation group. One important factor responsible for focal deactivations appears to be the presence of a slow wave, which can be the electrographic correlate of prolonged inhibition. Supported by CIHR grant MOP-38079.