Are Epilepsy-Related fMRI Components Dependent on the Presence of Interictal Epileptic Discharges in Scalp EEG?

Abstract

Spatial independent component analysis (ICA) is increasingly being used to extract resting-state networks from fMRI data. Previous studies showed that ICA also reveals independent components (ICs) related to the seizure onset zone. However, it is currently unknown how these epileptic ICs depend on the presence of interictal epileptic discharges (IEDs) in the EEG. The goal of this study was to explore the relation between ICs obtained from fMRI epochs during the occurrence of IEDs in the EEG and those without IEDs. fMRI data sets with co-registered EEG were retrospectively selected of patients from whom the location of the epileptogenic zone was confirmed by outcome of surgery (n = 8). The fMRI data were split into two epochs: one with IEDs visible in scalp EEG and one without. Spatial ICA was applied to the fMRI data of each part separately. The maps of all resulting components were compared to the resection area and the EEG-fMRI correlation pattern by computing a spatial correlation coefficient to detect the epilepsy-related component. For all patients, except one, there was a remarkable resemblance between the epilepsy-related components selected during epochs with IEDs and those without IEDs. These findings suggest that epilepsy-related ICs are not dependent on the presence of IEDs in scalp EEG. Since these epileptic ICs showed partial overlap with resting-state networks of healthy volunteers (n = 10), our study supports the need for new ways to classify epileptic ICs.

A prospective fMRI-based technique for localising the epileptogenic zone in presurgical evaluation of epilepsy

“…On the other hand, based on our experience, even in cases where EEG-fMRI shows multiple areas of BOLD activation, these activation clusters are represented in different independent components, representing different aspects of epileptic activity [21]. Similar findings were reported in a recent study (van Houdt et al, 2014). The authors argued that in patients where the EEG-fMRI patterns included more than one activation cluster, one of them reflected the onset area, while others were related to propagation.…”

Epileptic networks are strongly connected with and without the effects of interictal discharges

“…Our findings are concordant with those of other recent studies investigating connectivity patterns in the absence of scalp IEDs: a seed‐based FC study in patients with frontal lobe epilepsy revealed that the FC features of the epileptic network were similar when comparing fMRI dataset with visible IEDs in the simultaneous scalp EEG to dataset without IEDs. Another study, using ICA on resting‐state fMRI in patients with drug‐resistant epilepsy, showed that the BOLD components related to the epileptogenic region of a patient are similar in EEG‐fMRI data with and without visible concomitant scalp IEDs . Uniquely, our study included the whole BOLD dataset from which we regressed out the effect of the IEDs in order to avoid any confounding effect that could occur between consecutive recording sessions.…”

Abnormal cognitive network interactions in Lennox‐Gastaut syndrome: A potential mechanism of epileptic encephalopathy

“…One possibility is that networks involved in the expression of scalp epileptic activity show fMRI connectivity that is observable when discharges are absent, akin to the intrinsic correlations observed in physiologic networks in the absence of task performance 11 -that is, epileptic activity may represent the "tip of the iceberg" against a background of ongoing atypical network interactions. 30 Alternatively, it is possible that small sample size, electrical events not visible on scalp EEG, or fMRI signal changes preceding the onset of visible epileptic activity, 31 may have impeded detection of differences between fMRI periods with and without discharges. However, our results are consistent with previous EEG-fMRI studies in genetic generalized epilepsy 32 and localization-related epilepsy, 30 where persisting fMRI connectivity abnormalities were observed in the absence of scalp-recorded discharges.…”

“…30 Alternatively, it is possible that small sample size, electrical events not visible on scalp EEG, or fMRI signal changes preceding the onset of visible epileptic activity, 31 may have impeded detection of differences between fMRI periods with and without discharges. However, our results are consistent with previous EEG-fMRI studies in genetic generalized epilepsy 32 and localization-related epilepsy, 30 where persisting fMRI connectivity abnormalities were observed in the absence of scalp-recorded discharges.…”