Abstracts

Rationale: EEG-fMRI studies in patients with focal epilepsy and unilateral spikes often show positive BOLD responses (activations) not only ipsilateral but also contralateral to the spikes, in regions where there is no apparent EEG involvement. This study aimed to investigate possible EEG changes in these contralateral homologous areas by using spectral analysis of EEG signal. Methods: We retrospectively studied 19 patients with focal epilepsy and unilateral spikes. According to the pattern of BOLD activations, patients were divided into group 1 (activation ipsi and contralateral to the spikes) or group 2 (only ipsilateral activation). EEGs recorded outside the scanner were used to mark the spikes in order to have a signal free of artefacts. Markings were based on spike distribution and morphology that were similar to those recorded in the scanner. Spike epochs of 640 ms duration immediately after the peak of the marked spike were compared to baseline epochs located just before the spike. Spectral analysis was performed with EEGs in referential montage (FCz reference), and differences between baseline and spike epochs were analyzed by paired t-test. Results: Significant EEG changes in electrodes contralateral to the spikes were seen in 9/10 patients in group 1 and only 2/10 patients in group 2 (one patient had 2 types of spikes that were analyzed separately). Spectral changes were seen in delta and/or theta bands in all patients except one (from group 1) who had changes in delta, theta, alpha and beta bands. No changes in gamma band were seen. Conclusions: Subtle EEG changes, only seen by spectral analysis, occurred in 90% of contralateral homologous BOLD activations and in only 20% of patients showing no such contralateral responses. These subtle EEG changes might represent the neuronal changes causing the BOLD activations in contralateral areas. Whether these changes are due to an effect of the focus in the contralateral homologous region remains to be further investigated. Most EEG-fMRI studies in epilepsy have included the analysis of large EEG events such as spikes, but more subtle EEG events may also result in measurable metabolic changes.