EEG spectral changes underlying BOLD responses contralateral to spikes in patients with focal epilepsy

Montreal Neurological Institute and Department of Neurology and Neurosurgery, McGill University, Montreal, Canada.
Epilepsia (Impact Factor: 4.57). 05/2009; 50(7):1804-9. DOI: 10.1111/j.1528-1167.2009.02080.x
Source: PubMed


Simultaneous electroencephalogram and functional magnetic resonance imaging (EEG-fMRI) in patients with focal epilepsy and unilateral spikes often shows positive blood oxygenation level-dependent (BOLD) responses (activations), not only ipsilateral but also contralateral to the spikes. We aimed to investigate whether minimal EEG changes could underlie these contralateral BOLD responses by using EEG spectral analysis.
We studied 19 patients with focal epilepsy and unilateral spikes. According to the pattern of BOLD activation, patients were divided into Group 1 (ipsi- and contralateral to the spikes) or Group 2 (only ipsilateral). EEG from outside the scanner was used to mark spikes similar to those recorded in the scanner. Epochs of 640 ms before and after the peak of the spikes were chosen as baseline and spike epochs. Spectral analysis was performed in referential montage (FCz reference), and differences between baselines and spikes were analyzed by paired t-test.
Significant EEG changes in electrodes contralateral to the spikes were seen in 9 of 10 patients in Group 1 and in only 2 of 10 patients in Group 2 (one patient had two types of spikes that were analyzed separately). Spectral changes were seen in delta and/or theta bands in all patients except one (in Group 1) who had changes in all bands.
Significant contralateral EEG changes occurred in 90% of contralateral BOLD activations and in only 20% of patients without contralateral BOLD responses. The reason why these changes predominate in lower frequencies rather than in higher frequencies is unclear. These spectral changes in areas corresponding to contralateral activations might reflect poorly synchronized but possibly intense neuronal activity.

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Available from: Louise Tyvaert, Oct 22, 2014
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    • "Another explanation is that the more extensive inter-hemispheric synaptic connections between extra-temporal lobe regions facilitate the spread or propagation of epileptic activity between these hemispheres when the focus is in these regions. This is supported by evidence from EEG-fMRI studies (Yu et al., 2009; Kobayashi et al., 2006) (Tyvaert et al., 2008) as well as human and animal data (Keller and Roberts, 2008; Blumenfeld et al., 2007); (Holmes et al., 1999; Zilles et al., 1998; Silva-Barrat et al., 1986) which show regional as well as distant cortical and subcortical changes associated with focal interictal spikes, being more so in patients with extra-temporal foci. It also would correlate well with the described increased incidence of bilateral features in extra-temporal lobe seizure semiologies (Luders et al., 1998; Gastaut, 1970) and of bilateral interictal and ictal discharges seen in patients with extra-TLE (Bautista et al., 1998; Taylor et al., 2003; Gibbs and Gibbs, 1955) as opposed to TLE where most reports show that even if bilateral interictal discharges are recorded, they are predominant over the side with seizure onset in 60–70% of patients (Hirsch et al., 1991; Williamson et al., 1993). "
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    • "In animal and human EEG-fMRI studies, widespread and distant (both positive and negative) BOLD signal changes are seen with focal interictal epileptic spikes or brief seizures (Kobayashi et al., 2006a; Englot et al., 2008; Truccolo et al., 2011). Using EEG spectral analysis, Yu et al. (2009) revealed significant contralateral EEG changes (predominating in lower frequencies ) in 90% of contralateral BOLD activations triggered by spikes. These authors suggested that these spectral changes in areas corresponding to contralateral activations possibly reflected poorly synchronized but intense neuronal activity. "
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