Changes preceding interictal epileptic EEG abnormalities: comparison between EEG/fMRI and intracerebral EEG

Area of Science:

• Neuroimaging
• Epilepsy Research
• Neurophysiology

Background:

• Simultaneous electroencephalography (EEG) and functional magnetic resonance imaging (fMRI) can detect blood oxygen level dependent (BOLD) changes preceding epileptic spikes.
• The origin of these prespike BOLD signals remains unexplained.

Purpose of the Study:

• To investigate the origin of prespike BOLD signal changes observed in EEG/fMRI studies.
• To identify potential electrographic correlates of early BOLD responses using stereo-EEG (SEEG).

Main Methods:

• Four patients with drug-resistant focal epilepsy underwent EEG/fMRI with detectable prespike BOLD responses.
• Stereo-EEG (SEEG) was performed for presurgical localization.
• BOLD responses were analyzed using hemodynamic response functions (HRFs) peaking around the spike.
• SEEG data were analyzed for electrographic changes preceding scalp abnormalities.

Main Results:

• One patient showed a SEEG interictal discharge corresponding to the early BOLD response.
• In the other three patients, no SEEG electrographic changes preceded the early BOLD signal.
• This suggests early BOLD responses are not always linked to detectable synchronized neural discharges.

Conclusions:

• Early BOLD activity in epilepsy can sometimes be attributed to synchronized neural discharges detectable by SEEG.
• However, in most cases, prespike BOLD responses reflect metabolic phenomena.
• These metabolic changes may arise from neuronal or non-neuronal mechanisms, including glial activity.