The aim of this study was to investigate if sequential analysis of BOLD signal changes induced by seizures is useful for preoperative identification of the site of seizure onset in patients with pharmaco-resistant focal epilepsy.
Method: We analyzed BOLD raw data from 5 patients with focal medically refractory epilepsy who experienced partial seizures during fMRI as part of a preoperative evaluation. To sequence the changes in BOLD signal seizure-induced, each seizure epoch was divided into groups of five consecutive images (ten-second blocks). t-maps were calculated continuously from 120 s before the onset of clinical/EEG seizure onwards by comparing two consecutive groups of five images. Time lag between each comparison was 2 s. Relative changes in BOLD signal between two consecutive groups of five images along the seizure epoch were determined. Results were compared with those of subtraction ictal SPECT coregistered with MRI (SISCOM) and intracranial EEG (2 patients).
Results: A typical seizure was registered in each patient. After sequential analysis, a well-localized and statistically significant (t: 7-14) area of signal increase was consistently found at seizure initiation in each patient. This area invariably preceded the onset of clinical/electrical seizure by several seconds (6-52 s); was concordant with SISCOM results in all but one patient; and overlapped with the ictal onset zone determined by intracranial EEG in those 2 patients who underwent invasive-EEG recordings. Complete resection of this initial area of signal increase resulted in seizure remission. Three out of four patients who underwent epilepsy surgery remained seizure-free.
Conclusion: Sequential analysis of ictal-fMRI data may be useful to precisely and non-invasively delineate the ictal onset zone within the brain; and provide insights into the cerebral substrates involved in the generation and propagation of seizures.