Objective-High-frequency oscillations (HFOs) in the intracerebral electroencephalogram (EEG) have been linked to the seizure onset zone (SOZ). We investigated whether HFOs can delineate epileptogenic areas even outside the SOZ by correlating the resection of HFO-generating areas with surgical outcome.Methods-Twenty patients who underwent a surgical resection for medically intractable epilepsy were studied. All had presurgical intracerebral EEG (500Hz filter and 2,000Hz sampling rate), at least 12-month postsurgical follow-up, and a postsurgical magnetic resonance imaging (MRI). HFOs (ripples, 80 -250Hz; fast ripples, >250Hz) were identified visually during 5 to 10 minutes of slow-wave sleep. Rates and extent of HFOs and interictal spikes in resected versus nonresected areas, assessed on postsurgical MRIs, were compared with surgical outcome (Engel's classification). We also evaluated the predictive value of removing the SOZ in terms of surgical outcome.Results-The mean duration of follow-up was 22.7 months. Eight patients had good (Engel classes 1 and 2) and 12 poor (classes 3 and 4) surgical outcomes. Patients with a good outcome had a significantly larger proportion of HFO-generating areas removed than patients with a poor outcome. No such difference was seen for spike-generating regions or the SOZ.Interpretation-The correlation between removal of HFO-generating areas and good surgical outcome indicates that HFOs could be used as a marker of epileptogenicity and may be more accurate than spike-generating areas or the SOZ. In patients in whom the majority of HFOgenerating tissue remained, a poor surgical outcome occurred.Thirty percent to 40% of patients with focal epilepsy are medically intractable, 1 and for some, surgical removal of epileptogenic areas is the best option to gain seizure freedom. Intracranial electroencephalographic (iEEG) investigations are indicated for patients in whom noninvasive methods fail to identify a single focal seizure generator. CIHR Author Manuscript CIHR Author Manuscript CIHR Author ManuscriptiEEG is used to define the seizure onset zone (SOZ). 3 Removal of the SOZ alone, however, does not always predict the surgical benefit. 4,5 It is uncertain whether the outcome can be improved by removing areas of interictal spiking, often more widespread than the SOZ. 6,7 Intracranial studies also have limitations, as their results depend on electrode location and type of implantation (intracortical vs subdural). For instance, iEEG electrodes only record neuronal activity in their direct vicinity and are blind for other areas, 8 making it hard to judge whether the activity at seizure onset really represents the seizure generator or is the result of propagation from else-where. Thus the actual focus and its extent may be missed, leading to unsuccessful surgery.Microelectrode-recorded high-frequency oscillations (HFOs), ripples (80 -250Hz), and fast ripples (FRs, 250 -500Hz), were found predominantly in epileptogenic tissue. 9 -11 They can also be recorded with macroelectrodes duri...

SUMMARYPurpose: High-frequency oscillations (HFOs) known as ripples and fast ripples (250-500 Hz) can be recorded from macroelectrodes inserted in patients with intractable focal epilepsy. They are most likely linked to epileptogenesis and have been found in the seizure onset zone (SOZ) of human ictal and interictal recordings. HFOs occur frequently at the time of interictal spikes, but were also found independently. This study analyses the relationship between spikes and HFOs and the occurrence of HFOs in nonspiking channels. Methods: Intracerebral EEGs of 10 patients with intractable focal epilepsy were studied using macroelectrodes. Rates of HFOs within and outside spikes, the overlap between events, event durations, and the percentage of spikes carrying HFOs were calculated and compared according to anatomical localization, spiking activity, and relationship to the SOZ.Results: HFOs were found in all patients, significantly more within mesial temporal lobe structures than in neocortex. HFOs could be seen in spiking as well as nonspiking channels in all structures. Rates and durations of HFOs were significantly higher in the SOZ than outside. It was possible to establish a rate of HFOs to identify the SOZ with better sensitivity and specificity than with the rate of spikes. Discussion: HFOs occurred to a large extent independently of spikes. They are most frequent in mesial temporal structures. They are prominent in the SOZ and provide additional information on epileptogenicity independently of spikes. It was possible to identify the SOZ with a high specificity by looking at only 10 min of HFO activity.

Our objective was to evaluate the brain regions showing increased and decreased metabolism in patients at the time of generalized bursts of epileptic discharges in order to understand their mechanism of generation and effect on brain function. By recording the electroencephalogram during the functional MRI, changes in the blood oxygenation level-dependent signal were obtained in response to epileptic discharges observed in the electroencephalogram of 15 patients with idiopathic generalized epilepsy. A group analysis was performed to determine the regions of positive (activation) and negative (deactivation) blood oxygenation leveldependent responses that were common to the patients. Activations were found bilaterally and symmetrically in the thalamus, mesial midfrontal region, insulae, and midline and bilateral cerebellum and on the borders of the lateral ventricles. Deactivations were bilateral and symmetrical in the anterior frontal and parietal regions and in the posterior cingulate gyri and were seen in the left posterior temporal region. Activations in thalamus and midfrontal regions confirm known involvement of these regions in the generation or spread of generalized epileptic discharges. Involvement of the insulae in generalized discharges had not previously been described. Cerebellar activation is not believed to reflect the generation of discharges. Deactivations in frontal and parietal regions remarkably followed the pattern of the default state of brain function. Thalamocortical activation and suspension of the default state may combine to cause the actual state of reduced responsiveness observed in patients during spike-and-wave discharges. This brief lapse of responsiveness may therefore not result only from the epileptic discharge but also from its effect on normal brain function.absence ͉ epilepsy ͉ thalamus T he electroencephalogram (EEG) of patients with epilepsy presents paroxysmal discharges that depend on the type of epilepsy. In epilepsy that has been termed ''idiopathic generalized'' according to the Commission on Classification and Terminology of the International League Against Epilepsy (1), the most common type of discharge is the 2-to 3-Hz spike-and-wave burst, which occurs simultaneously over wide cortical regions, most often with an anterior predominance. The origin of this discharge and of the absence seizures that often accompany spike-and-wave bursts when they last several seconds has been a subject of investigation and controversy for many years (see ref.2 for a review), particularly with respect to the involvement of subcortical structures. The recently developed method of combined EEG and functional magnetic resonance imaging (fMRI) (EEG͞fMRI) allows the investigation of the brain regions, cortical and subcortical, that are involved in metabolic changes as a result of epileptic discharges seen in the scalp EEG. In our recent publication (3), we described for each individual the patterns of increases and decreases in blood oxygenation leveldependent (BOLD) signal resulting from bu...