Available online 9 March 2023

Author links open overlay panelAbstract

Some patients with medically intractable epilepsy are considered for surgical treatment. In some surgical candidates, the investigation includes the placement of intracerebral electrodes and long-term monitoring to find the region of seizure onset. This region is the primary determinant of the surgical resection but about one-third of patients are not offered surgery after electrode implantation and among those operated only about 55% are seizure free after five years. This paper discusses why the primary reliance on the seizure onset maybe suboptimal and may be in part responsible for the relatively low surgical success rate. It also proposes to consider some interictal markers that may have advantages over seizure onset and may be easier to obtain.

Section snippetsHistorical context

In the early days of electroencephalography (EEG), it was unusual to record epileptic seizures because the recordings only lasted a short time (typically 30 minutes) and it was unlikely that a seizure would occur during that time, unless seizures were extremely frequent. It was observed that the EEG of patients often showed interictal markers such as focal spikes and sharp waves in focal epilepsy, or widespread spike and wave discharges in generalized epilepsy. These became important diagnostic

Problems with the seizure onset zone

There are some conceptual and practical issues with relying on the SOZ as recorded on SEEG for delineating the region to be resected. We refer here to SEEG but subdural grid recordings have the same issues and the discussion below applies equally to SEEG and subdural grids or strips.

The first and probably most fundamental issue is that of poor spatial sampling. Each electrode contact is sensitive to a very small brain region, approximately a sphere of 5 mm in radius [7], [8]. This implies that

Surgery and the current recording of the seizure onset zone

Although surgery performed after an SEEG investigation does not rely exclusively on the SEEG-defined SOZ, this information plays a critical role. If the SOZ is widespread or if it is known to have been missed (clinical onset before EEG onset) most centers will not offer surgery. As described in large retrospective studies, surgery is offered only if the SOZ can be defined clearly [12], [13] to approximately 60 to 70% of investigated patients. The major issue is that close to half of the

EEG-fMRI and the spike onset zone

When EEG and functional MRI (fMRI) are recorded simultaneously, it is possible to analyze the fMRI sequence as a function of what happens in the EEG. It is in particular possible to measure the blood oxygen level-dependent (BOLD) signal changes occurring at the time of EEG spikes. The application of EEG-fMRI to different types of epilepsy was recently reviewed by Ikemoto et al. [14]. The BOLD measurement is made in the whole brain with equal sensitivity to deep and superficial structures. It is

Fast Ripples in SEEG

High frequency oscillations (HFOs) have shown great promise in the last 20 years as potential markers of the epileptogenic zone. Many studies seemed to demonstrate this potential at the group level, as discussed in the review by Frauscher at al. [20]), but at least one multicenter study could not confirm their utility at the individual patient level [21]. Several explanations have been proposed for this lack of effectiveness despite the early promises: in the lower frequency range (80 to 250 

Spike-gamma

Epileptic spikes are the traditional biomarker of epileptic tissue, but in intracerebral recordings they play a moderate role because they are often widespread and are considered to have low specificity for the epileptogenic zone. In our study [25], we revisited this paradigm and evaluated if some parameters of spikes could provide a more accurate prediction than just their presence. We measured many parameters related to spike morphology, spike propagation, activation during different sleep

Conclusion

To cure a patient of their seizures it seems logical to find the region from which they start and to remove it surgically. This approach, using SEEG, only yields seizure freedom in about 55% of operated patients (operated patients represent about 65% of those in whom electrodes are implanted). One must therefore examine if the premise of removing the seizure onset zone needs to be re-considered. We discussed above the limitations of intracerebral electrodes and the difficulty of separating the

Funding

The author's research is funded by grant FDN 143208.

Disclosure of interest

The author declares that he has no competing interest.

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