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Source Imaging in Drug Resistant Epilepsy - Current Evidence and Practice

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One third of patients with focal epilepsy are drug-resistant. There is compelling evidence from randomized controlled trials that epilepsy surgery is an efficient therapeutic option in these patients. However, the epileptogenic zone (EZ) cannot be localized in half of the patients who undergo presurgical ...

One third of patients with focal epilepsy are drug-resistant. There is compelling evidence from randomized controlled trials that epilepsy surgery is an efficient therapeutic option in these patients. However, the epileptogenic zone (EZ) cannot be localized in half of the patients who undergo presurgical evaluation, and it is erroneously localized (i.e. they do not become seizure-free) in more than one third of the operated patients.

The major challenge in epilepsy surgery is the accurate localization of the EZ. Since none of the diagnostic modalities, on its own, can localize the EZ with sufficient accuracy, a multimodal approach is used, integrating data on seizure semiology, EEG (long-term video-EEG monitoring) and neuroimaging (MRI, PET and SPECT). However, visual inspection of the EEG abnormalities can mislocalize the location of the source, due to volume conduction of the electric signals to areas on the scalp that are distant from the source.

During the last decades, technical development and advances in signal analysis triggered a rapid development of techniques that estimate the location in the brain of the source generating the epileptiform discharges recorded by EEG or MEG (magnetoencephalography). There are several methods available for electromagnetic source imaging (EMSI). In spite of the published evidence that these methods are accurate, EMSI is included into the presurgical evaluation in only a few centers. This is partly due to lack of knowledge of these relatively new methods: EMSI is lacking in from the training of epileptologists and neurophysiologists, interpreting EEG. The goal of this special issue on EMSI is to bridge this gap by providing a comprehensive update on the state-of the art of EMSI.
We suggest the following papers:
1. Electromagnetic source imaging in presurgical evaluation of patients with drug resistant focal epilepsy. An editorial / introduction to the topic, with emphasis on EMSI´s placement in the presugical evaluation process.
2. Taking the EEG back into the brain: The power of multiple discrete sources. This paper will explain how epileptiform EEG and MEG signals are generated, and how the signals propagate and volume-conduct to the surface. There will be much emphasis on the paradigm shift, from reading EEG and MEG in sensor space to reading in source-space.
3. Source imaging: overview of the available methods. This paper will describe in detail the analysis process and will give an overview of the available methods of EMSI.
4. Evidence for the accuracy of ESI. This paper will present a systematic review of the clinical validation studies, and it will detail the evidence for accuracy and for clinical utility of the various electric source imaging methods.
5. Evidence for the accuracy of MSI. This paper will present a systematic review of the clinical validation studies, and it will detail the evidence for accuracy and for clinical utility of the various magnetic source imaging methods.
6. Cortical mapping using EMSI. Besides localization of the EZ (i.e. what needs to be removed), an important aspect in presurgical evaluation is localization of the eloquent cortex (i.e. what must not be removed). This paper will give an overview on non-invasive mapping techniques, based on EMSI.
7. Connectivity analysis in source imaging. Our understanding of ictogenesis has changed in the last decades, and the concept of the epileptic focus is gradually shifting towards a model based on abnormal networks in the brain. Although these methods are not yet validated in clinical practice, they are imaging new techniques that potentially can change the way we do presurgical evaluation.
8. Localization of High Frequency Oscillations (HFO) in drug resistant Epilepsy: there is mounting evidence that HFOs are biomarkers for the EZ. This paper will review introduce the readers to this relatively new neurophysiological entity, and it will review the current evidence about this.


Keywords: Epilepsy, focus, presurgical evaluation, source imaging, source localization


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