Understanding the epileptogenicity of lesions: A correlation between intracranial EEG and EEG/fMRI

Role of High-Frequency Oscillation in Locating an Epileptogenic Zone for Radiofrequency Thermocoagulation

Radiofrequency thermocoagulation (RFTC) has been proposed as a first-line surgical treatment option for patients with drug-resistant focal epilepsy (DRE) that is associated with gray matter nodular heterotopia (GMNH). Excellent results on seizures have been reported following unilateral RFTC performed on ictal high-frequency-discharge, fast-rhythm, and low-voltage initiation areas. Complex cases (GMNH plus other malformations of cortical development) do not have good outcomes with RFTC. Yet, there is little research studying the effect of high-frequency oscillation in locating epileptogenic zones for thermocoagulation on unilateral, DRE with bilateral GMNH. We present a case of DRE with bilateral GMNH, treated using RFTC on unilateral GMNH and the overlying cortex, guided by stereotactic electroencephalogram (SEGG), and followed up for 69 months. Twenty-four-hour EGG recordings, seizure frequency, post-RFTC MRI, and neuropsychological tests were performed once yearly. To date, this patient is seizure-free, the electroencephalogram is normal, neuropsychological problems have not been found, and the trace of RFTC has been clearly identified on MRI. His dosage of antiepileptic medication has, furthermore, been significantly reduced. It is concluded that RFTC on unilateral DRE with bilateral GMNH may achieve good long-term effects, lasting up to, and perhaps longer than, 69 months. Ictal high-frequency oscillation (fast ripple) inside the heterotopia and the overlying cortex may be the key to this successful effect.

Epileptogenicity and pathology - Under consideration of ablative approaches

Besides resective epilepsy surgery, minimally invasive ablation using new diagnostic and therapeutic techniques recently became available. Optimal diagnostic approaches for these treatment options are discussed. The pathophysiology of epileptogenic networks differs depending on the lesion-types and location, requiring a differential use of non-invasive or invasive functional studies. In addition to the definition of epileptogenic zones, a challenge for pre-surgical investigation is the determination of three-dimensional epileptic networks to be removed.

RF-ablation in periventricular heterotopia-related epilepsy

Drug-resistant focal epilepsy is a common occurrence in patients with gray matter nodular heterotopia (NH), and surgical treatment is often considered in these cases. NH-related epileptogenicity is sustained by complex networks, which may involve the nodules and extralesional cortex in various combinations. Therefore, invasive EEG is usually required to identify the structures involved in seizure generation. It has been reported that surgery may be effective in cases with unilateral lesions, whereas bilateral cases are not optimal candidates for surgical success. Furthermore, violation of cortical and subcortical structures for approaching deep-seated nodules may result in neurological deficits. For these reasons, selective stereotactic ablation with radiofrequency thermocoagulation (RFTC) has been proposed as an alternative option in these patients. In particular, RFTC may be performed by using the same recording intracerebral electrodes implanted for stereo-electro-encephalo-graphy (SEEG) monitoring, with the advantage of a reliable electro-clinical guide. Excellent results on seizures have been initially reported following coagulation of single, unilateral NH. Subsequent experience has indicated that, basing on the evidence of SEEG recording, promising results may be obtained also in more extended unilateral and bilateral cases. In more complex cases, coagulation of both the nodules and of the involved extralesional cortical structures is often required. In a recently reported series, 67% of patients experienced sustained seizure freedom after the procedure. However, post RFTC seizure outcome in complex cases (NH plus other malformations of cortical development) is not as good as in other patterns of NH. RFTC, especially if guided by SEEG evaluation, should be considered as a first-line treatment option in NH-related epilepsy. Satisfactory results may be obtained also in cases not amenable to traditional surgery. The procedure is safe and does not prevent eventual resective surgery in case of failure in seizure control. For these reasons, in patients undergoing SEEG evaluation, electrode arrangement should be planned with the aim to cover as extensively as possible the heterotopic and extralesional areas, which will presumably be the targets of RFTC.

Stereo-EEG-guided radio-frequency thermocoagulations of epileptogenic grey-matter nodular heterotopy

OBJECTIVE

To retrospectively evaluate seizure outcome in a case-series of patients with nodular heterotopy (NH)-related epilepsy treated by stereo-EEG (SEEG)-guided radio-frequency thermocoagulation (RF-THC) of the NH.

METHODS

Five patients (three male, age 5-33 years) with drug-resistant focal epilepsy presented a single NH at brain MRI. Following video-EEG monitoring, patients underwent SEEG recording to better identify the epileptogenic zone. All patients received RF-THC of the NH, using contiguous contacts of the electrodes employed for recording. The contacts for RF-THC lesions were chosen according to anatomical (intranodular position) and electrical (intranodular ictal low-voltage fast activity) criteria.

RESULTS

At SEEG recordings, ictal discharge originated from the NH alone in three cases and from the NH and ipsilateral hippocampus in one case. In the remaining case, different sites of ictal onset, including the NH, were identified within the left frontal lobe. No adverse effects related to the RF-THC procedures were observed, apart from a habitual seizure that occurred during coagulation in one patient. Postprocedural sustained seizure freedom was detected in four cases (mean follow-up 33.5 months). In the case with left frontal multifocal ictal activity, RF-THC of the NH provided no benefit on seizures, and the patient is seizure-free after left frontal lobe resection.

CONCLUSIONS

SEEG-guided RF-THC proved to be a safe and effective option in our small case-series of NH-related focal epilepsy. The indications to this treatment were strictly dependent on findings of intracerebral recording by SEEG, which can define the role of the NH in the generation of the ictal discharge.

EEG-fMRI validation studies in comparison with icEEG: a review

Simultaneous EEG-fMRI is a non-invasive investigation technique developed to localize the generators of interictal epileptiform discharges (IED) in patients with epilepsy. Although the value of EEG-fMRI in epilepsy presurgical evaluation is being assessed clinically, its utility is still controversial. In this review, we considered EEG-fMRI applications in epilepsy presurgical evaluation with a focus on validation studies that compared the results of EEG-fMRI with those of the current "gold standard" intracranial EEG (icEEG) in order to assess its utility of seizure focus localization and the possibility for EEG-fMRI to reduce the need for invasive techniques such as icEEG. Since the advances of EEG-fMRI partially rely on the maturation of its data analysis, we also reviewed the methodological developments in EEG-fMRI analysis. It is possible that combining with other neuroimaging modalities such as MEG/MSI and ESI, EEG-fMRI may play a greater role in epilepsy presurgical evaluation.

Initiation of epileptiform activity in a rat model of periventricular nodular heterotopia

PURPOSE

Periventricular nodular heterotopia (PNH) is, in humans, often associated with difficult-to-control epilepsy. However, there is considerable controversy about the role of the PNH in seizure generation and spread. To study this issue, we have used a rat model in which injection of methylazoxymethanol (MAM) into pregnant rat dams produces offspring with nodular heterotopia-like brain abnormalities.

METHODS

Electrophysiologic methods were used to examine the activity of the MAM-induced PNH relative to activity in the neighboring hippocampus and overlying neocortex. Recordings were obtained simultaneously from these three structures in slice preparations from MAM-exposed rats and in intact animals. Bath application or systemic injection of bicuculline was used to induce epileptiform activity.

KEY FINDINGS

In the in vitro slice, epileptiform discharge was generally initiated in hippocampus. In some cases, independent PNH discharge occurred, but the PNH never "led" discharges in hippocampus or neocortex. Intracellular recordings from PNH neurons confirmed that these cells received synaptic drive from both hippocampus and neocortex, and sent axonal projections to these structures-consistent with anatomic observations of biocytin-injected PNH cells. In intact animal preparations, bicuculline injection resulted in epileptiform discharge in all experiments, with a period of ictal-like electrographic activity typically initiated within 2-3 min after drug injection. In almost all animals, the onset of ictus was seen synchronously across PNH, hippocampal, and neocortical electrodes; in a few cases, the PNH electrode (histologically confirmed) did not participate, but in no case was activity initiated in the PNH electrode. Interictal discharge was also synchronized across all three electrodes; again, the PNH never "led" the other two electrodes, and typically followed (onset several milliseconds after hippocampal/neocortical discharge onset).

SIGNIFICANCE

These results do not support the hypothesis that the PNH lesion is the primary epileptogenic site, since it does not initiate or lead epileptiform activity that subsequently propagates to other brain regions.