Identifying the primary epileptogenic hemisphere from electroencephalographic (EEG) and magnetoencephalographic dipole lateralizations in children with intractable epilepsy

Preoperative evaluation using magnetoencephalography: Experience in 382 epilepsy patients

OBJECTIVE

Identifying epilepsy patients for whom clinical MEG is likely to be beneficial avoids or optimizes burdensome ancillary investigations. We determined whether it could be predicted upfront if MEG would be able to generate a hypothesis about the location of the epileptogenic zone (EZ), and in which patients MEG fails to do so.

METHODS

MEG recordings of 382 epilepsy patients with inconclusive findings regarding EZ localization prior to MEG were acquired for preoperative evaluation. MEG reports were categorized for several demographic, clinical and MEG variables. First, demographic and clinical variables were associated with MEG localization ability for upfront prediction. Second, all variables were compared between patients with and without MEG location in order to characterize patients without MEG location.

RESULTS

Our patient group had often complex etiology and did not contain the (by other means) straightforward and well-localized cases, such as those with concordant tumor and EEG location. For our highly-selected patient group, MEG localization ability cannot be predicted upfront, although the odds of a recording with MEG location were significantly higher in the absence of a tumor and in the presence of widespread MRI abnormalities. Compared to the patients with MEG location, patients without MEG location more often had a tumor, widespread EEG abnormalities, non-lateralizing MEG abnormalities, non-concordant MEG/EEG abnormalities and less often widespread MRI abnormalities or epileptiform MEG activity. In a subgroup of 48 patients with known surgery outcome, more patients with concordant MEG and resection area were seizure-free than patients with discordant results.

CONCLUSIONS

MEG potentially adds information about the location of the EZ even in patients with a complex etiology, and the clinical advice is to not withhold MEG in epilepsy surgery candidates. Providing a hypothesis about the location of the EZ using MEG is difficult in patients with inconclusive EEG and MRI findings, and in the absence of specific epileptiform activity. More refined methods are needed for patients where MEG currently does not contribute to the hypothesis about the location of the EZ.

Pre-operative evaluation in pediatric patients with cortical dysplasia

INTRODUCTION

Focal cortical dysplasia (FCD) is an important cause of refractory seizures and catastrophic epilepsy in infants and children who had epilepsy surgery.

AIMS OF THE REVIEW

This manuscript will discuss age-related unique clinical characteristics in evaluation of infants and young children because the understanding of these age-related features is critical in selecting children who can benefit from epilepsy surgery. In addition, we will review the non-invasive tools available for the presurgical evaluation of children with FCD and their individual contribution to the formulation of the presurgical hypothesis.

Epilepsy surgery in children: why, when and how?

Epilepsy surgery is safe and effective treatment in children who fail to respond to antiepileptic medications. After failure of two appropriate antiepileptic medications, chances that the child will become seizure free with more or different medications is <5%, and she should be diagnosed with "refractory epilepsy". A consideration for surgical candidacy should be given to all children who fulfill the definition of refractory epilepsy. In appropriately selected children, epilepsy surgery offers a high chance of seizure freedom without incurring any new post-operative neurological deficits. No age is bar to epilepsy surgery. Even infants can safely have epilepsy surgery if they are surgical candidates. For most children, who are surgical candidates, a good history and physical examination, video EEG evaluation, and a high quality brain MRI are sufficient to make surgical decision. These tools are increasingly available all over the world. Better education of families, Pediatricians, Pediatric Neurologists and community care-givers is necessary to salvage children early from mortality and morbidity of untreated, sometimes life long, epilepsy.

Magnetoencephalography in pediatric lesional epilepsy surgery

This study was performed to assess the usefulness of magnetoencephalography (MEG) as a presurgical evaluation modality in Korean pediatric patients with lesional localization-related epilepsy. The medical records and MEG findings of 13 pediatric patients (6 boys and 7 girls) with localization-related epilepsy, who underwent epilepsy surgery at Seoul National University Children's Hospital, were retrospectively reviewed. The hemispheric concordance rate was 100% (13/13 patients). The lobar or regional concordance rate was 77% (10/13 patients). In most cases, the MEG spike sources were clustered in the proximity of the lesion, either at one side of the margin (nine patients) or around the lesion (one patient); clustered spike sources were distant from the lesion in one patient. Among the patients with clustered spike sources near the lesion, further extensions (three patients) and distal scatters (three patients) were also observed. MEG spike sources were well lateralized and localized even in two patients without focal epileptiform discharges in the interictal scalp electroencephalography. Ten patients (77%) achieved Engel class I postsurgical seizure outcome. It is suggested that MEG is a safe and useful presurgical evaluation modality in pediatric patients with lesion localization-related epilepsy.

The role of magnetoencephalography in epilepsy surgery

Epilepsy surgery requires the precise localization of the epileptogenic zone and the anatomical localization of eloquent cortex so that these areas can be preserved during cortical resection. Magnetoencephalography (MEG) is a technique that maps interictal magnetic dipole sources onto MR imaging to produce a magnetic source image. Magneto-encephalographic spike sources can be used to localize the epileptogenic zone and be part of the workup of the patient for epilepsy surgery in conjunction with data derived from an analysis of seizure semiology, scalp video electroencephalography, PET, functional MR imaging, and neuropsychological testing. In addition, magnetoencephalographic spike sources can be linked to neuronavigation platforms for use in the neurosurgical field. Finally, paradigms have been developed so that MEG can be used to identify functional areas of the cerebral cortex including the somatosensory, motor, language, and visual evoked fields.

The authors review the basic principles of MEG and the utility of MEG for presurgical planning as well as intra-operative mapping and discuss future applications of MEG technology.

MEG Predicts Outcome Following Surgery for Intractable Epilepsy in Children with Normal or Nonfocal MRI Findings

PURPOSE

To identify the predictors of postsurgical seizure freedom in children with refractory epilepsy and normal or nonfocal MRI findings.

METHODS

We analyzed 22 children with normal or subtle and nonfocal MRI findings, who underwent surgery for intractable epilepsy following extraoperative intracranial EEG. We compared clinical profiles, neurophysiological data (scalp EEG, magnetoencephalography (MEG) and intracranial EEG), completeness of surgical resection and pathology to postoperative seizure outcomes.

RESULTS

Seventeen children (77%) had a good postsurgical outcome (defined as Engel class IIIA or better), which included eight (36%) seizure-free children. All children with postsurgical seizure freedom had an MEG cluster in the final resection area. Postsurgical seizure freedom was obtained in none of the children who had bilateral MEG dipole clusters (3) or only scattered dipoles (1). All five children in whom ictal onset zones were confined to < or = 5 adjacent intracranial electrodes achieved seizure freedom compared to three of 17 children with ictal onset zones that extended over >5 electrodes (p = 0.002). None of six children with more than one type of seizure became seizure-free, compared to eight of 16 children with a single seizure type (p = 0.04). Complete resection of the preoperatively localized epileptogenic zone resulted in seizure remission in 63% (5/8) and incomplete resections, in 21% (3/14) (p = 0.06). Age of onset, duration of epilepsy, number of lobes involved in resection, and pathology failed to correlate with seizure freedom.

CONCLUSIONS

Surgery for intractable epilepsy in children with normal MRI findings provided good postsurgical outcomes in the majority of our patients. As well, restricted ictal onset zone predicted postoperative seizure freedom. Postoperative seizure freedom was less likely to occur in children with bilateral MEG dipole clusters or only scattered dipoles, multiple seizure types and incomplete resection of the proposed epileptogenic zone. Seizure freedom was most likely to occur when there was concordance between EEG and MEG localization and least likely to occur when these results were divergent.

Contralateral 80–280Hz EEG ripples and hippocampal single unit discharge inhibition in response to acute tetanization of rat right caudate putamen in vivo

Clinically, 4-8 Hz (or 30-80 Hz) stimulation of the caudate nucleus ceases (or enhances) the neocortical and hippocampal epileptiform activities of the epilepsy patients. Possibly, electric stimulation of the caudate nucleus could produce epilepsy. In order to prove this point we delivered the acute tetanization (60 Hz, 2s, 0.4-0.6 mA) into the rat right caudate putamen nucleus (ATRC) and examined bilateral neocortical EEG and hippocampal unit discharges in vivo. The results demonstrated that: (1) 80-280 Hz EEG ripples could be evoked bilaterally, and more stronger on the contralateral side. And the maximum amplitudes of the power spectra (microV2/Hz) have higher shifting variability among multiple contralateral EEG ripples. (2) The EEG ripples were coupled contralaterally with the hippocampal neuronal firing inhibition. (3) An episode of 10-15 Hz EEG oscillations was ipsilaterally coupled with rhythmic hippocampal neuronal bursts. It suggested that the hemispheric reactions of neocortical EEG and hippocampal neuronal discharges are lateralized in response to the stimulation. It implies that the epileptic network activities were reorganized by the ATRC. Neocortical EEG ripples, called as seizure-like fast oscillations, were repetitively evoked by the ATRC.