Siblings with refractory occipital epilepsy showing localized network activity on EEG-fMRI

Practical Fundamentals of Clinical MEG Interpretation in Epilepsy

Magnetoencephalography (MEG) is a neurophysiologic test that offers a functional localization of epileptic sources in patients considered for epilepsy surgery. The understanding of clinical MEG concepts, and the interpretation of these clinical studies, are very involving processes that demand both clinical and procedural expertise. One of the major obstacles in acquiring necessary proficiency is the scarcity of fundamental clinical literature. To fill this knowledge gap, this review aims to explain the basic practical concepts of clinical MEG relevant to epilepsy with an emphasis on single equivalent dipole (sECD), which is one the most clinically validated and ubiquitously used source localization method, and illustrate and explain the regional topology and source dynamics relevant for clinical interpretation of MEG-EEG.

Extrastriate visual cortex in idiopathic occipital epilepsies: The contribution of retinotopic areas to spike generation

OBJECTIVES

To provide insight into the pathophysiology of idiopathic childhood occipital epilepsies (ICOEs), by mapping the contribution of retinotopic visual areas to the generation and sustainment of epileptic activity.

METHODS

Thirteen patients affected by ICOEs (mean age = 10.9 years) underwent a video electroencephalography-functional magnetic resonance imaging (EEG-fMRI) study. A flexible-related fMRI analysis was applied to estimate the shape of the blood oxygen level-dependent (BOLD) response in each patient. Second-level analysis was performed using the interictal EEG discharge (IED)-specific response shape for the ICOE group. The resulting fMRI t-maps were warped to the Population-Average, Landmark- and Surface-based (PALS)-B12 atlas in Caret. For localization purposes, functional results were plotted and compared against 19 retinotopic areas for each hemisphere. A correlation analysis was performed between the hemodynamic maps and electroclinical variables.

RESULTS

The shape of the group-averaged hemodynamic response in ICOE patients showed an earlier time-to-peak and a more pronounced undershoot than the canonical hemodynamic response function (HRF). The random-effect analysis showed positive hemodynamic changes in the bilateral temporooccipital network. With regard to the retinotopic subdivision of the visual cortex, the primary visual area was consistently spared. Conversely, an extensive involvement of the occipitotemporal cortex, including the fusiform gyrus, and the occipitoparietal areas was observed. Moreover, a linear relationship was detected between the occipital spike-density and BOLD increases at the postcentral gyrus and temporooccipital cortex.

SIGNIFICANCE

Our data indicate that both the ventral and dorsal visual pathways are involved in spike generation in ICOEs, to extents that vary between patients, and reinforce the concept of benign childhood seizure susceptibility syndrome as a substrate for ICOEs. Finally, these results underscore the need for appropriate neuropsychological testing in these children, aimed at revealing selective impairments in functions subserved by both visual pathways.

An appraisal of the new operational definition of epilepsy--then and now

The focus to define epilepsy in the newly proposed classification has shifted from the conceptual perspective to practical application thought to better reflect that which is happening to the patient. Within the new definition, a single unprovoked or reflex seizure can be considered as epilepsy if the recurrence risk is similar to that following two unprovoked seizures. Epilepsy is considered to be resolved if the individual had an age-dependent epilepsy syndrome and has passed the applicable age or if the person has remained seizure-free for the last ten years without seizure medications for the last five years. This new operational definition of epilepsy may change the epileptologist's approach regarding when and how long to treat patients with seizures. The new definition also has significant psychosocial and employment-related implications for the patients. With regard to etiology, the terms idiopathic, symptomatic, and cryptogenic have been replaced by genetic, structural/metabolic, and unknown. This reflects a better understanding of the underlying cause of epilepsy based on genetic tests and better neuroimaging. The terms 'simple partial' and 'complex partial' seizures have been replaced by 'focal motor/sensory' and 'focal dyscognitive' seizures, thereby ending the ambiguity associated with the former terms and the difficulty encountered with definitions of altered states of consciousness. These changes, reflective of a better insight into the pathogenesis of seizures and epilepsy, are expected to be more pragmatic and assist when managing patients with epilepsy.