Magnetoencephalography reveals a unique neurophysiological profile of focal-onset epileptic spasms

Abstract

Epilepsy is defined as a disorder of the brain characterized by an enduring predisposition to experience epileptic seizures and the neurobiological, cognitive, psychological, and social difficulties relating to the condition. An epileptic spasm (ES) is a type of seizure characterized by clusters of short contractions involving axial muscles and proximal segments. However, the precise mechanism of ESs remains unknown. Despite the potential of magnetoencephalography (MEG) as a tool for investigating the neurophysiological mechanism of ESs, it has been difficult to use this methodology due to magnetic artifacts attributable to patient movement. We report on an 8-year-old girl suffering from intractable epileptic spasms from the age of 7 months. She was diagnosed with possible Aicardi syndrome [corrected] (AGS), characterized by the triad of callosal agenesis, infantile spasms, and chorioretinal lacunae. She is now intellectually delayed and suffers from intractable ES. We used both MEG and electroencephalography to investigate her epilepsy. The recording captured two series of spasm clusters. Spikes were clearly identified with MEG in about four-fifths of all spasms but were identified poorly or not at all in the remainder. MEG findings support previous studies that used intracranial electrodes to analyze patients with ESs and that showed variability in ES-associated spikes in terms of manner of cortical involvement and magnitude. Given the limitations of intracranial electrodes, such as sampling restrictions and invasiveness, MEG may be a helpful tool for non-invasively investigating the unique pathophysiological profile of focal-onset ESs.

Figure 1

Representative MRI of the patient. MRI shows multifocal dysplastic changes involving bilateral hemispheres with diffuse brain atrophy, which is more prominent in the left than the right hemisphere.

Figure 2

Representative EEG and MEG waveforms (top), their contour maps (middle), and all dipoles (bottom) of (a) interictal and (b) ictal epileptiform discharges. Contour maps. Arrows on the contour maps indicate the estimated dipole at the time point indicated by the broken line in the waveform. “Plus” indicates efflux of the magnetic field from the brain surface, and “minus” indicates influx of the magnetic field to the brain surface. MRI: equivalent current dipoles (ECDs) indicate location (circle) and orientation (bar) of spike sources. The ECDs of the interictal sharp wave and those of ictus are concordantly localized to the right frontal lobe (white broken circle). In an ictal recording, spikes were identified clearly with MEG in about four-fifths of spasms; they were identified poorly or not at all in the remainder even when the latter were in the same series of spasms as the former. These findings reflect the complex pathophysiology of ESs.