Localization of Abnormal Discharges Causing Insular Epilepsy by Magnetoencephalography

Abstract

The insula, one of the five cerebral lobes of the brain, is located deep within the brain and lies mainly beneath the temporal lobe. Insular epilepsy can be easily confused and misdiagnosed as temporal lobe epilepsy (TLE) because of the similar clinical symptoms and scalp electroencephalography (EEG) findings due to the insula location and neuronal connections with the temporal lobe. Magnetoencephalography (MEG) has higher sensitivity and spatial resolution than scalp EEG, and thus can often identify epileptic discharges not revealed by scalp EEG. Simultaneous scalp EEG and MEG were performed to detect and localize epileptic discharges in two patients known to have insular epilepsy associated with cavernous angioma in the insula. Epileptic discharges were detected as abnormal spikes in the EEG and MEG findings. In Patient 1, the sources of all MEG spikes detected simultaneously by EEG and MEG (E/ M-spikes) were localized in the anterior temporal lobe, similar to TLE. In contrast, the sources of all MEG spikes detected only by MEG (M-spikes) were adjacent to the insular lesion. In Patient 2, the sources of all MEG spikes detected simultaneously by EEG and MEG (E/M-spikes) were localized in the anterior temporal lobe. These findings indicate that MEG allows us to detect insular activity that is undetectable by scalp EEG. In conclusion, simultaneous EEG and MEG are helpful for detecting spikes and obtaining additional information about the epileptic origin and propagation in patients with insular epilepsy.

Magnetoencephalography in the presurgical evaluation of epilepsy

“…Magnetoencephalography can be used to confirm that the likelihood of insular epilepsy is high enough to justify invasive and potentially life-threatening monitoring with insular depths. Magnetoencephalography spikes in the insula can be seen in 60-100% of patients with insular epilepsy detected by other means [16,17,19]. These retrospective analyses probably overestimate MEG sensitivity since it is likely that insular epilepsy is frequently never detected.…”

“…However, distortion of the insular cortex by epileptogenic structures [17] and nonradial currents associated with normal insular gyrations make detection of spikes more probable than expected. Recent studies have convincingly demonstrated that epileptic activity from the insula can be detected by MEG [16][17][18][19]. • Posterior intrasylvian cortex including Heschl's gyrus and planum temporale: MEG sensitivity in these regions is better than expected.…”

“…Magnetoencephalography can detect spikes from this region if a large area (between 3 and 8 cm 2 ) is involved in epileptic activity [6,7,15]. • Insula: The insular cortex is relatively far from the MEG sensors and may produce mostly radial currents [16,17]. However, distortion of the insular cortex by epileptogenic structures [17] and nonradial currents associated with normal insular gyrations make detection of spikes more probable than expected.…”

“…• Insula: The insular cortex is relatively far from the MEG sensors and may produce mostly radial currents [16,17]. However, distortion of the insular cortex by epileptogenic structures [17] and nonradial currents associated with normal insular gyrations make detection of spikes more probable than expected. Recent studies have convincingly demonstrated that epileptic activity from the insula can be detected by MEG [16][17][18][19].…”

Multimodal investigation of epileptic networks

“…Among those, Heers et al studied three patients with cryptogenic epilepsy and hypermotor seizures (Heers et al, 2012). They showed that MEG can not only identify epileptic foci when other modalities fail but is also able to localize deep-seated foci such as in the insula (Park et al, 2012). We recently investigated 14 patients with insular seizures using MEG.…”