The utility of magnetoencephalography in the presurgical evaluation of refractory insular epilepsy

Abstract

SUMMARYPurpose: To study the utility of magnetoencephalography (MEG) in patients with refractory insular epilepsy. Covered by highly functional temporal, frontal, and parietal opercula, insular-onset seizures can manifest a variety of ictal symptoms falsely leading to a diagnosis of temporal, frontal, or parietal lobe seizures. Lack of recognition of insular seizures may be responsible for some epilepsy surgery failures. Methods: We retrospectively reviewed and analyzed MEG data in 14 patients with refractory insular seizures defined through intracranial electroencephalography (EEG) or by the presence of an epileptogenic lesion in the insula with compatible seizure semiology. MEG was performed as part of the noninvasive presurgical evaluation, using a 275-channel whole head MEG system. MEG data were analyzed using a single equivalent current dipole model. MEG localization was compared to interictal positron emission tomography (PET) and ictal single photon emission computed tomography (SPECT) results and to the resection margin. Key Findings: Three patterns of MEG spike sources were observed. Seven patients showed an anterior operculoinsular clusters and two patients had a posterior operculoinsular cluster. No spikes were detected in one patient, and the remaining four patients showed a diffuse perisylvian distribution. Spike sources showed uniform orientation perpendicular to the sylvian fissure. Nine patients proceeded to insular epilepsy surgery with favorable surgical outcome. Among patients with anterior operculoinsular cluster who proceeded to have surgery, MEG provided superior information to ictal SPECT in four of six patients and to interictal PET in five of six patients. Significance: MEG is useful in identifying patients who are likely to benefit from epilepsy surgery targeting the insula, particularly if a tight dipole cluster is identified even if other noninvasive modalities fail to produce localizing results.

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.…”

“…MEG can detect insular spikes [16][17][18]. Retrospective MEG-directed review of MRI may reveal previously undetected epileptogenic lesions in up to 50% of patients [18,40].…”

“…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.…”

Multimodal investigation of epileptic networks

“…We found six studies in which all patients (N ¼ 43) displayed some abnormality (Cukiert et al, 1998;Duffau et al, 2002;Heers et al, 2012;Kaido et al, 2006;Roper et al, 1993;von Lehe et al, 2008), two studies (N ¼ 23) with both MRI-positive and MRI-negative patients Mohamed et al, 2013), and seven studies (N ¼ 19) with exclusively MRI-negative patients (Dobesberger et al, 2008;Isnard et al, 2000Isnard et al, , 2004Kriegel et al, 2012;Nguyen et al, 2009;Ryvlin et al, 2006;Zhang et al, 2008). Overall, MRI had a sensitivity of 61%, corresponding to 87% positive predictive value when considering the results of surgery.…”