Single and Multiple Clusters of Magnetoencephalographic Dipoles in Neocortical Epilepsy: Significance in Characterizing the Epileptogenic Zone

PURPOSE

To characterize the epileptogenic zone in neocortical epilepsy (NE) by using magnetoencephalography (MEG).

METHODS

We defined and compared locations of single and multiple clusters of equivalent current dipoles (ECDs) for interictal spikes with MRI findings, ictal-onset zones (IOZs) from subdural electroencephalography (SDEEG), resected areas, and postsurgical outcomes of 20 patients who underwent cortical resection for medically intractable NE.

RESULTS

Fourteen patients had single clusters; six had multiple clusters. Overlap of clusters and IOZs defined group A (nine patients), in which a single cluster coincided with the IOZ; group B1 (four patients), in which a single cluster was within or partially overlapped the IOZ; group B2 (five patients), in which multiple-cluster sections overlapped IOZs; group C (two patients; one single; one multiple), in which no overlap was seen. More single clusters (nine of 14) than multiple clusters (none of six) coincided with the IOZ (p = 0.014). More patients with single clusters (10 of 14) than patients with multiple clusters (one of six) had seizure-free outcomes (p = 0.049). Eight of nine patients in group A, versus three of 11 in groups B1, B2, and C, achieved seizure-free outcomes (p = 0.0098). Correlations between MRI findings and postsurgical outcomes were not statistically significant; eight of 13 patients with single lesions, one of four with no lesions, and two of three with multifocal lesions had seizure-free outcomes.

CONCLUSIONS

In neocortical epilepsy, MEG ECD clusters correlated with SDEEG IOZs. Single clusters indicated discrete epileptogenic zones that required complete resection for seizure-free outcome. Multiple clusters necessitated that the multiple or extensive epileptogenic zones be completely identified and delineated by SDEEG.

Epileptic spikes: magnetoencephalography versus simultaneous electrocorticography

PURPOSE

To test the sensitivity of extracranial magnetoencephalography (MEG) for epileptic spikes in different cerebral sites.

METHODS

We simultaneously recorded MEG and electrocorticography (ECoG) by using subdural electrodes with 1-cm interelectrode distances for one patient with lateral frontal epilepsy and one patient with basal temporal epilepsy. We analyzed MEG spikes associated with ECoG spikes and compared the maximal amplitude and number of electrodes involved. We estimated and evaluated the locations and moments of the equivalent current dipoles (ECDs) of MEG spikes.

RESULTS

In patient 1, MEG detected 100 (53%) of 188 ECoG lateral frontal spikes, including 31 (46%) of 67 spikes that activated three subdural electrodes. MEG spike amplitudes correlated with ECoG spike amplitudes and the number of electrodes activated (p < 0.01). ECDs were perpendicular to the superior frontal sulcus. In patient 2, MEG detected 31 (26%) of 121 ECoG basal temporal spikes, but none that activated only three subdural electrodes. ECDs were localized in the entorhinal and parahippocampal gyri, oriented perpendicular to those basal temporal cortical surfaces. The ECD strength was 136.6 +/- 71.5 nAm in the frontal region, but 274.5 +/- 150.6 nAm in the temporal region (p < 0.01).

CONCLUSIONS

When lateral frontal ECoG spikes extend >3 cm2 across the fissure, MEG can detect >50%, correlating with spatial activation and voltage. In the basal temporal region, MEG requires higher-amplitude discharges over a more extensive area. MEG shows a significantly higher sensitivity to lateral convexity epileptic discharges than to discharges in isolated deep basal temporal regions.

Fusiform gyrus epilepsy: the use of ictal magnetoencephalography. Case report

The authors report successful presurgical identification of an epileptic focus in the fusiform gyrus by using ictal magnetoencephalography (MEG), which was performed with the aid of an advanced whole-brain neuromagnetometer. A 22-year-old man had suffered from medically refractory complex partial seizures since he was 10 years of age. Seizure symptoms, magnetic resonance imaging, and ictal single-photon emission computerized tomography examinations indicated right temporal lobe epilepsy; however, ictal electroencephalography, including sphenoidal recordings, failed even to lateralize the seizure focus. The MEG studies revealed that equivalent current dipoles of interictal activities were scattered bilaterally around the medial temporal structures, but those of ictal onset and postictal activities formed a cluster in the left fusiform gyrus. After confirmation of each ictal and interictal MEG finding by using long-term electrocorticography recordings, focal cortical resection of the left inferior temporal and fusiform gyri was performed. The histopathological diagnosis was cortical dysplasia, and the patient has achieved a good seizure outcome, now 15 months after the operation. Ictal and also postictal MEG may be more specific than interictal MEG for identifying the ictal onset zone.