A single tight MEG cluster may only represent a fragment of type I FCD

The value of magnetoencephalography for stereo-EEG-guided radiofrequency thermocoagulation in MRI-negative epilepsy

OBJECTIVE

Magnetoencephalography (MEG) is valuable for guiding resective surgery in patients with epilepsy. However, its value for minimally invasive treatment is still unknown. This study aims to evaluate the value of MEG for stereo-electroencephalogram (EEG)-guided radiofrequency thermocoagulation (SEEG-guided RF-TC) in magnetic resonance imaging (MRI)-negative epilepsies.

METHODS

An observational cohort study was performed and 19 MRI-negative patients who underwent SEEG-guided RF-TC in our epilepsy center were included. In addition, 16 MRI-positive patients were included as a reference group. Semiology, electrophysiology, and imaging information were collected. To evaluate the value of locating the MEG cluster, the proportion of the RF-TC contacts located in the MEG cluster out of all contacts used to perform RF-TC in each patient was calculated. All patients underwent the standard SEEG-guided RF-TC procedure and were followed up after the treatment.

RESULTS

Nineteen MRI-negative patients were divided into two groups based on the existence of MEG clusters; 10 patients with MEG clusters were in group I and nine patients without any MEG cluster were in group II. No significant difference was observed in terms of age, sex, type of seizures, or number of SEEG electrodes implanted. The median of the proportion of contacts in the MEG cluster was 77.0 % (IQR 57.7-100.0 %). The follow-up results showed that the probability of being seizure-free at one year after RFTC in MRI-negative patients with an MEG cluster was 30.0 % (95 % CI 11.6-77.3 %), significantly (p = 0.014) higher than that in patients without an MEG cluster; there was no significant difference when compared with MRI-positive patients.

CONCLUSION

This is the first study to evaluate the value of MEG in SEEG-guided RF-TC in MRI-negative epilepsies. MEG is a useful supplement for patients with MRI-negative epilepsy. MEG can be applied in minimally invasive treatment. MEG clusters can help identify better candidates and provide a valuable target for SEEG-guided RF-TC, which leads to better outcomes.

Surgical strategy for focal cortical dysplasia based on the analysis of the spike onset and peak zones on magnetoencephalography

OBJECTIVE

The aim of this study was to elucidate the surgical strategy for focal cortical dysplasia (FCD) based on the interictal analysis on magnetoencephalography (MEG). For this purpose, the correlation between the spike onset zone (Sp-OZ) and the spike peak zone (Sp-PZ) on MEG was evaluated to clarify the differences in the Sp-OZ and its correlation with Sp-PZ in FCD subtypes to develop an appropriate surgical strategy.

METHODS

Forty-one FCD patients (n = 17 type I, n = 13 type IIa, and n = 11 type IIb) were included. The Sp-OZ was identified by the summation of gradient magnetic-field topography (GMFT) magnitudes at interictal MEG spike onset, and Sp-PZ was defined as the distribution of the equivalent current dipole (ECD) at spike peak. Correlations between Sp-OZ and Sp-PZ distributions were evaluated and compared with clinical factors and seizure outcomes retrospectively.

RESULTS

Good seizure outcomes (Engel class I) were obtained significantly more often in patients with FCD type IIb (10/11, 90.9%) than those with type IIa (4/13, 30.8%; p = 0.003) and type I (6/17, 35.3%; p = 0.004). The Sp-OZ was significantly smaller (1 or 2 gyri) in type IIb (10, 90.9%) than in type IIa (4, 30.8%; p = 0.003) or type I (9, 53.0%; p = 0.036). Concordant correlations between the Sp-OZ and Sp-PZ were significantly more frequent in type IIb (7, 63.6%) than in type IIa (1, 7.7%; p = 0.015) or type I (1, 5.8%; p = 0.004). Complete resection of the Sp-OZ achieved significantly better seizure outcomes (Engel class I: 9/10, 90%) than incomplete resection (11/31, 35.5%) (p = 0.003). In contrast, complete resection of the Sp-PZ showed no significant difference in good seizure outcomes (9/13, 69.2%) compared with incomplete resection (11/28, 39.3%).

CONCLUSIONS

The Sp-OZ detected by MEG using GMFT and its correlation with Sp-PZ were related to FCD subtypes. A discordant distribution between Sp-OZ and Sp-PZ in type I and IIa FCD indicated an extensive epileptogenic zone and a complex epileptic network. Type IIb showed a restricted epileptogenic zone with the smaller Sp-OZ and concordance between Sp-OZ and Sp-PZ. Complete resection of the Sp-OZ provided significantly better seizure outcomes than incomplete resection. Complete resection of the Sp-PZ was not related to seizure outcomes. There was a definite difference in the epileptogenic zone among FCD subtypes; hence, an individual surgical strategy taking into account the correlation between the Sp-OZ and Sp-PZ should be considered.

Magnetoencephalographic Spike Analysis in Patients With Focal Cortical Dysplasia: What Defines a "Dipole Cluster"?

BACKGROUND

The purpose of this study is to clarify the source distribution patterns of magnetoencephalographic spikes correlated with postsurgical seizure-free outcome in pediatric patients with focal cortical dysplasia.

PATIENTS AND METHODS

Thirty-two patients with pathologically confirmed focal cortical dysplasia were divided into seizure-free and seizure-persistent groups according to their surgical outcomes based on Engel classification. In each patient, presurgical magnetoencephalography was reviewed. Dipole sources of magnetoencephalographic spikes were calculated according to a single dipole model. We obtained the following quantitative indices for evaluating dipole distribution: maximum distance over all pairs of dipoles, standard deviation of the distances between each dipole and the mean coordinate of all dipoles, average nearest neighbor distance, the rate of dipoles located within 10, 20, and 30 mm from the mean coordinate, and the rate of dipoles included in the resection. These indices were compared between the two patient groups.

RESULTS

Average nearest neighbor distance was significantly smaller in the seizure-free group than in the seizure-persistent group (P = 0.008). The rates of dipoles located within 10, 20, and 30 mm from the mean coordinate were significantly higher in the seizure-free group (P = 0.001, 0.001, 0.005, respectively). The maximum distance, standard deviation, and resection rate of dipoles did not show a significant difference between the two groups.

CONCLUSIONS

A spatially restricted dipole distribution of magnetoencephalographic spikes is correlated with postsurgical seizure-free outcomes in patients with focal cortical dysplasia. The distribution can be assessed by quantitative indices that are clinically useful in the presurgical evaluation of these patients.

MEG and EEG dipole clusters from extended cortical sources

Data from magnetoencephalography (MEG) and electroencephalography (EEG) suffer from a rather limited signal-to-noise-ratio (SNR) due to cortical background activities and other artifacts. In order to study the effect of the SNR on the size and distribution of dipole clusters reconstructed from interictal epileptic spikes, we performed simulations using realistically shaped volume conductor models and extended cortical sources with different sensor configurations. Head models and cortical surfaces were derived from an averaged magnetic resonance image dataset (Montreal Neurological Institute). Extended sources were simulated by spherical patches with Gaussian current distributions on the folded cortical surface. Different patch sizes were used to investigate cancellation effects from opposing walls of sulcal foldings and to estimate corresponding changes in MEG and EEG sensitivity distributions. Finally, white noise was added to the simulated fields and equivalent current dipole reconstructions were performed to determine size and shape of the resulting dipole clusters. Neuronal currents are oriented perpendicular to the local cortical surface and show cancellation effects of source components on opposing sulcal walls. Since these mostly tangential aspects from large cortical patches cancel out, large extended sources exhibit more radial components in the head geometry. This effect has a larger impact on MEG data as compared to EEG, because in a spherical head model radial currents do not yield any magnetic field. Confidence volumes of single reconstructed dipoles from simulated data at different SNRs show a good correlation with the extension of clusters from repeated dipole reconstructions. Size and shape of dipole clusters reconstructed from extended cortical sources do not only depend on spike and timepoint selection, but also strongly on the SNR of the measured interictal MEG or EEG data. In a linear approximation the size of the clusters is proportional to the inverse SNR.

Magnetoencephalography spike sources interrelate the extensive epileptogenic zone of tuberous sclerosis complex

OBJECTIVE

We hypothesized that the extensive epileptic network in patients with tuberous sclerosis complex (TSC) manifests as clustered and scattered distributions of magnetoencephalography spike sources (MEGSS).

METHODS

We retrospectively analyzed pre-surgical MEG in 15 patients with TSC. We performed single moving dipole analysis to localize and classify clustered and scattered MEGSS. We compared the number of electrodes within the resected area (RA) and the proportions of clustered and scattered MEGSS within RA with the seizure outcome.

RESULTS

The number of electrodes within RA ranged from 29 to 83 (mean=51). The MEGSS were distributed over multiple lobes (3-8; mean=5.9) and bilaterally in 14 patients. Clusters of MEGSS ranged from 1 to 4 (mean=1.4). The number of MEGSS ranged in total from 28 to 139 (mean=70); in the clusters, 10-128 (mean=49); and in the scatters, 0-45 (mean=21). Four patients achieved an Engel class I surgical outcome, four, a class II outcome; five, a class III outcome; and two, a class IV outcome. The proportion of MEGSS ranged in total from 0 to 92% (mean=57%) within RA; 0-100% (mean=67%) in the resection hemisphere; 0-100% (mean=63%) in the clusters; and 0-81% (mean=28%) in the scatters. Univariate ordinal logistic regression analyses showed that the proportion of scattered MEGSS within RA (p=0.049) significantly correlated with seizure outcomes. Multivariate analyses using three covariates (number of electrodes, proportions of clustered and scattered MEGSS within RA) showed that only the proportion of scattered MEGSS within RA significantly correlated with seizure outcomes (p=0.016).

SIGNIFICANCE

MEG data showed a wide distribution of multilobar MEGSS in patients with TSC. The seizure outcome was not related to the clustered MEGSS within RA, since the grids were essentially planned to cover and resect the clustered MEGSS surrounding tubers. The maximal possible resection of scattered MEGSS correlated with improved seizure outcome in TSC. Some parts of the epileptogenic zone disrupted by multiple tubers did not have a sufficiently large area to produce clustered MEGSS. Although the wide distribution of scattered MEGSS is not interpreted as epileptogenic, they might be interrelated with clustered MEGSS to project a complex epilepsy network and be part of the extensive epileptogenic zones found in TSC.