Analysis of ictal magnetoencephalography using gradient magnetic-field topography (GMFT) in patients with neocortical epilepsy
Introduction
Magnetoencephalography (MEG) is a powerful tool for noninvasive preoperative evaluation of intractable epilepsy. MEG provides information of the spatial distribution of epileptic foci, which corresponds well to the surgical results (Otsubo et al., 2001, Pataraia et al., 2004, Oishi et al., 2006, Knowlton et al., 2006). Analysis with the equivalent current dipole (ECD) method has been conventionally applied for interictal spikes in preoperative MEG. However, ictal events are only occasionally captured during MEG recording. Since Stefan et al. (1992) reported their analysis of ictal magnetoencephalographic activity, several reports have been published on the usefulness of ictal MEG (Shiraishi et al., 2001, Eliashiv et al., 2002, Oishi et al., 2002, Tilz et al., 2002, Assaf et al., 2003, Tanaka et al., 2004, Yoshinaga et al., 2004, Medvedovsky et al., 2012). Although these studies using ECD succeeded in localizing ictal magnetoencephalographic activity, ECD analysis of ictal MEG has been controversial due to the low signal-to-noise ratio (SNR) during ictal activity (Tanaka et al., 2004, Tanaka et al., 2009, Yagyu et al., 2010). Because ECD estimation requires enough SNR for statistically appropriate confidence, the application of ECD for ictal onset activity with low SNR is unreliable. Although reports on several spatial filters to localize the source of ictal onset have recently been published (Tanaka et al., 2009, Yagyu et al., 2010, Fujiwara et al., 2012), there are no adequate methods to validate their estimated results statistically.
Gradient magnetic-field topography (GMFT) was developed to represent the spatio-temporal dynamics of brain surface activity (Hashizume et al., 2007). Unlike ECD estimation or other spatial filters, with GMFT there is no need to solve the biomagnetic inverse problem. Moreover, because GMFT is less influenced by the SNR, it can delineate brain activity even at the early phase of epileptic spikes, which is usually not suitable for appropriate ECD localization (Shirozu et al., 2010). The spatio-temporal accuracy was proved by comparison between GMFT and cortical voltage activity simultaneously recorded with intracranial electroencephalography (EEG) (Shirozu et al., 2016). The aim of this study was to validate the usefulness of GMFT for analysis of ictal MEG. We hypothesized that GMFT is superior to ECD for analysis of ictal neuromagnetic dynamics, even during early ictal activity.
Section snippets
Patients
We retrospectively reviewed 13 patients (10 males and 3 females; 5–45 years old) with intractable neocortical epilepsy who had ictal events during preoperative MEG. All of them had undergone epilepsy surgery, including cortical resection and/or multiple subpial transection, between 2000 and 2012. Twelve of the 13 patients underwent implantations of intracranial electrodes and chronic intracranial EEG monitoring (CiEEG) to determine the epileptogenic foci and resection areas. At their last
Clinical profiles
The median ages of the patients at surgery and at epilepsy onset were 20 (range, 5–45 years) and 5.5 (range, 0.5–38 years), respectively. The median duration of epilepsy was 10 years (range, 1–42 years). The preoperative epilepsy diagnoses were temporal lobe epilepsy (n = 5) and frontal lobe epilepsy (n = 8). Simple partial seizures (n = 2), complex partial seizures (n = 10), and secondary generalized tonic-clonic seizures (n = 4) were found. Three patients presented with multiple seizure types, and 12
Discussion
In the present study, we demonstrated that GMFT could analyze ictal MEG, even in the case of the patient who failed ECD estimation, and could localize more precise distribution of ictal activity than with ECD analysis. Furthermore, the distributions of ictal GMFT were concordant with the IOZ derived from CiEEG, and the degree of concordance was at the gyral-unit level.
Conclusions
We demonstrated that GMFT is superior to ECD for analysis of ictal neuromagnetic dynamics, even at an early ictal event. The ictal GMFTs were concordant with the results of the IOZ derived from CiEEG at the gyral-unit level. In addition, GMFT is useful for analysis of ictal MEG in patients with neocortical epilepsy.
Funding
This study was supported in part by a Health Labour Sciences Research Grant from the Ministry of Health, Labour and Welfare of Japan.
Acknowledgements
Special thanks to our former colleagues, Hiroatsu Murakami and Ayataka Fujimoto, for their significant contributions to the recording and analysis of ictal MEG.
Conflict of interest: None of the authors have any conflicts of interest to disclose.
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