Abstract
Both magnetoencephalography and stereo-electroencephalography are used in presurgical epilepsy assessment, with contrasting advantages and limitations. It is not known whether simultaneous stereo-electroencephalography–magnetoencephalography recording confers an advantage over both individual modalities, in particular whether magnetoencephalography can provide spatial context to epileptiform activity seen on stereo-electroencephalography. Twenty-four adult and paediatric patients who underwent stereo-electroencephalography study for pre-surgical evaluation of drug-resistant focal epilepsy, were recorded using simultaneous stereo-electroencephalography–magnetoencephalography, of which 14 had abnormal interictal activity during recording. The 14 patients were divided into two groups; those with detected superficial (n = 7) and deep (n = 7) brain interictal activity. Interictal spikes were independently identified in stereo-electroencephalography and magnetoencephalography. Magnetoencephalography dipoles were derived using a distributed inverse method. There was no significant difference between stereo-electroencephalography and magnetoencephalography in detecting superficial spikes (P = 0.135) and stereo-electroencephalography was significantly better at detecting deep spikes (P = 0.002). Mean distance across patients between stereo-electroencephalography channel with highest average spike amplitude and magnetoencephalography dipole was 20.7 ± 4.4 mm. for superficial sources, and 17.8 ± 3.7 mm. for deep sources, even though for some of the latter (n = 4) no magnetoencephalography spikes were detected and magnetoencephalography dipole was fitted to a stereo-electroencephalography interictal activity triggered average. Removal of magnetoencephalography dipole was associated with 1 year seizure freedom in 6/7 patients with superficial source, and 5/6 patients with deep source. Although stereo-electroencephalography has greater sensitivity in identifying interictal activity from deeper sources, a magnetoencephalography source can be localized using stereo-electroencephalography information, thereby providing useful whole brain context to stereo-electroencephalography and potential role in epilepsy surgery planning.
Highlights
Magnetoencephalography (MEG) and stereo-electroencephalography (SEEG) can provide complementary information for the presurgical assessment of refractory focal epilepsy
For patients whose detected epileptogenic focus was superficial cortex, as defined by the SEEG peak amplitude occurring in an electrode contact greater than four, we found no statistical difference between number of spikes identified by SEEG versus MEG (Fig. 2)
For patients with a detected deep brain epileptogenic focus defined by SEEG peak amplitude occurring in an electrode contact smaller than three, we found that number of spikes identified by SEEG was significantly higher than MEG (Mann–Whitney U-test, T 1⁄4 56.5, P 1⁄4 0.002) (Fig. 4), indicating that SEEG is more sensitive than MEG in identifying interictal spikes from deep sources (Table 2)
Summary
Magnetoencephalography (MEG) and stereo-electroencephalography (SEEG) can provide complementary information for the presurgical assessment of refractory focal epilepsy. It is difficult to interpret whether the epileptogenic zone (EZ) and SEEG electrode contacts identifying the abnormal epileptiform activity truly co-localize, or whether the zone is situated in nearby functionally connected brain structures This is one of a number of reasons, including underlying pathology, age at time of surgery, and brain region where EZ was located (extra-temporal versus temporal), that surgical resection of the EZ as identified by SEEG results in a 60–70% chance of achieving seizure freedom.[9] MEG has previously been compared with SEEG non-concurrently, demonstrating that concordance between both modalities in identifying epileptiform activity was associated with a higher chance of seizure freedom post resection.[10,11] these studies acknowledged the limitation that MEG recordings are necessarily brief ($1 h) compared with SEEG telemetry over several days, leading to uncertainty in whether interictal activity captured by MEG and SEEG relate to identical epileptogenic foci. Simultaneous SEEG–MEG recording should be able to resolve such questions
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