Remote MEG dipoles in focal cortical dysplasia at bottom of sulcus

SummaryObjective To investigate whether the magnetoencephalography (MEG) single moving dipole (SMD) method could delineate the epileptic zone of focal cortical dysplasia (FCD) at the bottom of sulcus (FCDB). Methods We retrospectively analyzed 17 children (11 male; mean age 8.8 years, range 3–17 years) with FCD type II who underwent epilepsy surgery. We compared spatial congruence between the following: (1) MEG cluster and FCDB and (2) MEG cluster and FCD at the brain surface (FCDS). We measured the volume and depth of magnetic resonance imaging (MRI)–visible lesions to investigate whether they affect spatial congruence between MEG cluster and MRI-visible lesion. Results Eight children had FCDB and the other nine children had FCDS. The volume of MRI-visible lesions for FCDB ranged from 1,632 to 4,707 mm3 (mean ± standard deviation [SD] 3,095 ± 1,211 mm3). The depth of FCDB ranged from 19 to 33 mm (mean ± SD 26 ± 4 mm). The volume of MRI-visible lesion for FCDS ranged from 2,375 to 57,331 mm3 (15,470 ± 18,455 mm3). There was a tendency for a smaller volume of MRI-visible lesion for FCDB, relative to FCDS(p = 0.079). In FCDB, six children showed clusters of MEG dipoles and two children showed scattered MEG dipoles for interictal spikes. The spatial congruence between the MEG result and FCDB was partially overlapping in four children and discordant in another four children. In FCDS, eight children had MEG cluster and one child had MEG scatter alone. The spatial congruence between MEG result and FCDS was overlapping in eight of nine children (fully two; partially six) and discordant in one of nine children. Fifteen children (88%; FCDB eight; FCDS seven) became seizure-free after resective surgery. MEG spike dipole resection ratio in the cluster ranged from 4–100% (mean 67%) in 6 FCDB and 23–99% (mean 77%) in 8 FCDS. Significance The SMD method may drift MEG spike dipoles for FCDB. Lesionectomy can control seizures for four of eight patients in FCDB despite the remote MEG dipoles. The FCDB or FCDS partially overlapped with MEG cluster may have an extending/invisible epileptogenic zone consecutive to the MRI-visible lesion.