Characteristics of MEG and MRI between Taylor's focal cortical dysplasia (type II) and other cortical dysplasia: Surgical outcome after complete resection of MEG spike source and MR lesion in pediatric cortical dysplasia
Introduction
Taylor et al. (1971) coined the term “focal cortical dysplasia” (FCD) to describe a specific cortical abnormality found in patients who presented with refractory partial epilepsy and who were cured by excision of the affected area. Since then, FCD has been used extensively in the literature to refer to a wide range of abnormalities of the cortex, and various subclassifications have been proposed (Mischel et al., 1995, Palmini et al., 2004, Tassi et al., 2002). FCD came under the umbrella of malformation of cortical development. Palmini et al. (2004) proposed a modified classification of cortical dysplasia (CD), which consisted of mild malformation of cortical development, FCD type IA and IB, Taylor's FCD type IIA and IIB.
Because of its intrinsic epileptogenicity, CD frequently causes intractable epilepsy (Otsubo et al., 2001, Palmini et al., 1995). About 76% of patients with CD become refractory to antiepileptic drugs (Semah et al., 1998). CD is nearly the most common pathology in childhood epilepsy surgical series (Farrel et al., 1992, Jay et al., 1993, Kuzniecky et al., 1993, Tassi et al., 2002). Some studies have reported seizure-free outcomes of 50–70% after surgical resection of FCD (Bautista et al., 2003, Edwards et al., 2000, Hong et al., 2000, Kral et al., 2003). Surgical treatment offers a promising therapeutic option for pediatric patients with intractable epilepsy secondary to CD (Kloss et al., 2002, Hudgins et al., 2005, Rosenow and Luders, 2001).
Children with intractable epilepsy usually undergo MRI, EEG and, increasingly, magnetoencephalography (MEG) during presurgical evaluation. MRI of CD lesions characteristically show blurring of the gray-white matter, cortical thickening, and abnormal signals in the white matter. MEG has been used to localize the epileptogenic zone and map the functional eloquent cortex (Minassian et al., 1999, Morioka et al., 1999, Otsubo et al., 2001). MEG source localization of interictal spikes were closely concordant with the ictal onset zone recorded by intracranial video-EEG recordings (Papanicolaou et al., 2005, Minassian et al., 1999, Knowlton et al., 1997, Morioka et al., 1999). There has not been any report of characteristics of MEG spike sources (MEGSSs) in subtypes of CD.
The aims of this study were to determine whether MRI and MEG could reveal characteristics among subtypes of CD and whether surgical outcome could differ for subtypes of CD. We hypothesized that MRI and MEG could identify differences between Taylor's FCD and other CD and that removal of both clustered MEGSSs and MRI lesions would result in seizure control in both groups.
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Patient population
We retrospectively studied 27 pediatric patients who had MRI, MEG and surgery for intractable epilepsy with histological confirmation of CD from 1991 to 2005. Patients having tumors along with CD were excluded. The mean age of the 15 boys and 12 girls was 12.6 years (range, .92–19 years). This study was approved by the research ethics committee of the Hospital for Sick Children and the Scripps Clinic, and parents or responsible parties gave informed consent for all procedures used in this study.
Histological subtypes and lesion locations
Of the 27 patients, 13 (48%) had Taylor's FCD and 14 (52%) had other CD. Of the 13 patients with Taylor's FCD, 7 (54%) were type IIA and 6 (46%) were type IIB. Of the 14 patients with other CD, 3 (21%) were type IA, 10 (71%) had mild malformation of cortical development, and 1 (7%) had mixed type IB and mild malformation of cortical development. Two (7%) of the 27 patients, 1 with Taylor's FCD and 1 with other CD, had additional hippocampal sclerosis.
Of the 27 patients, 11 (41%) CDs were
MEG spike sources between Taylor's FCD and other CD
MEG clusters alone were more often seen in Taylor's FCD, while clusters and coexisting scattered MEGSSs were more likely seen in other CD. Furthermore, MEGSSs with distribution larger than MR lesion was more common in other CD than in Taylor's FCD. Taylor's FCD and other CD are thought to result from insults occurring at different stages of neuronal development (Barkovich et al., 2005). Taylor's FCD develops at an early stage during neuronal development. In comparison other CDs occur later,
Conclusions
We have found certain MR features and MEGSSs in subtypes of CD. MEG clusters alone were often seen in Taylor's FCD, while clusters and coexisting scatters were frequently seen in other CD. Despite differences in MR features and MEGSSs between Taylor's FCD and other CD, there was no difference in surgical outcomes following complete resection of both clustered MEGSSs and MR lesion. The area containing MR lesion and clustered MEGSSs is believed to identify the intrinsic epileptogenic zone in
Acknowledgments
We wish to thank Ms. Carol Squires for her editorial services for which she received her compensation. We would also like to thank Ms. Momoko Sugiyama for her assistance in checking the references and submitting the paper.
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