Optimizing MR imaging detection of type 2 focal cortical dysplasia: best criteria for clinical practice

Abstract

Background and purpose: Type 2 FCD is one of the main causes of drug-resistant partial epilepsy. Its detection by MR imaging has greatly improved surgical outcomes, but it often remains overlooked. Our objective was to determine the prevalence of typical MR imaging criteria for type 2 FCD, to provide a precise MR imaging pattern, and to optimize its detection.

Materials and methods: We retrospectively reviewed 1.5T MR imaging of 71 consecutive patients with histologically proved type 2 FCD. The protocol included millimetric 3D T1-weighted, 2D coronal and axial T2-weighted, and 2D or 3D FLAIR images. Two experienced neuroradiologists looked for 6 criteria: cortex thickening, cortical and subcortical signal changes, blurring of the GWM interface, the "transmantle" sign, and gyral abnormalities. The frequency of each sign and their combination were assessed. We compared the delay between epilepsy onset and surgery, taking into account the time of type 2 FCD detection by MR imaging.

Results: Only 42 patients (59%) had positive MR imaging findings. In this group, a combination of at least 3 criteria was always found. Subcortical signal changes were constant. Three characteristic signs (cortical thickening, GWM blurring, and transmantle sign) were combined in 64% of patients, indicating that MR imaging can be highly suggestive. However, typical features of type 2 FCD were overlooked on initial imaging in 40% of patients, contributing to a delay in referral for surgical consideration (17 versus 11.5 years when initial MR imaging findings were positive).

Conclusions: A combination of 3 major MR imaging signs allows type 2 FCD to be recognized in clinical practice, thereby enabling early identification of candidates for surgery.

Figures

Fig 1.

Typical MR imaging signs of type 2 FCD. Coronal 3D T1WI (A), coronal FLAIR (B), and coronal T2WI (C) in a 45-year-old man with left motor seizures, epilepsy onset at 6 years, and findings on several previous MR images considered normal. An unusually deep and straight sulcus in the precentral region (asterisk), with minimal cortical thickening at the bottom of the sulcus, and cortical signal increase in T2WI and FLAIR. Abnormal subcortical signal, marked at the bottom (arrow), surrounded by an area of subtle signal increase (arrowheads), responsible for a gradient of signal abnormalities from the periphery to the center of the dysplasia. Barely perceptible transmantle sign (C, double arrows). Coronal FLAIR (D) and magnification (E) of the left central region in a 19-year-old woman with right motor seizure, epilepsy onset at 16 years of age. These MR images show a marked increased signal, tapering gradually from the gray-white matter interface to the superolateral edge of the lateral ventricle (triple arrows), typical of a transmantle sign.

Fig 2.

Cortical pseudothickening. Axial T1WI (A and B) and coronal T2WI (C) in a 29-year-old man with left motor seizures, onset at 3 years of age. These images show cortical thickening (arrow) and blurring (asterisk) of the gray-white matter interface of the right central sulcus. D, Coronal FLAIR image allowing cortical to be distinguished from subcortical signal increase (arrowheads).

Fig 3.

Negative MR imaging findings. Axial and coronal 3D T1WI (A and C) and axial and coronal 3D FLAIR (B and D) in a 15-year-old boy with right frontal lobe epilepsy, nocturnal seizure predominance, and onset at 12 years. Absence of the 6 criteria and no minor sulcal abnormality were found. E, 18FDG-PET coregistered on MR imaging (axial section) allows recognition of a gyral hypometabolism corresponding to the anterior part of the right cingulate cortex (arrow). F and G, Histology slides show typical features of type 2b FCD with giant dysmorphic neurons in the cortex (double arrow) and balloon cells in the underlying white matter (arrowhead) (Klüver-Barrera, original magnification ×40).

Fig 4.

Negative MR imaging findings with minor sulcal abnormalities. Coronal T2WI (A), FLAIR (B), and T1WI (C) in a 36-year-old man with right nocturnal frontal lobe epilepsy, onset at 20 years of age. None of the 6 criteria were found. A minor sulcal abnormality is perceptible in the right superior frontal area, with an unusually large and deep sulcus (arrow). D and E, Cortical specimen. Deep part of the pathologic sulcus with typical type 2a FCD features: cortical disorganization and the presence of giant neurons (arrowhead) without balloon cells. Note that the good delineation of the gray-white matter interface (double arrow) is correlated with the absence of blurring on MR imaging (Klüver-Barrera, original magnification ×5 [D]; ×15 [E]).