The clinicopathologic spectrum of focal cortical dysplasias: a consensus classification proposed by an ad hoc Task Force of the ILAE Diagnostic Methods Commission

Abstract

Purpose: Focal cortical dysplasias (FCD) are localized regions of malformed cerebral cortex and are very frequently associated with epilepsy in both children and adults. A broad spectrum of histopathology has been included in the diagnosis of FCD. An ILAE task force proposes an international consensus classification system to better characterize specific clinicopathological FCD entities.

Methods: Thirty-two Task Force members have reevaluated available data on electroclinical presentation, imaging, neuropathological examination of surgical specimens as well as postsurgical outcome.

Key findings: The ILAE Task Force proposes a three-tiered classification system. FCD Type I refers to isolated lesions, which present either as radial (FCD Type Ia) or tangential (FCD Type Ib) dyslamination of the neocortex, microscopically identified in one or multiple lobes. FCD Type II is an isolated lesion characterized by cortical dyslamination and dysmorphic neurons without (Type IIa) or with balloon cells (Type IIb). Hence, the major change since a prior classification represents the introduction of FCD Type III, which occurs in combination with hippocampal sclerosis (FCD Type IIIa), or with epilepsy-associated tumors (FCD Type IIIb). FCD Type IIIc is found adjacent to vascular malformations, whereas FCD Type IIId can be diagnosed in association with epileptogenic lesions acquired in early life (i.e., traumatic injury, ischemic injury or encephalitis).

Significance: This three-tiered classification system will be an important basis to evaluate imaging, electroclinical features, and postsurgical seizure control as well as to explore underlying molecular pathomechanisms in FCD.

Figure 1. Histopathological findings in FCD Type Ia (abnormal radial lamination and abundant microcolumns)

11 year old girl with a 10 year history of drug-resistant seizures. A: Normal appearing neocortex adjacent to the lesion shown in B and C. Selective labelling of neuronal cell bodies using antibodies directed against NeuN reveals a characteristic layering of the human isocortex (L1-L6). Scale bar = 500 μm, applies also to B. B: Distinct microcolumnar arrangements of small diameter neurons can be detected in FCD Type Ia, when surgical specimen is cut perfectly perpendicular to the pial surface and 4 μm paraffin embedded sections were used. MRI showed smaller cortical (parieto-occipito-temporal) lobes in affected vs. non-affected hemispheres (Blumcke et al. 2010). High magnification in C reveals abundant microcolumns, which are composed of more than 8 neurons (arrow). In addition, Layer 4 is less clearly visible (arrowheads). Scale bar = 200 μm.

Figure 2. Histopathological findings in FCD Type Ib (abnormal tangential layer composition)

A: 3-year old girl with drug-resistant epilepsy originating from the left parieto-occipital lobe. The cortex is thin (hypoplastic) and no layering can be detected. NeuN immunoreactivity. MRI showed a smaller cortical region. Scale bar = 500 μm. B: NeuN immunoreactivity in a surgical case showing normal layer formation (L1-L6) with a sharp boundary between cortex and white matter (same image as Figure 1A). 4μm paraffin embedded section with haematoxylin counterstaining. Scale bar = 500μm, applies also to C. C: 23-year old male patient with drug-resistant focal epilepsy since birth and a hyperintense MRI signal at the parieto-occipital region. Note complete loss of Layer 4 (arrow). In addition, there is no distinction between supragranular Layers L2 and L3. The border towards the white matter is blurred.

Figure 3. Imaging and histopathological findings in FCD Type IIa

FCD Type IIa in an 18 year old female patient with refractory seizures from age 5 years, that would start with sensory disturbance in the left foot. A: ¹⁸F-FDG-PET showing an area of slight hypometabolism in the superior, medial right parietal lobe (arrowhead). B: Coronal T2 FLAIR did not reveal definitely abnormal signal intensities. C: Coronal T1-weighted MRI were also reported normal. Please note the different orientation of the planes of the PET and MR images. The MRI is oblique coronal, so that the inferior part of the image is posterior to that seen on the PET. D: Microscopic inspection of surgical specimen revealed severe cortical dyslamination (arrow) without distinguishable layer formation (except Layer 1). NeuN immunohistochemistry. Scale bar = 1000 μm. Section thickness = 15μm. E: Abundant dysmorphic neurons with dense accumulation of SMI32-neurofilament proteins can be identified. Scale bar = 100 μm. Section thickness = 7μm. F: High-power magnification of dysmorphic neurons (arrows) depicted from same area shown in E (H&E stain). Note their variable morphological appearance which may also result from plane of sectioning. No balloon cells can be recognized. Scale bar = 30 μm. Section thickness = 4μm.

Figure 4. Imaging and histopathological findings in FCD Type IIb

A: The “transmantle-sign” in T2 FLAIR imaging is characterized by a funnel-like hyperintensity (arrow) tapering from the gyrus to the ventricle. B: Inspection of the surgical specimen reveals a distinct correlation between T2 FLAIR hyperintensity and lack of normal myelin content (black arrow points to greyish subcortical areas), which can be identified from the subcortical white matter to the ventricle (red arrow). C: H&E staining combined with Luxol-Fast blue (H&E-LFB) allows visualization of a sharp boundary between neocortex (NCX) and white matter (WM) in a control subject. D: H&E-LFB. In this FCD Type IIb specimen, the myelin content is significantly reduced (see also macroscopic image in B). E: NeuN immunohistochemistry, 4μm paraffin embedded serial section to D. Severe cortical dyslamination is visible (with the exception of Layer 1). In addition, cortical thickness is increased and not distinguishable from WM border (same magnification as C and D). Scale bar = 1 mm. F: In FCD Type IIb, enlarged dysmorphic neurons present with a huge nucleus and abnormal intracytoplasmic Nissl aggregates. G: Antibodies to non-phosphorylated neurofilament proteins (SMI32) reveal aberrant NFP accumulation in a dysmorphic neuron. H: Balloon cells are another hallmark of this FCD variant. Scale bar = 50 μm, applies also to F, G and I. I: Balloon cells express the intermediate filament vimentin. E, G and I: 4 μm paraffin embedded sections, counterstained with haematoxylin.

Figure 5. Histopathological findings in FCD Type IIIa

A: Normal cortical layering (L1-L6) observed adjacent to lesion shown in B . NeuN immunoreactivity. B: A characteristic finding in approx. 10% of MTLE patients is ‘Temporal Lobe Slerosis’ at the interface between Layers 2 and 3 (arrow) (Thom et al. 2009). In this patient, MRI signals within the temporal lobe were reported normal. Scale bar = 200 μm, applies also to A and C. C: There is laminar astrogliosis below temporal lobe sclerosis (arrow), indicating neuronal cell loss in Layers 2/3. GFAP immunoreactivity.

Figure 6. Histopathological findings in FCD Type IIIa (small “lentiform” heterotopia and heterotopic neurons with blurring of white matter boundary)

A: The boundary between gray and white matter is very sharp in normal appearing neocortex. B: Heterotopic neurons are a rare finding in normal deep white matter (Rojiani et al. 1996). C: Blurring of the gray-white matter boundary in a surgical temporal lobe specimen obtained from a 39 year old female patient with drug-resistant MTLE and hippocampal sclerosis. MRI signalling within the temporal lobe was reported normal. D: Increased numbers of heterotopic neurons can be often observed in deep subcortical white matter (Emery et al. 1997). Same patient shown in C. E: A rare finding is the observation of small “lentiform” heterotopias in the white matter of the temporal lobe obtained from a patient with HS. This abnormality was not reported by MRI prior to operation. F: Synaptophysin staining of “lentiform” heterotopias shown in E. Scale bar in B= 200 μm, applies also to A, C and D. Scale bar in F = 500μm, applies also to E. MAP2-immunoreactivity in A-D. 4μm paraffin embedded sections counterstained with haematoxylin.

Figure 7. Histopathological findings in FCD Type IIIb

A: CD34 immunoreactivity demarcated a ganglioglioma (GG). Abundant CD34 positive tumor aggregates can be identified within adjacent neocortex (arrows). This frequent infiltration pattern should not be confused with the diagnosis of FCD (no FCD). Scale bar = 1 mm. B: Cortical dyslamination and hypoplasia adjacent to a ganglioglioma, but not infiltrated by tumor clusters, are a hallmark of FCD Type IIIb. C: Normal cortical lamination adjacent to lesion shown in B at same magnification. D: Histopathological analysis identified a Dysembryoplastic Neuroepithelial Tumor (DNT). Tumor aggregates can be detected in close proximity to the mass lesion (arrow). H&E staining. Scale bar = 500 μm, applies also to B and C. E: Adjacent neocortex (NCx) revealed severely compromised cortical lamination (NeuN immunoreactivity). However, these clear cell tumor infiltrates contribute to the disrupted cortical architecture and should not be diagnosed as FCD (no FCD). Scale Bar = 200 μm. 4 μm paraffin embedded sections.

Figure 8. Histopathological findings in FCD Type IIIc associated with a vascular malformation

21 year old male patient with drug-resistant seizures and a leptomeningeal vascular malformation in the right temporo-occipital lobe. A: Histopathological specimen showing a vascular malformation (VM) in the subarachnoidal space. H&E staining. B: 4 μm paraffin embedded serial section to A. NeuN immunohistochemistry. The neocortex below the vascular malformation is atrophic and revealed severe tangential dyslamination with almost complete loss of Layers 3 and 4 (FCD Type IIIc). C: Adjacent to the vascular malformation and tangential dyslamination shown in B, microcolumnar (radial) dyslamination was also recognized in this patient (arrows). Arrowheads point to Layer 4, which is less clearly distinguishable. NeuN immunohistochemistry. Scale bar = 200 μm, applies also to A and B.

Figure 9. Histopathological findings in FCD Type IIId associated with a glio-mesodermal scar

A: Glio-mesodermal scarring in a 9-year old patient with perinatal hemorrhagic brain injury. NCx = Neocortex. HE = Haematoxylin-Eosin staining. B: NeuN labelling revealed disruption of cortical layering and abundant microcolumnar arrangement of cortical neurons. C: Pronounced reactive astrogliosis (GFAP) is a common finding in glio-mesodermal scarring. Scale bar in A = 500μm, applies also to B and C.

Figure 10. Abnormal cell types in FCD

Representative examples of abnormal cell types in FCD variants. All images were taken at same magnification (scale bar in B = 50 μm) using recommended immunohistochemical markers (see supplementary Table 3). 4μm paraffin embedded and formalin fixed specimens. A – B: biopsy control samples from Layer 3 (in A) and Layer 2 (in B). C: A dysmorphic neuron accumulating nonphosphorylated neurofilaments (antibody SMI 32) in a FCD Type IIb specimen. Also note significantly enlarged nucleus with prominent nucleolus. D: This hypertrophic pyramidal neuron was observed at the border between Layer 2 and Layer 3 in a FCD Type IIIa specimen. E: Microcolumn with alignment of immature, small diameter neurons. FCD Type Ia specimen. F: In gangliogliomas, dysplastic neurons often show bizarre morphology and multiple nucleoli.