Focal cortical dysplasias (FCD) represent a heterogeneous group of disorders of cortical formation, which may demonstrate both architectural and proliferative features. They are one of the most common causes of epilepsy and can be associated with hippocampal sclerosis and cortical glioneuronal neoplasms.
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Epidemiology
Age of presentation, usually with epilepsy, in part, depends on the type of cortical dysplasia, with type I (see below) more frequently presenting in adulthood 4.
Clinical presentation
Focal cortical dysplasia is a frequent cause of drug-refractory epilepsy.
Pathology
Classification
Since the first description in 1971 by Taylor et al. 5, a number of classification systems for focal cortical dysplasia have been devised.
This includes a previously commonly used classification based on histopathology proposed by Palmini et al. 6 in 2004 and a genetic/imaging classification by Barkovich et al. 2 in 2005.
The consensus classification published by the International League Against Epilepsy's Diagnostic Methods Commission, lead author Blumcke, has incorporated and largely superseded other classification systems 9,13.
Unfortunately, as is the case with many classification systems that have developed in parallel with numerous iterations and revisions, there is significant overlap between the various classification systems with the same terminology used slightly differently. As such it is safest to explicitly state which classification system is being used (e.g. "Blumcke Type IIB").
Taylor dysplasia (1971)
ILAE (Blumcke) classification of focal cortical dysplasia (2011 and 2022) 9
Radiographic features
MRI
MRI is the modality of choice to assess patients with possible focal cortical dysplasias although not all histopathological proven areas of focal cortical dysplasia will be evident on MRI and at other times FCD will be found adjacent to other lesions (e.g. tumors) 11.
General features of focal cortical dysplasia include 4:
cortical thickening
blurring of white matter-grey matter junction with abnormal architecture of subcortical layer
T2/FLAIR signal hyperintensity of white matter with or without the transmantle sign
T2/FLAIR signal hyperintensity of grey matter
abnormal sulcal or gyral pattern
segmental and/or lobar hypoplasia/atrophy
there is no edema, calcification, or contrast enhancement 10
Also, each type of focal cortical dysplasia can exhibit more or less of these features. The types below refer to the ILAE (Blumcke) classification of focal cortical dysplasia.
Type I
generally not obvious on MRI
may seen subtle grey-white mater junction blurring due to neurons located in the subcortical u-fibers 11
the majority involve the temporal lobe 12
Type IIa
blurring of grey/white matter junction
cortical thickening
abnormal gyral and/or sulcal pattern 11
Type IIb
same as type IIa, with the addition of....
focal signal abnormality (increase T2) extending from cortex to ventricle (transmantle sign); seen in 94% of cases 4,11
Type III
Type III focal cortical dysplasia (according to the ILAE (Blumcke) classification) is FCD associated with adjacent other abnormalities (e.g. IIIa - hippocampal atrophy; IIIb - tumor (e.g. DNET or ganglioglioma); IIIc - vascular malformation; IIId - early childhood insult (e.g. gliosis)) and as such imaging appearances will be dominated by the associated abnormality rather than the dysplasia itself 11.
Treatment and prognosis
Surgical resection of the refractory epileptogenic area of focal cortical dysplasia typically leads to good seizure control. In the presence of transmantle sign better post-surgical outcomes have been reported 8.
Differential diagnosis
-
adult-type diffuse gliomas
-
glioneuronal and neuronal tumors
cortical harmatomas of tuberous sclerosis
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