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 refractory epilepsy.
Pathology
Classification
Some classification systems for focal cortical dysplasia have been devised over the years since the first description in 1971 by Taylor et al. 5.
The most common classification used until recently was the histopathological system proposed by Palmini et al. 6 in 2004 a genetic/imaging classification by Barkovich et al. 2 in 2005.
The most recent classification system is that suggested by Blumcke in 2011 and has been widely accepted. It has been subsequently updated in 2022 by the International League Against Epilepsy's Diagnostic methods commission9. This last update integrated the following new pathological classifications: mild malformation of cortical development (mMCD), mild malformation of cortical development with oligodendroglial hyperplasia (MOGHE) and "no definite FCD on histopathology". It also introduced a novel multi-layered classification scheme combining histopathological diagnosis, genetic and neuroimaging findings to provide an integrated final diagnosis.
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 consensus classification of focal cortical dysplasia (2022)
Radiographic features
MRI
MRI is the modality of choice to assess patients with possible focal cortical dysplasias. There is much overlap of imaging features between the different types of FCD, and in many instances, no MRI abnormality is evident (especially Blumcke mild FCD).
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 Blumcke classification of focal cortical dysplasia (2011).
Type I
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location
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type Ia: usually confined to temporal lobes 4
if associated with hippocampal atrophy (as is common), it is now classified as type IIIa in the Blumcke classification
type Ib: more frequently seen outside of the temporal lobes
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structure
blurring of grey/white matter junction (less marked than with Type II FCD)
prominent segmental or lobar atrophy/hypoplasia with loss of regional white matter volume
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signal
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white matter
moderately increased T2/FLAIR signal
decreased T1 signal
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Type II
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location
commonly found in frontal lobes
less likely to be in the temporal lobes compared to Type I FCD
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structure
abnormal gyri and sulci
marked blurring of grey/white matter junction
cortical thickening
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signal
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white matter
moderately increased T2/FLAIR signal, typically brighter than the adjacent cortex
decreased T1 signal
focal signal abnormality may extend from cortex to ventricle (transmantle sign): not seen in type I
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grey matter
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some increase in T2 signal
despite an increase in T2 signal, the cortex remains hypointense to much brighter adjacent white matter 4
more evident than in type I
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Type III
Type III focal cortical dysplasia (according to the Blumcke classification) as associated with adjacent other abnormalities (e.g. IIIa - hippocampal atrophy; IIIb - glioneuronal 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.
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
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adult-type diffuse gliomas
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glioneuronal and neuronal tumors
