Focal cortical dysplasia

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. 

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

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 was the histopathological system proposed by Palmini et al. ⁶ in 2004, a genetic/imaging classification by Barkovich et al. ² in 2005.

The 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 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)

• Palmini classification of focal cortical dysplasia (2004)

• Barkovich classification of focal cortical dysplasia (2005)

• 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

  • astrocytoma

  • oligodendroglioma

  • glioblastoma

• glioneuronal and neuronal tumors

  • ganglioglioma

  • dysembryoplastic neuroepithelial tumor (DNET)

See also

• bottom of sulcus focal cortical dysplasia