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 depends to a degree on the type of cortical dysplasia, with type I (see below) more frequently presenting in adulthood ⁴. 

Clinical presentation

It is a frequent cause of refractory epilepsy. 

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 a histopathological system proposed by Palmini et al ^{ref required} 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. 

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 classifications 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)
• Blumcke classification of focal cortical dysplasia (2011)

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 MCD). 

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 penetration through cortex (transmantle sign)
• T2/FLAIR signal hyperintensity of grey matter
• abnormal sulcal or gyral pattern
• segmental and/or lobar hypoplasia/atrophy

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

• location
  • type Ia: usually confined to temporal lobes⁴
    • 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
• 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
• signal
  • white matter
    • moderately increased T2/FLAIR signal 
    • decreased T1 signal 

Type II

• location
  • commonly found in frontal lobes
  • less likely to be in the temporal lobes compared to typeType I FCD
• structure
  • abnormal gyri and sulci
  • marked blurring of grey/white matter junction
  • cortical thickening
• signal
  • white matter
    • moderately increased T2/FLAIR signal 
    • decreased T1 signal 
    • focal signal abnormality may extend from cortex to ventricle (transmantle sign): not seen in type I
  • grey matter
    • moderate increasedsome increase in T2 signal
      • cortex remains hypointense compared to white matter
      • more evident than in type I

Type III

Type III focal cortical dysplasia (according to the Blumcke classification) as associated with adjacent other abnormalities (e.g. IIIa - hippocampal atrophy; IIIb - glioneural tumour (e.g. DNET or ganglioneuroma); 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.