Sections of cortex from 52 of 224 (23%) patients with cortical dysplasia, operated on for drug-resistant partial epilepsy, were retrospectively re-examined histologically at Niguarda Hospital, and Istituto Nazionale Neurologico ‘C. Besta’, Milan, Italy. Three subgroups were identified as follows: 1) architectural dysplasia (31 patients) with abnormal cortical lamination and ectopic neurons in white matter; 2) cytoarchitectural dysplasia (6 patients) with altered cortical lamination and giant neurofilament-enriched neurons; and 3) Taylor-type cortical dysplasia (15 patients) with cortical laminar disruption and giant dysmorphic neurons and balloon cells. Group 1 architectural dysplasia cases had a temporal lobe location for the epileptogenic zone, with focal hypoplasia, and a significantly lower seizure frequency than groups 2 and 3. Group 3 Taylor-type dysplasia patients had an extratemporal epileptogenic zone, interictal distinctive stereo-EEG with high frequency fast spikes and polyspikes interspersed with fast low amplitude activity, and MRI signal alterations with thickening of cortex, blurred grey-white matter junction and hyperintensity of subcortical white matter. Group 3 Taylor-type had the best outcome, with 75% seizure-free after one year follow-up, compared with 50% in type 2 cytoarchitectural cases, and 43% seizure-free in type 1 architectural dysplasia cases. The three-category classification has easily recognized histopathological findings, and clinically homogeneous characteristics. 
COMMENT. In the above series of focal cortical dysplasia, the video or stereo-EEG provided the best delineation of the epileptogenic zone, superior to the MRI. In one-third of cases, the MRI was unremarkable, and in those with MRI evidence of a lesion, the boundaries were ill-defined. Type 3 Taylor-type dysplasia had a better outcome than the type 1 architectural and type 2 cytoarchitectural dysplasias.