Since the original description by Taylor, the term focal cortical dysplasia has been used to refer to a wide range of alterations of the cortical mantle. More recently, these conditions have been described from neuroimaging, neuropathological and genetic standpoints, generating several classifications. It is widely recognized that these classifications are unsatisfactory. We propose a simplified classification of focal cortical dysplasias based on easily recognized neuropathological characteristics. We retrospectively re-examined histological sections of cortex from 52 of 224 (23%) patients operated on for drug-resistant partial epilepsy in which cortical dysplasia was present but not associated with other brain pathologies except hippocampal sclerosis. Three subgroups were identified: (i) architectural dysplasia (31 patients) characterized by abnormal cortical lamination and ectopic neurones in white matter; (ii) cytoarchitectural dysplasia (six patients) characterized by giant neurofilament-enriched neurones in addition to altered cortical lamination; and (iii) Taylor-type cortical dysplasia (15 patients) with giant dysmorphic neurones and balloon cells (all but two patients) associated with cortical laminar disruption. The patients with architectural dysplasia had lower seizure frequency than those with cytoarchitectural and Taylor-type dysplasia, and the epileptogenic zone was mainly in the temporal lobe. In patients with Taylor-type dysplasia, the epileptogenic zone was mainly extratemporal, and interictal stereo-EEG was distinctive. MRI was unrevealing in 34% of patients, but distinctive signal alterations characterized most patients with Taylor-type dysplasia, while focal hypoplasia with MRI abnormalities was found in architectural dysplasia. Patients with Taylor-type dysplasia had the best outcome, with 75% seizure-free (Engel class Ia) after at least a year of follow-up compared with 50% of cytoarchitectural dysplasia and 43% of architectural dysplasia patients seizure-free. This three-category classification is based on easily recognized histopathological characteristics and avoids complicated terminology, while the distinctive ensemble of other characteristics defines clinically homogeneous groups.

We present the results of a retrospective study on 10 patients operated on for intractable epilepsy associated with nodular heterotopia as identified by high resolution MRI. Seven patients had unilateral heterotopia, one patient had symmetric bilateral heterotopia and two patients had asymmetric bilateral heterotopia. By stereo-electroencephalogram (SEEG) (nine patients) interictal activity within nodules was similar in all cases, and ictal activity never started from nodules alone but from the overlying cortex or simultaneously in nodules and cortex. Excellent outcomes (Engel class Ia, 1987) were achieved in the seven patients with unilateral heterotopia, showing that surgery can be highly beneficial in such cases when the epileptogenic zone is carefully located prior to surgery by MRI and particularly SEEG. For the bilateral cases surgical outcomes were Engel IIa (one patient) or Engel IIIa (two patients). Histological/immunohistochemical studies of resected specimens showed that all nodules had similar microscopic organization, even though their extent and location varied markedly. The overlying cortex was dysplastic in nine patients, but of normal thickness. We suggest that nodule formation may be the result of a dual mechanism: (i) failure of a stop signal in the germinal periventricular region leading to cell overproduction; and (ii) early transformation of radial glial cells into astrocytes resulting in defective neuronal migration. The intrinsic interictal epileptiform activity of nodules may be due to an impaired intranodular GABAergic system.

SEEG is a useful and relatively safe tool in the evaluation of surgical candidates when noninvasive investigations fail to localize the epileptogenic zone. SEEG-based resective surgery may provide excellent results in particularly complex drug-resistant epilepsies.