Epileptogenic networks: cerebral dysplasias and cerebral ectopias

There are three areas in which functional imaging may be applied to malformations of cortical development (MCD) that give rise to epilepsy: the localization of epileptic activity; the identification of areas of functional abnormality; the mapping of normal cerebral functions, and how these may be displaced. Single photon emission computerized tomography (SPECT) commonly shows interictal hypoperfusion and ictal hyperperfusion. Positron emission tomography (PET), using (18)F-fluorodeoxyglucose, often shows glucose metabolism in ectopic grey matter and hypometabolism in MCD, compared with normal cortex. Cognitive activation tasks, using PET or fMRI, have shown that malformed cortex may participate in cerebral functions, but also widespread atypical cortical organization. Cognitive activation tasks using fMRI are useful when planning surgical resections close to eloquent cortex. Binding of (11)C-flumazenil to central benzodiazepine receptors (cBZR) is often abnormal in MCD, and in areas of cortex that appear normal on MRI, with increased and decreased binding. MR Spectroscopy also reveals abnormalities that are more extensive than the MRI visible lesion, implying more widespread structural and functional abnormalities. The recording of EEG during fMRI has allowed the latter to be used to localize cerebral areas involved in the generation of interictal epileptiform activity. Electroencephalograph source localization and magneto-encephalography may also prove to be useful in the localization of interictal epileptiform activity, particularly when fMRI data are used to constrain the solutions.