Advances in neuroimaging: management of partial epileptic syndromes

Abstract

Sch uble and Cascino review the techniques of structural and functional neuroimaging currently available for the presurgical evaluation of patients with partial epilepsies and discuss the clinical impact of the various techniques on the management of intractable partial epilepsies. Their article will be of interest to neurosurgeons, neurologists, paediatric neurologists, neuroradiologists, and colleagues in nuclear medicine who care for patients with partial epilepsies. Structural MRI is the procedure of choice in lesional epilepsies such as mesial temporal sclerosis, malformation of cortical development, and primary brain tumours, since patients with intractable symptomatic partial epilepsies are favourable candidates for surgical treatment. The authors describe several techniques of functional imaging that are useful in temporal lobe epilepsies (TLE). Unilateral hypometabolism of the affected temporal lobe as shown by positron emission tomography using deoxyglucose (FDG-PET) predicts good surgical outcome. Periictal single photon emission computed tomography (SPECT) has a high sensitivity in mesial TLE and is even superior to interictal FDG-PET in localising the foci of nonlesional TLE. Abnormal proton (1H) spectroscopy (1H MRS) suggests neuronal loss, gliosis, or functional alteration and is highly sensitive in lateralising the ictal onset zone in temporal lobe epilepsy. Unlike patients with temporal lobe epilepsies, many patients with extratemporal epilepsies have limited postoperative outcome and often are not regarded as favourable candidates for surgical treatment. Since this is particularly true in nonlesional extratemporal epilepsies, functional neuroimaging techniques are necessary and important in this group of patients. The results of FDG-PET in nonlesional extratemporal epilepsies are less favourable than in TLE, and the hypothesis that PET with benzodiazepine receptor ligands (FMZ-PET) may be more sensitive in extratemporal epilepsies is still under investigation. According to Sch uble and Cascino, the method of subtracting the ictal SPECT scans from interictal scans with subsequent coregistration to MRI (SISCOM) is a promising tool for patients with extratemporal epilepsies, and localised SISCOM abnormalities, which are concordant with the ictal onset zone, predict a good postoperative seizure outcome. I would suggest that neurophysiological techniques of functional imaging such as MEG, EEG source analysis, and possibly fMRI might be increasingly included in the presurgical evaluation of patients with extratemporal epilepsies. Although requiring further validation, these techniques, together with SISCOM, will likely enhance the diagnostic yield and improve postoperative outcome in such patients in the future, if they prove useful in guiding subdural electrode placement. They might become particularly useful for presurgical evaluation in children, who often suffer from nonlesional extratemporal epilepsies.