INTERICTAL REGIONAL SLOW ACTIVITY IN TEMPORAL-LOBE EPILEPSY CORRELATES WITH LATERAL TEMPORAL HYPOMETABOLISM AS IMAGED WITH (18)FDG PET - NEUROPHYSIOLOGICAL AND METABOLIC IMPLICATIONS

Objectives-The phenomenon of interictal regional slow activity (IRSA) in temporal lobe epilepsy and its relation with cerebral glucose metab olism, clinical data, MRT, and histopathological findings was studied. Methods-Interictal F-18-fluorodeoxyglucose positron emission tomograp hy (FDG PET) was performed under continuous scalp EEG monitoring in 28 patients with temporal lobe epilepsy not associated with intracranial foreign tissue lesions, all of whom subsequently underwent resective surgery. Regions of interest (ROIs) were drawn according to a standard template. IRSA was considered lateralised when showing a 4:1 or great er ratio of predominance on one side. Results-Sixteen patients (57%) h ad lateralised IRSA which was always ipsilateral to the resection and of maximal amplitude over the temporal areas. Its presence was signifi cantly related to the presence of hypometabolism in the lateral tempor al neocortex (p=0.0009). Logistic regression of the asymmetry indices for all measured cerebral regions confirmed a strong association betwe en IRSA and decreased metabolism of the posterior lateral temporal neo cortex only (p=0.009). No significant relation could be shown between slow activity and age at onset, duration of the epilepsy, seizure freq uency, and MRI evidence for hippocampal atrophy. Furthermore, IRSA was not specifically related to mesial temporal sclerosis or any other pa thology. Conclusions-Interictal regional slowing in patients with temp oral lobe epilepsy not associated with a mass lesion is topographicall y related to the epileptogenic area and therefore has a reliable later alising, and possibly localising, value. Its presence is irrelevant to the severity or chronicity of the epilepsy as well as to lateral deac tivation secondary to neuronal loss in the mesial temporal structures. Although slow EEG activity is generally considered as a non-specific sign of functional disturbance, interictal regional slowing in tempora l lobe epilepsy should be conceptualised as a distinct electrographic phenomenon which is directly related to the epileptogenic abnormality. The strong correlation between interictal regional slowing and latera l temporal hypometabolism suggests in turn that the second may delinea te a field of reduced neuronal inhibition which can receive interictal and ictal propagation.