Pre-operative extracranial and intracranial EEG investigation in patients with temporal lobe epilepsy: trends, results and review of pathophysiologic mechanisms.

A thorough understanding of the clinical and EEG correlates of complex partial seizures has been achieved over the last decade, mainly as a result of the utilization of long term EEG-video monitoring technique in the pre-operative investigation of patients with focal cerebral seizures who are considered for surgical therapy (1-17). Evidence derived from EEG-video recording of complex partial seizures of temporal lobe origin, suggest that the specificity of classical semiological signs traditionally attributed to the temporal lobe, is rather limited (12,18,19). Some classical ictal symp-tomatology remains, however, as a reliable pre-dictor of seizure onset in the temporal lobe. Such is the case with auditory, olfactory, and vestibular hallucinations , as well, as with experiental phenomena (9,18,19,20,21). Furthermore, EEG investigation of patients with poorly controlled complex partial seizures of temporal lobe origin with chronically implanted intracerebral electrodes, has permitted a more reliable identification of specific anatomical substrates within the temporal lobe responsible for the genesis of this ictal symptomatology (22). Complex partial seizures of temporal lobe origin normally do not provide clinical lateralizing clues Lateralization of seizure onset in temporal lobe epileptic patients remains a comer-stone problem in their pre-operative investigation, particularly if one considers that recording of bitemporal independent interictal epileptiform discharges in patients with complex partial seizures of temporal lobe origin who are surgical candidates, is a common phenomenon, ranging from 12 to 40% according to different series (13,23-28). Whether the etiopathogenesis of bitemporal independent epileptic foci is due to bitemporal damage (29,30) or due to a mechanism of secondary epileptogene-sis (mirror focus) (25,31,32), is a controversial issue and it is not always possible to ascertain despite the utilization of sophisticated long-term EEG monitoring and neuro-imaging techniques. Evidence supporting the existence of an underlying bitem-poral pathological damage, as opposed to a primary (lesional related) epileptic focus with secondary epileptogenesis (mirror focus) will be presented and discussed. Most of the pre-operative lateralizing and localizing problems in patients with complex partial seizures of temporal and frontal lobe origin, can be resolved by means of long-term EEG monitoring with extracranial or intracranial electrodes, (13 and 28 as review articles), a technique which can supply useful evidence deriving from three sources: identification of the clinical seizure pattern, recording of the ictal electrographic seizure onset as well as spread and recording of the interictal epileptic abnormalities. The current study deals mostly with the effectiveness of long term EEG monitoring with extracrani-a1 and intracranial electrodes …