Pure amnestic seizures in temporal lobe epilepsy. Definition, clinical symptomatology and functional anatomical...

Pure amnestic seizures (PAS) sometimes occur in patients with temporal lobe epilepsy. They never represent the only type of seizures in these patients. Pure amnestic seizures are defined as seizures during which the only clinical manifestation is the patients' inability to retain in memory what occurs during the seizure coupled with the preservation of other cognitive functions and the ability to interact normally with their physical and social environment. It is postulated that PAS result from selective ictal inactivation of mesial temporal (MT) structures without isocortical involvement. This occurs most often in patients with neuropsychological and electroencephalographic (EEG) evidence of bilateral dysfunction of MT structures (six out of eight patients in this study). In the few patients without such evidence as well as in some with bilateral MT dysfunction, PAS may result from seizure discharge limited to the MT structures of both temporal lobes. In the light of current anatomical knowledge, contralateral spread of seizure discharge from the MT structures of one side to those of the other through the dorsal hippocampal commissure is the only likely explanation for this situation. One observation with depth electrode stimulation of MT structures supports this view. In patients with evidence for bilateral MT dysfunction, a unilateral seizure may presumably suffice to induce a PAS, the contralateral MT structures being unable to ensure normal memory function. In most instances PAS can be distinguished from episodes of transient global amnesia on clinical grounds.