Amobarbital effects on the posterior hippocampus during the intracarotid amobarbital test

Objective: To relate functional effects on the hippocampus during the intracarotid amobarbital test (IAT) to its direct perfusion with amobarbital. Methods: In 17 patients with intractable temporal lobe epilepsy, 28 hemispheres were perfused with 2.3 mt of 10% solution of 200 mg amobarbital and 37 MBq Tc-99m-hexamethylpropylene amine oxime (HMPAO). For evaluation of amobarbital effects, data were combined from stereo-EEG (S-EEG) recordings from intrahippocampal depth electrodes and high-resolution SPECT after intracarotid injection of HMPAO. Results: Perfusion of the entire hippocampus was observed only in hemispheres with a fetal origin of the posterior cerebral artery (PCA). In 10 hemispheres, S-EEG recordings could not unequivocally be assigned to either the anterior or the posterior part of the hippocampus. In the remaining 18 hemispheres, only the two with a fetal type of PCA showed perfusion of the entire hippocampus. In both, hippocampal electrical activity changed under the influence of amobarbital but did not differ in anterior and posterior contacts. In 15 of 16 hemispheres in which SPECT demonstrated perfusion of the anterior hippocampus only, amobarbital injection resulted in significant S-EEG activity change in both the anterior and the posterior parts of the hippocampus. Conclusion: S-EEG effects on the posterior hippocampus during the IAT can occur without direct perfusion of those brain areas.