Evaluation of the thiopental test in epilepsy surgery patients

One hundred and three patients underwent a thiopental test during a presurgical evaluation for epilepsy surgery. Depth electrodes were implanted bilaterally into the mesial temporal and mesial frontal lobes. Thiopental was infused at 25 mg/30 s for adults and 0.3 mg/kg/20 s in children until loss of corneal reflexes or a total of 1 g. The absence of beta activity on the EEG, and activation of interictal spiking were two parameters monitored at all recording sites. Positive results were correlated with the anatomy of the ictal epileptic foci. Half of the patients produced increased interictal spiking and three-quarters of them showed absence of beta production in at least one lobe. Despite a reasonably high concordance between ictal foci and the two thiopental parameters, both measures yielded a low sensitivity and specificity. This work, as well as that reported in the literature, was unable to confirm the hypothesis that local damage within the temporal lobe was responsible for positive thiopental test results. An alternative hypothesis is proposed that the thalamus may be a distant source of these findings. Irrespective of the mechanism, the thiopental test must be used cautiously in the evaluation for epilepsy surgery.