MONITORING SODIUM METHOHEXITAL DISTRIBUTION WITH [TC-99M]HMPAO WITH SINGLE-PHOTON EMISSION COMPUTED-TOMOGRAPHY DURING WADA TEST

Twenty-five consecutive patients being considered for surgery for intr actable epilepsy had intracarotid sodium methohexital procedures (ISM) as part of their evaluation. The lipophilic brain SPECT agent, [Tc-99 m]hexamethylpropylene aminoxime (HMPAO), was administered intravenousl y during the procedure to determine the intracerebral distribution of methohexital. The validity of the ISM depends on consistent delivery o f the anesthetizing agent to one hemisphere, including the mesial stru ctures of the temporal lobe. To prevent postoperative language and mem ory deficits, correct interpreration of the test results supposes a re liable knowledge of which regions of the brain have been anesthetized. Currently, no absolute criteria allow determination of the level and topographical extent of the anesthesia. We compared results of HMPAO-S PECT with clinical and EEG video-monitoring data and with results of d igital subtraction angiography (DSA) performed during the test. In all patients, the effect of SM was ipsilateral cerebral hypoperfusion on SPECT and crossed cerebellar diaschisis. The distribution of HMPAO var ied from patient to patient. The delivery of SM to mesial temporal lob e structures was not constant. Using nonparametric tests, we demonstra ted a statistically significant relationship between hypoperfusion on SPECT and duration of hemiplegia but not with the duration of aphasia. Hypoperfusion on SPECT was also related to the onset and duration of drug-induced delta activity on EEG. SPECT showed a statistically diffe rent distribution of SM in the brain from that predicted with DSA. We present our experience with HMPAO-SPECT use for mapping the distributi on of methohexital's effects during the ISM (Wada test). We confirm th e results of previous studies that SPECT assessment may be an excellen t way of determining the distribution of barbiturate during the examin ation. It increases confidence in interpreting results of speech and m emory testing by detecting either contralateral diffusion of the drug due to crossflow between hemispheres or insufficient quantitative deli very to the homolateral hemisphere.