Regional analyses of CNS microdialysate glucose and lactate in seizure patients

Purpose: To correlate glucose (and lactate) results obtained from microdialysate to recent studies suggesting that glucose transporter activity may be significantly altered in seizures. Methods: We used a fluorometric technique to quantify glucose and lactate levels in microdialysates collected from two to four depth electrodes implanted per patient in the temporal and frontal lobes of a series of four patients. Hour-by-hour and day-to-day changes in brain glucose and lactate levels at the same site were recorded. Additionally we compared regional variations in lactate/glucose ratios around the predicted epileptogenic region. Results: Lactate/glucose ratios in the range of 1-2:1 were the most commonly seen. When the lactate/glucose ratio was <1: 1, we typically observed a relative increase in local glucose concentration (rather than decreased lactate), suggesting increased transport, perhaps without increased glycolysis. In some sites, lactate/glucose ratios of 3:1-15: 1 were seen, suggesting that a circumscribed zone of inhibition of tricarboxylic acid cycle activity may have been locally induced. In these dialysates, collected from probes closer to the epileptogenic region, the large increase in lactate/glucose ratios was a result of both increased lactate and reduced glucose levels. Conclusions: We conclude that regional variations in brain extracellular glucose concentrations may be of greater magnitude than previously believed and become even more accentuated in partial seizure patients. Data from concomitant assays of microdialysate lactate and glucose may aid in understanding cerebral metabolism.