Cerebral microdialysis values in healthy brain tissue - a scoping review.

Background: Intracerebral microdialysis is an advanced method to guide clinicians during intensive care of patients with severe acute brain injury. Using intracerebral microdialysis, markers of brain metabolism and homeostasis can be analysed. Currently, trends are considered more important in clinical decision-making than absolute values. Establishing absolute reference values in healthy brain tissue may facilitate an earlier detection of abnormal brain tissue metabolism and provide better decision support for clinicians. However, the current evidence on normal values in the uninjured human brain has not previously been summarized. The aim of this study was to summarise the literature regarding microdialysate concentrations of common markers of brain energy metabolism (glucose, lactate, pyruvate, glutamate, and glycerol) in vivo in healthy brain tissue of humans and gyrencephalic animals.

Method: MEDLINE, Embase, CENTRAL, CINAHL, and Web of Science were searched for published studies that report values of microdialysis in healthy brain tissue. In order to identify unpublished studies, we searched ClinicalTrials.gov, WHO International Clinical Trials Registry Platform (ICTRP), and EU Clinical Trials Register. Study quality was evaluated using a pre-specified protocol.

Result: Out of 3257 studies identified, 39 studies were included. Six of these studies were in humans (total n = 54), 26 in pigs/swine (n = 432), two on monkeys (n = 10), one in sheep (n = 15), and one in dogs (n = 10). We found a high degree of clinical and methodological heterogeneity in both human and gyrencephalic animal studies.

Conclusion: This scoping review identified studies that applied microdialysis to measure common biomarkers in healthy brain tissue. The clinical and methodological heterogeneity between the measured values was substantial, limiting any conclusions. Furthermore, the quality of several human studies was moderate at best. Methodologically comparable studies are warranted to establish reference values for markers of brain energy metabolism using intracerebral microdialysate.