The Cerebral Extracellular Release of Glycerol, Glutamate, and FGF2 Is Increased in Older Patients following Severe Traumatic Brain Injury

Pekka Mellergård, Florence Sjögren, Jan Hillman

Old age is associated with a poor recovery from traumatic brain injury (TBI). In a retrospective study we investigated if the biochemical response following TBI is age dependent. Extracellular fluids were continuously sampled by microdialysis in 69 patients admitted to our NSICU following severe TBI. The concentrations of glycerol, glutamate, lactate, pyruvate, and eight different cytokines (IL-1β, IL-6, IL-10, IL-8, MIP-1β, RANTES, FGF2, and VEGF) were determined by fluorescence multiplex bead technology. Patients in the oldest age group (≥65 years) had significantly higher microdialysate concentrations of glycerol and glutamate compared to younger patients: the mean microdialysate concentration of glycerol increased from 55.9 μmol/L (25-44 year) to 252 μmol/L (≥65 years; p<0.0001); similarly glutamate increased from 15.8 mmol/L to 92.2 mmol/L (p<0.0001). The lactate-pyruvate ratio was also significantly higher in the patients ≥65 years of age (63.9) compared with all the other age groups. The patterns of cytokine responses varied. For some cytokines (IL-1b, IL-10, and IL-8) there were no differences between age groups, while for others (MIP-1b, RANTES, VEGF, and IL-6) some differences were observed, but with no clear correlation with increasing age. For FGF2 the mean microdialysate concentration was 43 pg/mL in patients ≥65 years old, significantly higher compared to all other age groups (p<0.0001). Increased concentrations of glycerol and glutamate would indicate more extensive damaging processes in the elderly. An increase in concentration of FGF2 could serve a protective function, but could also be related to a dysregulation of the timing in the cellular response in elderly patients.