Extracellular glutamate in the brains of children with severe head injuries: a pilot microdialysis study.

Objective: To determine, using a microdialysis technique, the extracellular levels of glutamate and non-transmitter amino acids in the brain of children with severe head injuries.
Methods: A microdialysis probe was inserted along side the intracranial pressure (ICP) bolt in 9 children (age range 2-14 years) with severe head injuries (Glasgow coma scale<8). Dialysate samples were collected hourly and analysed, using High Performance Liquid Chromatography (HPLC), for glutamate, glutamine and various structural amino acids. Clinical monitoring parameters were also correlated with amino acid concentrations.
Conclusions: Mean glutamate concentrations in the dialysate varied from very low (<5 micro M) to very high (>30 micro M). No correlation with outcome at discharge was demonstrated. Structural amino acid levels did not follow the glutamate fluctuations. Low glutamine/glutamate ratio was associated with increased morbidity, while a high ratio was associated with clinical improvement. Glutamate metabolism may have a more significant role in the pathophysiology of paediatric head injury than has already been recognised.