In our unit, patients with severe head injury are monitored by a triple lumen access device including ICP (IntraCerebral Pressure), PtiO2 (oxygen pressure in the brain) and microdialysis. This last monitoring allows measurement of brain parenchymal concentrations of small molecules : glucose, lactate, pyruvate, glutamate, glycerol,.... It's a tool to evaluate the metabolic state of the brain divided into 4 categories : normal, hyperglycolysis, ischemia and metabolic crisis.