Dissociation of cerebral glucose metabolism and level of consciousness during the period of metabolic depression following human traumatic brain injury

Utilizing [F-18]fluorodeoxyglucose positron emission tomography (FDG-PET), we studied the correlation between CMRglc and the level of consciousness within the first month following human traumatic brain injury. Forty-three FDG-PET scans obtained on 42 mild to severely head-injured patients were quantitatively analyzed for the determination of regional cerebral metabolic rate of glucose (CMRglc). Reduction of cerebral glucose utilization, defined as a CMRglc of less than or equal to 4.9 mg/100 g/min, was present regionally in 88% of the studies. The prevalence of global cortical CMRglc reduction was higher in severely head-injured patients (86% versus 67% mild-moderate), although the absolute magnitude was similar across the injury severity spectrum (mean CMRglc 3.9 +/- 0.6 mg/100 g/min). The level of consciousness, as measured by the Glasgow Coma Scale, correlated poorly with the global cortical CMRglc value (r = 0.08; p = 0.63). With regards to severity of head injury, this correlation was worst for the severely injured (r = -0.11; p = 0.58) and better for the mildly injured patients (r = 0.50; p = 0.07). In most cases, intraparenchymal hemorrhagic lesions were associated with either focal CMRglc reduction or elevation. It is concluded that the etiologies of CMRglc reduction are likely multifactorial given the complex nature of traumatic brain injury and that the reduction of CMRglc represents a fundamental pathobiologic state following head injury that is not tightly coupled to level of consciousness.