Early Derangements in Oxygen and Glucose Metabolism Following Head Injury: The Ischemic Penumbra and Pathophysiological Heterogeneity

Introduction  Conclusive evidence of cerebral ischemia following head injury has been elusive. We aimed to use ¹⁵O and ¹⁸Fluorodeoxyglucose positron emission tomography (PET) to investigate pathophysiological derangements following head injury. Results   Eight patients underwent PET within 24 h of injury to map cerebral blood flow (CBF), cerebral oxygen metabolism (CMRO₂), oxygen extraction fraction (OEF), and cerebral glucose metabolism (CMRglc). Physiological regions of interest (ROI) were generated for each subject using a range of OEF values from very low (<10), low (10–30), normal range (30–50), high (50–70), and critically high (≥70%). We applied these ROIs to each subject to generate data that would examine the balance between blood flow and metabolism across the injured brain independent of structural injury. Discussion   Compared to the normal range, brain regions with higher OEF demonstrate a progressive CBF reduction (P < 0.01), CMRO₂ increase (P < 0.05), and no change in CMRglc, while regions with lower OEF are associated with reductions in CBF, CMRO₂, and CMRglc (P < 0.01). Although all subjects demonstrate a decrease in CBF with increases in OEF > 70%, CMRO₂ and CMRglc were generally unchanged. One subject demonstrated a reduction in CBF and small fall in CMRO₂ within the high OEF region (>70%), combined with a progressive increase in CMRglc. Conclusions  The low CBF and maintained CMRO₂ in the high OEF ROIs is consistent with classical cerebral ischemia and the presence of an ‘ischemic penumbra’ following early head injury, while the metabolic heterogeneity that we observed suggests significant pathophysiological complexity. Other mechanisms of energy failure are clearly important and further study is required to delineate the processes involved.