Characterization of edema by diffusion-weighted imaging in experimental traumatic brain injury

The objective of this study was to use diffusion-weighted magnetic resonance imaging (DWI) to help detect the type of edema that develops after experimental trauma and trauma coupled with hypotension and hypoxia (THH). Reduction in the apparent diffusion coefficients (ADCs) is thought to represent cytotoxic edema. In a preliminary series of experiments, the infusion edema model and middle cerebral artery occlusion models were used to confirm the direction of ADC change in response to purely extracellular and cytotoxic edema, respectively. The ADCs increased (p<0.05) in the case of extracellular edema and decreased (p<0.001) in cytotoxic edema. Following these initial experiments, a new impact acceleration model was used to induce traumatic brain injury. Thirty-six adult Sprague-Dawley rats were separated into four groups; sham, trauma alone, hypoxia and hypotension (HH), and THH. Following trauma, a 30-minute insult of hypoxia (PaO2 of 40 mm Hg) and hypotension (mean arterial blood pressure (MABP) of 30 mm Hg) were imposed and the animals were resuscitated. The DWI was carried out at four 1-hour intervals postinjury, and MABP, intracranial pressure (ICP), cerebral perfusion pressure (CPP), and cerebral blood flow (CBF) were monitored. The ADCs in the control and HH groups remained unchanged. The ADCs in the THH group rapidly decreased from a control level of 0.68 +/- 0.05 x 10(-3) mm2/second to 0.37 +/- 0.09 x 10(-3) mm2/second by 3 hours posttrauma (p < 0.001). In this group, the decreased CBF and CPP during secondary insult remained low despite resuscitation, with the ICP increasing to 56 +/- 7 mm Hg by 3 hours. In the trauma alone group, the rise in ICP reached a maximum value (28 +/- 3 mm Hg) at 30 minutes with a significant and sustained increase in CBF despite a gradual decrease in CPP. The ADCs in this group were not significantly reduced. The data lead the authors to suggest that the rise in ICP following severe trauma coupled with secondary insult in this model is predominately caused by cytotoxic edema and that ischemia plays a major role in the development of brain edema after head injury.