Figure 1. Computed tomographic scan showing an extraventricular drain and cerebral edema. The anterior horn of the right lateral ventricle appears smaller than the left but is within normal limits. The xenon flow study shows the left hemisphere with normal perfusion and flow from 30 to 60 mL/100 mg of tissue per minute. The right frontal/parietal cortex is hypoperfused with flows of 20 mL/100 mg of tissue per minute, with sparing of the occipital region on posterior parietal lobes, where flows are 30 to 50 mL/100 mg of tissue per minute.
Stable xenon CT cerebral blood flow (CBF) determination is effective to evaluate brain perfusion after traumatic brain injury. Stable (nonradioactive) xenon gas is inhaled and acts as a tracer. The CT scan is obtained with specialized software, and the attenuation of the signal reflects the perfusion within the brain while the regional blood flow is quantitated. The scans are performed when arterial PCO2 and mean arterial blood pressure are known. Measurement of CBF coupled with information about ICP have been used to guide clinical management of traumatic brain injury. Xenon CT has been used in the evaluation of CBF in nontraumatic brain injury1 but its efficacy in guiding therapy is not as well established.
Neurologic injury in bacterial meningitis is due to several factors, such as elevated ICP, vasculitis, sinus thrombosis, alterations in CBF, defective autoregulation of CBF, or alterations in cerebral metabolism.2 Elevated ICP may be due to cerebral edema,3 increased CSF volume,4,5 or increased cerebral blood volume.6 Cerebral blood flow is elevated, normal, or decreased during the course of bacterial meningitis.1 Many patients with bacterial meningitis have intact CBF/PCO2 reactivity; hence, hyperventilation may further reduce CBF in patients whose CBF is already below normal.1 Our patient likely had cerebral edema and decreased compliance, which was not evident on the initial CT. Since his CBF was low, increased cerebral blood volume was not a factor in the increased ICP.
Computed tomography and magnetic resonance imaging have demonstrated a wide variety of cerebral abnormalities in children with bacterial meningitis. The incidence of abnormality varies among series, and recent studies have shown a low incidence of cerebral edema without any relationship to neurologic examination or outcome4,7. While CT scans are useful if they reveal specific focal abnormalities or edema to indicate the presence of elevated ICP, a normal CT scan does not preclude increased ICP.
If the physical examination or initial imaging study suggests that ICP is elevated, internal ICP monitoring should be considered. If ICP is found to be elevated, a xenon CT aids in determining the treatment strategy by delineating the pattern of CBF. In our case, since the patient had decreased CBF, the treatment strategy focused on improving cerebral perfusion pressure and minimizing cerebral metabolic needs while avoiding hyperventilation (which would have decreased already compromised blood flow). While there is no direct evidence that this treatment strategy improves outcome, there is evidence that patients with lower cerebral perfusion pressure (mean arterial pressure − ICP) have poorer outcomes, and that patients with low CBF have poorer outcomes.1 The only way to manipulate CBF and cerebral perfusion pressure is with simultaneous monitoring of arterial blood pressure and ICP, paired with quantitative CBF to guide management.
Accepted for publication June 15, 1999.
Corresponding author: Laura M. Ibsen, MD, Oregon Health Sciences University, Division of Pediatric Critical Care Medicine, Department of Pediatrics, 3181 SW Sam Jackson Park Rd, Portland, OR 97201-3098.
Radiological Case of the Month. Arch Pediatr Adolesc Med. 2002;156(3):294. doi: