Bilateral dilating pupils and coma in cerebral compression, and misconception about uncus herniation

Background: Uncus herniation is a concept believed to cause bilaterally dilated pupils and deterioration of consciousness in cerebral compression and high intracranial pressure. This study indicates that the concept is based on assumptions and is misleading. Methods: Without new animal experiments, data were obtained from our previous National Institutes of Health- supported laboratory experiments on cerebral compression in cats caused by extradural balloons, and in rats caused by virus-induced brain tumors. The experiments in cats were conducted before the Institutional Animal Care and Use Committee requirement. The status of uncus herniation in humans was studied through a literature review. Results: As cerebral compression increased in cats, cerebral microvessels circulation diminished and gradually stopped in the entire brain, including the brainstem. Simultaneously, pupils became dilated and fixed, and vital signs gradually stopped. Similar microvessel alterations occurred in rats when drowsiness and coma occurred. Sequential in vivo arterial injections of vital dyes confirmed gradual cerebral microvessel obstruction. Postmortem examination revealed no uncus herniation. Light and electron microscopy revealed severe ischemic changes in the entire brain. Conclusion: In cerebral compression, bilateral dilating pupils and coma represent diminishing and arresting cerebral circulation in the entire brain. Uncus herniation may or may not occur in humans. When it does, it is associated, but not the cause, and may produce unilateral pupil dilation and local symptoms.