Pearls• Cerebral air emboli (CAE) are scattered, small, distally located, and characteristically of low density on CT scan. • CAE should be considered as a stroke etiology in appropriate cases.• Prevention is a key treatment strategy.

Decompressive hemicraniectomy can be a life-saving treatment for uncontrolled elevations in intracranial pressure (ICP) due to trauma or space-occupying ischemic or hemorrhagic stroke. 1 Herniation in the direction opposite the site of the craniectomy-paradoxical herniation-is an underrecognized and potentially fatal complication of this procedure. It can occur spontaneously or may be precipitated by a lumbar puncture (LP) or a CSF shunt. It is critical to recognize this treatable entity because standard interventions aimed at lowering ICP are harmful.Case report. A 24-year-old man with a right-sided subdural hemorrhage (SDH) and temporal contusion from head trauma underwent SDH evacuation, anterior temporal lobectomy, and a right-sided hemicraniectomy. Subsequently, a medium pressure ventriculoperitoneal shunt (VPS) was placed for hydrocephalus. Two months later, he followed simple commands and had a left-sided hemianopsia and hemiparesis. At this time, his scalp was slightly sunken over the skull defect. He was transferred to our hospital for rehabilitation, where he developed fevers and decreased responsiveness. An LP revealed an opening pressure of 11 cm H 2 O. There were two white blood cells, and CSF cultures were negative. Additional workup revealed a urinary tract infection.Two days after the LP, his condition markedly deteriorated over 4 hours. He became unarousable and was extensor posturing in all extremities. The right pupil became dilated and fixed. There also was a dramatic (3 cm) increase in the concavity of the scalp from baseline. A brain CT scan showed marked midline shift in the direction opposite the craniectomy site with subfalcine herniation and effacement of the peripontine cisterns ( figure, A and B).Given the concern for paradoxical herniation, 5 hours after his initial deterioration, he was placed in the Trendelenberg position, IV fluids were administered, and the VPS was tied off. Within 1 hour, his condition improved. He again followed simple commands, the skull flap was no longer concave, and the pupillary abnormalities resolved. Follow-up brain CT scan confirmed resolution of the tissue shifts and herniation (figure, C and D).