Pneumocephalus after posterior fossa exploration in the sitting position.

Entrainment of air following exploration of posterior cranial fossa in the sitting position was studied in five patients. Intracranial pressure was monitored through a ventriculostomy catheter after closure of the dura. In three patients nitrous oxide was added to the breathing mixture only after the baseline intracranial pressure had stabilised following closure of the dura. A marked rise in intracranial pressure was observed immediately. A rapid decrease in intracranial pressure occurred when nitrous oxide administration was stopped. Two patients were nitrous oxide from the beginning. No change in intracranial pressure was noted. Computerised tomogram on the first postoperative day revealed a significant amount of air in eight cases.