Pneumocephalus is a pathologic collection of gas within the cranial cavity. Patients undergoing neurosurgical procedures may be at increased risk for the development of tension paneumocephalus if nitrous oxide(N₂O) is used during a subsequent anesthetic.

Thirty-seven patients undergoing cerebral aneurysm surgery had a computed tomographic scan of the head performed on or after the day of their surgery. 64 scans were examined for the presence of intracranial air. The magnitude of pneumocephalus was recorded as A-P(mm), width(m),& numbers of section.

Air was seen in all scans obtained in the first three postoperative days, During the second postoperative weeks, the incidence and the size of pneumocephalus decreased. A significant number of patients have an intracranial air collection in the first two weeks after the procedure.

These data indicate that all patients have pneumocephalus immediately after a cerebral aneurysm surgery. This information should be considered in the evaluation of the patient and the selection of anesthetic agents during a second anesthetic in the first 2 weeks after the first procedure.