Gross pneumocranium post functional endoscopic sinus surgery (FESS) for chronic paranasal sinus inflammatory disease. Defect in the roof of the right anterior ethmoidal aircells with a fistula into the subarachnoid space indicating disruption to the dura. Gas has thus passed from the nasal cavity into the cranium via the ethmoidal aircells. Note the compression of this gas on the frontal lobes (arrows). This indicates mass effect and suggests that the gas (air) is under tension.