Tension pneumocephalus:A rare complication of FESS

There is a bone defect at the fovea ethmoidalis (red arrow).
Also there are post-operative changes indicating that the patient had undergone FESS.
The intracranial air is a complication of FESS.

With this complication, usually the patient goes home feeling fine, and then shows up approximately two weeks later with CSF leak and meningitis, due to the defect in the bone and dura.
Tension pneumocephalus occurs when air in the head acts like a mass: there is a bony defect which lets air in but not out (valve-like function).
Every time the patient sneezes, air is forced through the defect into the intra-cranial space, and remains trapped there.
At a certain moment the amount of air is sufficient to cause mass effect on the surrounding intra-cranial structures.