Pneumocephalus is defined as the presence of air or gas in the cranial cavity. Typical causes include trauma, neoplasm, infection and surgical intervention, but some cases are idiopathic. However, idiopathic pneumocephalus is rare.
We present a very rare case of idiopathic pneumocephalus with a widely pneumatized mastoid. The patient was a 20-year-old man who complained of right orbital pain but there were no other specific neurological symptoms. Initially, he was treated by neurosurgery to close a bone defect only. However, right orbital pain recurred after one and a half months after surgery. Otolaryngological findings (pure tone audiometry, tympanometry, and measurement of eustachian tube function) were normal. Hyperpneumatized mastoid air cells and a bone defect of the posterior cranial fossa were detected on computed tomography scan. We performed second surgery, mastoidectomy, obliteration of the mastoid and bone defect, and blockade aditus using free fascia from the abdomen, bone powder and abdominal fatty tissue. Follow-up CT 9 month after surgery demonstrated that there was no recurrence of pneumocephalus. To cure it, it is important to identify the route of air invasion and block it. We thought that the mechanism of pneumocephalus in our case involved the presence of “a check valve effect” formed in the widely pneumatized mastoid.