<s>[INST] About a week ago I had a CT scan of my brain due to an auto accident.  The scan showed I have an air pocket in my brain. My primary doctor has ordered a second CT scan to be done in about 2 weeks to see if there is any change in the air pocket. What is the usual causes of an air pocket and is having one something serious? [/INST] Hello, thanks for writing to Chat Doctor. Your CT scan has showed an air pocket in the brain-also known as pneumocephalus. The most common causes are trauma- as is in your case and neurosurgery. It occurs when there is a fracture of the skull/para-nasal sinus, and outside air enters the brain cavity. It is not usually serious, unless something called a tension pneumocephalus develops(signs of increased pressure in brain). Otherwise, this is asymptomatic in most patients, and the air will gradually get resorted. Hope I have answered your query. Do write in if you have any more questions. Wishing you a speedy recovery. </s>