Causes of pneumocephalus and when to be concerned about it.

Intracranial gas is commonly detected on neuroimaging. The recognition of this finding can at first blush be unsettling. Being able to localize this gas to a specific compartment: intraarterial, intravenous, intraparenchymal, subdural, epidural, subarachnoid and intraventricular, is the first step in determining the importance of the gas. Determination of the route of entry: through the skull, extension from a paranasal sinus or the mastoid air cells, via the spine, or trans-vascular, is the other important factor in determining potential consequences. Understanding these parameters allows for a confident determination of etiology. More importantly, it generally provides guidance as to what must be done: either to disregard (e.g., subarachnoid gas following lumbar puncture and intravenous gas following IV placement), obtain follow-up (e.g., postoperative gas), or administer emergent treatment (e.g., intraarterial gas and epidural abscess). In this review, we use gas location and route of entry to classify the various causes of pneumocephalus and provide examples of each of these etiologies.