Pneumocephalus, pneumoencephalus, intracranial pneumatocele or aerocele are all terms for air in the head, where it is not meant to be. It can be a normal consequence of brain surgery and is usually not a major thing, except were there is too much air, or when it ends up under tension. This condition has never come up in the exam until a relatively recent radiology question (Question 14.2 from the first paper of 2016). The college presented us with a CT slice depicting the classical "Mount Fuji" sign, in a patient who has lost consciousness following a C5-6 epidural abscess drainage.
Clinical features of pneumocephalus
To answer the question "what would it be like to have a lot of air around my brain", Markham (1967) performed a survey of around 300 patient cases. The patients had the following symptoms:
- Headaches in 38%
- Nausea and vomiting
- Depressed neurological status
In addition to these, one might expect tension pneumocephalus to present with all the characteristic features of increased intracranial pressure.
Causes of pneumoencephalus
The list below is copied directly from Schirmer et al (2010)
Other causes of pneumocephalus not mentioned above may include:
- Use of nitrous oxide during surgery
- Transfer by fixed-wing aircraft (gas expands in the depressurised cabin)
Radiological manifestations of pneumoencephalus
This image, used for Question 14.2 from the first paper of 2016, was stolen shamelessly from Eric Miller's Emergency Medical Minute (Podcast #93). Specifically, it is tension subdural pneumoencephaly, which can be identified by the "Mount Fuji" sign (S.Michel, 2004). Observe how the intracranial air is under pressure: the frontal lobes have been squished and separated, giving a twin peak appearance. The lateral venticle is somewhat squashed-looking, which also suggests that there is increased intracranial pressure.
Causes of tension pneumoencephalus
Tension pneumoencephalus is a situation where air is able to enter the skull, but not exit it. This occurs when some sort of a valve-like mechanism is formed. This is called the "ball-valve" or the "inverted bottle" mechanisms.
The ball-valve effect is where air enters the cranial cavity through a defect when it is forced there under prssure (eg. coughing, sneezing etc.). Then, it cannot passively escape (the pressure is not great enough).
The inverted bottle effect is due to CSF drainage, and is the most likely explanation for tension pneumocephalus following spinal surgery. It is also the most likely mechanism to explain the pathology in Question 14.2 from the first paper of 2016. Lundsford et al described this in 1979. In essence, the drainage of CSF from the spine creates a negative intracranial pressure, which entrains air. " The mechanism for entry of air into the intracranial compartment is analogous to the entry of air into an inverted soda-pop bottle", the authors muse. "As the fluid pours out, air bubbles to the top of the container".