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
  • Seizures
  • Dizziness
  • 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)

  • Skull defects
    • Post surgical
    • Craniotomy [5–8]
    • Transsphenoidal or endoscopic sinus surgery
    • Shunt insertion [10, 11]
    • Twist-drill drainage of chronic subdural hematomas
  • Post-traumatic
    • Fracture through air sinus or skull base
    • Open fracture though cranial convexity with dural laceration
  • Congenital skull or tegmen tympani defects
  • Neoplasms causing tumor erosion through the skull or skull base
    • Osteomas, Epidermoid, Pituitary tumors
  • Infection with gas-producing organisms
    • Post invasive procedures
    • Lumbar puncture
    • Ventriculostomy
    • Spinal anesthesia 
  • Barotrauma

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

pneumoencephaly

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".

Management of pneumocephalus

This is well covered in the excellent free article by Dabdoub et al (2015), the salient features of which are summarised below. 

In summary, these are the management options:

  1. Do nothing. The vast majority (85%) are reabsorbed spontaneously, without any intervention and with few clinical manifestations.  Karavelioglu (2014) suggest that patience is required, as the process of passive reabsorption may take a couple of weeks. 
  2. Conservative management: This consists of putting the patient head-up (30°) and  avoiding Valsalva maneuvers (coughing, sneezing, straining to open bowels). As an aside, this preventative strategy also includes avoiding aeromedical retrieval. According to Donovan et al (2008), the intracranial gas pocket predictably expands in a depressurised cabin.
  3. Isobaric oxygen: after 24 hours of 100% oxygen by mask, Gore et al (2008) found that the mean volume of their patients' pneumocephalus decreased more than in those patients who only had room air.
  4. Hyperbaric oxygen seems to have a good effect: Paiva et al (2014) found that 1-hour sessions at 2.5 atmospheres of O2 resolved the pneumocephalus much faster than standard 5L/min nasal prong oxygen.
  5. Surgical management is mainly indicated in the context of symptomatic or tensioning pneumocephalus. "Drilling of burr holes, needle aspiration, and closure of the dural defect" is advocated.

References

Nelson, A. S., et al. "Pneumoencephaly following lumbar puncture in association with an ethmoidal osteoma and porencephalic cyst." Journal of Neurology, Neurosurgery & Psychiatry 78.10 (2007): 1149-1151.

Michel, Steven J. "The Mount Fuji Sign 1." Radiology 232.2 (2004): 449-450.

Turgut, Mehmet, and Orhan Akyüz. "Symptomatic tension pneumocephalus: an unusual post-operative complication of posterior spinal surgery." Journal of clinical neuroscience 14.7 (2007): 666-668.

Schirmer, Clemens M., Carl B. Heilman, and Anish Bhardwaj. "Pneumocephalus: case illustrations and review." Neurocritical care 13.1 (2010): 152-158.

Satapathy, G. C., and H. H. Dash. "Tension pneumocephalus after neurosurgery in the supine position." British journal of anaesthesia 84.1 (2000): 115-117.

Markham, J. W. "The clinical features of pneumocephalus based upon a survey of 284 cases with report of 11 additional cases." Acta neurochirurgica 16.1-2 (1967): 1-78.

Lunsford, L. Dade, et al. "Subdural tension pneumocephalos: Report of two cases." Journal of neurosurgery 50.4 (1979): 525-527.

Dabdoub, Carlos B., et al. "Review of the management of pneumocephalus." Surgical neurology international 6 (2015).

Karavelioglu, Ergun, Olcay Eser, and Alpay Haktanir. "Pneumocephalus and pneumorrhachis after spinal surgery: Case report and review of the literature." Neurologia medico-chirurgica 54.5 (2014): 405-407.

Gore, Pankaj A., et al. "Normobaric oxygen therapy strategies in the treatment of postcraniotomy pneumocephalus." (2008). Journal of Neurosurgery May 2008 / Vol. 108 / No. 5 / Pages 926-929

Paiva, Wellingson Silva, et al. "Effects of hyperbaric oxygenation therapy on symptomatic pneumocephalus." Therapeutics and clinical risk management 10 (2014): 769.

Donovan, Daniel J., et al. "Aeromedical evacuation of patients with pneumocephalus: outcomes in 21 cases." Aviation, space, and environmental medicine 79.1 (2008): 30-35.