Pneumocephalus refers to the presence of intracranial gas, and in the vast majority of cases the gas is air. Rarely a gas forming infection can result in pneumocephalus 4. The term encompasses air in any of the intracranial compartments, and is most commonly encountered following trauma or surgery.

Pneumocephalus can be divided by location:

  • extra-axial
    • epidural
    • subdural
    • subarachnoid
  • intra-axial
    • parenchymal
    • intra-ventricular
    • intra-vascular

Clearly, the demographics of affected patients depends on the underlying cause:

  • mechanical trauma (common)
  • instrumentation, e.g. neurosurgery, external ventricular drain insertion, sinus surgery, peridural anaesthesia
    • after supratentorial craniotomy, pneumocephalus may persist in a minority of patients in the 3rd postoperative week, but is not expected to persist beyond this 6
  • barotrauma 3
  • otogenic pneumocaphalus
  • pneumosinus dilatans
  • meningitis from a gas forming organism (rare) 2, 4

In the vast majority of cases, pneumocephalus is asymptomatic. Tension pneumocephalus, trapped expansion on intracranial air due to a ball valve effect resulting in mass effect, can result in a headache and signs and symptoms of increased intracranial pressure 1,4

In either case, a minority of patients describe 'bruit hydro-aerique' (a splashing noise on head movement, equivalent to the succussion splash of pyloric stenosis4. This noise may also be audible to the examiner with the aid of a stethoscope. 

The diagnosis is simple provided care is taken to ensure that air density/intensity is actually confirmed. 

Air on CT will have a very low density (near -1024HU) but care needs to be taken in ensuring that it is not fat which although of much higher density (-90HU) also appear completely black on routine brain windows. 

On MRI the diagnosis may be trickier as there is no 'objective' density measurement. Air will appear completely black on all sequences, but depending on the location and morphology can be mistaken for blood product or flow voids 5

Treatment depends on the cause, and in many instances, no treatment is necessary with the air being gradually resorbed. This is the case in the vast majority of post operative pneumocephalus, an expected finding in essentially all post-craniotomy patients. 

In cases of tension pneumocephalus then a burr hole may need to be performed to relieve pressure. 

When pneumocephalus results from a CSF leak then identification of the leak site and surgical repair is usually required. 

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Article information

rID: 9879
Section: Pathology
Synonyms or Alternate Spellings:
  • pneumocranium

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Cases and figures

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    Case 1: post traumatic injury
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    Case 4: post sinus surgery
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    Case 5: tension pneumocephalus post FESS
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