Extra-axial collection (summary)

Dr Jeremy Jones et al.

Extra-axial collections are collections of fluid within the skull, but outside the brain parenchyma. They may be comprised of CSF, blood or pus and may exist in the extradural, subdural or subarachnoid space. 

Reference article

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  • pathophysiology
    • fluid collection in the extradural, subdural or subarachnoid space
      • blood, e.g. post-traumatic extradural haemorrhage
      • fluid, e.g. chronic subdural haemorrhage
      • pus, e.g. subdural empyema
  • investigation
    • CT head
      • best initial test, especially in an unwell patient
    • MRI brain
      • may be helpful for further assessment

An extra-axial collection will sit between the brain parenchyma and the skull vault. It will therefore be in one of three spaces:

  • extradural
    • lentiform (lens-like) shape
    • don't cross cranial sutures
  • subdural
    • more crescent-like shape
    • may cross sutures
  • subarachnoid
    • extends down into the sulci
    • extends into the basal cisterns

With all collections, it is important to make an assessment for:

  • midline shift
  • hydrocephalus

The appearance of the collection and the features that clarify its location will not be different on an MRI scan. However, it may be easier to determine which space the collection is in if it is small and, using different sequences, it should be possible to determine the type of fluid and narrow the differential diagnosis.

Medical student radiology curriculum
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Article information

rID: 51212
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Cases and figures

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    Case 1: extradural haematoma
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    Case 2: subdural empyema
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    Case 3: chronic subdural haemorrhage (MRI)
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