Pseudosubarachnoid hemorrhage

Last revised by Hoe Han Guan on 10 May 2023

Pseudosubarachnoid hemorrhage describes an apparent increased attenuation within the basal cisterns simulating true subarachnoid hemorrhage. It is usually due to cerebral edema.

The most common cause is cerebral edema where there is a decrease in parenchymal attenuation and engorgement and dilatation of the superficial venous structures due to an increased intracranial pressure 1,2. This is seen in hypoxic-ischemic brain injury and recent resuscitation from cardiopulmonary arrest.

Other causes include:

  • usually, symmetrical density confined to the basal cisterns (i.e. no sulcal density)
  • 30-40 HU (cf. true acute subarachnoid hemorrhage ~60 HU)
  • often seen with generalized cerebral edema or basal cistern effacement
  • the appearances are thought to be due to a combination of
    • cisternal effacement
    • distention +/- thrombosis of vessels
    • adjacent brain hypoattenuation accentuating contrast difference

Given et al. reviewed 7 cases of generalized cerebral edema accompanied by increased basal cisternal attenuation which were all found not to have subarachnoid blood at lumbar puncture or autopsy 1.

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