Pseudosubarachnoid haemorrhage describes an apparent increased attenuation within the basal cisterns simulating true subarachnoid haemorrhage. It is usually due to cerebral oedema.
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Pathology
Aetiology
The most common cause is cerebral oedema 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-ischaemic brain injury and recent resuscitation from cardiopulmonary arrest.
Other causes include:
- severe meningitis: breakdown of the blood-brain barrier allowing mildly hyperdense proteinaceous material to leak into the subarachnoid space 3
- venous sinus thrombosis
- bilateral large subdural haemorrhage producing effacement of sulci and basal cisterns and the false impression of blood in the subarachnoid space 5
- intrathecal contrast
Radiographic features
CT
- usually, symmetrical density confined to the basal cisterns (i.e. no sulcal density)
- 30-40 HU (cf. true acute subarachnoid haemorrhage ~60 HU)
- often seen with generalised cerebral oedema 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 generalised cerebral oedema accompanied by increased basal cisternal attenuation which were all found not to have subarachnoid blood at lumbar puncture or autopsy 1.
Differential diagnosis
- true subarachnoid haemorrhage
- acute meningitis mimicking a subarachnoid haemorrhage 3
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