Pseudosubarachnoid haemorrhage is a sign related to apparent increased attenuation within the basal cisterns which simulates a true subarachnoid haemorrhage (SAH).
Causes and associations
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 anoxia 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
- usually symmetrical density confined to the basal cisterns (i.e. no sulcal density)
- 30-40 HU (compared with true acute SAH ~60HU)
- 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.
- 1. Given CA, Burdette JH, Elster AD et-al. Pseudo-subarachnoid hemorrhage: a potential imaging pitfall associated with diffuse cerebral edema. AJNR Am J Neuroradiol. 2003;24 (2): 254-6. AJNR Am J Neuroradiol (full text) - Pubmed citation
- 2. Yuzawa H, Higano S, Mugikura S et-al. Pseudo-subarachnoid hemorrhage found in patients with postresuscitation encephalopathy: characteristics of CT findings and clinical importance. AJNR Am J Neuroradiol. 2008;29 (8): 1544-9. doi:10.3174/ajnr.A1167 - Pubmed citation
- 3. Mendelsohn DB, Moss ML, Chason DP et-al. Acute purulent leptomeningitis mimicking subarachnoid hemorrhage on CT. J Comput Assist Tomogr. 18 (1): 126-8. - Pubmed citation
- 4. Opeskin K, Silberstein M. False positive diagnosis of subarachnoid haemorrhage on computed tomography scan. J Clin Neurosci. 2012;5 (4): 382-6. Pubmed citation
- 5. Rabinstein AA, Pittock SJ, Miller GM et-al. Pseudosubarachnoid haemorrhage in subdural haematoma. J. Neurol. Neurosurg. Psychiatr. 2003;74 (8): 1131-2. Free text at pubmed - Pubmed citation