Pseudosubarachnoid hemorrhage

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Pseudosubarachnoid haemorrhage is a sign related to apparent increased attenuation within the basal cisterns which simulates a true subarachnoid haemorrhage (SAH).

Pathology

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~~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 (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.

Differential diagnosis

true subarachnoid haemorrhage
acute leptomeningitis mimicking a subarachnoid haemorrhage ³

-<p><strong>Pseudosubarachnoid haemorrhage</strong> is a sign related to apparent increased attenuation within the basal cisterns which simulates a true <a href="/articles/subarachnoid-haemorrhage">subarachnoid haemorrhage</a> (SAH).</p><h4>Pathology</h4><h5>Causes and associations</h5><p>The most common cause is <a href="/articles/cerebral-oedema-1">cerebral oedema</a> where there is a decrease in parenchymal attenuation and engorgement and dilatation of the superficial venous structures due to an increased intracranial pressure <sup>1,2</sup>. This is seen in anoxia and recent resuscitation from cardiopulmonary arrest.</p><p>Other causes include:</p><ul>
+<p><strong>Pseudosubarachnoid haemorrhage</strong> is a sign related to apparent increased attenuation within the basal cisterns which simulates a true <a href="/articles/subarachnoid-haemorrhage">subarachnoid haemorrhage</a> (SAH).</p><h4>Pathology</h4><h5>Causes and associations</h5><p>The most common cause is <a href="/articles/cerebral-oedema-1">cerebral oedema</a> where there is a decrease in parenchymal attenuation and engorgement and dilatation of the superficial venous structures due to an increased intracranial pressure <sup>1,2</sup>. This is seen in <a title="hypoxic brain damage" href="/articles/hypoxic-brain-damage">hypoxic ischaemic brain injury</a> and recent resuscitation from cardiopulmonary arrest.</p><p>Other causes include:</p><ul>

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