A pseudosubarachnoid hemorrhage is an apparent increased attenuation on CT scans within the basal cisterns that mimics a true subarachnoid hemorrhage.^[1] This occurs in cases of severe cerebral edema, such as by cerebral hypoxia. It may also occur due to intrathecally administered contrast material,^[2] leakage of high-dose intravenous contrast material into the subarachnoid spaces, or in patients with cerebral venous sinus thrombosis, severe meningitis, leptomeningeal carcinomatosis,^[3] intracranial hypotension, cerebellar infarctions, or bilateral subdural hematomas.^[4]

In a true subarachnoid hemorrhage, there is higher attenuation on CT scans of the basal cisterns, and blood that has leaked from a vessel or formed a hematoma is more highly attenuated due to the absorption of plasma.^[5] Pseudosubarachnoid hemorrhages have been observed in as much as 20% of patients resuscitated from non-traumatic cardiopulmonary arrest. Patients with pseudosubarachnoid hemorrhages may have worse prognoses than those with true subarachnoid hemorrhages because of underlying disease processes and decreased cerebral perfusion with elevated intracranial pressure.^[6] The identification of a pseudosubarachnoid hemorrhage as opposed to a true subarachnoid hemorrhage may therefore change a patient's treatment plan.

References