False positive diagnosis of subarachnoid haemorrhage on computed tomography scan

Five cases are presented in which subarachnoid haemorrhage (SAH) was diagnosed by clinicians and/or radiologists on computed tomography (CT) scan. No macroscopic SAH was present on neuropathologic examination. In retrospect it was considered that the neurologic signs and the neuropathologic features close to the time of CT scan were in keeping with the patients being brain dead, i.e. had no cerebral blood flow at the time of the scans. On review of the CT scans it was considered the hyperdense material seen in the subarachnoid space must have been blood in congested subarachnoid blood vessels. The cases demonstrate that if a patient presents comatose and CT scan shows cerebral oedema then the presence of high attenuation material in the subarachnoid space should not necessarily be considered to represent SAH. The value of seeking radiological opinion is highlighted but even then diagnosis may be difficult.