UKMLA PLAB AMC USMLE Prometric MCQ

A 32-year-old man is admitted one hour after the sudden onset of severe headache, which he described as like 'being hit on the head with a hammer'. You suspect subarachnoid haemorrhage, but a CT scan of the brain is reported as normal.

Correct answer: A

The investigation of choice in suspected subarachnoid haemorrhage (SAH) is immediate CT scan without contrast, taking very thin cuts through the base of the brain to optimise the chances of seeing small collections of blood. Imaging within 12 hours using modern scanners has a 98-100% sensitivity for SAH.
Lumbar puncture should be performed in suspected SAH if the CT scan is not diagnostic. The CSF specimen should be centrifuged without delay and examined by spectrophotometry for the presence of xanthochromia due to the presence of oxyhaemoglobin and bilirubin. Note, however, that xanthochromia may not be present if the CSF is examined within 12 hours of haemorrhage occurring, so lumbar puncture should be delayed for 12 hours.