If a non-contrast CT scan is negative but SAH is suspected, what is the next best diagnostic step?

If a non-contrast CT scan is negative but SAH is suspected, what is the next best diagnostic step?

• A. Digital subtraction angiography
• B. MRI
• C. EEG
• D. Lumbar puncture

Answer: (D)

When subarachnoid hemorrhage is strongly suspected but the initial noncontrast CT is negative, the next step is to examine the cerebrospinal fluid with a lumbar puncture. CT scans are very good at picking up fresh blood early, but their sensitivity drops over time. If symptoms started hours ago and SAH is still likely, CSF analysis can reveal red blood cells or xanthochromia from bilirubin that indicates prior bleeding. Xanthochromia generally becomes detectable after several hours due to hemoglobin breakdown, so LP can confirm SAH when CT misses it. Checking multiple CSF tubes helps distinguish true hemorrhage from a traumatic tap, since true SAH will show red blood cells in successive samples, whereas a traumatic tap tends to decrease in later tubes. Other tests like imaging to locate an aneurysm are important after SAH is confirmed, and MRI can detect SAH but is not the most practical immediate step in this scenario. EEG isn’t used to diagnose SAH.

When subarachnoid hemorrhage is strongly suspected but the initial noncontrast CT is negative, the next step is to examine the cerebrospinal fluid with a lumbar puncture. CT scans are very good at picking up fresh blood early, but their sensitivity drops over time. If symptoms started hours ago and SAH is still likely, CSF analysis can reveal red blood cells or xanthochromia from bilirubin that indicates prior bleeding. Xanthochromia generally becomes detectable after several hours due to hemoglobin breakdown, so LP can confirm SAH when CT misses it. Checking multiple CSF tubes helps distinguish true hemorrhage from a traumatic tap, since true SAH will show red blood cells in successive samples, whereas a traumatic tap tends to decrease in later tubes.

Other tests like imaging to locate an aneurysm are important after SAH is confirmed, and MRI can detect SAH but is not the most practical immediate step in this scenario. EEG isn’t used to diagnose SAH.