Intern Review – Can CT Angiography Rule Out Subarachnoid Hemorrhage?

The bottom line – CTA may be able to reasonably exclude SAH when pre-test probability is relatively low and LP is non-diagnostic.

A 55 year old male with PMH significant for HTN and DM presents with new onset worst headache of his life.  He is hemodynamicly stable and physical examination is without focal neurologic deficit.  You suspect SAH and proceed with non-contrast CT head which is negative.  An LP is attempted but fails.  What are your options?

Given the significant morbidity and mortality of this condition and up to 10% are misdiagnosed on initial visit to the ED (1, 2), a careful workup is warranted when to rule out SAH.

The sensitivity for detecting SAH with a non-contrast head CT decreased with time:

  • <6 hours – 100% (3)
  • 6-12 hours – 98%
  • 12-24 hours – 93% (4)
  • 24 hours to 5 days – <60%

After 10 days, subarachnoid blood is resorbed and the study becomes useless (5).  When CT is negative, the next move is an LP to look for RBCs and xanthrochromia in the CSF.  CSF xanthrochromia takes about 2-12 hours to develop but can be detected for up to 3 weeks or longer (6, 7).  In one study, however, traumatic taps may produce a false positive report if not processed within 2 hours of CSF collection (8).

When the CSF is analysed, the RBC count in the 3rd or 4th tube is used to make the diagnosis.  While no specific cutoff exists for the number of RBCs that may constitute a negative tap, a reduction of RBCs by 25% from tubes 1 to 4 is sometimes used.  A recent publication does suggestion that less than 2000 RBCs and no xanthochromia is sensitive enough to rule out SAH (12). This method, however, may itself be folly to definitively say no SAH exists, as at least one study has shown this reduction in RBC count may occur when in fact an SAH is present (9).  Therefore, only a negative CT and LP combined can reliably rule out the process, a process that that has improved with the newer generation CT scanners (10).

Back to our case.  So what do we do when we can’t obtain an LP?  In a small study of 116 patients, CT angiography was used in combination with CT and LP for the diagnosis of SAH (11).  in this study, CT angiography added diagnostic utility to a negative CT and LP by finding 2 cases that would have been missed by a negative CT and LP alone.  So what about just doing CT and CTA?

In a study published in Academic Emergency Medicine in 2010, a mathematical probability model using a pretest probability of 15% or less for SAH (acute-onset headache, non-focal neurological exam) was utilized to determine posttest probability of excluding aneurysmal or AVM related SAH with CT/CTA alone.  In this model, combining CT/CTA was able to exclude SAH with a 99% posttest probability.  Mathematically, the pretest probability for a negative CT/CTA with LP puts the risk of a missed SAH at less than 1%.

Why not just do CT/CTA then for everyone?  For the majority of individuals whom we are considering SAH as a diagnosis, the pretest probability is higher than the aforementioned study for an ED population.  Simply relying on a CT/CTA alone therefore puts us a risk of missing some of the lower risk patient population.  That said, it is likely a reasonable strategy in those unable to undergo LP or will not consent to the procedure.  An alternative in our case might be to perform an fluoroscopic guided LP – as long as the resources are available.  Using ultrasound could potentially improve LP success when landmarks are not easily palpated. When these options are not available, ordering a follow up CTA is a reasonable next step.  At least then you may be reassured that the inability to get an LP decreases the overall chances of missing something sinister.

Written by Dr. Daniel “Zach” Adams, Intern at OSU EM.

Edited by Dr. Michael Barrie, Assistant Professor at OSU EM.


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