Brandon Haefke, MD
Dr. Haefke is a PGY-2 Emergency Medicine resident in Rochester, MN at the Mayo Clinic.
Intro Case: A 36-year-old male with no significant previous medical history presents to the ED with a headache. He describes a convincing thunderclap onset. You obtain a head CT without contrast that is normal. However, his headache started 8 hours prior to the CT being obtained. What now? You
consider (perhaps dread) performing a lumbar puncture. But what about this CT head angiogram that you’ve been hearing about? What’s the best way to rule out subarachnoid hemorrhage in this patient?
The diagnosis of non-traumatic Subarachnoid Hemorrhage (SAH) is challenging, as it is a low-incidence but high-mortality disease. Traditionally, ruling out this condition required a lumbar puncture to ensure the absence of xanthochromia. Recent literature has given EM physicians evidence-based support for using a single head CT, or sometimes only good history and physical exam, to rule out this disease. Even more recently, the emergence of CT angiography has given physicians another diagnostic option, but also created a clinical controversy and divergent practice patterns. This article will provide a deep dive into the various means of ruling out SAH, and outline the advantages and disadvantages of various diagnostic methods.
Incidence and Presentation: Needle in a Haystack
Non-traumatic headaches are a common causes of ED visits (in one study the #5 cause of ED visits overall and 2.8% of all ED visits) 1 . The vast majority of these patients are not found to have a specific diagnosis and are ultimately discharged without event. However, these clinical encounters are haunted by the spectre of SAH, and with good reason. Patients with a true SAH may initially present with a normal neurologic exam and vitals in 40% of cases. 2 These patients can be clinically indistinguishable from those with a benign tension headache.
If these patients are discharged without appropriate recognition, they are at high risk for re-bleeding; a devastating condition with a 70% mortality rate 2 . This is a situation where we could potentially discharge a well-appearing patient and they could suffer a catastrophic death hours to days later. This appropriate concern needs to be balanced with the incredibly low incidence of the disease; it occurs in approximately 6.9 per 100,000 patients per year in the US 3 . Some quick math would estimate approximately 260 cases diagnosed in the US each year.
Excluding with H&P: Ottawa Subarachnoid Hemorrhage Rule
In 2013, Dr. Perry et al. used retrospective data from several tertiary care centers to identify a profile of patients that were low risk of SAH and did not need any ancillary testing 4 . They derived the following parameters:
This group of low-risk patients can be safely ruled out with H&P alone. This clinical decision rule was prospectively validated in 2017 and found to have 100% sensitivity in ruling out SAH; it was endorsed by ACEP clinical policy in 2019 with a “Level A” recommendation.5 ,1
Excluding with Head CT: The 6-Hour Rule
A non-contrast head CT Has always been useful in the diagnosis of SAH, but has been traditionally considered to be insufficiently sensitive to rule out the disease. However, newer generation CT scanners provide much better resolution and are less likely to miss subarachnoid bleeding. In 2011, Dr. Perry et al. (same author group as above), conducted a prospective trial to
determine the sensitivity of head CT using a “modern” scanner. They found an overall sensitivity of 92.9%, with a sensitivity of 100% if they study was performed within 6 hours of headache onset 6 . This provides convincing evidence that current generation CT scans can rule out SAH if performed
promptly. This practice pattern was also endorsed by ACEP clinical policy in 2019 with a “Level A” recommendation.1
The Bleeding Edge
The above studies have greatly limited the number of unnecessary LPs performed in the emergency department. However, many patients with a concerning HPI will not present within the 6 hour window. A negative head CT outside of this time frame would be expected to miss 7% of patients
with SAH, therefore additional testing for this high-mortality condition is warranted.
Lumbar puncture (looking for xanthochromia) is considered to be the gold standard for ruling out SAH in both EM and Neurosurgical literature, with a sensitivity generally considered to be 100%. 7 However, this procedure has several drawbacks, including patient discomfort, post-LP headaches, and
being time-consuming for providers. Could there be another way?
Rolling the Dice: CT Angiography
Since the majority of non-traumatic SAH are caused by cerebral aneurysms, it seems logical that performing CT angiography of the head would be a well-suited test for ruling out the disease. And indeed, several preliminary studies have demonstrated just that. The only prospective study to date
was performed by Carstairs et all in 2006. They enrolled 106 patients who potentially had SAH, and performed both CT and CTA on all subjects, followed by LP on all patients with a negative non-contrast CT. The 6 patients in this study with LP-confirmed SAH were all identified by CTA. 8
Subsequent meta-analysis using retrospective data have estimated the sensitivity of CTA for SAH to be 97-99%. 9,10 While more prospective trials need to be performed, available data suggests that CTA has excellent sensitivity for ruling out SAH.
However, using CTA also has several drawbacks. The most concerned is the high rate of false positives. Approximately 3% of the adult population have cerebral aneurysms, the vast majority of which never reach clinical significance. 11 In other words, CTA is 4000x more likely to show a
false-positive aneurysm than find a true SAH.
If a CTA shows an aneurysm without clear evidence of subarachnoid hemorrhage, an LP must still be performed to rule out SAH. Even if SAH is ruled out, these patients are subsequently burdened with the knowledge of their (likely inconsequential) cerebral aneurysm, which may lead to unnecessary anxiety and increased healthcare utilization. 12, 1
Other drawbacks of CTA include increased healthcare cost, and increased radiation exposure. These are balanced by the potential benefits of being a non-invasive modality, not posing a risk of post-LP headache, and probably patient preference. 1 Additionally, CTA has the very attractive feature of requiring minimal work for ED providers (it’s very easy to click “CT head angio” on an EMR). ACEP did endorse this practice in their 2019 policy with a “Level C” recommendation. 1
The Choice is Yours (Or Your Patient’s)…
In the diagnostic workup of any high-risk, low-incidence disease, it is best to involve the patient in decision making. Are they an anxious person who wants the most sensitive test possible, or are worried about radiation exposure? They would be best served by an LP. Are they highly averse to the discomfort of LP? Perhaps the CTA is the way to go. Or perhaps they are a person with a high risk tolerance, who would rather accept the 7% miss rate of a non-contrast head CT and stop there.
You can also help guide the patient by estimating the likelihood of either a false-positive CTA or difficult/non-diagnostic LP (see below).
● Aneurysmal SAH is a low-incidence but high-mortality disease that in many instances cannot be safely ruled out by H&P alone.
● Conversely, there are situations when either an H&P or simple CT head without contrast can effectively rule out the disease.
● LP is highly sensitive, but has several drawbacks including being time-consuming and causing patient discomfort
● CTA has comparable sensitivity, but has a poor PPV and increases cost and radiation exposure.
● In the situation where either a CTA or LP is needed, there are compelling reasons for either practice, but the best course of action is probably to involve the patient in their own decision making when clinically appropriate.