We've all heard the classic story of subarachnoid hemorrhage (SAH): the worst headache of a patient's life, starting suddenly during exertion. But how do we determine which "thunderclap headaches" are really indicative of SAH? And what if the presentation is more subtle, as is often the case?

What is the best diagnostic approach to be sure we are not missing SAH?

Headache is a common chief complaint in emergency departments, accounting for 2-3% of visits. Subarachnoid hemorrhage (SAH) represents 1-3% of those cases and is an important life-threatening cause of headache. Timely diagnosis of SAH is of critical importance, due to high morbidity and mortality of missed aneurysmal bleeds.

Subarachnoid hemorrhage most commonly occurs when an aneurysm ruptures in the brain, causing bleeding into the subarachnoid space. Saccular (berry) aneurysms are the most common subtype, most often found at arterial bifurcations in the anterior circulation of the Circle of Willis. The blood from an aneurysmal rupture can cause irritation and damage of brain tissue, as well as delayed vasospasm. Traumatic SAH also occurs, but this presentation focuses on the diagnosis of non-traumatic SAH.

The classical presentation of SAH is a severe, sudden onset headache that peaks in intensity immediately. The headache may be associated with nausea, vomiting, meningismus, lower back pain, altered mental status and/or brief loss of consciousness. Seizures occur in 10% of cases and signal a poor prognosis. The range of presentations of SAH overlaps with more common and benign causes of headache, which sometimes makes it more difficult to diagnose.

SAH has an incidence of 10 in 100,000 person-years but remains a source of significant morbidity and mortality. 10-15% of patients with SAH die before making it to the emergency room. Patients can also suffer from potentially fatal vasospasm 5-8 days after the initial hemorrhage. The overall mortality rate of SAH is 20-40%, despite recent advances in management. In patients who survive, up to 50% are severely disabled. A clear diagnostic approach can reduce the risk of poor outcomes by allowing for rapid intervention.

No clear consensus approach to evaluating SAH exists. There are many diagnostic studies to help rule out SAH:

-noncontrast CT (NCCT) of head
-NCCT/LP in combination
-NCCT/CTA in combination
-MRI/MRA

Clinical decision rules have also been used. NCCT is the standard test, but the need for follow-up tests - and which ones - is debated. There is literature supporting all of the diagnostic strategies.

One cost-effectiveness analysis (Maholta et al) showed the combination of CT and LP is generally the most cost-effective approach in patients with thunderclap headache. That same analysis showed CTA has poorly defined utility as follow-up for negative CT and is much more expensive.

In contrast, clinical guidelines published in April 2016 in the Journal of Emergency Medicine (Meurer et al) promote CTA as a reasonable alternative to LP as follow-up to a negative CT. The debate is ongoing.

The American College of Emergency Physicians (ACEP) Clinical Policy on Acute Headache from 2008 offers a Level B* recommendation in favor of using LP after negative CT to rule out SAH. However, there are situations where that might not be the best approach. It is important to risk stratify patients when deciding whether to perform an LP after a negative CT scan, since it is not a totally benign procedure.

*Level B recommendations reflect moderate clinical certainty.

Sensitivity analyses (Maholta et al) showed LP should be performed after negative CT, except when CT sensitivity exceeds 99.2% and the SAH prevalence is below 3.2%, where no follow-up may be considered.

For a patient with a negative CT scan

Lumbar puncture is an invasive procedure. Potential complications include post-LP headache, back pain, CSF infection, uncal or transtentorial herniation, and spinal hematoma.

A risk-benefit analysis of CT/LP in diagnosing SAH (Migdal et al) showed that the incidence of LP complications was low, but higher than that of new SAH diagnoses. The benefit of LP may be limited for patients with a low pretest probability of SAH.

High risk clinical characteristics associated with SAH include:

-prior SAH
-known intracranial aneurysm
-a priori altered mental status
-arrival by ambulance
-age ≥40
-complaint of neck stiffness/pain
-onset with exertion
-vomiting
-witnessed loss of consciousness
-raised blood pressure

The utility of performing LP after negative NCCT is debated as CT technology and sensitivity improve. Nevertheless, LP remains a useful, cost-effective follow-up study in certain cases of potential SAH, particularly if the patient is high-risk or if the CT was performed >6h after the onset of headache.

Link to Video: https://www.youtube.com/watch?v=weoY_9tOcJQ

Because the literature surrounding the diagnosis of SAH is complex, risk stratification and a shared decision making approach is best. Patients at high risk for SAH based on clinical characteristics are more likely to benefit from an LP after a negative NCCT than low-risk patients.

Discussing the risks and benefits of various diagnostic studies with the patient and explaining the options along the way is key.

-Long B, Koyfman A. Controversies in the Diagnosis of Subarachnoid Hemorrhage. J Emerg Med. 2016 May 20.

-Malhotra A, Wu X, Kalra VB, Schindler J, Forman HP. Cost-effectiveness Analysis of Follow-up Strategies for Thunderclap Headache Patients With Negative Noncontrast CT. Acad Emerg Med. 2016;23(3):243-50.

-Manhas A, Nimjee SM, Agrawal A, et al. Comprehensive Overview of Contemporary Management Strategies for Cerebral Aneurysms. World Neurosurg. 2015;84(4):1147-60.

-Mark DG, Sonne DC, Jun P, et al. False-negative Interpretations of Cranial Computed Tomography in Aneurysmal Subarachnoid Hemorrhage. Acad Emerg Med. 2016;23(5):591-8.

-Meurer WJ, Walsh B, Vilke GM, Coyne CJ. Clinical Guidelines for the Emergency Department Evaluation of Subarachnoid Hemorrhage. J Emerg Med. 2016;50(4):696-701.

-Migdal VL, Wu WK, Long D, Mcnaughton CD, Ward MJ, Self WH. Risk-benefit analysis of lumbar puncture to evaluate for nontraumatic subarachnoid hemorrhage in adult ED patients. Am J Emerg Med. 2015;33(11):1597-601.

-Perry JJ, Stiell IG, Sivilotti ML, et al. High risk clinical characteristics for subarachnoid haemorrhage in patients with acute headache: prospective cohort study. BMJ. 2010;341:c5204.

-Singer RJ, Ogilvy CS, Rordorf G. Clinical manifestations and diagnosis of aneurysmal subarachnoid hemorrhage. In: UpToDate, Biller J (Ed), UpToDate, Waltham, MA. (Accessed on May 28, 2016).

-Westafer LM, Carpenter CR, Milne WK. Hot Off the Press: An Observational Study of 2248 Patients Presenting with Headache, Suggestive of Subarachnoid Hemorrhage, that Received a Lumbar Puncture Following a Normal CT Head. Acad Emerg Med. 2016;