Subarachnoid Hemorrhage: Why do we miss it?

Authors: John J. Campo, MD (EM Resident Physician, Department of Emergency Medicine, Harbor-UCLA Medical Center) and Manpreet Singh, MD, MBE, FACEP (Director, Undergraduate Medical Education, Department of Emergency Medicine, Harbor-UCLA Medical Center; Assistant Professor of Emergency Medicine, David Geffen School of Medicine at UCLA) // Reviewed by: Joe Ravera, MD (@PEMUVM1); Alex Koyfman, MD (@EMHighAK); Brit Long, MD (@long_brit)


A previously healthy 55-year-old female presents to the ED complaining of a headache. She describes it as an “intense” headache that started abruptly 10 hours ago. She has a history of migraines, but states this is different. She took two acetaminophen with no improvement which prompted her to come to the ED. You are concerned that the patient has a subarachnoid hemorrhage (SAH) and immediately order a non-contrast CT scan of the head (NCCTH). The patient asks you if the scan is normal does that mean she can go home safely.


Headache is one of the most common reasons for emergency department visits, accounting for 3% of all visits in the United States (1). The vast majority, about 90%, are benign primary headaches (i.e. migraine, tension, cluster headaches). The other 10% are due to secondary causes, which include potentially life-threatening processes. Subarachnoid hemorrhage (SAH) is relatively rare, comprising about 1% of all headaches presenting to the ED (2). Although it is uncommon, SAH is one cause of headache that no one wants to miss because misdiagnosis can be catastrophic, with case fatality rates as high as 50% (3). The implications for rapid, accurate diagnosis and intervention can be profound. One year mortality for individuals with an untreated SAH is 65% whereas one year mortality for those with a timely diagnosed and treated SAH is 18% (4, 5).

Securing the diagnosis is not always straightforward. This is reflected by the fact that rates of misdiagnosis have been estimated to be as high as 7% (6). This is called “failure to consider” in the medical malpractice/error world. The most common mistake providers make is failing to obtain a NCCTH which occurs in 73% of misdiagnosed cases. This stems in part from an overreliance on the classic presentation and failing to appreciate that there is a spectrum of disease presentation (7). Patients suffering a SAH do not always appear “sick” and do not always have neurological deficits. The reality is about 40% have a normal mental status and no neurological deficits. Additional reasons for misdiagnosis include failing to understand the limitation of CT in diagnosing SAH and failing to perform a lumbar puncture.

Epidemiology, Risk Factors, and Pathogenesis

Most cases of non-traumatic or spontaneous SAH are caused by rupture of an aneurysm (8). About 15% of patients presenting with SAH do not have an aneurysm found on imaging. Causes of nonaneurysmal, non-traumatic SAH include perimesencephalic hemorrhage, vascular malformations, intracranial arterial dissection, cerebral venous thrombosis, pituitary apoplexy, and cocaine abuse.

Figure 1. Left PCOM Aneurysm Rupture

Figure 2. Perimesencephalic Hemorrhage

Within the general population, the prevalence of an aneurysm is estimated to be 5%, or 10 to 15 million people in the United States (9). Risk factors for formation of aneurysms include genetic disorders (autosomal dominant polycystic kidney disease, Ehlers-Danlos syndrome), hypertension, cigarette smoking, and alcohol consumption (10). Most aneurysms do not rupture, as there are about 30,000 cases of SAH in the US each year. Risk factors associated with rupture of an aneurysm include hypertension, cigarette smoking, alcohol consumption, use of sympathomimetics, and aneurysm size and location. Most individuals who have an aneurysmal SAH are between 40 and 60 years of age. Physical exertion may trigger some aneurysmal ruptures, but most occur without an identifiable trigger (11, 12). Rupture of an aneurysm results in bleeding directly into the subarachnoid space. As blood accumulates and mixes with cerebrospinal fluid (CSF), secondary complications begin to develop (i.e. increased intracranial pressure, vasospasm, and hydrocephalus).

Figure 3. Subarachnoid Hemorrhage

Clinical Presentation

The classic presentation of an aneurysmal SAH is middle ago person with a sudden onset, severe headache described as the “worst headache of my life”. 75% of patients with SAH describe it as abrupt and maximal at onset, the so called “thunderclap headache”. Some patients experience a sentinel headache which is a sudden, severe headache that precedes a SAH by days or weeks. Common associated symptoms include loss of consciousness (9%), vomiting (61%), and neck pain or stiffness (75%) (13). One recent meta-analysis found that objective neck stiffness (LR+ 6.6) and a history neck pain (LR+ 4.1) were findings most strongly associated with SAH (14). Depending on the location of the bleed, focal neurological deficits may be seen.

Ottawa Subarachnoid Hemorrhage Rule

The Ottawa Subarachnoid Hemorrhage (SAH) Rule for Headache is a prospectively validated clinical decision tool developed to aid in risk stratifying patients. It has 100% sensitivity for SAH, but only a 15.3% specificity (15). Table 1 lists the criteria used in the decision rule. If the answer to each is no, the patient can be ruled out for SAH. Inclusion criteria include patients ≥15 years old, Glasgow coma scale score of 15, new severe atraumatic headache, maximum intensity within 1 hour. Exclusion criteria include new neurologic deficits, prior aneurysm, prior SAH, known brain tumors, or chronic recurrent headaches (≥3 headaches of the same character and intensity for >6 months).

Diagnostic Imaging

All patients presenting with a complaint of a sudden onset severe headache (i.e. thunderclap) should be evaluated immediately for possible SAH with a NCCTH. The major limitation of using CT to diagnose SAH is that the sensitivity is time dependent. If performed within 6 hours of headache onset, read by an attending radiologist, and the patient’s hematocrit is > 30%, a negative NCCTH effectively rules out SAH (LR- 0.01) (16). If the CT is performed after 6 hours of headache onset, the sensitivity decreases from 98.7% to 85.7% (*Please see editor’s note below).

If outside the 6-hour window, further evaluation with lumbar puncture (LP) should be pursued if the CT is negative, though CTA can be used. LP findings consistent with a diagnosis of SAH include an opening pressure > 20 cm H2O, elevated red blood cells (RBCs) in CSF that do not decrease when tubes 1 and 4 are compared, and xanthochromia. No guidelines exist for a cutoff number of RBCs to diagnose SAH, but one prospective study found that patients with <2000 RBCs and no xanthochromia were highly unlikely to have SAH (17).

An alternative approach to CT plus LP in patients presenting after 6 hours of symptom onset is to perform a computed tomography angiography (CTA) of the brain after a negative NCCTH. If the CTA shows no evidence of an aneurysm, the likelihood of a SAH is low (post-test probability of < 1%) (18). There are limitations to this approach, namely the identification of an asymptomatic aneurysm and the negative downstream effects it can have. Additionally, identifying an aneurysm on CTA would require a lumbar puncture to be performed. Of note, ACEP guidelines recommend use of LP or CTA in those with negative head CT but suspicion for SAH remains. AAEM also provides further guidance on use of LP and CTA.

*Editor’s Note: A retrospective study conducted between 2008 and 2017 published in November 2021 in the Emergency Medicine Journal found sensitivity of multislice CT for aneurysmal SAH approached 100% even at 24 hours. At 6, 12, 24, 48, 72 and 96 hours post symptom onset, the sensitivities were 100% (95% CI 98.0-100), 100% (98.2-100), 100% (98.3-100), 99.6% (97.6-100), 99.6% (97.6-100) and 98.7% (96.4 -99.7), respectively. For all cases of SAH, sensitivities were 100% (98.3-100), 99.2% (97.2-99.9), 99.3% (97.5-99.9), 99.0% (97.1-99.8), 99.0% (97.2-99.8) and 97.8% (95.5-99.1), respectively. This study included 347 patients with 260 cases of SAH. While this is a retrospective study, this lends further credence to the argument for shared decision making with patients regarding LP.

Vincent A, Pearson S, Pickering JW, Weaver J, Toney L, Hamill L, Hurrell M, Than M. Sensitivity of modern multislice CT for subarachnoid haemorrhage at incremental timepoints after headache onset: a 10-year analysis. Emerg Med J. 2021 Nov 24:emermed-2020-211068. doi: 10.1136/emermed-2020-211068.

Initial Management

Once the diagnosis of SAH is established, immediate evaluation of the patient’s airway, breathing, circulation, and mental status is necessary. Patients should be placed in a monitored setting. A neurosurgical consultation and CTA of the brain should be completed urgently. The patient’s head should be elevated to 30° to improve venous drainage. Analgesia should be provided, which can reduce blood pressure prior to starting antihypertensives. For an aneurysmal SAH, the American Stroke Association guidelines recommend targeting a goal SBP of 160 mm Hg with a titratable agent, such nicardipine or clevidipine (19). Nimodipine has been shown to improve neurological outcomes if given to patients with aneurysmal SAH. Reversing anticoagulation and antiplatelets should be done in consultation with the neurosurgery team. A short course of seizure prophylaxis can be considered. The definitive management for a ruptured aneurysm is surgical coiling or clipping. The earlier this is performed, the less chance there is for rebleeding.

Take Home Points

  • All patients presenting with a thunderclap headache should undergo evaluation for a SAH starting with a NCCTH.
  • If performed within 6 hours of onset of the headache, a negative NCCTH essentially rules out a SAH.
  • If outside the 6-hour window, further evaluation with an LP (or CTA) should be pursued if the CT is negative.
  • The most common mistake made by providers leading to misdiagnosis is not getting a NCCTH.


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