What is the role of imaging in diagnosing subarachnoid hemorrhage (SAH)?

From the Guidelines

For suspected subarachnoid hemorrhage (SAH), the recommended initial imaging is a non-contrast head CT scan, performed as soon as possible after symptom onset, as it has a high sensitivity of 98.7% and specificity of 99.9% when performed within 6 hours of symptom onset 1. The Ottawa SAH Rule can be used to identify patients who require additional investigation for SAH, including those with age ≥40 y, neck pain or stiffness, witnessed loss of consciousness, onset during exertion, thunderclap headache, or limited neck flexion on examination 1. If the CT is negative but clinical suspicion remains high, a lumbar puncture should be performed to analyze the cerebrospinal fluid for the presence of blood, especially if the patient presents more than 6 hours from ictus 1. The following are key points to consider when evaluating a patient for SAH:

• Non-contrast head CT scan is the initial imaging of choice
• CT angiography (CTA) or magnetic resonance angiography (MRA) may be considered if the initial CT is negative and performed more than 6 hours after symptom onset, or if clinical suspicion remains high despite a negative CT
• Cerebral angiography is the gold standard for detailed vascular imaging and is essential for treatment planning in cases where SAH is confirmed
• The Ottawa SAH Rule can be used to identify patients who require additional investigation for SAH Some key considerations when interpreting imaging results include:
• The sensitivity of CT decreases as time progresses, as blood products are reabsorbed
• CTA and MRA can detect most aneurysms, while cerebral angiography remains the gold standard for detailed vascular imaging
• The use of MRI is not currently indicated as a primary evaluation tool for acute SAH, but it may be considered in certain cases, such as when results on non-contrast CT are normal and there are persistent unexplained neurologic findings 2.

From the Research

Role of Imaging in Diagnosing Subarachnoid Hemorrhage (SAH)

• Imaging plays a crucial role in diagnosing SAH, with non-enhanced CT of the head being the initial imaging modality for detection of ruptured intracranial aneurysms 3
• Computed tomography (CT) of the head with the assistance of the Ottawa subarachnoid hemorrhage rule should be utilized as an initial diagnostic measure for suspected SAH 4
• If further investigation is needed, a CT angiography of the head or a lumbar puncture can be considered, keeping risks and limitations in mind 4, 3
• Digital subtraction angiography (DSA) remains the reference exam for diagnosing SAH, but multidetector row CT angiography may potentially replace DSA in the emergency setting 3
• MRI and MRA can be added to the diagnostic work-up for SAH unrelated to aneurysm rupture to exclude other differential diagnoses such as venous thrombosis or angiitis 3

Diagnostic Yield and Complications of Lumbar Puncture

• Lumbar puncture (LP) has a low diagnostic yield for SAH in patients with a negative head CT, with only 1% of patients being deemed positive for SAH in one study 5
• LP is associated with serious complications, a significant false positive rate, and extended ED length of stay 5
• However, LP still has a role in diagnosing SAH, particularly in the setting of significant anemia, despite the high sensitivity of non-contrast head CT 6

Management Strategies for Suspected SAH

• The most effective follow-up strategy for evaluation of patients with thunderclap headache and negative initial non-contrast CT for acute SAH is CT with lumbar puncture (LP) follow-up, with the highest expected utility of 0.79926 quality-adjusted life-year (QALY) 7
• CT with no follow-up is the best strategy only when the sensitivity of CT is very high (99.6%) or the pre-test probability of SAH in a patient with thunderclap headache with negative initial CT is low (1.6%) 7