What is the best imaging modality for diagnosing Subarachnoid Hemorrhage (SAH)?

From the Guidelines

Computed tomography (CT) without contrast is the best initial imaging modality for diagnosing subarachnoid hemorrhage (SAH). A non-contrast head CT should be performed as soon as possible after symptom onset, ideally within 6 hours, as it has a sensitivity of approximately 98.7% during this timeframe, as shown in a 2016 meta-analysis 1. The sensitivity decreases significantly after 24 hours as blood products become isodense with brain tissue. If clinical suspicion remains high but the initial CT is negative, a lumbar puncture should be performed to look for xanthochromia or red blood cells in the cerebrospinal fluid, especially in patients presenting > 6 hours from ictus 1. In cases where CT is negative but clinical suspicion remains high, CT angiography (CTA) or magnetic resonance imaging (MRI) with FLAIR sequences may be considered as additional diagnostic tools. MRI with FLAIR is more sensitive than CT for detecting SAH after 24 hours. Digital subtraction angiography remains the gold standard for identifying the underlying cause of SAH, particularly for detecting aneurysms, but is typically performed after initial diagnosis with CT, as it can aid in decision-making on the choice of optimal treatment modality 1. The rapid diagnosis of SAH is critical as early intervention can significantly reduce mortality and improve outcomes.

Some key points to consider when diagnosing SAH include:

• The Ottawa SAH Rule, which can help identify patients who are unlikely to have SAH and thereby avoid additional imaging and workup 1
• The importance of using high-quality CT scanners and fellowship-trained, board-certified neuroradiologists to interpret the images 1
• The limitations of CTA in detecting small aneurysms, and the potential need for digital subtraction angiography (DSA) in certain cases 1
• The role of MRI in detecting SAH after 24 hours, and its potential use as an additional diagnostic tool 1

Overall, the choice of imaging modality for diagnosing SAH depends on the clinical presentation and the availability of resources, but CT without contrast remains the best initial imaging modality, as supported by the most recent and highest quality study 1.

From the Research

Imaging Modalities for Diagnosing Subarachnoid Hemorrhage (SAH)

• Non-enhanced CT of the head is the initial imaging modality in suspected SAH for the detection of ruptured intracranial aneurysms 2
• Digital subtraction angiography (DSA) remains the reference exam 2
• Multidetector row CT angiography may potentially replace DSA in the emergency setting 2
• MRI and MRA can be added to the diagnostic work-up in order to exclude other differential diagnoses such as venous thrombosis or angiitis 2
• Noncontrast computed tomography is highly sensitive in detecting subarachnoid blood, especially within 6 hours of hemorrhage 3
• Computed tomography angiography is slowly replacing digital subtraction angiography as the first-line technique for the diagnosis and treatment planning of cerebral aneurysms 3
• FLAIR and GE T2* MRI pulse sequences, and CT scans, are all statistically significant indicators of acute SAH 4
• GE T2*-weighted images are statistically significant indicators of subacute-to-chronic SAH, whereas other MRI pulse sequences, and CT scans, are not 4
• Noncontrast CT imaging exhibits inadequate sensitivity to serve as a sole diagnostic modality in detecting spontaneous subarachnoid hemorrhage caused by aneurysm or arteriovenous malformation 5
• Cerebral CT, combined with intracranial CT angiography, is recommended to make a positive diagnosis of SAH, to identify the cause and to investigate for an intracranial aneurysm 6

Comparison of Imaging Modalities

• CT is insensitive in the detection of subacute or chronic SAH 2
• FLAIR imaging is more sensitive than CT to demonstrate a subarachnoid hemorrhage and offers greater degrees of sensitivity for the diagnosis of restricted subarachnoid hemorrhage in cortical sulcus 6
• The sensitivity of noncontrast cranial CT in detecting all spontaneous subarachnoid hemorrhages is 93% (95% confidence interval [CI] 88% to 97%) 5

Clinical Practice

• Transcranial color-coded duplex sonography, CT, or MRI are used in clinical practice in order to detect aggravating factors of SAH like hydrocephalus or vasospasm 2
• A lumbar puncture should be performed if these investigations are normal while clinical suspicion is high 6