Imaging Study for Subarachnoid Hemorrhage

Initial Imaging Protocol

The American College of Radiology recommends a non-contrast CT scan of the head as the initial study for suspected subarachnoid hemorrhage, due to its superiority in detecting acute subarachnoid blood, with a sensitivity of 93-98% for subarachnoid hemorrhage in general 1, 2, 3
Non-contrast CT is the modality of choice for detecting acute subarachnoid hemorrhage because it is superior to MRI in detecting acute subarachnoid blood 1, 2, 3
The American College of Radiology considers non-contrast CT, Angio-CT with contrast, and Angio-MR without contrast as equivalent modalities for initial evaluation, meaning only one needs to be ordered initially 4, 5

Angio-CT with contrast is highly appropriate as a follow-up study to identify aneurysms or malformations after a positive non-contrast CT scan 4, 5
Cerebral angiography is the gold standard with sensitivity and specificity >98% for detecting aneurysms or malformations 4
Angio-MR without contrast is a valid alternative for vascular evaluation 1, 2

The American College of Radiology recommends against using contrast in the initial CT scan because the goal is to detect blood, not characterize vessels 3
Acute blood appears hyperdense (white) on non-contrast CT, making contrast unnecessary for its detection 1, 2
The recommended diagnostic sequence is: non-contrast CT → Angio-CT with contrast or cerebral angiography → identification of aneurysm in 70% of non-traumatic subarachnoid hemorrhages 1, 4
Angio-CT has advantages over conventional angiography, including being faster and less invasive, with a sensitivity >95% for aneurysms ≥3mm 4, 6, 7

The American Heart Association recommends that a lumbar puncture be performed when there is a high clinical suspicion of subarachnoid hemorrhage (SAH) with a negative or non-conclusive computed tomography (CT) scan, especially if the CT scan was performed after 6 hours of symptom onset 8, 9
The International Stroke Society suggests that a lumbar puncture should be done when the initial CT scan is negative but there is a high clinical suspicion of SAH, such as sudden severe headache 10, 11
The sensitivity of CT scans for diagnosing SAH decreases significantly after 6 hours of symptom onset, making lumbar puncture necessary 12, 13

If a CT scan is performed after 6 hours of symptom onset, the sensitivity decreases significantly, making lumbar puncture necessary 12, 13
In patients with acute headache suggestive of SAH, a third-generation or higher CT scan performed within 6 hours of headache onset and interpreted as normal by a neuroradiologist does not require lumbar puncture 10, 11

Patients with SAH should be treated in centers with neurosurgical experience that regularly manage aneurysms using endovascular and surgical techniques 10, 11

The diagnostic algorithm for SAH includes performing a non-contrast CT scan immediately upon suspicion of SAH, and if the CT scan shows SAH, lumbar puncture is not necessary 10, 11, 12, 13
If the CT scan is negative, consideration should be given to performing a lumbar puncture, especially if the CT scan was not performed within 6 hours of symptom onset or was not interpreted by a neuroradiologist 10, 11

The patient presents with sudden-onset severe "thunderclap" headache triggered by physical exertion, associated symptoms such as photophobia, nausea, and vomiting, and first occurrence of these symptoms, which are classic features of subarachnoid hemorrhage (SAH) 14
The Canadian Stroke Best Practice Recommendations explicitly state that when CT is performed after 6 hours, is not read by an experienced neuroradiologist, or clinical suspicion remains high despite negative CT, lumbar puncture should be performed 14

CT sensitivity for SAH decreases significantly after 6 hours from symptom onset, and even third-generation CT scans have limitations, especially in detecting small bleeds 14
Lumbar puncture has high diagnostic yield, eliminating the need for further neurosurgical investigation in almost 90% of cases when negative, although the exact statistic is not provided in the cited reference 14
MRI brain with SWI is less sensitive than CT for acute SAH (50-94% sensitivity vs 98% for CT) and is not the recommended second-line test after negative CT 15, 16
The American Heart Association recommends lumbar puncture when there is high clinical suspicion with negative or non-conclusive CT, although the strength of evidence for this recommendation is not provided in the cited reference 14

Xanthochromia (yellow discoloration of CSF from bilirubin breakdown) is the key diagnostic finding, not just bloodstained CSF, and spectrophotometric analysis has high sensitivity when performed 12 hours to 2 weeks after symptom onset 14
The strength of evidence for the use of spectrophotometric analysis is not explicitly stated, but it is recommended for maximum sensitivity 14
Timing is critical: Xanthochromia evaluation is most sensitive after a minimum delay of 12 hours from headache onset, though earlier testing may still be clinically appropriate 14

SAH has >40% mortality within 30 days if untreated, with high early rebleeding risk, and missing this diagnosis has catastrophic consequences for morbidity and mortality 14