What is the best initial study to rule out subarachnoid hemorrhage (SAH)?

Best Initial Study to Rule Out Subarachnoid Hemorrhage

Non-contrast CT of the head is the best initial study to rule out subarachnoid hemorrhage, and when performed within 6 hours of symptom onset using modern high-quality scanners (16-slice or better), a negative result is sufficient to exclude SAH without requiring lumbar puncture. 1

Timing-Based Diagnostic Algorithm

Within 6 Hours of Symptom Onset

• Non-contrast head CT alone is sufficient when performed on high-quality scanners and interpreted by experienced radiologists 1
• Sensitivity approaches 98.7-100% in this time window, missing fewer than 1.5 in 1000 cases 1, 2
• Modern 16-slice or better CT scanners have demonstrated sensitivity of 97.8% (95% CI 93.1-99.4%) 2
• No lumbar puncture is needed after negative CT in this early window 1, 3

Beyond 6 Hours from Symptom Onset

• Lumbar puncture for xanthochromia evaluation becomes necessary if CT is negative or non-diagnostic 1
• LP should be performed at least 6-12 hours after symptom onset to allow xanthochromia development 1
• Spectrophotometric analysis for xanthochromia has 100% sensitivity and 95.2% specificity when combined with negative CT 1
• The combination of negative CT plus negative LP has demonstrated 100% sensitivity for ruling out SAH 4

Critical Caveats and Pitfalls

Scanner Quality Matters

• These high sensitivities apply only to high-quality CT scanners (16-slice or better) with images interpreted by fellowship-trained, board-certified neuroradiologists 1, 2
• Equipment must meet American College of Radiology specifications 1
• Older generation scanners may miss up to 5% of cases 5

Atypical Presentations Require Different Approach

• The 6-hour rule does not apply to patients presenting with atypical features 1:
  • Primary neck pain without headache
  • Syncope as presenting symptom
  • New seizure
  • New focal neurological deficits
• These patients require full workup (CT plus LP if negative) regardless of timing 1

When to Skip Imaging Entirely

• The Ottawa SAH Rule identifies low-risk patients who may not need any imaging 1
• Patients must be alert, >15 years old, with new severe non-traumatic headache reaching maximum intensity within 1 hour 1
• If they meet none of these criteria, imaging may be unnecessary: age ≥40 years, neck pain/stiffness, witnessed loss of consciousness, onset during exertion, thunderclap headache, or limited neck flexion 1
• However, this rule has very low specificity (7.6-15.3%), so most patients still require imaging 1

Role of Advanced Imaging

CTA After Positive CT

• Once SAH is confirmed on non-contrast CT, CTA is the preferred next step to identify the bleeding source 1
• CTA has 96.5% sensitivity and 88% specificity for aneurysms overall 1
• Sensitivity increases to 98.4% with 100% specificity for aneurysms >3 mm 1

CTA Limitations Requiring DSA

• Small aneurysms <3 mm may be missed (sensitivity drops to 61%) 1, 6
• Overlying bone at skull base can obscure visualization 6
• Vessel tortuosity can decrease specificity 7
• DSA with 3D rotational angiography remains the gold standard and should be performed when CTA is negative but SAH pattern suggests aneurysmal source 6, 7

MRI Has Limited Role

• MRI is not an initial imaging study for suspected SAH 1
• Susceptibility-weighted imaging (SWI) can detect subarachnoid blood but is inferior to CT for acute diagnosis 1
• MRI with FLAIR and gradient echo sequences may help when CT is negative but clinical suspicion remains high 6