How to differentiate between traumatic and non-traumatic subarachnoid hemorrhage (SAH) on a non-contrast computed tomography (CT) head scan?

Differentiating Traumatic vs Non-Traumatic SAH on Non-Contrast CT Head

The distinction between traumatic and non-traumatic SAH on non-contrast CT relies primarily on the distribution pattern of blood, clinical history, and presence of associated injuries—with non-traumatic SAH typically showing blood concentrated in the basal cisterns and sylvian fissures, while traumatic SAH demonstrates blood over the cerebral convexities with associated skull fractures or parenchymal contusions. 1

Key CT Distribution Patterns

Non-Traumatic (Aneurysmal) SAH

• Blood distribution is characteristically concentrated in the basal cisterns, sylvian fissures, and interhemispheric fissure 1
• Diffuse basal cistern and sylvian fissure SAH patterns strongly suggest an underlying aneurysm and warrant vascular imaging regardless of CTA results 1
• The hemorrhage pattern often appears symmetric and follows the expected distribution of ruptured aneurysms based on location 1
• Absence of traumatic parenchymal injuries (contusions, diffuse axonal injury) supports non-traumatic etiology 2

Traumatic SAH

• Blood is typically located over the cerebral convexities in a superficial cortical/sulcal distribution 3
• Associated findings include skull fractures, scalp hematomas, parenchymal contusions, subdural hematomas, or epidural hematomas 4
• The hemorrhage pattern is often asymmetric and corresponds to sites of direct impact 3
• Small-volume focal cortical SAH is more consistent with trauma than diffuse basal SAH 1

Clinical Context Integration

Clinical history is essential and must be actively sought, as imaging alone may not definitively distinguish etiologies in all cases:

• Non-traumatic presentation: Sudden-onset "thunderclap" headache reaching maximal intensity within 1 hour, onset during exertion, witnessed loss of consciousness, neck pain/stiffness, or photophobia 1
• Traumatic presentation: Clear history of head trauma, mechanism of injury, loss of consciousness at time of impact 4
• Warning signs for non-traumatic SAH: Sentinel headache in preceding 2-8 weeks, age ≥40 years, no history of trauma 1

Important Diagnostic Pitfalls

When CT Findings Are Ambiguous

• Small amounts of sulcal SAH can occur in both traumatic and non-traumatic settings, making differentiation challenging 2
• In cases of sulcal-only SAH without clear trauma history, consider underlying etiologies: cerebral amyloid angiopathy (CAA) in older patients, reversible cerebral vasoconstriction syndrome (RCVS) in younger patients, posterior reversible encephalopathy syndrome (PRES), or cortical venous thrombosis 2
• These conditions collectively represent >50% of non-aneurysmal, non-traumatic sulcal SAH cases 2

Timing Considerations

• CT sensitivity for SAH is 98.7% when performed within 6 hours of symptom onset, but decreases significantly after this window 1
• If CT is performed >6 hours after onset or clinical suspicion remains high despite negative CT, lumbar puncture for xanthochromia evaluation should be performed 1, 5
• Xanthochromia analysis by spectrophotometry has 100% sensitivity and 95.2% specificity when performed >6-12 hours after symptom onset 1, 5

Algorithmic Approach

1. Assess blood distribution pattern on CT:

   • Basal cisterns/sylvian fissures → suspect non-traumatic (aneurysmal) SAH 1
   • Convexity/superficial cortical → suspect traumatic SAH 3

2. Evaluate for associated traumatic injuries:

   • Skull fractures, contusions, subdural/epidural hematomas → traumatic 4, 3
   • Absence of traumatic findings → non-traumatic 2

3. Correlate with clinical presentation:

   • Thunderclap headache, no trauma history → non-traumatic 1
   • Clear trauma mechanism → traumatic 4

4. Proceed with appropriate vascular imaging:

   • For diffuse basal/sylvian SAH: CTA or catheter angiography is mandatory to identify aneurysm, as 70% of non-traumatic SAH cases have an identifiable aneurysm 5
   • CTA has 96.5% sensitivity and 88% specificity for aneurysms of all sizes, with 98.4% sensitivity for aneurysms >3mm 1
   • Digital subtraction angiography remains the gold standard with >98% sensitivity and specificity 5

Critical Caveat

In equivocal cases where imaging pattern is atypical or clinical history is unclear, err on the side of pursuing vascular imaging to exclude aneurysm, as missing aneurysmal SAH carries >40% mortality within 30 days and high early rebleeding risk 5. The consequences of missed non-traumatic SAH are catastrophic, justifying a lower threshold for additional workup when uncertainty exists 1.