What is the most likely cause of non-traumatic diffuse subarachnoid hemorrhage (SAH) in a patient?

Non-Traumatic Diffuse Subarachnoid Hemorrhage: Etiology

Ruptured intracranial aneurysm is the most likely cause of non-traumatic diffuse SAH, accounting for 85% of cases, with the remaining 15% attributed to non-aneurysmal causes including perimesencephalic hemorrhage (10%) and rare vascular malformations (5%). 1

Primary Causes by Frequency

Aneurysmal SAH (85% of cases)

• Ruptured cerebral aneurysm is responsible for 70-85% of all non-traumatic SAH and requires immediate vascular imaging to identify the source 1
• The most common aneurysm locations include:
  • Anterior communicating artery (35.5%) 2
  • Posterior communicating artery (17.1%) 2
  • Middle cerebral artery (15.7%) 2
  • Internal carotid artery (11.8%) 2
  • Basilar artery (2.6%) 2

Non-Aneurysmal SAH (15% of cases)

Perimesencephalic SAH (10% of cases):

• Blood restricted to perimesencephalic cisterns with negative angiography 3
• Excellent prognosis with minimal complications 4
• Likely caused by small venous or capillary ruptures not detectable on conventional angiography 3

Other rare causes (5%):

• Arteriovenous malformations (4.2%) 2
• Moyamoya disease (0.8%) 2
• Ruptured small terminal artery aneurysms with subsequent parent vessel occlusion (presents with characteristic localized infarcts) 5
• Cerebral amyloid angiopathy 1
• Reversible cerebral vasoconstriction syndrome 1

Distinguishing Diffuse vs. Perimesencephalic Patterns

The distribution pattern on CT is critical for determining etiology:

• Diffuse SAH involving basal cisterns and Sylvian fissures strongly suggests aneurysmal rupture and requires urgent vascular imaging 1, 2
• Sylvian fissure SAH is most frequently associated with aneurysmal causes 2
• Tentorial cerebelli SAH is most commonly associated with non-aneurysmal causes 2
• Perimesencephalic-only distribution suggests benign non-aneurysmal hemorrhage 3, 4

Risk Factors for Aneurysmal SAH

Key risk factors that increase likelihood of aneurysmal cause:

• Hypertension (strongest predictor in multivariate analysis) 2
• Cigarette smoking 6
• Heavy alcohol use 6
• Personal or family history of aneurysm or hemorrhagic stroke 6

Diagnostic Approach

Immediate vascular imaging is mandatory for diffuse SAH:

• CT angiography (CTA) should follow positive non-contrast CT showing diffuse SAH 1, 7
• CTA has sensitivity >95% for aneurysms ≥3mm but may miss smaller aneurysms 7
• Catheter cerebral angiography remains the gold standard with sensitivity and specificity >98%, including for aneurysms <3mm 1, 7
• If initial angiography is negative with aneurysmal pattern SAH, repeat DSA in days to weeks (yield 10%) 1
• 3D rotational angiography identifies aneurysms in 25% of patients with previously negative 2D angiography 1

Critical Clinical Pitfall

Angiogram-negative non-perimesencephalic SAH requires careful evaluation:

• Unlike benign perimesencephalic SAH, diffuse angiogram-negative SAH has worse prognosis similar to aneurysmal SAH 5
• Look for small brain infarctions suggesting ruptured terminal artery aneurysm with subsequent vessel occlusion 5
• These patients require intensive management similar to aneurysmal SAH, including CSF diversion and vasospasm monitoring 5