Most Common Cause of Subarachnoid Hemorrhage
Ruptured intracranial aneurysm is the most common cause of spontaneous subarachnoid hemorrhage, accounting for 85% of all cases. 1
Etiology and Epidemiology
- Aneurysmal rupture causes 85% of spontaneous SAH cases, making it by far the predominant etiology 1
- The remaining 15-20% of cases are angiogram-negative, with some representing perimesencephalic hemorrhage (generally benign) and others non-perimesencephalic patterns that may result from small terminal artery aneurysms with subsequent parent vessel occlusion 2
- SAH affects approximately 6 in 100,000 individuals annually in developed countries, with an estimated incidence of 9/100,000 inhabitants/year in Spain 1, 3
Risk Factors and Pathophysiology
In adults with hypertension, the risk profile is particularly important:
- Hypertension is a major modifiable risk factor for aneurysmal SAH, though the direct relationship remains somewhat uncertain 1, 3
- Smoking represents the other primary modifiable risk factor 1, 3
- Preexisting hypertension increases the risk of rebleeding after initial aneurysm rupture, with rebleeding rates of 7-26% before aneurysm repair 1
- The combination of hypertension and aneurysm rupture creates a particularly dangerous scenario, as elevated blood pressure increases transmural pressure across the aneurysm wall 1
Clinical Implications
- The high mortality and morbidity from aneurysmal SAH (30-50% mortality rate) makes early recognition and treatment critical 1
- Aneurysmal SAH typically presents with sudden-onset severe headache ("worst headache of my life" in 80% of patients), often with loss of consciousness, nausea/vomiting, and nuchal rigidity 4, 5
- Rebleeding risk is highest in the first 24 hours (3-4% rate) and remains elevated at 1-2% per day for the first 4 weeks if untreated 1, 4
Other Causes (Non-Aneurysmal)
While aneurysmal rupture dominates, other causes include:
- Perimesencephalic hemorrhage (venous origin, better prognosis) 2
- Arteriovenous malformations 6
- Trauma (excluded from spontaneous SAH definition) 7
- Small terminal artery aneurysms with parent vessel occlusion (may present as angiogram-negative non-perimesencephalic SAH) 2
Critical pitfall: In hypertensive patients presenting with SAH, do not assume the hypertension is merely reactive to the hemorrhage—chronic hypertension is both a risk factor for aneurysm formation and a predictor of rebleeding 1, 2. Blood pressure should be controlled to systolic <160 mmHg using titratable agents to prevent rebleeding while maintaining adequate cerebral perfusion 4, 5.