In a middle‑aged woman with no significant past medical history who presents with a sudden severe headache and CT evidence of subarachnoid hemorrhage, what is the most likely cause of the spontaneous SAH?

Most Likely Cause of Spontaneous Subarachnoid Hemorrhage

A ruptured intracranial aneurysm is the most likely cause of this patient's spontaneous subarachnoid hemorrhage, accounting for 85% of all spontaneous SAH cases. 1, 2

Epidemiology and Etiology

The clinical presentation—sudden severe "worst headache ever," neck stiffness, photophobia, and CT-confirmed SAH—strongly points to aneurysmal rupture as the underlying cause. The evidence hierarchy for spontaneous SAH etiology is clear:

• Ruptured intracranial aneurysm: 85% of cases 1, 2
• Non-aneurysmal causes: 15% of cases, including:
  • Perimesencephalic hemorrhage (10% of all SAH) 2
  • Rare vascular malformations 2
  • Cerebral amyloid angiopathy 2
  • Reversible cerebral vasoconstriction syndrome 2
  • Cerebral venous sinus thrombosis (very rare) 3
  • Spinal arteriovenous fistulas (extremely rare) 4

Why Aneurysmal Rupture is Most Likely in This Case

The diffuse pattern of SAH involving basal cisterns and Sylvian fissures (typical for this presentation) strongly suggests aneurysmal rupture rather than perimesencephalic or other non-aneurysmal patterns. 2 The patient's middle age, female sex (unexplained female predominance exists), and hypertension (BP 165/95) align with typical aneurysmal SAH demographics. 1

Diagnostic Confirmation via Cerebral Angiogram

The planned cerebral angiogram is the appropriate next step:

• Digital subtraction angiography (DSA) remains the gold standard with sensitivity and specificity >98%, including for aneurysms <3mm 2
• CT angiography has >95% sensitivity for aneurysms ≥3mm but may miss smaller lesions 2
• If initial angiography is negative with an aneurysmal pattern of SAH, repeat DSA in days to weeks yields an additional 10% detection rate 1, 2
• 3D rotational angiography identifies aneurysms in 25% of patients with previously negative 2D angiography 2

Common Pitfalls to Avoid

Do not assume a benign cause if the initial angiogram is negative. Angiogram-negative non-perimesencephalic SAH (which this diffuse pattern represents) carries significantly worse outcomes than perimesencephalic SAH, with an odds ratio of 6.23 for poor outcome compared to perimesencephalic patterns. 5 These cases may represent:

• Small terminal artery aneurysms with subsequent parent vessel occlusion (look for small infarcts on imaging) 6
• Aneurysms <3mm that escaped initial detection 1
• Rare causes like spinal vascular lesions (if SAH pattern doesn't match intracranial findings, consider spinal MRI) 4

The presence of a low-grade fever (100°F) and tachycardia (HR 102) are consistent with the inflammatory response to blood in the subarachnoid space, not necessarily infection, though meningitis can complicate SAH. 1