A 70-year-old hypertensive man presents with sudden severe headache. Non-contrast CT brain shows hyperdensity in the basal cisterns and interpeduncular fossa. MRI confirms subarachnoid hemorrhage. Subsequent CT angiography shows no aneurysm. Repeat MRI 3 days later shows T1 hyperintensity only in the perimesencephalic cisterns. What is the most likely diagnosis?
- A Anterior communicating artery aneurysm rupture — aneurysm below CTA resolution
- B Perimesencephalic non-aneurysmal SAH — benign, likely venous origin ✓
- C Posterior communicating artery aneurysm — CTA false negative
- D Spinal dural arteriovenous fistula — remote hemorrhage
Explanation
Perimesencephalic non-aneurysmal subarachnoid hemorrhage (PMeSAH) is a benign subset of SAH where blood is centered in the perimesencephalic cisterns without anterior extension or intraventricular blood. It accounts for ~10% of SAH, has a characteristic distribution suggesting a venous or perforating artery source, consistently negative CTA and DSA, excellent prognosis, and no rebleed risk. The diagnosis requires: CT showing strictly perimesencephalic distribution, no aneurysm on quality CTA, and no other etiology. This contrasts with aneurysmal SAH which often shows anterior (sylvian, basal) distribution and carries high mortality.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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Written and medically reviewed by the StethoPrep medical team.