Overview
This lecture reviews the natural history studies of unruptured intracranial aneurysms and how these data inform decisions on whether or not to treat aneurysms based on rupture risk factors.
Natural History Studies of Aneurysms
- The natural history of an aneurysm is key in deciding if treatment is necessary.
- The ISUIA (International Study of Unruptured Intracranial Aneurysms) provided prospective data but had selection bias as higher-risk aneurysms were often treated and excluded from analysis.
- Earlier retrospective ISUIA data underestimated rupture risk; its low numbers are not accepted today.
- The prospective ISUIA study followed 1,692 untreated patients for a mean of 4.1 years, stratified by prior subarachnoid hemorrhage.
- ISUIA reported a 0% 5-year rupture rate for anterior circulation aneurysms <7mm (but actual risk is likely higher due to cohort bias).
- Posterior circulation aneurysms <7mm had a 2.5% 5-year rupture risk (0.5% per year).
- Large posterior circulation or posterior communicating artery (PCom) aneurysms (>25mm) had up to 50% 5-year rupture risk.
- Basilar apex and T-con locations were identified as higher risk for rupture.
The UCAS Japan Study
- The UCAS study in Japan followed over 5,000 patients with aneurysms ≥3mm.
- Irregular shape, daughter sac, size, ACom (anterior communicating artery), and PCom locations were significant risk factors for rupture.
- Annual rupture risk was about 1% overall, but smaller ICA aneurysms had much lower risk.
- Highest risk: small (3-4mm) ACom aneurysms (~1% per year), PCom, and distal ACA (pericallosal) aneurysms—these often rupture before reaching large size.
- Factors like previous SAH, smoking, family history, and multiple aneurysms were not significant in this particular study.
Location and Rupture Risk
- Superior hypophyseal and ophthalmic artery aneurysms have low rupture risk.
- PCom, ACom, and pericallosal aneurysms have higher rupture risk, even when small.
- Basilar tip aneurysms also show a steep increase in rupture risk as size increases.
Clinical Decision Factors: To Treat or Not to Treat
- All ruptured and symptomatic aneurysms (causing nerve palsy or vision loss) should be treated.
- Patient age influences treatment; young patients may benefit more from intervention.
- Size threshold for intervention is typically >7mm; <5mm are generally observed unless other risk factors are present.
- Location is crucial; high-risk sites include ACom, PCom, and pericallosal even if small or irregular.
- Low-risk locations (e.g., superior hypophyseal) are often overtreated.
Key Terms & Definitions
- Aneurysm — An abnormal bulge in the wall of a blood vessel, often in the brain.
- Subarachnoid Hemorrhage (SAH) — Bleeding into the space around the brain, often due to ruptured aneurysm.
- ISUIA — Major study on the natural history of unruptured intracranial aneurysms.
- UCAS — Japanese cohort study on unruptured cerebral aneurysms.
- PCom — Posterior Communicating Artery.
- ACom — Anterior Communicating Artery.
- ICA — Internal Carotid Artery.
Action Items / Next Steps
- Review ISUIA and UCAS study tables for rupture risk by size and location.
- Memorize high-risk aneurysm features for exams (small ACom, PCom, pericallosal, irregular shape).
- Prepare for case discussions on aneurysm treatment indications.