What is the risk of rupture of an actively growing right Middle Cerebral Artery (MCA) bifurcation aneurysm that has increased in size from 8 mm to 10 mm over the past 6 months?

Risk of Rupture for an Actively Growing 10 mm MCA Bifurcation Aneurysm

The risk of rupture for a 10 mm right MCA bifurcation aneurysm that has grown from 8 mm over 6 months is significantly elevated, with an annual rupture rate of approximately 1% based on size alone, but the demonstrated growth pattern increases this risk substantially. 1

Size-Related Risk Factors

• Aneurysms ≥10 mm in diameter have a significantly higher annual rupture rate (approximately 1%/year) compared to those <10 mm (0.05%/year) in patients without prior subarachnoid hemorrhage (SAH) 1
• The International Study of Unruptured Intracranial Aneurysms (ISUIA) found that aneurysm size was the strongest independent predictor of future rupture 1
• For MCA aneurysms specifically, the 7.5-year rupture risk for 10-24 mm aneurysms is approximately 3% 1

Growth as a Critical Risk Factor

• Aneurysm growth is strongly associated with increased rupture risk, with studies showing that 90% of ruptured aneurysms demonstrated enlargement before rupture 2
• The observed growth from 8 mm to 10 mm over just 6 months represents significant and rapid enlargement, which substantially increases rupture risk beyond what would be expected from size alone 2
• Aneurysms with larger absolute diameter growth have been shown to have higher rupture rates compared to those with minimal or no growth (3.89 ± 2.34 mm vs 1.79 ± 1.02 mm) 2

Location-Specific Considerations

• MCA bifurcation aneurysms have specific morphological and hemodynamic characteristics that influence rupture risk 3
• Studies of MCA bifurcation aneurysms have identified that certain parameters like the ratio of longest dimension to width (D/W) and energy loss (EL) are significant predictors of rupture 3
• Sharper bifurcation angles and wider inclination angles between the M1 and M2 arteries correlate with higher aneurysmal inflow rate coefficient (AIRC), which is a significant independent predictor of MCA aneurysm growth 4

Cumulative Risk Assessment

• The 1-, 2-, and 3-year cumulative growth rates for unruptured aneurysms have been calculated as 2.5%, 8%, and 17.6% respectively, with growth being a precursor to potential rupture 5
• The combination of the aneurysm reaching the 10 mm threshold and demonstrating active growth creates a compound risk profile that exceeds the risk of either factor alone 2, 5
• While the ISUIA study reported a 1% annual rupture rate for aneurysms ≥10 mm, this rate is likely higher for aneurysms demonstrating active growth 1, 2

Clinical Implications

• The American Heart Association guidelines recognize that aneurysm growth of ≥0.5 cm in 6 months may be a reasonable indication for intervention to reduce rupture risk 1
• The demonstrated growth of 2 mm over 6 months, combined with the current size of 10 mm, places this aneurysm in a higher risk category that warrants serious consideration for treatment 1, 2
• The natural history of actively growing aneurysms suggests that continued surveillance without intervention carries a significant risk of rupture 2, 5

In summary, this 10 mm MCA bifurcation aneurysm with documented growth of 2 mm over 6 months has crossed the critical size threshold of 10 mm and demonstrated significant growth, both independent risk factors for rupture. The combined effect of these factors suggests a substantially elevated rupture risk that exceeds the 1% annual risk typically associated with aneurysms of this size.