What is the risk of bleeding from a 9mm pericallosal Anterior Cerebral Artery (ACA) aneurysm?

Risk of Bleeding from a 9mm Pericallosal ACA Aneurysm

A 9mm pericallosal anterior cerebral artery (ACA) aneurysm carries a moderate risk of bleeding, with an estimated annual rupture rate of approximately 1-2%. This risk is significant enough to warrant consideration for treatment in most patients, particularly those who are younger or have other risk factors for rupture.

Risk Assessment Based on Size and Location

The risk of rupture for intracranial aneurysms is primarily determined by:

1. Size of the aneurysm

   • The 9mm size places this aneurysm in a moderate-to-high risk category
   • According to AHA guidelines, aneurysms ≥10mm have a clearly established higher rupture risk, but those approaching this size (like 9mm) also warrant strong consideration for treatment 1
   • Studies have shown that aneurysms in the 7-10mm range have demonstrated significant bleeding risk 1

2. Location of the aneurysm

   • Pericallosal artery aneurysms, despite being relatively rare (2-9% of all intracranial aneurysms), have a higher propensity to rupture compared to other anterior circulation aneurysms 2
   • These aneurysms are typically small but have a higher tendency to bleed compared with other supratentorial aneurysms 3

Specific Rupture Risk Data

• In the International Study of Unruptured Intracranial Aneurysms (ISUIA), aneurysms <10mm in the anterior circulation had a rupture rate of approximately 0.05% per year 1
• However, this general statistic may underestimate the risk for pericallosal aneurysms specifically
• Studies focusing on distal anterior cerebral artery aneurysms suggest they may have higher rupture rates than similarly sized aneurysms in other locations 4
• In a study by Yasui et al, 22 of the ruptured aneurysms were <9mm in diameter at initial diagnosis, indicating that even smaller pericallosal aneurysms can rupture 1

Risk Factors That May Increase Bleeding Risk

Several factors may increase the risk of rupture for a pericallosal ACA aneurysm:

• Morphological features: Irregular shape, daughter sacs, or multilobulated appearance 1, 5
• Hypertension: History of poorly controlled blood pressure 1
• Prior subarachnoid hemorrhage from another aneurysm significantly increases risk 1
• Family history of aneurysmal subarachnoid hemorrhage 1
• Growth on serial imaging: Aneurysms that demonstrate growth over time have higher rupture risk 1

Clinical Implications

Based on the AHA guidelines, a 9mm pericallosal aneurysm would fall into a category that warrants strong consideration for treatment, particularly because:

1. It approaches the 10mm threshold that clearly indicates treatment 1
2. It is in a location known to have higher rupture rates 2, 4
3. The consequences of rupture are severe, with approximately 50% of pericallosal aneurysm ruptures resulting in intracerebral hematoma 2

Treatment Considerations

For a 9mm pericallosal aneurysm:

• Microsurgical clipping is often preferred over endovascular coiling due to the distal location and morphologic features typical of these aneurysms 4
• The decision for treatment should consider patient age, with stronger indications for treatment in younger patients who have longer life expectancy and therefore cumulative rupture risk 1
• If observation is chosen, regular follow-up imaging is essential to monitor for growth or morphological changes 1

Important Caveats

• The natural history of individual aneurysms can be unpredictable
• While size is the strongest predictor of rupture risk, even small aneurysms can rupture
• The risk-benefit analysis for treatment must consider both the risk of rupture and the risks associated with intervention
• Surgical treatment of pericallosal aneurysms can be technically challenging but has good outcomes in experienced centers 3