What is the risk of rupture and re-bleed for posterior circulation aneurysms, and how are they graded and treated?

Brain Aneurysm Types, Posterior Circulation Rupture Risk, Grading, and Rebleeding

Why Posterior Circulation Aneurysms Rupture More Often

Posterior circulation aneurysms carry substantially higher rupture risk than anterior circulation aneurysms, with small posterior circulation aneurysms (<7 mm) demonstrating approximately 2.5% annual rupture risk compared to near 0% for similar-sized anterior circulation aneurysms without prior subarachnoid hemorrhage history. 1, 2

Key Anatomic and Hemodynamic Factors

• Location-specific vulnerability: Basilar apex and vertebrobasilar junction aneurysms demonstrate the highest rupture risk, with relative risk 13.8 times higher than anterior circulation aneurysms 1
• Perforating artery involvement: Posterior circulation aneurysms at the basilar apex are intimately associated with midbrain perforating arteries, making them both more prone to rupture and more dangerous when they do 1
• Hemodynamic stress: The posterior circulation experiences different flow dynamics and wall stress patterns that predispose to rupture 1

Aneurysm Classification by Size

Standard Size Categories

• Small: <7 mm (extremely low rupture risk for anterior circulation without prior SAH) 1, 2
• Medium: 7-10 mm (intermediate risk requiring individualized assessment) 1
• Large: 10-24 mm (approximately 1% annual rupture risk) 1
• Giant: ≥25 mm (highest surgical risk with 20-50% combined mortality/morbidity for posterior circulation) 1

Location-Based Classification

• Anterior circulation: Internal carotid, anterior cerebral, middle cerebral arteries
• Posterior circulation: Vertebral, basilar, posterior cerebral, posterior inferior cerebellar arteries 1
• Posterior communicating artery: Grouped with posterior circulation for rupture risk stratification despite anatomic proximity to anterior circulation 1

Grading Systems for Ruptured Aneurysms

Clinical Severity Assessment

• Hunt and Hess Grade: Standard clinical grading scale used to determine severity and predict outcome after subarachnoid hemorrhage 1
• Fisher Grade: Radiographic scale assessing subarachnoid blood burden, predicting vasospasm risk 3
• MGH Grade: Composite system incorporating age, Hunt and Hess grade, aneurysm size, and Fisher grade to predict surgical risk 3

Outcome Prediction

• Clinical scales are recommended by the 2023 AHA/ASA guidelines to determine severity and predict outcome 1
• MGH grading demonstrates strong correlation with outcomes: 96% excellent/good outcomes for Grade 0, declining to 0% for Grade 4 with surgical treatment 3

Risk of Rebleeding

Acute Rebleeding Risk (Unsecured Aneurysm)

The highest risk period for rebleeding is immediately after initial rupture, making urgent aneurysm securing the priority intervention. 1

• Blood pressure control is recommended for unsecured aneurysms, avoiding severe hypotension, hypertension, and blood pressure variability 1
• Routine antifibrinolytic therapy is not recommended by 2023 guidelines 1
• Emergency anticoagulation reversal should be performed if applicable 1

Long-Term Rebleeding After Treatment

After Coil Embolization

• Overall rerupture rate: 0.9% per year for ruptured aneurysms in all locations after coiling 1
• Posterior circulation specific: 1.4% annual rerupture rate, primarily from distal basilar artery aneurysms 1
• Large aneurysms (>2 cm): 2.7% annual rerupture rate after coiling 1
• Incomplete occlusion: Significantly higher rebleeding risk, with 49% showing aneurysm growth in one series 1

After Surgical Clipping

• Limited data available, but rerupture rates appear lower than coiling with complete occlusion 1
• Surgical clipping achieves 95.6% complete occlusion versus 32.3% complete occlusion with endovascular treatment in posterior circulation 3
• No reruptures documented in surgically clipped distal basilar aneurysms during 32.5 patient-years follow-up 1

Treatment Recommendations for Posterior Circulation Aneurysms

Primary Treatment Modality

Coiling is preferred over clipping for posterior circulation aneurysms when amenable, based on 2023 AHA/ASA guidelines prioritizing improved functional outcomes. 1, 4

Treatment Decision Algorithm

1. Ruptured aneurysms: Urgent securing required regardless of size 1

   • Evaluate by both endovascular and neurosurgical specialists 1
   • Prefer coiling for posterior circulation when technically feasible 1, 4
   • Consider clipping for middle cerebral artery aneurysms 2

2. Unruptured posterior circulation aneurysms:

   • <7 mm without prior SAH: Consider treatment given 2.5% annual rupture risk (versus observation for anterior circulation) 1, 2
   • ≥7 mm: Strong consideration for treatment 1, 2
   • Basilar apex location: Favor treatment regardless of size due to elevated rupture risk 1, 2

3. Incomplete obliteration: Partial treatment securing rupture site during acute phase is reasonable, with retreatment in 1-3 months 4

Critical Pitfalls to Avoid

• Don't assume size thresholds apply equally: The 10 mm threshold for anterior circulation does not apply to posterior circulation—smaller aneurysms warrant treatment 2
• Don't treat at low-volume centers: Treatment should only occur at high-volume centers (>100 UIA consultations annually) by operators performing >30 aneurysm procedures yearly 2
• Don't ignore incomplete occlusion: Aneurysms with residual filling require close angiographic follow-up and consideration for retreatment 1, 4
• Don't use phenytoin for seizure prophylaxis: Associated with excess morbidity and mortality 1

Post-Treatment Management

• Hydrocephalus: Urgent CSF diversion for acute symptomatic hydrocephalus; permanent diversion for chronic symptomatic hydrocephalus 1, 4
• Nimodipine: Early enteral administration recommended for vasospasm prevention 1
• VTE prophylaxis: Initiate once aneurysm is secured 1
• Avoid hypervolemia: Induction of hypervolemia is potentially harmful 1