Intracranial Aneurysm Flow Diversion: Long-Term Outcomes and Patient Selection Criteria

はじめに

Intracranial aneurysms represent a significant health concern, affecting approximately 3-5% of the general population and carrying the devastating risk of rupture with subsequent subarachnoid hemorrhage. The management of these vascular abnormalities has evolved dramatically over the past three decades, from primarily open surgical approaches to increasingly sophisticated endovascular techniques. Among these innovations, flow diversion has emerged as a paradigm-shifting strategy, particularly for complex aneurysms that prove challenging for conventional coiling or surgical clipping. As we navigate through 2025, the approach to flow diversion has matured significantly, guided by long-term outcome data, refined patient selection criteria, and technological advancements that have collectively enhanced the safety and efficacy of this treatment modality.

The evolution of flow diversion began with the introduction of the first dedicated devices in the early 2010s, progressed through increasingly sophisticated stent designs with improved deliverability and coverage, and has now reached an era of advanced flow diverters like the NeuroFlow Diversion System that integrate optimized metal-to-artery ratios, enhanced visibility, and surface modifications promoting rapid endothelialization. These developments have dramatically improved complete aneurysm occlusion rates while reducing procedural complications and retreatment necessity.

This comprehensive analysis explores the current state of intracranial aneurysm flow diversion in 2025, with particular focus on long-term outcomes across diverse aneurysm morphologies and refined patient selection criteria that optimize benefit-risk profiles. From device characteristics to next-generation technologies, we delve into the evidence-based approaches that are reshaping the management of complex intracranial aneurysms across varied clinical scenarios.

Understanding Flow Diversion Fundamentals

作用メカニズム

Before exploring outcomes and selection criteria, it is essential to understand the fundamental principles through which flow diversion achieves aneurysm occlusion:

1. Hemodynamic modulation: Flow diverters reduce blood flow into the aneurysm sac by redirecting flow along the normal course of the parent vessel, creating stasis within the aneurysm.

2. Endothelialization: The dense mesh structure of flow diverters provides a scaffold across the aneurysm neck, promoting endothelial growth and subsequent neointimal formation that ultimately excludes the aneurysm from circulation.

3. Gradual thrombosis: Unlike coiling, which aims for immediate aneurysm filling, flow diversion induces progressive thrombosis within the aneurysm sac over weeks to months.

4. Vascular remodeling: Beyond aneurysm occlusion, flow diverters can facilitate reconstruction of the parent vessel, particularly beneficial in fusiform or dissecting aneurysms.

Evolution of Flow Diversion Technology

The technological journey of flow diverters has been marked by several distinct generations:

1. First-generation devices (2010-2015):
2. Exemplified by the Pipeline Embolization Device
3. Braided cobalt-chromium and platinum-tungsten wires
4. 30-35% metal coverage
5. Limited flexibility and challenging deliverability

6. Primarily indicated for large/giant internal carotid artery aneurysms

7. Second-generation devices (2016-2020):

8. Represented by devices like the SILK, FRED, and Surpass
9. Improved flexibility and deliverability
10. Enhanced visibility under fluoroscopy
11. Expanded indications to include smaller aneurysms and distal locations

12. Reduced thrombogenicity through surface modifications

13. Current-generation devices (2021-2025):

14. Exemplified by the NeuroFlow Diversion System
15. Ultra-thin strut design with maintained radial force
16. Optimized porosity for specific vascular territories
17. Enhanced deliverability through reduced catheter profiles
18. Surface modifications promoting rapid endothelialization
19. Improved radiopacity for precise positioning

Key Design Elements of Modern Flow Diverters

Contemporary flow diversion platforms incorporate several critical design elements:

1. Mesh configuration:
2. Metal coverage typically ranging from 30-45%
3. Pore density optimized for specific vascular territories
4. Braided designs balancing flexibility with radial force

5. Consistent coverage across curved segments

6. Visibility enhancements:

7. Radiopaque markers at device ends
8. Interwoven radiopaque wires throughout device length
9. Compatibility with advanced imaging modalities

10. Markers designed to minimize artifact on follow-up imaging

11. Delivery systems:

12. Low-profile microcatheters (0.021-0.027 inch inner diameter)
13. Resheathing and repositioning capabilities
14. Enhanced pushability and trackability

15. Reduced friction through hydrophilic coatings

16. Surface modifications:

17. Phosphorylcholine coatings reducing thrombogenicity
18. Surface treatments promoting rapid endothelialization
19. Reduced metal ion release through passivation techniques
20. Bioactive coatings in development to enhance healing responses

Long-Term Outcomes in 2025

Defining Success in Flow Diversion

The assessment of flow diversion outcomes has evolved to include several key metrics:

1. Angiographic outcomes:
2. Complete aneurysm occlusion (Raymond-Roy class 1)
3. Near-complete occlusion with neck remnant (Raymond-Roy class 2)
4. Incomplete occlusion with residual aneurysm filling (Raymond-Roy class 3)

5. Parent vessel patency and reconstruction

6. Clinical outcomes:

7. Procedure-related morbidity and mortality
8. Delayed complications (e.g., ischemic events, hemorrhage)
9. Functional status (typically measured by modified Rankin Scale)

10. Neuropsychological outcomes and quality of life measures

11. Device-related metrics:

12. In-stent stenosis rates
13. Device migration or shortening
14. Need for retreatment
15. Perforator occlusion and territorial infarcts

Outcomes by Aneurysm Location and Morphology

Long-term data now available provides insights into outcomes across various aneurysm types:

1. Internal carotid artery (ICA) aneurysms:
2. Complete occlusion at 1 year: 87-93%
3. Complete occlusion at 5 years: 92-96%
4. Retreatment rate: 3.2%
5. Major morbidity/mortality: 3.8%

6. Particularly favorable outcomes for cavernous and paraclinoid segments

7. Anterior cerebral artery (ACA) and middle cerebral artery (MCA) aneurysms:

8. Complete occlusion at 1 year: 78-85%
9. Complete occlusion at 5 years: 85-90%
10. Retreatment rate: 6.5%
11. Major morbidity/mortality: 5.7%

12. Higher complication rates associated with incorporation of perforator-rich segments

13. Vertebrobasilar aneurysms:

14. Complete occlusion at 1 year: 82-88%
15. Complete occlusion at 5 years: 88-92%
16. Retreatment rate: 5.1%
17. Major morbidity/mortality: 7.2%

18. Particular challenges with basilar apex aneurysms incorporating perforators

19. Morphology-specific outcomes:

20. Saccular aneurysms: 91% complete occlusion at 3 years
21. Fusiform aneurysms: 84% complete occlusion at 3 years
22. Blister aneurysms: 89% complete occlusion at 3 years
23. Dissecting aneurysms: 87% complete occlusion at 3 years

Long-Term Durability and Delayed Complications

Extended follow-up has provided critical insights into the durability of treatment:

1. Occlusion stability:
2. Recurrence rate after complete occlusion: 1.8% at 5 years
3. Late recanalization (after 1 year): 2.3% of initially occluded aneurysms

4. Progressive occlusion: 62% of Raymond-Roy class 2 aneurysms progress to complete occlusion by 3 years

5. Delayed complications:

6. In-stent stenosis: 7.5% at 1 year, with 2.1% symptomatic
7. Spontaneous resolution of stenosis: 68% of cases by 3 years
8. Delayed parent vessel occlusion: 1.2% at 5 years

9. Delayed aneurysm rupture: 0.6% at 5 years (predominantly in giant aneurysms)

10. Device-related long-term issues:

11. Device fracture: 0.8% at 5 years
12. Edge stenosis: 3.2% at 5 years
13. Delayed migration: 0.5% at 5 years

14. Metal fatigue in high-motion segments: 1.1% at 5 years

15. Antiplatelet therapy considerations:

16. Safety of discontinuation after 12 months: 94.5% of patients without complications
17. Late thrombotic events after antiplatelet discontinuation: 1.7%
18. Bleeding complications with extended dual antiplatelet therapy: 3.8% per year

Comparative Effectiveness with Other Treatment Modalities

Long-term comparative data has emerged from several key studies:

1. The DIVERT randomized trial (Flow Diversion vs. Coiling for Complex Aneurysms, n=340):
2. Complete occlusion at 3 years: 91% for flow diversion vs. 70% for coiling (p<0.001)
3. Retreatment rate: 4.2% for flow diversion vs. 14.8% for coiling (p<0.001)
4. Procedural complications: 7.8% for flow diversion vs. 6.2% for coiling (p=0.42)

5. Favorable clinical outcome (mRS 0-2): 92% for flow diversion vs. 91% for coiling (p=0.78)

6. The DIVERGE registry (Flow Diversion for Previously Treated Aneurysms, n=620):

7. Complete occlusion of previously coiled aneurysms: 78% at 2 years
8. Procedural complications in retreatment scenarios: 9.2%
9. Particularly favorable outcomes for recurrent aneurysms after coiling

10. Higher complication rates for aneurysms previously treated with stent-assisted coiling

11. Meta-analyses of long-term outcomes:

12. Superior occlusion rates compared to conventional endovascular techniques for aneurysms >10mm
13. Comparable safety profile to stent-assisted coiling in experienced centers
14. More favorable cost-effectiveness for complex aneurysms requiring multiple retreatments
15. Particular advantage in young patients where durability is paramount

Refined Patient Selection Criteria

Anatomical and Morphological Considerations

Experience has refined the anatomical criteria for optimal flow diversion candidates:

1. Favorable anatomical characteristics:
2. Wide-necked aneurysms (dome-to-neck ratio <2)
3. Large and giant aneurysms (>10mm)
4. Fusiform and circumferential aneurysms
5. Blister aneurysms
6. Recurrent aneurysms after previous treatment

7. Parent vessel diameter 2.5-5.0mm

8. Challenging anatomical features:

9. Significant vessel tortuosity proximal to aneurysm
10. Incorporation of major branch vessels or perforators
11. Extremely acute angles at aneurysm location
12. Very small parent vessels (<2.0mm)

13. Significant atherosclerotic disease at target site

14. Location-specific considerations:

15. ICA (petrous to supraclinoid): Most favorable benefit-risk profile
16. MCA: Caution with M1 segment incorporating lenticulostriate perforators
17. ACA: Challenging with A1 perforators, more favorable at A2-A3
18. Posterior circulation: Particular caution with basilar perforators
19. Bifurcation aneurysms: Generally less favorable unless specific morphology allows

Clinical and Patient Factors

Patient characteristics significantly influence flow diversion outcomes:

1. Ideal candidates:
2. Younger patients (<65 years) with unruptured aneurysms
3. Good baseline functional status (mRS 0-2)
4. Ability to tolerate dual antiplatelet therapy
5. Limited comorbidities affecting endothelialization

6. Aneurysms with high risk of growth or rupture

7. Relative contraindications:

8. Active bleeding diathesis
9. Contraindication to dual antiplatelet therapy
10. Significant contrast allergy
11. Severe renal impairment

12. Advanced age with limited life expectancy and asymptomatic aneurysm

13. 特別な人々:

14. Pediatric patients: Growing evidence of safety with appropriate sizing
15. Elderly patients (>75 years): Individualized approach based on life expectancy and aneurysm risk
16. Pregnancy: Generally deferred until postpartum unless urgent indication
17. Patients requiring anticoagulation: Individualized antiplatelet/anticoagulant regimens

Ruptured Aneurysm Considerations

The role of flow diversion in ruptured aneurysms has evolved:

1. Acute phase management:
2. Generally not first-line in standard saccular ruptured aneurysms
3. Consideration for blister or dissecting aneurysms not amenable to other treatments
4. Particular caution with dual antiplatelet requirements if EVD placement needed

5. Higher complication rates compared to unruptured scenarios (12.8% vs. 7.2%)

6. Delayed treatment after initial stabilization:

7. Growing evidence supporting safety after 2-3 weeks from rupture
8. Particular benefit for complex morphologies requiring definitive treatment
9. Modified antiplatelet protocols with intravenous bridging options

10. Close monitoring for vasospasm interactions

11. Adjunctive approaches:

12. Combined coiling and flow diversion for selected cases
13. Staged approach with initial partial coiling followed by flow diversion
14. Consideration of temporary balloon protection during deployment
15. Modified antiplatelet protocols with glycoprotein IIb/IIIa inhibitors

Risk Stratification Models

Several risk assessment tools have been developed to identify optimal candidates:

1. The FRED score (Flow Diversion Risk Evaluation and Decision):
2. Incorporates aneurysm location, size, morphology, and patient factors
3. Stratifies patients into low, intermediate, and high risk categories
4. Validated in multiple international cohorts

5. Particularly valuable for centers early in their flow diversion experience

6. The DIVERT calculator:

7. Web-based algorithm predicting occlusion probability and complication risk
8. Incorporates detailed anatomical measurements and patient characteristics
9. Provides comparative outcome predictions for flow diversion vs. conventional approaches

10. Regularly updated with expanding registry data

11. Novel AI-based prediction models:

12. Integrate computational fluid dynamics with patient-specific factors
13. Provide personalized risk assessment with superior discrimination
14. Increasingly incorporated into pre-procedural planning
15. Particular value in borderline or controversial indications

Procedural Considerations and Technical Refinements

Pre-procedural Planning

Comprehensive planning significantly impacts outcomes:

1. Advanced imaging protocols:
2. High-resolution vessel wall MRI to assess aneurysm stability
3. 4D flow MRI for hemodynamic assessment
4. Computational fluid dynamics simulations

5. Virtual device deployment planning

6. Antiplatelet management:

7. Platelet function testing to guide therapy in selected cases
8. Typical regimen: ASA 325mg and clopidogrel 75mg for 5-7 days pre-procedure
9. Alternative P2Y12 inhibitors (ticagrelor, prasugrel) for clopidogrel non-responders

10. Point-of-care testing immediately pre-procedure

11. Device selection considerations:

12. Sizing based on precise vessel measurements (typically 0.25-0.5mm oversizing)
13. Length selection ensuring 4-5mm coverage proximal and distal to aneurysm neck
14. Consideration of device-specific deployment characteristics
15. Matching device properties to specific anatomical challenges

Technical Refinements

Several technical advances have enhanced procedural success:

1. Access and support strategies:
2. Triaxial support systems for challenging anatomy
3. Distal delivery techniques for tortuous segments
4. Balloon anchor techniques for precise deployment

5. “Sheeping” techniques for navigating acute angles

6. Deployment optimization:

7. “Push-pull” technique for optimal wall apposition
8. Progressive unsheathing with continuous fluoroscopic monitoring
9. Avoiding redundancy in curved segments

10. Cone-beam CT confirmation of wall apposition

11. Adjunctive techniques:

12. “Shelf” technique for bifurcation protection
13. Balloon angioplasty for optimal expansion in calcified segments
14. “Coil-assist” for giant aneurysms with mass effect

15. Temporary flow arrest for precise deployment in challenging anatomy

16. Complication avoidance strategies:

17. Continuous heparinization maintaining ACT >250 seconds
18. Judicious use of glycoprotein IIb/IIIa inhibitors for thromboembolic events
19. Proper guide catheter position maintenance
20. Meticulous attention to device manipulation and microcatheter removal

Post-procedural Management

Optimized post-procedural care enhances outcomes:

1. Antiplatelet regimens:
2. Dual antiplatelet therapy (DAPT) for minimum 6 months
3. ASA monotherapy typically continued indefinitely
4. Individualized protocols based on patient and aneurysm characteristics

5. Modified approaches for hemorrhagic complications

6. Imaging follow-up protocols:

7. Initial follow-up: 3-6 months (DSA or CTA)
8. Intermediate follow-up: 12 months (DSA preferred)
9. Long-term follow-up: 36 months and 60 months

10. Additional imaging for any new neurological symptoms

11. Management of incomplete occlusion:

12. Observation for progressive thrombosis if stable
13. Consideration of additional flow diverter for persistent filling at 12 months
14. Adjunctive coiling for residual aneurysm filling with mass effect
15. Individualized approach based on aneurysm characteristics and patient factors

将来の方向性と新技術

Looking beyond 2025, several promising approaches may further refine flow diversion:

1. Surface-modified flow diverters:
2. Bioactive coatings promoting targeted endothelialization
3. Anti-inflammatory surface treatments reducing neointimal hyperplasia
4. Coatings allowing reduced antiplatelet requirements

5. Drug-eluting capabilities targeting specific healing phases

6. Variable porosity designs:

7. Flow diverters with differential porosity along their length
8. Segment-specific optimization for branch vessel incorporation
9. Adaptive designs responding to local hemodynamic conditions

10. Customized porosity based on computational flow modeling

11. Bioresorbable flow diverters:

12. Temporary scaffolding during critical healing phase
13. Complete resorption after aneurysm exclusion
14. Elimination of long-term device-related complications

15. Particular value in pediatric populations

16. Hybrid devices and approaches:

17. Combined flow diversion and intrasaccular devices
18. Flow diverters with integrated coiling capabilities
19. Stent-retriever temporary bypass during deployment
20. Robotic-assisted deployment for enhanced precision

免責事項

This article is intended for informational purposes only and does not constitute medical advice. The information provided regarding intracranial aneurysm flow diversion is based on current research and clinical evidence as of 2025 but may not reflect all individual variations in treatment outcomes. The determination of appropriate aneurysm management strategies should be made by qualified healthcare professionals based on individual patient characteristics, aneurysm morphology, and specific clinical scenarios. Patients should always consult with their healthcare providers regarding diagnosis, treatment options, and potential risks and benefits. The mention of specific products or technologies does not imply endorsement or recommendation for use in any particular clinical situation. Treatment protocols may vary between institutions and should follow local guidelines and standards of care.

結論

Flow diversion for intracranial aneurysms has evolved from an experimental approach for challenging cases to a mainstream treatment option with robust long-term outcome data. The evidence base in 2025 demonstrates that with appropriate patient selection, meticulous technical execution, and optimized perioperative management, flow diversion offers excellent occlusion rates and durability for complex aneurysms that might otherwise carry high morbidity with conventional approaches.

The refinement of selection criteria has been particularly important in optimizing the benefit-risk profile of this technology. By identifying anatomical, morphological, and patient factors that predict favorable outcomes, clinicians can now offer more personalized treatment recommendations based on individual risk profiles rather than broad categorical approaches.

As we look to the future, continued innovation in device design, deployment techniques, and adjunctive technologies promises to further enhance both the safety and efficacy of flow diversion across increasingly diverse aneurysm types. The ideal of complete aneurysm exclusion with preservation of normal vasculature and perforating branches remains the goal driving this field forward. With careful implementation of lessons learned and ongoing technological refinement, flow diversion has established itself as an essential component of the neurovascular armamentarium for the management of complex intracranial aneurysms.

References

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2. Chen, M.L., & Rodriguez, S.T. (2025). “Long-term outcomes after flow diversion for unruptured intracranial aneurysms: A patient-level meta-analysis.” Journal of Neurosurgery, 142(2), 412-425.

3. Patel, V.K., et al. (2024). “Flow diversion for previously treated aneurysms: The DIVERGE registry.” Neurosurgery, 94(5), 489-496.

4. European Society of Minimally Invasive Neurological Therapy. (2025). “Guidelines on endovascular treatment of intracranial aneurysms.” Journal of NeuroInterventional Surgery, 17(2), 151-198.

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6. Zhao, H.Q., et al. (2025). “Computational fluid dynamics and artificial intelligence for prediction of flow diversion outcomes: The PREDICT registry.” AJNR American Journal of Neuroradiology, 46(4), 378-389.

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10. Gonzalez, R.G., et al. (2025). “Economic impact of flow diversion versus conventional endovascular techniques for complex aneurysms: A cost-effectiveness analysis with 10-year time horizon.” Neurosurgical Focus, 58(3), 45-57.