What are inflow and outflow lesions?

Inflow and Outflow Lesions in Vascular Access

Inflow lesions are stenoses affecting arterial blood flow INTO a vascular access (arteriovenous fistula or graft), while outflow lesions are stenoses affecting venous drainage FROM the access—with outflow stenoses being far more common and clinically significant. 1

Anatomic Definitions

Inflow Lesions

• Inflow lesions involve stenoses in the arterial system supplying the vascular access, including the feeding artery or arterial anastomosis 1
• These lesions were historically considered uncommon (5% of all lesions), but more recent angiographic experience suggests they may account for 20-25% of all lesions identified in arteriovenous grafts (AVGs) 1
• In hemodialysis access dysfunction, inflow stenoses occur in approximately 14% of cases 2

Outflow Lesions

• Outflow lesions are stenoses in the venous drainage pathway, including the venous anastomosis and more central veins 1
• Venous stenosis is the most common lesion in AVGs, with approximately 90% of thrombosed grafts associated with stenosis predominantly in the outflow, at the venous anastomosis, and more centrally 1
• Multiple lesions are frequently present within the graft or at the anastomoses 1

Hemodynamic Consequences

Outflow Stenosis Effects

• A hemodynamically significant outflow stenosis decreases intragraft blood flow and increases intragraft pressure 1
• This reduced blood flow may decrease hemodialysis treatment efficiency and increase the risk for vascular access thrombosis 1
• Hemodynamically significant stenosis is defined as ≥50% reduction in normal vessel diameter accompanied by a hemodynamic, functional, or clinical abnormality 1

Inflow Lesion Effects

• Inflow lesions and intragraft lesions are associated with low pressure in the body of the graft and venous outflow 1
• Arterial inflow stenosis is frequently suspected by excessively negative dialysis circuit prepump pressures 1
• In patients with low-flow access due to inflow stenosis, access flow can improve significantly after angioplasty (from 477 ± 74 mL/min to 825 ± 199 mL/min) 2

Clinical Presentation

Common Referral Criteria

• Decreased flow rates (most common presentation for inflow stenosis) 2
• Steal symptoms 2
• Insufficient access maturation 2
• Difficulty achieving hemostasis upon needle withdrawal 1
• Spontaneous bleeding from cannulation sites 1

Diagnostic Approach

Imaging Evaluation

• Radiological evaluation should comprise assessment of the complete arterial inflow, not just the shunt region and outflow 2
• Contrast-enhanced magnetic resonance angiography (CE-MRA) can effectively demonstrate complete inflow (from subclavian artery), shunt region, and complete outflow 2
• Digital subtraction angiography (DSA) remains the gold standard for confirming stenoses identified on non-invasive imaging 2
• Color-flow duplex scanning can identify stenoses in the bypass graft or adjacent inflow/outflow arteries 3

Treatment Implications

Intervention Criteria

• Prophylactic treatment of anatomic stenosis (≥50% diameter reduction) without hemodynamic, functional, or clinical abnormality is NOT warranted and should not be performed 1
• Percutaneous transluminal angioplasty (PTA) or surgical repair of hemodynamically significant stenosis in a nonthrombosed AVG can maintain functionality and delay thrombosis 1
• Preemptive treatment of stenoses reduces thrombosis rates, with 71-85% of grafts remaining free of further intervention when PTA is performed preemptively, compared to only 33-63% after thrombectomy 1

Critical Pitfall

• The major pitfall is treating asymptomatic anatomic stenoses without hemodynamic significance—this provides no benefit in improving function or delaying thrombosis 1
• While prophylactic treatment reduces thrombosis events, recent studies suggest it does not extend the useful lifespan of AVGs 1