An endoleak is blood leaking around or through a stent graft that was placed inside an aortic aneurysm. When surgeons repair an aortic aneurysm using a minimally invasive approach called endovascular aneurysm repair (EVAR), they thread a fabric-lined tube (the stent graft) through blood vessels and position it inside the weakened section of the aorta. The graft is meant to seal off the aneurysm completely, redirecting blood flow through the tube and relieving pressure on the fragile artery wall. An endoleak means that seal isn’t perfect, and blood is still finding its way into the space between the graft and the aneurysm wall.
This matters because the whole point of the repair is to stop blood from pressing against the weakened artery. If blood continues to flow into that space, called the aneurysm sac, the aneurysm can keep growing and, in rare cases, rupture. Endoleaks are the most common complication after EVAR, but not all of them are dangerous. The type of endoleak determines how urgently it needs to be treated.
The Five Types of Endoleaks
Endoleaks are classified into five types based on where the blood is coming from and how it enters the aneurysm sac. The distinction is important because some types create high pressure inside the sac (mimicking the original problem) while others produce only low-pressure trickles that often resolve on their own.
Type I: Seal Failure at the Graft Edges
A Type I endoleak occurs when blood leaks around the top (Type Ia) or bottom (Type Ib) of the stent graft where it meets the artery wall. Essentially, the graft isn’t sitting flush against the vessel. This creates a direct channel between full-pressure blood flow and the aneurysm sac, which is the most dangerous scenario. The Society for Vascular Surgery recommends that surgeons resolve a Type I endoleak before the patient leaves the operating room if at all possible. When one shows up on later imaging, it should be treated promptly to cut the aneurysm off from pressurized circulation.
Type II: Backflow From Branch Arteries
Type II is the most common endoleak, found in roughly 19% of EVAR patients. Small arteries that once branched off the section of aorta now covered by the graft can allow blood to flow backward into the aneurysm sac. When only a single branch vessel is involved (Type IIa), the leak is low-pressure and often seals itself without treatment. When multiple branches feed the sac (Type IIb), the flow pattern is more complex but still low-pressure. About two-thirds of patients with Type II endoleaks are managed conservatively with monitoring alone, and only around 22% end up needing a procedure.
Type III: Defect in the Graft Itself
A Type III endoleak means blood is getting through the graft, either at a junction where two pieces of the device overlap (Type IIIa) or through a tear in the graft fabric (Type IIIb). Like Type I, this exposes the aneurysm sac to high-pressure blood flow and requires repair. Fabric tears tend to appear years after the original procedure as the material wears over time.
Type IV: Graft Porosity
Type IV endoleaks are caused by tiny amounts of blood seeping through the microscopic pores of the graft material. These are low-pressure, clinically insignificant, and resolve on their own within 30 days as the body’s clotting process seals the fabric.
Type V: Endotension
Type V, also called endotension, is the most puzzling. The aneurysm sac keeps expanding even though no leak can be found on any imaging study. Several theories exist for why this happens: pressure might transmit through blood clot inside the sac, fluid may slowly filter through the graft material, or there could be a leak too small for current imaging to detect. Because the cause is unclear, treatment decisions are made on a case-by-case basis.
Why the Type Matters So Much
The key distinction is pressure. Type I and Type III endoleaks allow full arterial blood pressure into the aneurysm sac. This essentially re-creates the original problem the surgery was meant to fix, and the aneurysm can continue to grow or even rupture. These leaks are treated aggressively, usually as soon as they’re discovered.
Type II endoleaks, by contrast, involve low-pressure backflow. Many shrink or disappear without intervention. The trigger for treatment is typically growth of the aneurysm sac. Per vascular surgery guidelines, a sac diameter increase of 5 millimeters or more is considered significant expansion. Research has shown that this degree of growth at one year after surgery is independently associated with increased long-term mortality, regardless of whether an endoleak is visible, and generally warrants closer monitoring or intervention.
How Endoleaks Are Detected
Most endoleaks produce no symptoms at all. You won’t feel blood leaking around the graft. That’s why regular imaging after EVAR is essential. The standard surveillance schedule calls for a CT scan with contrast dye at 30 days, 6 months, and 1 year after the procedure. If everything looks good at the one-year mark, annual CT scans are recommended for life.
CT angiography is the primary tool because it shows both the graft’s position and any contrast dye pooling outside it. Contrast-enhanced ultrasound is sometimes used as an alternative, particularly for patients who need to limit their exposure to radiation or contrast dye. Duplex ultrasound can serve as a screening tool but is less sensitive for detecting slow or small leaks.
How Endoleaks Are Treated
Treatment depends entirely on the type of endoleak and whether the aneurysm sac is growing.
For Type I endoleaks, the most common fix is placing an extension cuff, essentially an additional piece of stent graft that extends the original device further along the artery to create a better seal. If that approach has already failed, surgeons can inject a sealing material into the gap. For Type III leaks, the solution is similar: placing a new graft component to bridge the junction gap or cover the fabric tear.
Type II endoleaks that need treatment are typically addressed by blocking the branch arteries feeding blood into the sac. This is done through embolization, a minimally invasive procedure where a doctor threads a thin catheter to the leak and deploys tiny coils or injects a liquid sealing agent that hardens in place. The catheter can reach the aneurysm sac through several routes: through an artery, through a vein that runs alongside the aorta, or by direct needle puncture through the back or abdomen. The choice depends on the anatomy and location of the leak.
One important finding from long-term studies: for Type II endoleaks, intervention doesn’t always improve outcomes. A retrospective study following patients for over a decade found that procedures to close Type II leaks did not improve overall survival, reduce the need for later open surgery, or increase the rate of leak closure compared to conservative monitoring, even in patients whose sac was growing. The strongest predictor of eventually needing open surgical conversion wasn’t whether a Type II leak was treated, but whether a different type of endoleak developed later. About 12% of patients with early Type II endoleaks ultimately required conversion to open surgery.
Living With Long-Term Monitoring
If you’ve had EVAR, the reality is that surveillance imaging becomes a permanent part of your healthcare. Endoleaks can develop years after the initial procedure as the graft shifts, the artery remodels, or graft materials degrade. A scan that’s clean at year two doesn’t guarantee year five will look the same.
The roughly 80% of EVAR patients who never develop a detectable endoleak still need annual imaging, because late-onset leaks are well-documented. For the roughly 20% who do develop a Type II endoleak, most will simply continue with regular monitoring. The small subset who show sac growth will have a conversation with their vascular surgeon about whether intervention is likely to help, weighed against the reality that these procedures don’t always produce lasting results and may need to be repeated.