UPJ Obstruction • Pyeloplasty and approaches

If There's a Crossing Vessel: How the Operation Changes

Name: If There's a Crossing Vessel
Creator: Prof. Dr. Ali Avanoğlu
Published: 2026-08-19

A crossing vessel is an extra vessel running to the kidney's lower pole that presses on the ureteropelvic junction (UPJ) from outside. It's a common cause of episodic flank pain in older children and adolescents; this page explains how the vessel changes the operation, and whether it is cut.

Who this is forFamilies and adolescents whose MR urography or operative report mentions a "crossing vessel"

Reading≈5 min

ByProf. Dr. Ali AvanoğluUpdated19 August 2026

This page is part of UPJ obstruction.Read the full treatment overview on the hub page

three causes: intrinsic narrow segment · crossing vessel · high ureteral insertion (crossing vessel highlighted)

01 · What is a crossing vessel?02 · In which children is it suspected, and how is it found?03 · What changes during surgery? Is the vessel cut?04 · Does the pain go away after surgery?05 · Questions that carry into adulthood

In brief5 madde

What is a crossing vessel?: Normally a single main artery enters the kidney; in some people, an extra (accessory) vessel running to the lower pole passes in front of the point where the ureter…
In which children is it suspected, and how is it found?: A crossing vessel typically stays silent in infancy and shows itself in older children or adolescents.
What changes during surgery? Is the vessel cut?: The basic rule: a crossing vessel is not cut — it is preserved, and the ureter is moved to lie in front of it.
Does the pain go away after surgery?: In children who present with episodic pain and are found to have a crossing vessel, the pain resolves in the large majority after pyeloplasty.
Questions that carry into adulthood: A crossing vessel is more common in adolescents and adults, so this anatomy also affects the treatment decision for adult UPJ obstruction.

What is a crossing vessel?

Normally a single main artery enters the kidney; in some people, an extra (accessory) vessel running to the lower pole passes in front of the point where the ureter leaves the renal pelvis.

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This vessel can press on the ureter from outside like a bridge, or cause the ureter to kink over it. UPJ obstruction (ureteropelvic junction obstruction; informally, kidney outlet narrowing) caused this way is an "external compression" type; in most cases, an intrinsic (wall-related) narrowing in the junction beneath the vessel is present as well. The vessel itself is normal, not diseased — the problem is its position.
A crossing vessel is reported in roughly 20–40% of childhood UPJ obstruction in the literature; in older children and adolescents, especially those presenting with episodic pain, that figure approaches half. In infants, the main cause is instead a narrow segment in the junction itself.

In which children is it suspected, and how is it found?

A crossing vessel typically stays silent in infancy and shows itself in older children or adolescents.

Severe, episodic flank pain with nausea and vomiting, coming on after heavy fluid intake or exercise (Dietl's crisis); ultrasound may be normal between episodes.
On ultrasound, marked hydronephrosis (kidney swelling) during pain, mild during pain-free periods.
Intermittent or borderline drainage impairment on MAG3 diuretic renography (a kidney scan).
New-onset hydronephrosis in an older child whose ultrasound was previously normal.

Doppler ultrasound can sometimes show the vessel; MR urography or CT angiography gives a clearer picture. In children, MR urography is preferred because it involves no radiation. That said, most surgeons can identify the vessel safely during surgery itself, so imaging beforehand isn't mandatory in every case — it's requested when it's thought it may change the surgical plan or approach.

What changes during surgery? Is the vessel cut?

The basic rule: a crossing vessel is not cut — it is preserved, and the ureter is moved to lie in front of it.

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The vessel supplies the lower pole; cutting it could cause permanent tissue loss in that area. Anderson–Hynes (dismembered) pyeloplasty has a particular advantage for this anatomy: because the ureter is completely detached from the renal pelvis, it can be moved from behind the vessel to in front of it (transposition), the narrow segment is removed, and the new connection is made in front of the vessel, without tension. This corrects both the external compression and any accompanying intrinsic narrowing in a single step.
Techniques that only lift the vessel away and suspend it (a vascular hitch), without dividing the junction, have been described for selected cases with no intrinsic narrowing; this can be done laparoscopically and may not require a stent. However, if narrowing is found in the junction during surgery, switching to a dismembered repair becomes necessary — which is why dismembered pyeloplasty is the first choice at most centers. Which technique is used becomes clear once the anatomy is seen during surgery, and these possibilities are explained to you beforehand.
The magnified view of minimally invasive approaches (laparoscopic, robotic) makes separating the vessel easier, so these are often preferred in such cases; the open approach is applied with equal confidence. The choice of approach is made through the joint assessment of two specialists.

Does the pain go away after surgery?

In children who present with episodic pain and are found to have a crossing vessel, the pain resolves in the large majority after pyeloplasty.

See a doctor: fever (38 °C/100.4 °F and above) after surgery, severe pain that doesn't ease, vomiting, reduced urine output, visible blood in the urine.

The pain attacks come from the obstruction itself, so they stop once the obstruction is relieved. That said, the widened renal pelvis takes time to shrink, and in the first weeks, stent-related complaints (frequent urination, burning, flank pain when voiding) can be mistaken for an attack. If pain continues after the stent is removed, or comes with fever, re-evaluation is needed.

Questions that carry into adulthood

A crossing vessel is more common in adolescents and adults, so this anatomy also affects the treatment decision for adult UPJ obstruction.

In adults, endoscopic incision from the inside (endopyelotomy) is not recommended when a crossing vessel is present — success is lower, and there is a risk of vessel injury. In these cases, laparoscopic or robotic pyeloplasty is the first choice. When narrowing repaired in childhood recurs in adulthood, it matters how the vessel was handled in the first operation; this is why keeping your childhood operative report is important.

UPJ Obstruction video guide3 short videos, in order

Frequently asked questions

Is a crossing vessel congenital?

Yes; the vessel is an anatomical variant and can be found in healthy people too. The problem is that it passes in front of the ureter, with the pressure becoming more noticeable as the child grows. The vessel's presence alone is not a reason for surgery — obstruction and symptoms are needed.

Is MR urography necessary before surgery?

Not mandatory; most surgeons identify the vessel during surgery. It's requested in cases that may affect the surgical approach or technique, in redo operations, or when the anatomy is complex. MR urography is preferred over CT in children because it involves no radiation.

What happens to the kidney if the vessel is cut?

Cutting the vessel supplying the lower pole can cause permanent tissue loss in that area, which is why it is preserved. In dismembered pyeloplasty, the ureter is moved in front of the vessel, solving the problem without touching it.

Is surgical success lower in children with a crossing vessel?

No; with dismembered pyeloplasty, success is roughly 90–95% and above in the literature, the same as in other cases. Episodic pain resolves in the large majority. What matters is that the vessel is recognized during surgery and the ureter moved in front of it.

←Previous • Pyeloplasty and approachesOpen vs. Laparoscopic vs. Robotic

Related pagesFull index →

BasicsWhy Does UPJ Obstruction HappenUPJ obstruction (ureteropelvic junction obstruction) is most often congenital, arising from a narrow or poorly contracting segment in the wall of the junction itself. In older children and adolescents, a crossing vessel compressing the junction from outside becomes more prominent; high ureteral insertion and acquired causes are rarer. The cause shapes the surgical plan.Pyeloplasty and approachesAnderson–Hynes (Dismembered) PyeloplastyAnderson–Hynes pyeloplasty is the "dismembered" repair technique in which the narrow UPJ is removed and the ureter is reattached to the renal pelvis. It is the gold standard in both children and adults; this page explains why the technique is preferred and its steps.Diagnosis and evaluationMR Urography in Children: When Is It Needed, What Does It Show?MR urography is a contrast-enhanced MRI scan that shows the detailed anatomy of the kidneys and urinary tract, along with function, in the same session. It is not routine for suspected UPJ obstruction; it comes into play for selected questions that ultrasound and MAG3 cannot answer. This page explains when it's needed, how it's done, and its limits.By age, and special situationsUPJ Obstruction in AdolescentsUPJ obstruction in adolescents looks different from the silent picture seen in infants: it most often appears as severe flank pain with vomiting, coming on after heavy fluid intake or exercise. This page explains why diagnosis can be delayed in adolescence, the role of a crossing vessel, and how the treatment decision is made.

UPJ Obstruction topic clustersMain treatment page →

Basics 7 Diagnosis and evaluation 6 Follow-up or surgery? 7 Conservative follow-up 5 Pyeloplasty and approaches 7 After surgery and follow-up 6 By age, and special situations 7

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