Why Does UPJ Obstruction Happen? Congenital Narrowing, Vessel Compression, High Insertion

Name: Why Does UPJ Obstruction Happen
Creator: Prof. Dr. Ali Avanoğlu
Published: 2026-08-19

UPJ 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.

Who this is forFamilies whose child has been diagnosed with UPJ obstruction and are asking "why did this happen — did we do something?"

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

01 · How the narrowing arises 02 · Intrinsic narrowing: a problem in the junction's own wall 03 · Crossing vessel: compression from outside 04 · High ureteral insertion, kinks, and bands 05 · Acquired and secondary causes

In brief5 madde

How the narrowing arises: The kidney and the ureter develop from separate buds in the womb, and the upper end of the ureter joins the renal pelvis.
Intrinsic narrowing: a problem in the junction's own wall: In infants, the main cause is intrinsic narrowing: a short segment at the junction is either narrow, or its muscle fibers are disorganized so that it cannot contract…
Crossing vessel: compression from outside: An extra artery or vein supplying the lower pole of the kidney sometimes passes right in front of where the ureter begins and compresses the junction from outside.
High ureteral insertion, kinks, and bands: Normally the ureter leaves from the very lowest point of the funnel-shaped renal pelvis.
Acquired and secondary causes: More rarely, narrowing develops later: a previous kidney or ureter operation, injury from passing a stone, scarring left by repeated infections, or, rarely, external…

How the narrowing arises

The kidney and the ureter develop from separate buds in the womb, and the upper end of the ureter joins the renal pelvis. If this junction does not develop perfectly, it can end up narrow, poorly contracting, or open to compression from outside.

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For this reason, UPJ obstruction is largely a developmental condition; it is not the result of anything done or not done in pregnancy, of diet, or of medication. Parents often feel guilty about it — there is no reason to.
There are several structural types of narrowing. More than one factor can be present in the same child, and which one predominates changes with age.

Intrinsic narrowing: a problem in the junction's own wall

In infants, the main cause is intrinsic narrowing: a short segment at the junction is either narrow, or its muscle fibers are disorganized so that it cannot contract in the same wave-like fashion as the rest of the ureter. Urine cannot be pushed through this "aperistaltic" segment and backs up above it.

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This segment can sometimes look entirely normal from the outside; the problem lies in the structure of the wall. In some cases a true narrowing in diameter, or an internal fold or membrane-like structure, is found at the junction.
When treatment is needed for intrinsic narrowing, the narrow segment is removed and the ureter is re-attached to the pelvis with a wide opening. This is the basic logic behind Anderson–Hynes (dismembered) pyeloplasty.

Crossing vessel: compression from outside

An extra artery or vein supplying the lower pole of the kidney sometimes passes right in front of where the ureter begins and compresses the junction from outside. This is called a crossing vessel (a lower-pole accessory vessel).

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The literature reports a crossing vessel in roughly 20–40% of children with UPJ obstruction; in older children and adolescents — especially those presenting with episodic pain attacks — this figure approaches half. The vessel usually causes no trouble in infancy; as the child grows and urine flow increases, the compression becomes more evident.
The vessel itself is not cut — it supplies part of the kidney. In pyeloplasty, the ureter is moved in front of the vessel and re-attached to the pelvis in its new position. Before surgery, the vessel's presence is confirmed with MR urography or seen directly during the operation.

High ureteral insertion, kinks, and bands

Normally the ureter leaves from the very lowest point of the funnel-shaped renal pelvis. In some children the ureter leaves from higher up, on the side wall of the pelvis; as the pelvis widens, this exit folds over like a valve and flow becomes difficult. This is called high ureteral insertion.

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Fibrous bands and adhesions around the junction, or a sharp kink at the start of the ureter, can produce a similar effect. These types are usually seen together with intrinsic narrowing or a wide pelvis.
Dismembered pyeloplasty addresses all of these problems at the same time: the pelvis is reduced, the exit is moved to the lowest point, and the funnel is reshaped.

Acquired and secondary causes

More rarely, narrowing develops later: a previous kidney or ureter operation, injury from passing a stone, scarring left by repeated infections, or, rarely, external compression from another tissue. These are called secondary (acquired) UPJ obstruction.

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In severe kidney reflux, the ureter can become very wide and tortuous, and a kink at the junction that hampers flow can form; in that case reflux and obstruction are evaluated together.
Knowing the cause helps in planning the surgical approach, the incision, and expectations; but what determines the decision to follow or operate is not the cause — it is the kidney's function and the course of the widening. In our practice, two pediatric urology specialists make this plan together.

UPJ Obstruction video guide3 short videos, in order

Frequently asked questions

Is UPJ obstruction inherited from a parent?

Most cases arise on their own, with no one else in the family affected. Hydronephrosis has been reported in more than one child in the same family; however, no clear pattern of inheritance has been defined, and routine screening of siblings is not needed. You can ask your doctor if you have concerns.

Could something I did in pregnancy have caused this?

No. UPJ obstruction is a structural condition that arises while the kidney and ureter are developing in the early weeks of pregnancy; no link has been shown with diet, work, stress, or commonly used medications.

Does a crossing vessel go away without surgery?

The vessel itself is permanent, but not every crossing vessel causes compression. If there are no symptoms and no loss of function, it can be followed. If there is recurring pain, poor emptying, or declining function, pyeloplasty moves the ureter in front of the vessel and corrects the problem permanently.

Can surgery go ahead without knowing the exact cause?

Yes, often. Ultrasound and MAG3 do not always show the exact type of narrowing; the precise type is frequently seen only during the operation. Because dismembered pyeloplasty addresses intrinsic narrowing, a crossing vessel, and high insertion within the same operation, this does not disrupt the plan.

Prof. Dr. Ali Avanoğlu's publications on this topic · 1

Avanoglu A, Tiryaki S (2020). Embryology and Morphological (Mal)Development of UPJ. Frontiers in pediatrics. PubMed ↗

←Previous • BasicsWhat Is Kidney Swelling (Hydronephrosis)Next • BasicsSymptoms of UPJ Obstruction→

Related pagesFull index →

BasicsWhat Is UPJ Obstruction?UPJ obstruction (ureteropelvic junction obstruction; informally, kidney outlet narrowing) is a narrowing at the point where the renal pelvis opens into the ureter. Urine passes through this point slowly, the pelvis widens, and this appears on ultrasound as hydronephrosis (kidney swelling). In most children it is congenital, and a large share can be followed without surgery.Pyeloplasty and approachesIf There's a Crossing VesselA 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.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.By age, and special situationsUPJ Obstruction in Adults: Symptoms, Evaluation, TreatmentUPJ obstruction in adults is most often a narrowing that has been present since childhood and only becomes symptomatic late — showing up as flank pain, recurrent infection, or a kidney stone. This page summarizes evaluation and treatment options in adults, noting that it draws on review data rather than a formal guideline.

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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