Prof. Giacomo Novara — Urology, University of Padova
Ureteropelvic Junction Obstruction
UPJ Obstruction — Congenital hydronephrosis from junction obstruction
Ureteropelvic junction (UPJ) obstruction is a congenital or acquired obstruction to the drainage of the renal pelvis into the ureter. It leads to progressive accumulation of urine within the collecting system (hydronephrosis) which, if untreated, can result in irreversible renal parenchymal damage. Diagnosis is clinical and radiological; treatment is surgical.
🔬 Pathophysiology and causes
The ureteropelvic junction is the transition point between the renal pelvis and the proximal ureter. When this passage is obstructed, urine accumulates in the pelvis, generating increasing pressures that — over time — compress and atrophy the surrounding renal parenchyma. The rate of damage depends on the severity of the obstruction.
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Aperistaltic or stenotic segment
The most common cause, often congenital. A segment of the proximal ureter fails to contract normally (aperistalsis) or is anatomically narrowed (true stenosis). Urine is not propelled distally with adequate force.
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Compression by a crossing lower pole vessel
An aberrant lower pole arterial branch crosses the UPJ externally, compressing it. The pelvis and ureter are intrinsically normal, but the vessel "strangulates" them from outside. Visible on contrast-enhanced CT in the arterial phase.
Both forms may coexist. In neonates, congenital aperistaltic segments predominate; in young adults, a crossing vessel is frequently identified and may explain late-onset symptoms.
🩺 Clinical presentation
UPJ obstruction may present at very different ages depending on severity:
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Neonate / infant
Perinatal diagnosis on routine obstetric ultrasound. Detection of foetal hydronephrosis requires neonatal and urological follow-up. Often asymptomatic at birth.
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Young adult
Atypical renal colic or progressive flank pain. Non-contrast CT shows hydronephrosis but no calculi. This is the most characteristic presentation: the patient is investigated for suspected urolithiasis and a UPJ obstruction is found instead.
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With associated urolithiasis
Urinary stasis within the dilated collecting system promotes stone formation. In this case both the calculus and the underlying anatomical defect must be treated — removing only the stone would leave a dilated cavity in which it would reform.
📌 Diagnostic key: a patient with flank pain + hydronephrosis on CT with no visible calculi = UPJ obstruction until proven otherwise. Colic is not always present: the condition is sometimes discovered incidentally on imaging performed for other reasons.
🔍 Diagnostic workup
1
Renal ultrasound
First-line screening: demonstrates dilatation of the pelvicalyceal system (hydronephrosis). Not definitive for the cause, but indicates an obstruction at the level of the renal outlet.
2
CT abdomen without and with contrast
Rules out urolithiasis (unenhanced phase), evaluates perirenal vascular anatomy (arterial phase: crossing vessel?), and documents the degree of dilatation and residual parenchymal thickness. Essential for surgical planning.
An indispensable functional study: distinguishes the dilated-but-non-obstructed kidney from the dilated-and-obstructed kidney. Only functionally obstructed kidneys require surgical intervention.
🔬 How MAG3 diuretic renography works
MAG3 is a radiolabelled tracer filtered and secreted by the kidney. The uptake/excretion curve allows assessment of:
Differential renal function: the percentage contribution of each kidney to total function
Drainage curve: after administration of furosemide (diuretic), the normal kidney rapidly washes out the tracer; the obstructed kidney retains it
Practical interpretation: dilated kidney that drains normally after furosemide = dilatation without functional obstruction → surveillance. Dilated kidney that fails to drain after furosemide = functional obstruction → surgical indication. Kidney with no uptake = destroyed parenchyma → consider nephrectomy.
Surgical indication
Hydronephrosis + functional obstruction on renography
Flat drainage curve after furosemide. The kidney does not drain. Risk of progressive parenchymal damage. Surgical repair is indicated.
Surveillance
Hydronephrosis + normal renography
The kidney drains normally. The dilatation is anatomical but not functionally significant. Periodic ultrasound follow-up without intervention.
🔧 Surgical treatment — Anderson-Hynes pyeloplasty
Robotic pyeloplasty is the gold standard. Open surgery is no longer acceptable: it requires a large incision. Laparoscopy is technically demanding because of the multiple sutures required. With the robot, anatomical structures are better visualised, suturing is more precise, and the probability of success is higher.
⚙️ Robotic pyeloplasty (Anderson-Hynes technique)
Principle: excision of the obstructed UPJ segment (stenotic or aperistaltic) and reconstruction of continuity between the renal pelvis and ureter. If a crossing vessel is present, it is transposed so that it no longer compresses the reconstructed junction.
Robotic transperitoneal access (3–4 ports), patient in lateral decubitus.
Exposure of the renal pelvis and proximal ureter. Identification of any compressive crossing vessel.
Resection of the stenotic/aperistaltic segment. Concurrent removal of intrarenal calculi if present.
Pelvi-ureteric anastomosis (continuous absorbable suture) according to the Anderson-Hynes technique.
Placement of an internal DJ ureteral stent to support the anastomosis during healing.
Verification of anastomotic integrity. Perinephric drain placement.
Postoperative course: hospital stay 2 days. Discharged with an internal DJ stent in situ, which is removed cystoscopically as an outpatient procedure 4–6 weeks later. Renography at 3 months to confirm resolution of obstruction.
⚠️ If associated urolithiasis is present: stones must be removed during the same pyeloplasty procedure. Treating the stone in isolation while leaving the obstruction untreated would be ineffective: urinary stasis in the dilated collecting system would cause the stone to reform within months. Treat the cause, not the consequence.
⏱️ Consequences of delayed treatment
UPJ obstruction is not a static condition: chronic intrapelvic pressure causes progressive and irreversible atrophy of the renal parenchyma. The rate of damage is proportional to the severity of the obstruction.
📋 Illustrative case
A young patient with right hydronephrosis and documented functional obstruction on renography. Robotic pyeloplasty was recommended. For family reasons (living abroad) the patient did not undergo surgery.
Two years later, follow-up renography: the kidney that had previously shown reduced uptake was now entirely non-functioning. The renal parenchyma had been destroyed by prolonged obstruction.
Outcome: laparoscopic nephrectomy. A kidney that could have been saved with elective pyeloplasty was lost due to delay in surgical treatment.
The clinical message: when renography documents functional obstruction, treatment should not be postponed indefinitely.
🚨 Non-functioning kidney on renography = already destroyed parenchyma. A kidney with no tracer uptake has no residual salvageable function. In this setting the indication is nephrectomy — not pyeloplasty, which would have no effect on an already atrophic parenchyma.
❓ Frequently asked questions
Not necessarily. Renal dilatation (hydronephrosis) does not in itself imply the need for surgery. The decisive test is diuretic renography: if the kidney, despite being dilated, drains the tracer normally after furosemide, it means it is not functionally obstructed and can be monitored over time without surgery. If, however, the kidney fails to drain, it means the obstruction is already causing progressive damage and intervention is indicated to preserve renal function.
Pyeloplasty is a procedure that removes the obstructed or non-functioning segment of the ureteropelvic junction and reconstructs it to restore free urinary passage. It is performed using robotic laparoscopy — through 3 or 4 small abdominal incisions — without opening the abdomen. The hospital stay is approximately 2 days. At discharge, a temporary internal stent (a flexible tube keeping the reconstructed segment open) is left in place; it is removed as an outpatient procedure via a brief cystoscopy approximately one month later. The outcome of surgery is assessed by renography at 3 months.
The stone is removed during the same pyeloplasty procedure. There is no point treating the stone in isolation while leaving the UPJ obstruction intact: the urinary stasis persisting in the dilated collecting system would cause the stone to reform rapidly. The correct approach is to simultaneously correct the anatomical defect and remove the stones, thereby eliminating both the current problem and the substrate that generated it.
Yes. Flank pain, sometimes colicky in character, is the typical presentation in young adults. The mechanism differs from urolithiasis (there is no stone obstructing the ureter), but the clinical result is similar: pressure build-up within the collecting system generates pain, often very severe. The picture becomes suspicious when CT shows no calculi but reveals hydronephrosis: at that point renography is performed to determine whether the obstruction is functionally significant.
It depends on the severity of the obstruction and how long it has been present. A mild obstruction may remain stable for years. A severe obstruction progressively and irreversibly damages the renal parenchyma: atrophied renal tissue cannot recover. For this reason, when renography documents significant functional obstruction, surgery should not be deferred for a long period. Periodic renographic surveillance is essential: if renal function deteriorates, the surgical indication becomes urgent.