Hydronephrosis and Pelvi-ureteric Junction Obstruction

Introduction

Hydronephrosis is when the kidney's urine-collecting area (called the renal pelvis) swells up with extra fluid, like a balloon filling with water. It happens in about 1 in 100 babies. Most patients with hydronephrosis have no symptoms, so the condition is usually picked up by chance during other check-ups. It can show up before birth on a routine ultrasound that checks the baby's growth, or after birth during standard scans.

Causes and Risk Factors

Hydronephrosis often happens when something blocks the flow of urine from the kidney, or when urine flows backward from the bladder up to the kidneys (called vesicoureteric reflux, or VUR). Sometimes, scans do not show a clear blockage or reflux, and the swelling just goes away on its own over time without any treatment—the exact reason stays a mystery.

The common causes of obstruction in the urinary tract include but are not limited to the following conditions:

• Pelvi-ureteric junction obstruction (PUJO): A narrow spot where the kidney meets the ureter (urine tube)
• Vesicoureteric junction obstruction (VUJO): Narrowing where the ureter joins the bladder
• Posterior urethral valve: Extra tissue flaps in the boy's urethra that block urine outflow
• Ureterocele: A balloon-like swelling at the ureter's end inside the bladder
• Ectopic ureter: Ureter in the wrong place, causing poor drainage

Signs and Symptoms

Most infants and children with hydronephrosis have no symptoms. In older children who do notice issues, they might feel belly pain or pain in the side/back (called loin pain), a lump in the tummy (abdominal mass), vomiting or signs of urinary tract infection, like fever or cloudy urine.

Diagnosis

The first step in helping your child is figuring out exactly what is causing the hydronephrosis with the right tests. Key tests doctors may use:

• Ultrasound of urinary system: A safe scan using sound waves to check if swelling is in one kidney or both, and how bad it is—like taking a picture of the kidneys and tubes.
• Micturating cystourethrogram (MCU) / contrast-enhanced voiding urosonography (ceVUS): A small, soft tube goes into the bladder through the urethra (pee tube) to add special dye. As the bladder fills and your child pees, X-ray or ultrasound pictures show if urine is flowing backward (reflux).
• Isotope renal scan: Uses a tiny, safe amount of radioactive tracer (like DMSA, DTPA, or MAG3) to see how well each kidney works and check for blockages. It compares the kidneys' strength and measures any flow problems.

After these tests, our experts will review the results together with you and your family in a clear discussion. They will explain what the scans show and recommend the best plan for your child—often just careful watching over time if it is mild, or surgery if there is a fixable blockage. This personalized approach ensures the kidneys stay healthy with the least intervention needed. Your doctor will answer all your questions.

Pelvi-ureteric junction obstruction

One common reason kids get hydronephrosis is pelvi-ureteric junction obstruction (PUJO).

What is a PUJO?

PUJO happens when urine cannot flow properly from the kidney's collecting area (renal pelvis) to the ureter tube, causing hydronephrosis. In bad cases, urine backs up and stretches the kidney, which can hurt its function forever if not fixed. Usually, it is from a narrow or kinked spot in the tube present at birth, or sometimes an extra blood vessel crossing over and squishing it. For serious blockages, surgery is the best fix—it opens up the path so urine drains well and protects the kidney's health.

Treatment of PUJO

The most common surgery to fix PUJ obstruction is called pyeloplasty. Surgeons cut away the narrow or blocked part of the ureter tube, then reconnect it properly to the kidney's collecting area so urine can flow freely. All are done under full sleep anesthesia.

Types of pyeloplasty

• Laparoscopic pyeloplasty: Uses a tiny camera (laparoscope) and special tools through 2-3 small cuts near the belly button or side. The belly is filled with safe gas to make space. A temporary tube may stay in to help healing. It takes longer (3-4 hours) but means less pain, shorter hospital stays (a few days), and quicker return to play/school.
• Robotic-assisted laparoscopic: Same as above but with robot arms controlled by the surgeon for extra precision—still keyhole-sized cuts.
• Open pyeloplasty: If keyhole is not possible (e.g. tricky anatomy), they use one bigger cut on the tummy. Recovery takes a bit longer, but it works just as well.

Follow-up Care

After pyeloplasty surgery, your child will stay in the hospital ward for close monitoring, usually a few days before heading home. Feeding starts soon after waking up, and antibiotics shall be given to help prevent infections. A urinary catheter (a tube from the bladder to a collection bag) drains pee for 1-3 days and gets removed before discharge. Many kids also get a temporary JJ stent (plastic tube inside the ureter) to keep things open—it is removed 4-8 weeks later with a quick camera check in the bladder under short general anaesthesia.

We will schedule clinic visits with ultrasound and MAG3 scans about 1-3 months later to confirm the swelling (hydronephrosis) has improved and urine flows better.

Reference

• Cai PY, Lee RS. Ureteropelvic Junction Obstruction/Hydronephrosis. Urol Clin North Am. 2023 Aug;50(3):361-369. doi: 10.1016/j.ucl.2023.04.001. Epub 2023 May 22. PMID: 37385700.
• Tam YH, Pang KKY, Wong YS, Chan KW, Lee KH. From Laparoscopic Pyeloplasty to Robot-Assisted Laparoscopic Pyeloplasty in Primary and Reoperative Repairs for Ureteropelvic Junction Obstruction in Children. J Laparoendosc Adv Surg Tech A. 2018 Aug;28(8):1012-1018. doi: 10.1089/lap.2017.0561. Epub 2018 Mar 13. PMID: 29641368.
• Wong YS, Pang KKY, Tam YH. Comparing Robot-Assisted Laparoscopic Pyeloplasty vs. Laparoscopic Pyeloplasty in Infants Aged 12 Months or Less. Front Pediatr. 2021 Jun 14;9:647139. doi: 10.3389/fped.2021.647139. PMID: 34195160; PMCID: PMC8236621.

Acknowledgement

Principal author: Dr Wong Yuen-shan Sammi on behalf of Department of Paediatric Surgery, HKCH

Initial posting: Oct 2026

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