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Posterior urethral valves (PUV)
Posterior urethral valves are the commonest cause of congenital lower urinary tract obstruction in boys; in pregnancy there is a large thick-walled bladder with bilateral hydroureteronephrosis and oligohydramnios, and after birth the bladder is drained with a thin tube, the diagnosis is made by VCUG and the valves are ablated endoscopically.
Why it matters in the exam
The incidence is about 1 in 5000 to 8000 boys. Prenatal ultrasound shows a dilated bladder with a thick wall, a dilated posterior urethra (the "keyhole sign"), bilateral dilatation of the ureters and kidneys, and oligohydramnios — which can cause pulmonary hypoplasia, the main cause of neonatal death. The exam shows a fetus or newborn with these findings and asks about the first step after birth or about prognosis.
Immediately after birth a thin tube (a 5 to 8 Fr feeding tube) is placed in the bladder — preferred to a balloon catheter, which may cause bladder spasms. VCUG is performed, showing a dilated, elongated posterior urethra and sometimes reflux, and creatinine and electrolytes are checked. When the baby is stable the valves are ablated at cystoscopy; when the urethra is too small — a temporary vesicostomy. Reflux is present in about half of the children. Intrauterine intervention (a vesicoamniotic shunt) is still controversial.
The important predictor of future renal function is the lowest creatinine in the first year (nadir): a value below 1.0 mg per dL predicts a better prognosis. About a third of the children reach chronic kidney disease, and some need transplantation. "Pop-off" mechanisms — unilateral reflux into a dysplastic kidney, a urinoma or a large diverticulum — protect the other kidney. Even after successful ablation a "valve bladder" with low compliance and polyuria often remains, so lifelong urological and nephrological follow-up is needed.
Key points
- The commonest cause of congenital urethral obstruction in boys.
- Keyhole and oligohydramnios in pregnancy.
- Diagnosis by VCUG.
- Endoscopic valve ablation.
- Nadir creatinine below 1.0 — better prognosis.
Summary table
| Stage | Finding or action | Note |
|---|---|---|
| In pregnancy | Large thick bladder, keyhole, oligohydramnios | Risk of pulmonary hypoplasia |
| Right after birth | Thin tube into the bladder | 5 to 8 Fr, no balloon |
| Diagnosis | VCUG | Dilated posterior urethra |
| Treatment | Endoscopic valve ablation | Vesicostomy if the urethra is small |
| Prognosis | Nadir creatinine in the first year | Below 1.0 — better |
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Sources
- EAU Guidelines on Paediatric Urology, 2025 edition — Posterior urethral valves
- Campbell-Walsh-Wein Urology, 12th edition (2020) — Posterior Urethral Valves
- Nelson Textbook of Pediatrics, 22nd edition (2024) — Obstruction of the Urinary Tract
Sources are given for further reading; the text is written afresh and not copied from them.
Reviewed by the Examqo AI Professor · 2026-11-06. The text may contain errors; when in doubt, check the source.