• Hydronephrosis: dilation of the renal pelvis (pelviectasis) and calyces (caliectasis) due to excess urine in the collecting system of the kidney
  • Hydroureteronephrosis: dilation of the renal collecting system and the ureter to the level of the bladder


  • Incidence of hydronephrosis noted on routine prenatal ultrasound is 0.4–5%.
  • 10–30% of fetuses with hydronephrosis are due to ureteropelvic junction obstruction.
  • Posterior urethral valves and triad syndrome account for 1–2% of cases.


  • Ureteropelvic junction obstruction
    • Partial obstruction at the region where the renal pelvis drains into the ureter
  • Megaureter
    • Partial obstruction due to an intrinsic narrowing or an aperistaltic segment of distal ureter at the ureterovesical junction
  • Vesicoureteral reflux
    • In primary reflux (grades I to V depending on the severity), it is due to an insufficient flap valve–type mechanism at the ureterovesical junction which allows urine to flow retrograde from the bladder up into the ureters.
    • Hydroureteronephrosis is usually seen only with higher grades of reflux (grades III to V) or secondary reflux (reflux in the presence of an abnormal bladder, in which the reflux is often due to high storage or voiding pressures within the bladder). Secondary reflux is not graded.
  • Ureterocele
    • Hydroureteronephrosis secondary to obstruction of the ureter from a cystic dilation of the intravesical portion of the distal ureter
    • Most often associated with the upper pole ureter in duplicated collecting system; less frequently associated with a single system
    • Ureterocele is further classified as intravesical (contained completely within the bladder) or ectopic (extending down the bladder neck and often into the urethra).
  • Ectopic ureter
    • A ureter that drains into an abnormal location outside of the bladder
    • The hydroureteronephrosis can be the upper pole ureter of a duplicated collecting system or a single system.
    • Ectopic ureters can drain at various sites along the lower urinary tract depending on the sex of the child. In boys, they can drain into the bladder neck, prostatic urethra, vas deferens, seminal vesicle, or epididymis. In girls, they can drain into the bladder neck, urethra, introitus, and vagina.
    • The ectopic locations often require passage through the bladder neck or urogenital diaphragm, which produces obstruction of the distal ureter.
  • Urolithiasis
    • Obstructing calculi often produce dilation of the urinary tract proximal to its location.
    • Stone disease is rare in infancy except in preterm infants who receive furosemide.
    • The hydronephrosis is usually associated with renal colic.
  • Posterior urethral valves
    • Hydroureteronephrosis: nearly always bilateral, produced by outflow obstruction of the bladder from an obstructing membrane in the prostatic urethra
    • Because both kidneys are affected, there is a significant risk of chronic kidney disease and eventual progression to end-stage renal disease.
  • Triad syndrome
    • Hydroureteronephrosis, often with massively dilated ureters and a large bladder
    • Also known as prune belly syndrome or Eagle-Barrett syndrome
    • These boys have a triad of hypoplastic abdominal wall musculature (leading to a prunelike appearance), bilateral undescended testes, and a dilated urinary tract.
    • May have associated urethral atresia, imparting worse renal function prognosis
    • Significant risk of renal insufficiency and bladder dysfunction in these patients

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