• Hydronephrosis refers to a structural finding: dilatation of the renal calyces and pelvis.
  • Can be accompanied with hydroureter (dilatation of the ureter)
  • Hydronephrosis should not be used interchangeably with obstructive uropathy, which refers to the damage to renal parenchyma resulting from urinary tract obstruction (UTO).


  • More common in children than adults due to congenital anomalies.
  • Hydronephrosis is more common in women for adults <60 years old and in men for adults >60 years old.

Etiology and Pathophysiology

  • Hydronephrosis develops with increased pressure in the urinary collecting system, most commonly from some form of obstruction.
  • Nonobstructive hydronephrosis can occur in the setting of very high urinary output such as with diabetes insipidus or as a physiologic change in pregnancy.
  • Hydronephrosis may be acute/chronic, partial/complete, and uni-/bilateral.
  • Obstruction may occur at an level of the GU system:
    • Kidney: nephrolithiasis, transitional cell carcinoma, sloughed renal papillae, congenital ureteropelvic junction (UPJ) obstruction, blood clot, fungal ball
    • Ureter: nephrolithiasis, transitional cell carcinomas, strictures, sloughed renal papillae, retroperitoneal fibrosis, extrinsic compression
    • Bladder: neurogenic bladder, extrinsic compression, posterior urethral valves
    • Urethra: prostatic hypertrophy or cancer, strictures
  • Hydronephrosis in a transplanted kidney is more common than in native kidneys, due to ureteral reflux, strictures, ureteral compression (from peritransplant lymphoceles, hematomas) and bladder dysfunction.

Pediatric Considerations

  • Antenatal hydronephrosis is diagnosed in 1–5% of pregnancies, usually by US, as early as the 12th to 14th week of gestation.
  • Children with antenatal hydronephrosis are at greater risk of postnatal pathology.
  • Postnatal evaluation begins with US exam; further studies, such as voiding cystourethrogram (VCUG), based on the severity of postnatal hydronephrosis
  • In neonates, it is the most common cause of abdominal mass.
  • Common etiologies in children are VUR, congenital UPJ obstruction, neurogenic bladder, and posterior urethral valves.
  • Pediatric diagnostic algorithm differs from adult due to different differential diagnosis necessitating age-appropriate testing.

Pregnancy Considerations

  • Physiologic hydronephrosis in pregnancy is more prominent on the right than left and can be seen in up to 80% of pregnant women.
  • Dilatation is caused by hormonal effects, external compression from expanding uterus, and intrinsic changes in the ureteral wall.
  • Despite high incidence, most cases are asymptomatic.
  • If symptomatic and refractory to medical management, ureteric calculus should be considered and urinary infection must be excluded.

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