• Ureteropelvic junction (UPJ) obstruction: obstruction of ureter at the narrow junction with the renal pelvis, resulting in dilatation of the upper collecting system proximal to the obstruction; usually unilateral; may be due to intrinsic fibrotic narrowing of the ureter or due to extrinsic compression (i.e., accessory renal artery to the lower pole)
• Ureterovesical junction (UVJ) obstruction: usually due to a ureterocele that consists of cystic dilatations of the terminal ureter, often protruding into bladder; usually unilateral but can obstruct contralateral side as well as the bladder neck, associated with duplex anomalies of upper urinary tract (usually associated with the upper pole but may be associated with a single system)
• Vesicoureteral reflux (VUR): passage of urine from bladder to kidneys during voiding secondary to an incompetent valvular mechanism at the ureterovesical junction; may be unilateral or bilateral; usually no oligohydramnios; may be primarily due to incompetent vesicoureteral valve or secondary to bladder outlet obstruction
• Posterior urethral valves (PUVs): most common form involves mucosal folds protruding from the distal aspect of the verumontanum, which fuse to form an oval-shaped diaphragm, resulting in bladder outlet obstruction; may have primary reflux due to short intravesical tunnels or secondary reflux due to paraureteral diverticulae causing obstruction; may have renal dysplasia in severe cases; most common cause of urinary ascites or urinoma (1%–10% of infants with PUV have urinary ascites)
• Prune-belly syndrome: hypoplastic abdominal muscles associated with undescended testes and urinary tract abnormalities; associated with oligohydramnios, chronic renal failure, and pulmonary hypoplasia, VUR, and malrotation with a universal mesentery may be present
• Hydronephrosis may be associated with other congenital anomalies: urethral atresia, cloacal plate anomalies, midureteral stricture, ectopic ureter, multicystic kidney, retrocaval ureter, obstructive megaureter, hydrocolpos, pelvic tumor

• UPJ obstruction: 1 in 1,500 births, 65% male, 5% bilateral, most common cause of hydronephrosis
• UVJ obstruction secondary to ureterocele: Those associated with duplex systems affect females seven times more than males; single-system ureteroceles usually affect males; 10% of cases are bilateral.
• VUR: Prevalence is unknown, but genetic factors may be involved; 33% of siblings of index cases and 66% of children of parents with VUR have VUR
• PUV: 1 in 5,000 to 8,000 males; most common cause of lower urinary tract obstruction in males
• Prune-belly syndrome: 1 in 40,000 births

• Renal tumors, such as Wilms tumor, may present as an abdominal mass; ultrasonography or abdominal computed tomography should distinguish a tumor.
• Multicystic dysplastic kidney, unilateral; most common renal cystic disease in infants, and second most common cause of unilateral abdominal mass; DMSA scan reveals no renal tissue, and intravenous urography shows no kidney function.

• For congenital hydronephrosis, there is increased mortality if associated with nonrenal congenital anomalies: 52% versus 17% in isolated hydronephrosis.
• Oligohydramnios is a poor prognostic factor.
• Normal amniotic fluid volumes and unilateral hydronephrosis are associated with a better neonatal course.
• Long-term survival is related to the extent of preservation of renal function: 50% of long-term survivors of prune-belly syndrome require renal replacement therapy or transplantation.

• In general: prenatal hydronephrosis, oligohydramnios, neonatal abdominal mass, urinary ascites, urinoma, pulmonary hypoplasia with respiratory distress, particularly if seen in full-term baby
  • UPJ: abdominal, flank, or back pain; hematuria after minimal trauma; flank pain after large volume load
  • Ureterocele: Sonography detects hydronephrosis, a double- or single-collecting system, and possible dysplasia.
  • VUR: upper tract infection, sibling with VUR, symptoms of voiding dysfunction
  • PUV: poor urinary stream or dribbling seen in approximately 30% of cases

• In general: abdominal mass, signs of renal insufficiency (i.e., hypertension, failure to thrive, acidosis)
• PUV: palpable walnut-shaped bladder, hydroceles due to urinary ascites
• Prune-belly: wrinkled appearance of abdominal wall due to lack of abdominal wall musculature
  • Ten percent with cardiac anomalies, 50% with anomalies of the musculoskeletal system (i.e., limb anomalies and scoliosis)
  • Three percent of infants with prune-belly syndrome are female: anomalies of urethra, uterus, and vagina usually associated

• Serial serum creatinine and electrolytes, especially if bilateral hydronephrosis is present—90% of infants with PUV present with some degree of renal insufficiency; children may have renal insufficiency from dysplasia or acquired damage due to VUR and infection.
  • At birth, the infant’s creatinine is the same as the mother’s, but it should decrease to 0.4 by the end of the first week of life.
  • Creatinine levels in premature babies may not decrease until 34 to 35 weeks’ gestation.
  • If hydronephrosis is unilateral, document normal creatinine and electrolytes.

• Prenatal ultrasound can detect hydronephrosis and oligohydramnios; US performed later detects more anomalies.
  • Neonates have transient oliguria, so the collecting system may appear falsely normal; 72 hours of life is the best time to perform US.
  • UPJ obstruction: theoretically, dilated cysts communicate but can be difficult to distinguish from multicystic renal dysplasia; distal ureter should not be seen, and bladder appears normal.
  • Ureterocele: cystic dilatation of distal ureter
  • VUR: findings of intermittent hydronephrosis
  • PUV: hypertrophied and thickened bladder and bladder neck
  • Serial US is helpful. Progressive hydronephrosis indicates the presence of obstruction, while resolution of hydronephrosis indicates absence of obstruction.
• Voiding cystourethrography, in general, detects degree of VUR and dilatation or tortuosity of the lower collecting system.
  • PUV thickened and trabeculated bladder wall, 50% with diverticulae, VUR in 60% of cases
• Intravenous urography, in general, evaluates excretory function of renal parenchyma.
  • Ureterocele appears as a radiolucent circular shadow in bladder; if there is poor function, a duplicated upper pole may not be visualized, producing the drooping lily appearance of the lower collecting system.
• A DMSA scan, in general, identifies the presence of renal tissue which is found in the case of UPJ obstruction with communicating cysts. Multicystic renaldysplasia has noncommunicating cysts with no renal parenchyma. A DMSA scan is also important in detecting renal scarring that may be a long-term consequence of infection and reflux.
  • Forty percent with VUR show scarring at baseline.
• Diuretic renography is useful in assessing renal function. Mercaptoacetyl triglycine (MAG-3) (excreted by renal tubules) is more accurate in newborns and infants than is diethylenetriamine pentaacetic acid (DTPA).
  • Uptake is used to calculate differential renal function; normally each side is 50%.
  • Then furosemide is administered to assess the efficiency of drainage of kidneys. This is useful only if there is brisk washout, which indicates that there is no anatomic obstruction.
  • In unilateral neonatal hydronephrosis, if RUS and VCUG show no definite signs of anatomic obstruction, serial diuretic renography is useful.
  • If the first study shows function greater than 40%, repeat in 3 months; if 30% to 40%, repeat in 2 months; if 20% to 30%, repeat in 1 month; if less than 20%, repeat in 2 weeks.
  • If there is no deterioration in function of the involved kidney and no compensatory growth in the contralateral kidney, there is no evidence of anatomic obstruction.
  • If there is any decrement in renal function, indicating obstruction, surgical intervention is indicated.
  • Since less than 15% of infants will require surgery for obstruction, careful observation with serial diuretic renography is a reasonable option.

• Prenatal surgery: carries a high rate of complication; consider only if the condition is life-threatening (i.e., PUV) or if hydronephrosis is unilateral or there is normal amniotic fluid volume and pulmonary development; prenatal intervention is not indicated.
• Circumcise boys to decrease the risk of UTI.
• UPJ obstruction: Surgical excision (pyeloplasty) of obstructed portion of UPJ with reanastomosis is 91% to 99% successful if performed as soon as diagnosis is made; perform nephrectomy if no renal function is evident.
• Ureterocele: endoscopic incision with flap to minimize VUR; nephrectomy if poor renal function; in duplicated system, if upper pole is nonfunctional, may perform upper segment nephrectomy; if upper pole functions, may anastomose upper pole ureter with the lower pole ureter
• VUR: reimplantation of ureter for high-grade reflux, persistent VUR, or associated with breakthrough upper tract infections on antibiotic prophylaxis
• PUV: Drain the bladder with a urethral catheter or vesicostomy. Definitive treatment involves transurethral ablation of valves with resectoscope or fulguration. The prognosis is better if there was a normal US at less than 24 weeks’ gestation.
• Prune-belly: orchiopexy in the first year of life

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