DESCRIPTION

Ureteropelvic junction (UPJ) obstruction is a partial blockage of the kidney at the point where the renal pelvis transitions into the proximal ureter.

EPIDEMIOLOGY

• 45% of all cases of significant prenatal hydronephrosis are due to UPJ obstruction.
• Occurs more commonly in males (M/F 2:1)
• Left-sided lesion more common (66%)
• Bilateral in 10–40%

PATHOPHYSIOLOGY

• The obstruction can cause varying degrees of hydronephrosis.
• Mild forms of UPJ obstruction result in dilation of the renal pelvis without loss of function.
• More severe forms result in dilation of the renal pelvis and calyces with loss of renal parenchyma and decreased function.
• In the most severe cases, the kidney may have cystic dysplasia and very poor function.
• Congenital hydronephrosis owing to an intrinsic narrowing is nearly always asymptomatic.
• When the obstruction is intermittent owing to a crossing vessel, the renal pelvis becomes distended (most commonly owing to a transient increase in urine output), which drapes it over the vessel and kinks the ureter, resulting in an acute obstruction. The acute distention of the renal pelvis results in severe pain (renal colic).

ETIOLOGY

• Intrinsic: a congenital narrowing of the UPJ, which is most commonly due to abnormal musculature and fibrosis of this area, resulting in an adynamic segment
• Extrinsic: kinking at the UPJ, which is most commonly due to the renal pelvis draping over a lower pole crossing vessel. This type of obstruction can be intermittent.

ASSOCIATED-CONDITIONS

• Patients with UPJ may have an additional genitourinary malformation, most commonly
  • Vesicoureteral reflux
  • Contralateral UPJ obstruction
  • Multicystic dysplastic kidney
  • Renal agenesis
• Of patients with VATER association, 21% have UPJ obstruction and thus should be screened with renal ultrasound. (VATER stands for vertebral defects, anal atresia, tracheoesophageal fistula with esophageal atresia, and radial and renal anomalies.)