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KEY POINTS
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Key Points

  • Ureteropelvic junction obstruction (UPJ) prevents urinary flow from the renal pelvis to the ureter, and is the most common cause of significant neonatal hydronephrosis.

  • The timing of onset of obstruction determines the clinical presentation, ranging from multicystic dysplastic kidney to pelvicaliceal dilation for first trimester compared with third trimester obstruction respectively.

  • Most cases are unilateral, with bilateral cases, or cases in which the contralateral kidney has multicystic dysplastic changes, having much worse prognosis.

  • Pregnancy management is generally unchanged in unilateral UPJ cases, while fetal intervention by means of urinary shunting may be necessary in rare cases of severe bilateral UPJ.

  • Management of the neonate usually includes delaying imaging until 3–7 days of life to allow completion of normal physiologic diuresis and initiation of prophylactic antibiotics.

  • Indications for surgical intervention include renal function less than 40% expected, worsening hydronephrosis, renal pelvic diameter greater than 5 cm, or febrile morbidity.

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CONDITION
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Obstruction at the ureteropelvic junction (UPJ) is the most common cause of significant neonatal hydronephrosis (Lebowitz and Griscom, 1986). Because 85% to 90% of affected newborns appear entirely normal on physical examination at birth, prenatal recognition permits treatment of a condition that may otherwise be unrecognized (Grignon et al., 1986). This form of hydronephrosis is characterized by obstruction to the flow of urine from the renal pelvis to the ureter. UPJ obstruction may be classified as either primary or secondary. Primary causes of UPJ obstruction would include intrinsic problems such as intraluminal valves, polyps, congenital mucosal folds, muscular hypertrophy, and functional obstruction and extrinsic causes of obstruction such as aberrant crossing vessels. Secondary causes of UPJ obstruction would include vesicoureteral reflux or more distal obstruction. Among the suggested causes are intrinsic valves at the UPJ, abnormally thickened or oriented muscular bands at the UPJ, high insertion of the ureter on the renal pelvis, anomalous crossing bands or vessels at the UPJ, ischemia, and segmental ureteral dismotility (Williams and Karlaftis, 1966; Kelalis et al., 1971; Johnston et al., 1977; Hendren et al., 1980; Maizels and Stephens, 1980). In the majority of cases, however, a patent UPJ is found at the time of surgical correction. In most cases therefore, the obstruction appears to be more functional than mechanical.

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While the pathogenesis of UPJ obstruction is poorly understood, some aspects of the prenatal history are known. Complete obstruction at the UPJ before 8 to 10 weeks of gestation results in severe dysplastic changes in the developing kidney (Scholtmeijer and van der Harten, 1975; Potter, 1976; McGrory, 1980; Sanders and Hartman, 1984). The result is a multicystic dysplastic kidney (see Chapter 78). In contrast, incomplete UPJ obstruction that occurs during the second trimester does not result in multicystic dysplastic kidney but may result in variable degrees of renal dysplasia, in addition to pelvicaliceal dilation. In contrast, UPJ obstruction ...

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