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.

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 that occurs during the third trimester may result in marked pelvicaliceal dilation, but usually does not cause renal dysplasia.

Most cases of antenatal hydronephrosis are nonobstructive and will resolve spontaneously in the postnatal period (Belarmino and Kogan, 2006; Becker and Baum, 2006; Hanna, 2006). The diagnosis of UPJ obstruction in the fetus is based on pelvicaliceal dilation that exceeds proposed criteria for minimal fetal hydronephrosis (see Chapter 80). There is no agreement on the absolute cut-off limit for pathologic antenatal pelvic dilation. Most reports suggest postnatal investigation for pelvic dilation of more than 5 mm (Woodward and Frank, 2002; Becker and Baum, 2006). To distinguish primary UPJ obstructions, an anterior-posterior (AP) pelvic diameter of the renal pelvis ≥ 10 mm or the presence of caliectasis could be used (Arger et al., 1985; Kleiner et al., 1987). To distinguish UPJ obstruction from other causes of obstructive uropathy, there must also be pelvic dilation and an absence of findings suggestive of obstructive uropathy at a lower level, such as ureterectasis, vesicomegaly, ectopic ureterocele, and dilated posterior urethra. Although UPJ obstruction is, in a large part, a diagnosis of exclusion, ultrasound examination has proven reliable in determining the level of obstruction (Kleiner et al., 1987).

The overall incidence of genitourinary defects diagnosed by prenatal ultrasound examination has been estimated to be between 0.2% and 0.9%. The incidence of UPJ obstruction is estimated at 0.001% with a male to female ratio of 3:1 (Woodward and Frank, 2002). In newborns the left side is affected in almost two thirds of cases. Bilateral UPJ obstruction is seen postnatally in only 15% of cases (Johnston et al., 1977) but may be more common in prenatally diagnosed cases, as observed by Flake et al. (1986).

In unilateral UPJ obstruction the renal pelvis and infundibulum are dilated (Figure 81-1). Dilation of the calices may also be seen. The anterior to posterior pelvis to kidney ratio is greater than 50% (Arger et al., 1985). The following findings are not seen in primary UPJ obstruction: dilated ureter, ectopic ureterocele, dilated or thickened urinary bladder, or a dilated posterior urethra. In more severe cases of primary UPJ obstruction only a single fluid-filled structure may be seen, which is the dilated pelvis with only a thin rim of surrounding cortex (Figure 81-2) (Mahony, 1994). In even more severe cases, the renal pelvis may be so dilated that it appears as an abdominal cyst in which not even a rim of renal parenchyma is seen. Such severe UPJ obstructions can reach a significant size, distending the fetal abdomen and elevating the diaphragm (Jaffe et al., 1987). In rare instances, both the infundibulum and the renal pelvis are stenotic and only caliceal dilation is seen (Lucaya et al., 1984). In the most severe form of UPJ obstruction, the collecting system can rupture, with the formation of a perinephric urinoma or urinary ascites. Although some reports suggest it is unusual to have salvageable renal function in a kidney with a perinephric hematoma, in some instances this rupture may be protective (Callen et al., 1983; Friedland et al., 1983; Adzick et al., 1985; Harrison and Filly, 1991). In unilateral UPJ obstruction, the contralateral kidney produces a normal volume of amniotic fluid and the bladder fills and empties normally, even in cases in which the UPJ obstruction has resulted in renal dysplasia. It is important to recognize that UPJ obstruction may be seen frequently in association with contralateral multicystic dysplastic kidney or renal agenesis. Either condition, in association with high-grade UPJ obstruction, may produce profound oligohydramnios and Potters syndrome features (Mahony, 1994).

Bilateral UPJ obstruction is present in 15% to 50% of cases, depending on the series reported (Flake et al., 1986; Kleiner et al., 1987). Fortunately, the involvement is often asymmetric and severe bilateral obstruction is rare. However, as reported by Flake et al. up to 5% mortality can be seen with bilateral UPJ obstruction, which progresses to oligohydramnios and secondary pulmonary hypoplasia. UPJ obstruction paradoxically results in polyhydramnios in up to 25% of cases (Kleiner et al., 1987). The pathophysiology of the polyhydramnios is uncertain but is thought to be due to hyperfiltration occurring in the partially obstructed kidneys.

The differential diagnosis of UPJ obstruction includes all other causes of hydronephrosis, including vesicoureteral reflux, megaureter, obstructed duplicated collecting system, and bladder-outlet obstruction (Table 81-1). A detailed sonographic evaluation of the kidneys should differentiate the anatomic level of obstruction causing hydronephrosis. Bladder outlet obstruction, whether due to urethral atresia or posterior urethral valves, should be associated with vesicomegaly with a thick and often trabeculated bladder wall. Cases of ureterovesical obstruction, such as megaureter, are distinguished by the lack of vesicomegaly, with a dilated tortuous ureter extending from the renal pelvis to the ureterovesical junction. A duplex collecting system frequently results in hydronephrosis due to obstruction at the site of the ureteral insertion, often at an ectopic site. This may occur in association with an ureterocele (see Chapter 83). In such cases, the ureter is uniformly dilated up to the superior pole of the kidney, which may have cystic dysplastic changes from long-standing high-grade obstruction.

Vesicoureteral reflux may occur in association with UPJ obstruction and may be difficult to distinguish from UPJ obstruction as a cause of hydronephrosis. One sonographic observation that is helpful in distinguishing between these two diagnoses is observing the degree of pelvic dilation that occurs with bladder emptying. UPJ obstruction is minimally affected by voiding; however, vesicoureteral reflux is usually evidenced by marked fluctuations in the size of the ureter and renal pelvis associated with bladder contraction.

The most important determinants of the outcome of UPJ obstruction in the fetus are the gestational age at onset, severity of obstruction, and whether the UPJ obstruction is unilateral or bilateral. In early gestation, high-grade obstruction at the ureteropelvic junction that occurs during the first trimester results in a multicystic dysplastic kidney. In UPJ obstruction that occurs during the second trimester, the fetal kidneys are at risk for renal dysplasia and compromised renal function at birth. In contrast, UPJ obstruction that occurs during the last trimester rarely causes renal dysplasia, even in cases of high-grade obstruction. This vulnerability of the developing kidney to obstruction-induced dysplasia likely reflects the effect of increased pressure during the nephrogenic phase of renal development between 20 and 30 weeks of gestation (Harrison and Filly, 1991).