Renal Pelvis Dilatation

Definition

Fetal renal pelvis dilatation is also called urinary tract dilation or hydronephrosis , though these terms are less precise. It encompasses dilatation confined to the renal pelvis, termed pyelectasis or pelviectasis, and that which also involves the calyces, termed caliectasis or pelvicaliectasis . The pelvis is measured anterior to posterior in the transverse plane. Of the various thresholds used to diagnose dilatation, the most common are 4 mm in the second trimester (up to 20 weeks), and 7 mm at approximately 32 weeks in the third trimester. The second-trimester threshold is used to identify fetuses that need additional US evaluation in the third trimester, and it is also considered a minor marker that confers slightly increased risk for trisomy 21 (see Chapter 152 ). The third-trimester threshold is used to identify cases that need postnatal evaluation. Criteria have been established for mild, moderate, and severe renal pelvis dilatation, based on the risk of neonatal renal abnormality in a metaanalysis of more than 100,000 screened pregnancies. These definitions have been endorsed by the Society for Fetal Urology and are listed in Table 12.1 .

Prevalence and Epidemiology

Renal pelvis dilatation is observed in 1% to 5% of second-trimester pregnancies. The male-to-female ratio is 2:1. In 1.5% of pregnancies, approximately one-third of pregnancies with renal pelvis dilatation, a urinary abnormality is confirmed in the neonatal period. However, 40% to 90% of cases are transient and do not represent pathology. The overall likelihood of an underlying abnormality increases with the degree of dilatation: 12% with mild, 45% with moderate, and 88% with severe hydronephrosis.

The most common associated abnormality is UPJ obstruction, which occurs in 10% to 30% of cases. UPJ obstruction is identified in approximately 1:1000 to 1:2000 births, has a male-to-female ratio between 3:1 and 4:1, and is predominantly left-sided. The prevalence of UPJ obstruction increases with worsening dilatation, occurring in 5% with mild, 17% with moderate, and more than 50% with severe hydronephrosis. It is bilateral in 20% to 25% of cases.

VUR is also associated with renal pelvis dilatation. VUR occurs in approximately 4% with mild renal pelvis dilatation and 8% to 14% with moderate or severe pelvis dilatation. Although VUR is a less common cause of fetal hydronephrosis than UPJ obstruction, it is more frequent in the general population, occurring in approximately 1% of individuals. When VUR comes to attention in the neonatal period because renal pelvis dilatation was diagnosed prenatally, most affected infants are males, and VUR often resolves by early childhood. In contrast, when VUR is identified only after children have developed symptoms, the majority are females, and there is greater risk for renal insufficiency.