Chapter 44

### INTRODUCTION

The urinary system is essential for the elimination of metabolic wastes, maintenance of physiologic pH, and the preservation of fluid and electrolyte homeostasis. It also contributes to vital functions like erythropoiesis and blood pressure control. Obstruction in the upper or lower urinary tract system can lead to disruption in these vital body physiologies and can lead to significant morbidity, which may be irreversible and ultimately lead to mortality. In neonates and young children, reports have shown that obstruction of the urinary tract is the most common cause of renal insufficiency, especially in boys under the age of 1, as well as the number 1 cause of renal transplantation in children.1, 2 The majority of these obstructive uropathies are congenital in nature. The neonatal kidney is still developing, therefore obstruction can lead to alterations in the normal growth pattern and differentiation of the renal cells beginning in the intrauterine period and progressing postpartum, ultimately resulting in the development of fibrosis and renal failure. Obstruction can be limited to the upper urinary tract, the lower urinary tract, or a combination.

The main sign of obstructive uropathy is hydronephrosis, which involves dilation of the renal collecting system, including the renal pelvis and the renal calyces. Even the ureters may be dilated from the accumulation of urine in the collecting system. Genitourinary anomalies can be found in up to 0.2%–5% of pregnancies. Hydronephrosis comprises up to 87% of the identified anomalies. From prenatal ultrasound screening, varying degrees of hydronephrosis can be seen in up to 1.4% of fetuses, detected as early as the 12th week of pregnancy. More than half of these resolve spontaneously after birth. It is estimated that 48% are transient in nature (ie, from fetal folds in the proximal ureter during organogenesis), 15% are normal physiologic findings, 11% are from ureteropelvic junction (UPJ) obstruction, 9% are from vesicoureteral reflux (VUR), 4% are associated with congenital megaureters, and the others derive from miscellaneous causes (ie, from ureteroceles, posterior urethral valve [PUV], prune-belly syndrome [PBS]). There is a 2:1 male-to-female predominance of hydronephrosis. It can also be bilateral in 20%–40% of neonates.

Prenatal sonogram is mostly the means by which obstruction in the urinary tract is picked up in the developed world. If prenatal ultrasound was not obtained, neonatal presentations can range from palpable abdominal mass, urinary tract infections (UTIs), hematuria, failure to thrive, and occasionally renal failure. The risk of postnatal pathology from a prenatal finding of hydronephrosis has been correlated with the degree of the hydronephrosis seen. One meta-analysis reported 11.9% in cases of mild hydronephrosis and 45.1% in moderate and 88.3% in severe antenatal hydronephrosis.3 We examine some of the most common obstructive uropathies in neonates.

### URETEROPELVIC JUNCTION OBSTRUCTION

#### Epidemiology

As the name suggests, UPJ obstruction is an obstruction in the urinary tract at the junction between the renal pelvis and the ...

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