Chapter XIII.7. Hydronephrosis and Renal Anomalies
Daniel M. Bender, MA
Robert G. Carlile, MD
August 2022

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The 2004 first edition version of this chapter included vesicoureteral reflux and urinary tract problems. For the current third edition, these topics have been separated into two chapters. See Chapter XIII.8. Vesicoureteral Reflux and Urinary Tract Problems.

A term newborn male infant is noted to have unilateral hydronephrosis on prenatal ultrasound. At 2 days of age, a renal and bladder ultrasound shows a normal right kidney, moderately severe left renal hydronephrosis, no dilation of the ureter, and a normal bladder, consistent with P2 urinary tract dilation (UTD). A voiding cystourethrogram (VCUG) obtained at 6 weeks of age shows no evidence of reflux and no posterior urethral valves. Urinalysis, complete blood count, electrolytes, BUN, and creatinine are normal. At 4 weeks of age, a MAG3 nuclear medicine renal scan done under propofol sedation with furosemide washout half-time (T-1/2) shows equal split function (right kidney 50%, left kidney 50%). The T-1/2 is normal on the right and prolonged on the left.

The patient is observed with serial renal ultrasounds and MAG3 renal scans for the next two years. At 2 years of age, he develops left-sided abdominal pain, nausea, and vomiting, without fever or chills. A renal ultrasound shows worsening left hydronephrosis and a MAG3 renal scan shows diminished left renal split function to 35% (the right split function is now 65%), and markedly prolonged left renal washout half-time. The right renal washout half-time is normal.

He undergoes a left pyeloplasty at 2 years of age and does well post-operatively. A MAG3 renal scan is done 3 months postoperatively, which shows the left renal split function returned to 45% (right 55%), and a normalized washout half-time. A renal ultrasound postoperatively shows only minimal residual hydronephrosis.

Hydronephrosis, or urinary tract dilation (UTD), refers to the enlargement of the renal pelvis and calyces. In children, hydronephrosis may occur secondary to transient urinary tract dilation, vesicoureteral reflux (VUR), or congenital anomalies which predispose to obstruction. For the pediatrician, a diagnosis of hydronephrosis is often made long before a child enters their care, as many cases are discovered incidentally via prenatal ultrasonography. Urinary tract dilation represents one of the most common prenatal ultrasound findings, occurring in 1% to 2% of all pregnancies (1). Most cases of antenatal hydronephrosis are transient and resolve spontaneously before the end of pregnancy or within the first year of life. However, about 20% of diagnosed antenatal hydronephrosis cases may be clinically significant postnatally (2).

In the pediatric population, hydronephrosis is most often a consequence of abnormal ureteral development. During the fifth week of gestation, the ureteral bud develops as a posterior diverticulum of the mesonephric duct. The ureteral bud penetrates the adjacent metanephric blastema, inducing the formation of the metanephric kidney. The ureteral bud then goes on to develop the collecting system consisting of the ureter, renal pelvis, calyces, and collecting ducts. As the ureters develop, they become temporarily obstructed and then undergo physiologic recanalization. Meanwhile, the acquisition of circular smooth muscle begins at the ureterovesical junction (UVJ) and proceeds cephalad. The ureteropelvic junction (UPJ) is the proximal (superior) portion of the ureter that connects to the renal pelvis (not the bony pelvis), which is the last segment of the fetal ureter to canalize and acquire smooth muscle. Urine formation begins shortly thereafter at 12 weeks of gestation (3). It is believed that failure of canalization with abnormal development of circular musculature and/or collagen fibers is responsible for many of the dilatations observed in the neonatal period (2,4). The most implicated sites are the ureteropelvic junction, followed by the ureterovesical junction; these two conditions comprise the most common postnatal causes of congenital hydronephrosis.

Other genitourinary abnormalities causing urinary tract dilation include vesicoureteral reflux, urethral valves or stricture, ectopic ureter, ureterocele, megacalycosis, and prune-belly syndrome. Any non-genitourinary pathology resulting in compression or obstruction of the urinary system, such as peripelvic cysts, sacrococcygeal teratoma, or bowel duplication, can also result in hydronephrosis. Furthermore, it should be noted that renal anomalies such as multicystic dysplastic kidney and simple renal cysts may be confused with a hydronephrotic kidney on ultrasonography and should therefore be included in the differential.

Before the extensive use of prenatal ultrasonography, neonatal hydronephrosis could present as flank or abdominal pain, urinary tract infections, hematuria, abdominal mass, feeding difficulties, and/or failure to thrive. However, it was much more common for patients to present later, between the ages of 6 to 15 years (5). Today, hydronephrosis in the older child is often found incidentally during the workup for nonspecific abdominal complaints.

For the infant noted to have antenatal hydronephrosis, a postnatal ultrasound should be performed to confirm the diagnosis (6). Ultrasound is the best next diagnostic step as it is the least invasive, uses no radiation, and can provide excellent morphological evaluation of the kidney, ureter, and bladder (7). Visualization of the kidneys allows the clinician to determine the degree of hydronephrosis and caliectasis (dilation of the renal calyces), as well as to assess kidney size, parenchymal thickness, and cortical texture. The presence of the contralateral kidney, cortical cysts, calcifications, and the corticomedullary junction can also be noted. Assessing the morphology of the ureter and bladder wall can help to uncover congenital urinary tract abnormalities, such as ureteral duplication or ureteroceles. A careful evaluation can also rule out distal ureteral dilation, eliminating the need for further contrast studies of the ureter (2).

The Urinary Tract Dilation (UTD) classification system was developed in 2014 to standardize the grading and management of prenatal and postnatal hydronephrosis. It incorporates ultrasonographic parameters such as the anteroposterior renal pelvic diameter (APD), calyceal dilation, and appearance of the kidney parenchyma, ureters, and bladder to stratify patients into resolved, low-risk, intermediate-risk, and high-risk groups. If the urinary tract dilation has resolved on postnatal ultrasound, then a repeat ultrasound is done between 3 and 9 months of age. If no hydronephrosis is present at follow-up, the evaluation can then stop. Low-risk patients (P1 UTD) are those with an APD diameter of 10 to 15mm with or without central calyceal dilation. A follow-up ultrasound is recommended between 3 to 6 months in addition to outpatient urology/nephrology care. Ultrasound findings that place the child in the intermediate-risk group (P2 UTD) include an APD diameter of greater than 15mm with or without peripheral calyceal or ureteral dilation. High-risk patients (P3 UTD) share the same findings as P2, with the addition of renal parenchymal or bladder abnormalities. Those in the intermediate or high-risk groups should be urgently referred to urology/nephrology for closer follow-up and management (6).

Traditionally, a voiding cystourethrogram (VCUG) was performed if the hydronephrosis persisted after birth. This recommendation was based on data that suggests that up to 30% of prenatally diagnosed UTD is associated with vesicoureteral reflux; however, routine ordering of VCUG has fallen out of favor, since the natural history of VUR-associated antenatal hydronephrosis indicates that many cases resolve spontaneously (6). A VCUG should still be obtained in those in the high-risk group, and in any child with evidence of ureteral dilation, a thick-walled bladder, and/or suspected posterior urethral valves (2,6).

Renal scintigraphy (or diuretic renography), a nuclear medicine study, is another diagnostic tool indicated for those with intermediate and high-risk hydronephrosis. It uses radiolabeled isotopes MAG3 (mercaptoacetyltriglycine) and DMSA (dimercaptosuccinic acid, also known as succimer) to assess renal anatomy and function. In MAG3 diuretic renography, uptake of the tracer by the renal parenchyma is analyzed and compared, allowing for the assessment of differential renal function. The normal kidney with preserved GFR uptakes the radioisotope more readily than the hydronephrotic kidney, which indicates separate renal function (the split function). Diminished renal function is present when the split function is less than 40%, while good renal function is demonstrated by split function values of 45% to 50% (5). Renal excretion is then evaluated with the administration of intravenous furosemide 20 minutes after administration of the initial radiotracer. Sequential images, computer generated time activity curves, and calculated half-times then determine the degree of washout of the tracer in the area of interest. Prolonged washout times (washout half-times, abbreviated as T-1/2) greater than 20 minutes are often associated with true urinary tract obstruction (2). A DMSA renal scan is similar to the MAG3 scan and is useful in the evaluation of cortical renal scarring and patients with VUR.

Less commonly performed diagnostic tests for the evaluation of hydronephrosis include magnetic resonance (MR) urography, CT scan, intravenous pyelography, and urodynamics. MR urography has the advantage of providing detailed anatomic information while simultaneously evaluating renal function and drainage; however, high costs and the need for sedation limit its widespread use in infants and pediatric patients. A CT scan is also useful for visualizing renal and ureteral anatomy. CT angiography can be used to image vessels traversing the ureteropelvic junction. Excretory urography (IVP-intravenous pyelography) performed in older infants and children defines the collecting systems well and can be very useful with ectopic kidneys, duplicated kidneys, and ureters, as well as megaureters. Urodynamics (bladder function studies) are indicated when a functional obstruction is suspected (neurogenic, or non-neurogenic). Patients with spinal dysraphism should be evaluated with urodynamics.

Serum chemistries, especially creatinine, are also useful in hydronephrosis patients and should be obtained early on to help establish baseline renal function. Imaging studies must be evaluated in conjunction with the other imaging, laboratory, and clinical findings over time, especially with a period of observation (with serial studies), before definitive treatment options are considered.

One point of controversy in the management of prenatally diagnosed urinary tract dilation is whether to administer continuous antibiotic prophylaxis (CAP) for the prevention of recurrent urinary tract infection (UTI). The rate of UTI in patients with antenatal hydronephrosis is between 8% and 22% (6). To date, many of the studies aimed at answering this question have yielded inconsistent results. Of note, a meta-analysis from the European Association for Pediatric Urology did not find evidence supporting the widespread use of continuous antibiotic prophylaxis (1). As of February 2022, the American Urological Association recommends that physicians assess each patient’s unique risk for developing UTI when considering whether to initiate antibiotic prophylaxis (6). Important risk factors for treatment consideration include female sex, intact foreskin, severe hydronephrosis (P3 UTD), ureteral dilation greater than 7 mm, and the presence of urinary tract obstruction (7).

The widespread use of obstetrical ultrasound has resulted in the detection of antenatal hydronephrosis as a common presentation of congenital renal, ureteral, bladder, and urethral anomalies. Prompt evaluation and treatment of urinary tract anomalies in the neonatal period have allowed for increased long-term preservation of kidney function and urinary continence than was possible prior to the advent of prenatal ultrasonography. Patients with hydronephrosis have an excellent prognosis today.

Ureteropelvic Junction Obstruction

The most common cause of postnatal congenital hydronephrosis is ureteropelvic junction (UPJ) obstruction, which accounts for 10% to 30% of all cases (8). It may result from failure of physiologic ureteral recanalization or external compression by a renal vessel. Renal ultrasound will reveal a dilated renal pelvis and calyces without ureteral dilation. Diuretic renography/renal scan will show an obstructive pattern with prolonged washout half time (4).

Treatment options for UPJ obstruction range from active surveillance to pyeloplasty. Neonates with greater than 40% renal function are typically followed with serial renal US studies, since the spontaneous resolution of UPJ obstruction has been demonstrated. Antibiotic prophylaxis for patients with isolated UPJ-associated hydronephrosis has no proven benefits (6). Ureteropelvic junction obstruction repair (pyeloplasty) is recommended in a minority of cases when there is evidence of UPJ obstruction and deteriorating renal function (less than 40% split function), and no sign of distal ureteral distention (9). The procedure involves excision of the stenotic segment and reattaching the normal ureter and renal pelvis. Success rates for pyeloplasties range from 91% to 98% (2,9).

Ureterovesical Junction Obstruction

Ureterovesical junction (UVJ) obstruction, or primary obstructed megaureter, is the second most common cause of congenital hydronephrosis. It arises when collagenous tissue replaces muscular tissue along the distal ureter, disrupting normal ureteral peristalsis (2).

Diagnostic imaging is essential to evaluate the cause of a dilated ureter (megaureter). Primary dilated ureters are divided into three categories: refluxing megaureters, obstructed megaureters, and non-obstructed, non-refluxing megaureters. An uncommon fourth variety is the refluxing, obstructed megaureter. Megaureters can also develop secondary to tumors, retroperitoneal fibrosis, vascular malformations, and neurogenic bladder in those with spinal dysraphism. Renal ultrasound of a megaureter reveals hydronephrosis with predominant dilation of the distal ureter. On VCUG, a normal bladder and the absence of evident obstructive factors (e.g., urinary calculi) indicate a primary megaureter. A renal scan may also show an obstructive pattern (2,4).

In the newborn with preserved renal function (greater than 45%), ureterovesical junction obstruction is managed with observation (serial ultrasounds and renal scans) and antibiotic prophylaxis (2). Similar to ureteropelvic junction obstruction, many cases may resolve on their own over time. Indications for surgical repair (ureteral reimplantation, with or without tapering) include deterioration of renal function, breakthrough pyelonephritis, pain, or calculus formation. Ureteral reimplantation involves surgically repositioning the ureter in the bladder wall and removal of the non-peristaltic segment.

Renal Agenesis

Renal agenesis describes the congenital absence of one or both kidneys due to impaired embryologic development. It may result from the failed development of the ureteric bud from the Wolffian duct, or the failed interaction between the ureteric bud and metanephric blastema, both of which prevent the formation of the nephrons and subsequent kidney(s) (10). The incidence of unilateral renal agenesis is 1 in 1000 births while bilateral renal agenesis occurs in 1 in 3000 births (11).

Unilateral renal agenesis is often asymptomatic and discovered incidentally on imaging or during the workup of other congenital VACTERL abnormalities. The remaining kidney is also frequently affected, with vesicoureteral reflux (30%) and ureteropelvic junction obstruction (11%) being the most prevalent congenital associations (10). Genital anomalies are also commonly associated with renal agenesis due to the close interdependence of renal and genital development. This includes Mullerian agenesis in females and an ipsilateral absent vas deferens, ejaculatory duct, seminal vesicle, and epididymis in males (11). The prognosis for those diagnosed with a congenital solitary functioning kidney is uncertain due to a current lack of longitudinal studies that persist beyond adolescence (12). Long-term monitoring of those with a solitary functioning kidney should include annual blood pressure and microalbuminuria measurements. Clinical monitoring of GFR should occur every few years in patients with a healthy contralateral kidney and more often in those with an impaired solitary functioning kidney. ACE (angiotensin converting enzyme) inhibitors and dietary modifications (decreased sodium and protein intake) may also help slow the eventual decline in GFR due to prolonged hyperfiltration injury (10).

Bilateral renal agenesis is generally incompatible with life and results in Potter sequence. The characteristic lung, limb, and facial findings are a consequence of reduced amniotic fluid volume which is normally generated by the fetal kidneys. The diagnosis is made prenatally with ultrasonography demonstrating oligohydramnios and the absence of fetal kidneys. For those fetuses who are not stillborn, death typically occurs 24 to 48 hours after delivery from respiratory distress secondary to pulmonary hypoplasia (10). Although controversial, intrauterine intervention via serial amnioinfusions has been successful in preventing pulmonary hypoplasia in some cases of bilateral renal agenesis. There are now several documented cases of patients with bilateral renal agenesis who have survived into childhood and successfully undergone renal transplantation (13).

Renal Dysplasia

Renal dysplasia refers to a specific type of dysgenesis (or maldevelopment) in which primitive ductal structures persist, affecting the overall size, shape, or structure of the kidney (11). Two conditions are necessary for the development of renal dysplasia: a primary abnormal ureteric bud-metanephric blastema interaction, and a secondary defect resulting in impaired urine outflow (14). Although the diagnosis of dysplasia is a histological one, dysplastic kidneys have a distinct appearance on ultrasonography. A dysplastic kidney is usually smaller than average, lacks typical corticomedullary differentiation, and is harder to distinguish from the surrounding perirenal fat (14). Cysts may or may not be present on imaging. Monitoring should include routine blood pressure, microalbuminuria, and renal function tests. In addition, periodic renal ultrasounds are indicated to evaluate the growth of both kidneys.

For more on cystic kidney diseases, see chapter XIII.3. Cystic Kidneys.

Renal Ectopia

During normal renal development, the permanent kidneys migrate upwards from the pelvis to the adult position (T12-L3). The ascent ends when the kidneys reach the adrenal glands, which hinder further cephalad movement. This process is driven by decreasing body curvature and enlargement of the lumbar and sacral regions of the growing fetus (3). An ectopic kidney occurs in 1 in 900 people and results when the normal ascent of the kidney is incomplete or excessive (11). Common locations for an ectopic kidney include the pelvis (most common), iliac region, thorax, or contralateral side. Most patients with renal ectopia are asymptomatic and diagnosed incidentally from unrelated imaging studies. Hydronephrosis may occur when simultaneous kidney malrotation results in an anteriorly positioned renal pelvis and impaired urinary drainage (4).

Horseshoe Kidney

A horseshoe kidney is a fusion anomaly in which two separate functioning kidneys on either side of the midline are joined at their lower poles via an isthmus. It is the most common congenital kidney anomaly, occurring in 1 in 400 people (4). Fusion occurs when the two metanephric blastemas come into close contact in the fetal pelvis. Later in development when the horseshoe kidney migrates cephalad, the fused segment (isthmus) becomes trapped underneath the inferior mesenteric artery, impeding further ascent and medial rotation (4). Most patients are asymptomatic. When present, symptoms can include urolithiasis (20-60%), UTI (30%), or hydronephrosis secondary to UPJ obstruction from abnormal crossing vessels (10-30%) (4,10). The diagnosis is usually made incidentally via imaging showing low-lying kidneys and the presence of an isthmus connecting the two lower poles. There is an increased incidence of renal malignancy in those with a horseshoe kidney, particularly Wilms tumor, which is four times greater than that of the general population (11). Except in symptomatic cases and malignancy, a horseshoe kidney does not require treatment and carries a good prognosis.

1. What is the most common congenital urologic condition detected by prenatal ultrasound?
2. What initial imaging study should be done to evaluate a newborn with a history of antenatal hydronephrosis?
3. What are the two most common causes of newborn hydronephrosis and how are they distinguished one from another?
4. What further tests should be ordered for the infant, with a history of prenatal hydronephrosis which persists postnatally?
5. What are the options for the treatment of UPJ and UVJ obstruction?
6. How should a patient with a congenital solitary functioning kidney be managed clinically?

1. Rickard M, Dos Santos J, Keunen J, Lorenzo AJ. Prenatal hydronephrosis: Bridging pre- and postnatal management. Prenat Diagn. 2022;10.1002/pd.6114. doi:10.1002/pd.6114
2. Elder JS. Obstruction of the Urinary Tract. In: Kliegman RM, St. Geme JW, Blum NJ, et al (eds). Nelson Textbook of Pediatrics, 21st edition. 2020, Elsevier, Philadelphia, PA. pp. 2800-2810.
3. Sadler TW. Urogenital System. In: Sadler TW (ed). Langman's Medical Embryology. 14th ed. Philadelphia, PA: Wolters Kluwer; 2019.
4. Houat AP, Guimarăes CTS, Takahashi MS, et al. Congenital Anomalies of the Upper Urinary Tract: A Comprehensive Review. Radiographics. 2021;41(2):462-486. doi:10.1148/rg.2021200078
5. Bayne CE, Majd M, Rushton HG. Diuresis renography in the evaluation and management of pediatric hydronephrosis: What have we learned? J Pediatr Urol. 2019;15(2):128-137. doi:10.1016/j.jpurol.2019.01.011
6. Lockwood GM, Herndon CDA. Update on the Postnatal Management of Antenatal Hydronephrosis. American Urological Association. Published February 1, 2022. Accessed May 15, 2022.
7. Oliveira EA, Oliveira MC, Mak RH. Evaluation and management of hydronephrosis in the neonate. Curr Opin Pediatr. 2016;28(2):195-201. doi:10.1097/MOP.0000000000000321
8. Vemulakonda VM. Ureteropelvic junction obstruction: diagnosis and management. Curr Opin Pediatr. 2021;33(2):227-234. doi:10.1097/MOP.0000000000000994
9. Krajewski W, Wojciechowska J, Dembowski J, Zdrojowy R, Szyde?ko T. Hydronephrosis in the course of ureteropelvic junction obstruction: An underestimated problem? Current opinions on the pathogenesis, diagnosis and treatment. Adv Clin Exp Med. 2017;26(5):857-864. doi:10.17219/acem/59509
10. VanderBrink BA, Reddy PP. Anomalies of the Upper Urinary Tract. In: Partin AW, Dmochowski RR, Kavoussi LR, Peters CA (eds). Campbell-Walsh-Wein Urology, 12th ed. 2021. Philadelphia, PA: Elsevier. pp.714-740.
11. Elder, JS. Congenital Anomalies and Dysgenesis of the Kidneys. In: Kliegman RM, St. Geme JW, Blum NJ, et al (eds). Nelson Textbook of Pediatrics, 21st edition. 2020, Elsevier, Philadelphia, PA. pp. 2786-2789.
12. Schreuder MF. Life with one kidney. Pediatr Nephrol. 2018;33(4):595-604. doi:10.1007/s00467-017-3686-4
13. Sheldon CR, Kim ED, Chandra P, et al. Two infants with bilateral renal agenesis who were bridged by chronic peritoneal dialysis to kidney transplantation. Pediatr Transplant. 2019;23(6):e13532. doi:10.1111/petr.13532
14. Pope JC. Renal Dysgenesis and Cystic Disease of the Kidney. In: Partin AW, Dmochowski RR, Kavoussi LR, Peters CA (eds). Campbell-Walsh-Wein Urology, 12th ed. 2021. Philadelphia, PA: Elsevier. pp. 741-775.

Answers to questions
1. Hydronephrosis represents one of the most common abnormalities detected with prenatal ultrasound.
2. A renal and bladder ultrasound should be obtained postnatally. If it is normal, then the ultrasound should be repeated sometime between age 3 and 9 months. If normal, the hydronephrosis is considered to have resolved.
3. Ureteropelvic junction (UPJ) obstruction is the most common cause, with ureterovesical junction (UVJ) obstruction being the second most common cause of congenital hydronephrosis. They are distinguished by the fact that with UPJ obstruction, the ureter is not dilated, whereas the ureter is dilated with UVJ obstruction.
4. Electrolytes, BUN, and creatinine should be obtained to determine a baseline of renal function. A diuretic renal scan should be performed on patients with intermediate-risk or high-risk hydronephrosis. A VCUG should be obtained in those with severe hydronephrosis, suspected VUR, ureteral dilation, thick-walled bladder, and/or bladder outlet obstruction.
5. In infants noted to have good (40% or greater) split function on the renal scan, serial ultrasounds and diuretic renal scans may be used to follow the patient non-surgically. If there is renal function deterioration, breakthrough UTIs, or symptoms of renal colic, then surgery (pyeloplasty for UPJ obstruction, and ureteral reimplantation for UVJ obstructions) is indicated. Most children with UPJ and UVJ obstructions will not require surgical management. Continuous antibiotic prophylaxis is indicated for those with UVJ obstruction and those with additional risk factors (female sex, uncircumcised male, severe hydronephrosis).
6. A solitary functioning kidney may result from a congenitally absent or functionally impaired kidney, such as in the case of unilateral renal agenesis or severe unilateral renal dysplasia. To compensate for the absent kidney, the solitary kidney must hypertrophy to filter the same amount of blood as two normal kidneys. If this compensatory mechanism fails, hyperfiltration will occur, which will damage the kidneys over time and result in hypertension, albuminuria, and chronic kidney disease. Blood pressure, microalbuminuria, and GFR should be assessed regularly to monitor for disease progression. Periodic renal ultrasounds are useful in assessing normal kidney growth. The addition of ACE inhibitors and dietary modification may also help to slow functional renal decline.

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