Intrinsic causes are attributed to dysfunctional development. During the development of the upper ureter, the lumen of the ureteral bud solidifies, followed by ureteral lengthening and subsequent recanalization. The failure to completely recanalize is believed to be the cause of most intrinsic UPJ obstructions. Histologically, there is often a reduction or absence of smooth muscle fibers at the UPJ, with increased collagen deposition, fibrosis, and disrupted muscle continuity. ^, Ureteral valves, polyps, and leiomyomas present very rare intrinsic causes.

Extrinsic causes are typically aberrant or crossing vessels. Secondary UPJ obstruction can arise from severe vesicoureteral reflux (VUR), which leads to elongation and dilation of the ureter, causing proximal kinking or narrowing at the UPJ. This association requires a patient-specific approach, as simultaneous correction of VUR and UPJ obstruction is not recommended due to the risk of compromising the ureteral blood supply.

In cases of intrinsic stenosis, hydronephrosis is typically detected prenatally, often as part of routine ultrasound screenings. ( Fig. 53.1 ) However, the prevalence of hydronephrosis significantly exceeds the incidence of clinically relevant obstruction. A population-based study in the United States reported a postnatal hydronephrosis prevalence of 1.1% (1.4% in males and 0.7% in females), with only 7.7% of these cases having UPJO. Thus, the overall prevalence of UPJO in this population was 0.08%. Most of these infants are asymptomatic. If the intrinsic stenosis is not identified by prenatal ultrasound, UPJO often presents with a febrile urinary tract infection (UTI) during childhood. Extrinsic stenosis caused by crossing vessels typically presents in older children and teenagers with vague and nonspecific symptoms, such as recurrent abdominal or flank pain, nausea, and vomiting. These children are often evaluated by gastroenterologists and only referred when an ultrasound reveals hydronephrosis. The exact cause of intermittent obstruction remains unclear, but renal function is generally preserved. Hematuria following minor trauma or vigorous exercise may be a presenting symptom, likely due to the rupture of mucosal vessels in the dilated collecting system. Diagnosing intermittent extramural UPJ obstruction can be challenging, as renal pelvis dilation may not be visible during pain-free intervals.

Normal kidneys are typically identifiable by 18 weeks in all fetuses. Dilated kidneys can be seen as early as 12–14 weeks of gestation. The arrow demonstrates left-sided caliectasis on a coronal fetal image.

Prenatal dilation is detected in the second trimester with a sensitivity of up to 100%. Findings are graded by the classification of the Society for Fetal Urology (SFU) or the Urinary Tract Dilation (UTD) Classification ( Fig. 53.2 ). The documentation of the Anterior-Posterior Renal Pelvic Diameter (APRPD or APD), calyceal dilation, parenchymal thickness and echogenicity, ureter and bladder evaluation, and amniotic fluid assessment are essential. These parameters enable risk stratification for postnatal management. Postnatal ultrasound should be conducted after the third day of life to avoid false-negative studies resulting from the transitional nephrology of the newborn ( Fig. 53.3 ). Clinical examination and the initial ultrasound evaluation needs to assess the whole abdomen and urinary tract to rule out megaureter, signs of urethral valves, and associated congenital anomalies (i.e., VACTERL). ^,

Schematic drawing of the Society for Fetal Urology (SFU) grading system and the Urinary Tract Dilatation (UTD) classification system. APRPD: Anterior-posterior renal pelvic diameter.

From HAN, Miran et al. Conversion and reliability of two urological grading systems in infants: the Society for Fetal Urology and the urinary tract dilatation classifications system. Pediatr Radiol , 2017, 47. Jg., S. 65–73.

These neonatal ultrasound images come from infants with a history of prenatally detected renal dilation. (A) This ultrasound is normal for comparison purposes. There are dark renal pyramids ( arrow ) and no renal pelvic dilation. (B) This image shows isolated renal pelvic dilation ( arrow ) (SFU grade I). (C) This image shows dilation of the renal pelvis (solid arrow) and upper- and lower-pole calyces ( dotted arrows ) (SFU grade II). (D) Calyceal dilation and cortical thinning are seen (SFU grade IV). (E) Hydronephrosis with peripheral cysts ( arrow ) indicating dysplasia is seen. This kidney had no function on renal scan.

If the initial examination reveals an APD >15 mm or a urinary transport disturbance greater than grade III with dilated calyces and significant parenchymal thinning and/or increased parenchymal echogenicity, assessment of renal function and drainage should be conducted. As the tubular system is still maturing before the 4th to 6th week of life, and drainage after furosemide administration may be inadequate, the examination should not be performed before the 4th week of life.

Traditionally, renal function and drainage are evaluated with a diuretic isotopic renogram, where the transit of an injected radioisotope (99mTc-MAG3) through the urinary tract is monitored by a gamma camera. Persistence of more than 50% of the tracer in the renal pelvis 20 minutes after diuretic administration is typically diagnostic of obstruction, although the applicability of this threshold in pediatric patients remains debatable ( Fig. 53.4 ).

This scintigraphy is from a 2-month-old boy with left UPJO. 99mTc-MAG3 scintigraphy shows accumulation of the nucleotide in the left kidney and a rising curve. Left renal function is unimpaired (52%).

Functional MR urography (fMRU) has emerged as an alternative to the 99mTc-MAG3 scintigraphy (99m Technetium-Mercaptoacetyltriglycine) for dynamic renal function assessment. Established MR techniques involve intravenous contrast administration to show time-resolved renal perfusion and excretion. Standardized hydration of the child immediately before the examination, an empty bladder, and administration of furosemide are required. Using freely available analysis software, semiquantitative assessments of kidney function, including glomerular filtration rate (GFR) and drainage, can be made, typically matching the results of nuclear medicine procedures. In the parenchymal phase, structural changes in the kidney can be assessed. To detect scarring after pyelonephritis, the MRI with its high spatial resolution and multidimensional capabilities is a valid alternative to 99mTc-DMSA scintigraphy (Dimercaptosuccinic acid).

A significant advantage of fMRU is its superior anatomical and morphological resolution, allowing precise identification, especially in cases of renal duplication. With the capability to optimally visualize high-resolution morphology (e.g., ectopic ureteral orifice) and vascular structures (e.g., ureteropelvic junction obstruction due to vascular compression), it is ideal for surgical planning. Indications for this time-consuming, personnel-intensive, and technically demanding method should be decided in a multidisciplinary manner with input from pediatric nephrology, surgery, urology, and radiology ( Fig. 53.5 ).

Functional MRU (fMRU) showing gross hydronephrosis on the right with thinning of the parenchyma in a 2-month-old. Functional analysis showed 11% partial function.

Noncontrast methods for functional assessment such as BOLD MRI (“blood oxygenation level dependent”) to assess oxygenation or DTI (“diffusion tensor imaging”) assessing directed diffusion are being tested but have not yet become routine in clinical practice.

Voiding cystourethrography or voiding ultrasound are not generally recommended as the coincidence of reflux and UPJO is only about 7%, and these children typically are symptomatic, with ureteral dilatation or a febrile UTI.

In the past, intravenous pyelograms, invasive pressure flow studies (Whitaker test), and retrograde ureteropyelographies were routinely performed. These invasive tests have been replaced by advancements in the imaging techniques described above and are rarely applied today.

Research is focused on identifying biomarkers, and some authors consider certain results to be promising. However, no reliable serum or urine markers have yet been identified for the accurate detection of obstructive uropathy. ^,

Routine antibiotic prophylaxis was given to all infants with prenatal pelvic dilation in the past, but clinical experience shows infants with isolated hydronephroses have a low risk of UTI and systematic reviews have shown that the level of evidence in favor of continuous antibiotic prophylaxis in these children is low. Therefore, continuous antibiotic prophylaxis for isolated unilateral hydronephrosis is not justified.

The aim of repeated imaging and functional assessment is to detect cases of significant UPJ obstruction that will lead to deterioration of kidney function. To date, there is no universally accepted definition of significant obstruction, and the discussion on the management of neonatal hydronephrosis and UPJ obstruction is ongoing.

There is consensus that the morphologic appearance of a dilated renal pelvis alone is not a good indication for operation as the hydronephroses can resolve spontaneously. However, there are reliable data that most children with SFU grades 1–2 rarely progress to significant obstruction and typically have stable renal function. These children are followed with ultrasounds and if hydronephroses stays stable or decreases, functional assessment may be avoided.

Children with postnatal SFU grade 3–4 hydronephrosis and renal function below 40% are more likely to be diagnosed with clinically relevant obstruction requiring surgical intervention.

Most authors agree that a significant obstruction combined with an ipsilateral renal function of less than 35%–40% on first assessment is an indication for surgery, even though there is no evidence-based data to support the threshold renal function. Additionally, children presenting with a loss of ipsilateral function in follow-up assessments, and symptomatic children with recurrent febrile UTI, require surgical treatment. The goal of surgery is to remove the obstruction and thus stabilize renal function. Some of these kidneys will regain some of the lost function.

A different indication to treat arises from children who present with recurrent flank pain and in whom hydronephrosis and a lower-pole vessel are detected. In these, typically adolescent patients, renal function is usually symmetrical. The indication for surgery is based on the symptoms, which promptly resolve after surgical treatment.

Once the decision for operative treatment is made, repair of UPJ obstruction should be scheduled electively ensuring the availability of experienced surgeons and pediatric anesthesiologists.

The insertion of a percutaneous nephrostomy catheter prior to the repair is rarely necessary. It may be considered in cases of pyonephrosis to facilitate pus drainage in addition to antibiotic therapy, or in preterm infants with contraindications to early reconstruction.

Another rare indication for a temporary nephrostomy catheter is older children that present with an ipsilateral renal function <10%. Repeated functional assessment after a period of continuous drainage enables evaluation of renal function recovery, aiding in the decision-making process between pyeloplasty and nephrectomy. ^,

Typically, an ultrasound follow-up is scheduled a few weeks after the procedure or JJ-stent removal to rule out significant stenosis of the anastomosis. Functional assessment, typically 2–3 months after the procedure, is mandatory to prove successful treatment. Ultrasound alone does not allow to objectively measure ipsilateral renal function and adequate drainage.

International guidelines currently do not provide a clear recommendation regarding the duration of long-term follow-up. Recent literature emphasizes the importance of longitudinal monitoring, given the growing evidence of long-term complications such as arterial hypertension and chronic kidney disease in adults following surgical correction of UPJ obstruction during childhood.

A horseshoe kidney occurs when the lower or upper poles of the kidneys are connected in front of the spine, but the kidneys remain on their respective sides.

Horseshoe kidney is the most common type of kidney fusion anomaly, with an incidence of about 1 in 400, slightly more common in males. It differs from the “cake kidney,” where both kidneys are completely fused at their medial aspects.

In over 90% of cases, the lower poles of the right and left kidneys are connected, with about 40% of the isthmus lying at the midline, but more often slightly to the left. The renal pelvis usually faces ventrally, and the ureters descend over the renal parenchyma. The isthmus is located at or below the level of L3/L4 and often contains renal parenchyma with its own blood supply. The vascular supply is highly variable, with arteries and veins originating from the aorta, inferior vena cava, iliac, or sacral vessels. Occasionally, the isthmus is a fibrous band. The renal calyces are normal in number but face posteriorly due to the lack of kidney rotation. The ureters can emerge high and lateral from the renal pelvis.

Horseshoe kidney is associated with various congenital anomalies and syndromes, some outside the urogenital tract. Over 50% of individuals with a horseshoe kidney will remain asymptomatic. Symptoms (flank pain, nonspecific lower abdominal discomfort, micro-/macrohematuria) are typically due to urinary obstruction (extrinsic/intrinsic subpelvic stenosis) and/or stone formation with or without UTIs. Up to 36% of symptomatic individuals develop stones, 80% of which are associated with UTIs. Ultrasound often detects renal pelvis dilation, but scintigraphy typically shows nonobstructive findings with slightly delayed radiotracer drainage. Kidney tumors can occur at any age in horseshoe kidneys, with nephroblastoma in children and renal cell carcinomas in adults. ^, Although the incidence is not significantly increased, surgical treatment is more complex due to anatomical challenges.

Horseshoe kidney can be suspected on ultrasound when the kidney axis and lower poles point medially. The isthmus is often visible. Further diagnostics are only indicated if symptoms or significant renal pelvis dilation are present. DMSA scintigraphy can show function and scarring in both kidneys and the isthmus. MAG3 scintigraphy assesses renal function and drainage. With the introduction of fMRU, simultaneous anatomical, functional, and vascular assessment can be performed with one examination.

Symptomatic or significant urinary transport obstruction due to subpelvic stenosis should be surgically corrected. Standard pyeloplasty may be successful if there is no vascular anomaly or parenchymal bridge involvement. However, due to anatomical variability, more complex surgeries like bridge transection, nephropexy to the psoas muscle, and pyeloplasty with relocating the UPJ might be necessary. For stone treatment without obstruction, techniques like URS (ureteroscopy), Mini-PNL (mini percutaneous nephrolithotomy), and RIRS (retrograde intrarenal surgery) are effective. The success of extracorporeal shock wave lithotripsy (ESWL) is unclear. Percutaneous nephrolithotomy (PNL) should consider vascular and kidney positioning, and combined reconstructive surgery may be needed if obstruction is present.

Routine monitoring is unnecessary for an incidentally detected, nonpathological horseshoe kidney. However, patients should be informed about the condition to avoid unnecessary future examinations. Pregnancy is not negatively affected by an incidental horseshoe kidney diagnosis. Transplantation of a nonpathological horseshoe kidney is possible as one or two separate kidneys without impacting graft survival rates. After stone removal, regular ultrasound checks are needed to detect recurrences early. After reconstructive surgery, ultrasound monitoring should continue until renal pelvis dilation resolves.

A duplex system features a duplication of the upper urinary tract, distinguished from an accessory kidney by having a single renal capsule. The vascular supply is variable, with each part potentially receiving blood from different vessels. The renal pelvis and calyx system are duplicated; the ureters can merge or enter the bladder separately ( Fig. 53.9 ). Many of the duplex systems are accompanied by congenital dysplasia and hydronephrosis (mostly of the upper pole) as well as with an increased incidence of UTI due to both obstruction and VUR.

Types of duplication. (A) Bifid pelvis. (B) “Y” ureter. (C) “V” ureter. (D) Complete duplication with ectopic orifice.

Duplex kidneys are relatively common, with an incidence of 0.8%–4%. Girls are affected twice as often as boys with both sides affected equally. Most duplex kidneys are incidental findings without clinical relevance.

Ureteral duplex systems result from anomalies in the development of the urinary tract during embryogenesis. These anomalies occur when, in the 4th week of gestation, two ureteric buds arise from the mesonephric duct instead of one, leading to the formation of two separate ureters that may drain into a single kidney. The upper bud typically migrates more medially, giving rise to the upper-pole ureter, while the lower bud becomes the lower-pole ureter. Genetic and molecular factors, such as mutations in genes PAX2 and RET, have been identified as contributing to this process.

Recent studies suggest that the persistence of embryonic structures, such as Chwalla’s membrane, and disruptions in signaling pathways, like those involving GDNF and Hox genes, contribute to the formation of duplex systems. ^, These disruptions can additionally result in incomplete or abnormal migration and insertion of the ureteral buds, leading to ureteral ectopy, ureterocele, and primary megaureter. Environmental factors and maternal influences, such as teratogens and infections during pregnancy, may also play a role in the etiology of these congenital anomalies.

A partial or complete duplication of the ureter occurs when a single bud branches prematurely or when two ureteral buds arise from the mesonephric duct, respectively. A bifid renal pelvis is the highest level of bifurcation and occurs in 10% of the population. Other incomplete duplications occur throughout the ureter. An inverted-Y ureter is the rarest of all branching anomalies. This is presumably the result of separate ureteral buds that fuse before entering the metanephros.

Pathologies can occur in both the upper and at the bladder level. In the upper tract, subpelvic stenosis usually affects the lower segment of the duplex kidney. The indications for surgical intervention and access routes do not significantly differ from those for subpelvic stenosis in a single kidney, though the technique must be tailored to the individual situation. Standard Anderson-Hynes pyeloplasty can be used, while pyelopyelostomy is preferred for high ureter fissures with a short lower-pole ureter. Any obstruction-relieving anastomosis of the upper urinary tract is possible, including reversed pyeloplasty, ureteroureterostomy, and ureteropyelostomy.

For distal pathologies, the Meyer-Weigert rule is crucial: the upper segment’s ureter crosses the lower segment’s ureter before the bladder, which may cause reflux in the lower segment due to a short intramural course and lateralization of the ostium. Reflux is identified in up to two-thirds of children with duplex systems that develop UTI. Additionally, reflux may occur into the upper-pole ureter if the ureteral orifices are immediately adjacent, or if the upper ureter is located distally at the level of the bladder neck without any submucosal support. Diagnostic evaluation is similar to those for reflux in a single system ( Fig. 53.10 ).

( A ) VCUG with reflux into both lower poles, already suggestive for duplex kidneys due to the “ drooping flower ” sign. (B) fMRU visualizing the duplex kidneys, and (C) cystoscopic view onto the two left ureters.

The treatment of VUR in duplicated ureters follows the same principles as for a single system. Initial treatment includes preventive antibiotics and ultrasound surveillance. Low-grade VUR is associated with a similar resolution rate, but it will take longer compared to single systems. For reflux greater than grade II into duplex systems, the chance of resolution is believed to be marginal. Subureteral injection is a valid minimally invasive therapeutic option. To date, good quality studies analyzing suitable patients and success rates do not exist. Assessing these parameters is complicated due to the wide variety of anatomic situations, making comparison or randomization nearly impossible.

The distal ureters share a common vascular supply, so reimplantation involves mobilization and reimplantation of both ipsilateral ureters within their common sheath. If a combined obstructive-refluxive lower pole is noted, ipsilateral end-to-side pyeloureterostomy is an effective management for both obstruction and reflux. Even if significant scarring is present in the lower pole, reimplantation is usually effective unless major ureteral dilation is present. In the latter case, lower-pole nephroureterectomy may be warranted.

The upper segment’s ureter can have an obstructive/afunctional distal segment, leading to a primary obstructive megaureter. Additionally, it may terminate ectopically, causing incontinence in girls or presenting as an ectopic ureterocele. ^, Depending on the partial function of the moiety and the anatomy, ureteroureterostomy, ureteral ligation, common sheath reimplantation, or heminephrectomy are therapeutic options. ^, Each case must be assessed individually, and treatment decisions justified on case-based details.

The term “ectopy” derives from the Greek words “ektos” (outside) and “topos” (place), indicating that an ectopic ureter has its ureteral orifice located outside the bladder’s trigone. It may open into the bladder neck, urethra, or even outside the urinary tract.

In an autopsy study, the incidence of ectopic ureter was 1 in 1900, with a female: male ratio of 5:1. Over 80% of female cases are associated with a duplex kidney, compared to 50% in males.

In girls, continuous urinary incontinence is the most common symptom. In infancy, an ectopic megaureter is usually detected via ultrasound and remains asymptomatic as long as the child wears diapers. Later, incontinence (urine dribbling) is noted. Ectopic pyuria or pyelonephritis may also lead to diagnosis. In boys, clinical signs include epididymitis or pain in the epididymis or seminal vesicles, occasionally presenting as cystic dilation of the seminal vesicles.

An ectopic ureter opening in the vestibule can sometimes be detected by physical examination due to urine secretion. Typically, a dilatation of one ureter is incidentally found on pre- or postnatal ultrasound. If clinical symptoms suggest an ectopic ureter, fMRU can provide morphological and functional information. Often, the location of the ectopia cannot be detected on imaging and cystoscopy and ureteral catheterization is required to verify the exact location. Fig. 53.11 illustrates possible locations of the ectopic orifice.

(A) Ureteral ectopia in a boy. Possible sites are above the external sphincter (1–3), or into the seminal vesicle (4, 5), or anorectal (6). (B) Ureteral ectopia in a girl may be located at the bladder neck (1), or beyond the continence mechanism in the urethra (2), or on the perineum (3). Uterine or vaginal insertion (4–6) may also cause incontinence. Anorectal insertion (7) can also occur.

A ureterocele is a cystic dilation of the distal ureter that can extend into the bladder, bladder neck, or urethra. The embryological origin is unclear. The classic hypothesis involves the persistence of Chwalla’s membrane, but current molecular studies on trigone development may provide better explanations for different manifestations. Ureterocele can be associated with a single kidney (about 20%) but is more commonly seen with duplex systems (80%). It is bilateral in approximately 10% of cases. In duplex systems, the ureterocele usually involves the upper-pole ureter and opens distal to the lower-pole ureter. It can be orthotopic, residing in the bladder without affecting the bladder neck or outlet, or ectopic, extending into the bladder neck or urethra.

The incidence of ureterocele is about 1 in 4000, with girls being 4–7 times more frequently affected than boys.

Most children with ureteroceles are asymptomatic. Large ureteroceles may prolapse into the urethra, causing urinary retention, or rarely, it may extend to the urethral meatus and present as a smooth mass covered with mucosa in between the labia of a newborn girl ( Fig. 53.13 ). The obstruction in the associated ureter may cause infection or stone formation.

This 2-week-old baby presented with sepsis and was found to have this prolapsing ectopic ureterocele. The ureterocele was aspirated with return of purulent debris and underwent prompt decompression. Recovery was uneventful.

An obstructive ureterocele can be detected on prenatal or postnatal ultrasound as a cystic formation in the bladder, typically associated with megaureter and hydronephrosis of the upper moiety. Differentiation from a megaureter, which protrudes into the bladder, is essential. For functional assessment, DMSA and MAG3 scintigraphy as well as fMRU are available. To exclude or confirm ipsilateral or contralateral reflux, VCUG is recommended. Urethrocystoscopy can further clarify the diagnosis and allow for therapeutic intervention during the procedure.

Follow-up for all patients with duplex kidney pathologies should include ultrasound monitoring of kidney growth and dilatation. Annual blood pressure measurements and tests for proteinuria are recommended postoperatively to detect and manage hypertension early. VCUG should only be considered in cases of recurrent febrile UTIs. The need for follow-up functional assessments (scintigraphy or fMRU) should be determined on a case-by-case basis.

MUs can be divided into primary and secondary forms. Primary MUs are caused by morphological or functional anomalies within the vesicoureteral junction itself, whereas secondary MUs can be caused by various etiologies. Two different classifications are being used to describe the pathology: The classification by Smith et al. focuses on resulting pathology, with a) obstructive MU, b) refluxing MU, c) refluxing and obstructing MU, and d) nonrefluxing and nonobstructing MU. Pfister and Hendren based their classification on morphological appearance: grade I: partial ureteral dilatation with normal pyelocaliceal system; grade II: dilated ureter and accompanying caliectasis; and grade III: ureteral dilatation and significant hydronephrosis.

The ureterovesical junction forms between the 4th and 8th gestational week when the ureteral bud arises from the Wolffian duct and merges into the bladder at the trigonal level. Vitamin A-induced apoptosis and epithelial remodeling leads to final integration.

The molecular factors influencing the ureterovesical junction are still poorly understood. Mutations in Gene 1, variations in the RET tyrosine-kinase signaling system, and variations in the effectors YAP and TAZ are believed to play a role in forming MUs. Histologic findings include an increase in collagen fibers and a decrease in interstitial cells of Cajal.

With advances in prenatal imaging, most primary MUs are diagnosed prenatally. Although a significant proportion of postnatal diagnosed MUs are incidental findings, possible clinical symptoms include recurrent flank pain, UTIs, hematuria, and urolithiasis. ^,