Kidneys and Ureters

The initial phase of embryonic kidney development is the interaction between 2 tissues that are derived from the intermediate mesoderm: the metanephrogenic mesenchyme and the mesonephric duct. Kidney development proceeds in 3 successive steps: the pronephros, mesonephros, and adult metanephros. Each of these steps involves mesenchymal-to-epithelial transformation of cells that are derived from the intermediate mesoderm. Among the genes expressed in the mesenchyme that are important in early kidney development are Six1, Six2, Sall1, Pax2, Eya1, Foxc1, Wt1, and Hox11, which encode transcription factors, and Gdf11 and Gdnf, which encode transforming growth factors. A variety of renal abnormalities occur with mutations of these genes. Some of these genes also are involved in the formation of other organs, such as eye and muscle.¹

The pronephric duct arises from the intermediate mesoderm early during embryogenesis. This soon becomes the mesonephric, or Wolffian, duct. The metanephrogenic mesenchyme induces the formation of an epithelial branch from each of the paired mesonephric ducts; these branches are the ureteric buds (metanephric diverticula). The ureteric bud emerges from the mesonephric duct and enters the metanephrogenic mesenchyme where it branches and induces the mesenchyme to condense and differentiate into nephrons. Therefore, renal development involves reciprocal interactions between 2 intermediate mesodermal tissues. The ureteric bud fails to branch and elongate in the absence of metanephrogenic mesenchyme; likewise, the mesenchymal tissue undergoes apoptosis in the absence of the ureteric bud. Failure of development of, or early degeneration of, the ureteric bud leads to renal agenesis.

Diffusible signaling molecules produced by the mesenchyme, such as Gdnf (glial-cell-line-derived neurotrophic factor), stimulate budding and elongation of the mesonephric ducts. As the embryonic ureter grows into metanephric blastema, it undergoes a series of dichotomous divisions that eventually form the infundibula, calyces, and collecting ducts of the kidney. The size and functional capacity of the kidney are ultimately determined by the complexity of ureteric bud branching and the number of individual nephrons that have been induced by the time the pool of metanephric stem cells has been consumed and nephrogenesis terminates. Duplication anomalies of the urinary tract result from early division of the ureteric bud. The extent of the duplication relates to the developmental stage at which division of the diverticulum occurred.

The segment of the mesonephric duct that extends from the site of origin of the developing ureter to the primitive cloaca is termed the common excretory duct. The common excretory duct incorporates into the bladder base and proximal urethra. By the end of the sixth gestational week, the ureter and the mesonephric duct have separate openings into the urogenital sinus. As the common excretory duct is absorbed into the urogenital sinus, the original meatus on the mesonephric duct migrates in a cephalic and lateral direction. As the bladder and urethra develop, the ureteral orifice migrates in a lateral and cephalic direction and the opening of the mesonephric duct migrates medially and caudally. The terminal end of the mesonephric duct that was proximal to the ureteral bud develops into the ejaculatory duct in males. In females, the mesonephric duct involutes except for contribution to the Gartner duct. Alterations of this process of ureteral and mesonephric duct orifice migration lead to various genital and urinary tract anomalies, such as ectopic ureter.²

The fetal kidneys are initially located in the lower lumbar to upper sacral region, approximately at the level of the future S2 vertebra. Cephalad migration of the metanephros and caudal growth of the body cause progressive superior “migration.” The kidneys normally are located at the level of L2 or L3 by the end of the third month of gestation, and have reached their normal locations at L1 to T12 by the time of birth. Failure of migration or, rarely, excessive migration results in an ectopic kidney. During ascent, the fetal kidneys are supplied by a succession of lateral stem arteries from the iliac arteries and abdominal aorta; there is atrophy of inferior vessels as new cephalad vessels develop. Permanent renal arteries develop once renal migration has ceased. This developmental pattern of fetal vascular supply of the kidneys likely accounts for the common normal variations in postnatal renal arterial anatomy, for example, accessory or multiple renal arteries. Likewise, the arterial supply to an ectopic kidney is determined by the degree of fetal renal ascent.

Developmental abnormalities of the urinary tract frequently occur in association with anomalies of other organ systems. Renal ectopia, fusion anomalies, and agenesis can occur in combination with cardiac, extremity, GI, and vertebral anomalies. When most or all of these organ systems are involved, the term VACTERL association is utilized. This refers to a combination of vertebral, anorectal, cardiac, tracheoesophageal, renal, and limb anomalies. The most common vertebral anomalies in these children are segmentation anomalies, dysraphism, and sacral agenesis. Esophageal atresia, tracheoesophageal fistula, and imperforate anus are common in these children. Males with the VACTERL association are at increased risk for urethral anomalies. Other multiple congenital anomaly syndromes that often include clinically significant renal abnormalities include CHARGE (coloboma of the eye, heart defects, choanal atresia/stenosis, cranial nerve abnormalities, genital abnormalities, growth deficiency, ear anomalies) association, Townes-Brocks syndrome, branchio-oto-renal syndrome, Nager syndrome, Miller syndrome, and diabetic embryopathy. Many children with malformations of the external ears have renal anomalies.³

Bladder

The cloaca is an expanded terminal portion of the embryonic hindgut. The allantois extends into the ventral aspect of the cloaca. The urorectal septum is a wedge of mesenchyme that grows to divide the cloaca into the rectum and urogenital sinus by about the seventh week of gestation. The epithelium of the urinary bladder derives from the cranial aspect of the urogenital sinus. The caudal ends of the mesonephric ducts contribute to formation of the trigone region. The remainder of the bladder wall arises from adjacent splanchnic mesenchyme. The dome contains some tissue from the allantois. The vestigial allantois forms the urachus, an epithelial-lined canal that extends from the bladder dome to the umbilicus. The urachal lumen obliterates and the urachus eventually becomes a fibrous cord, the median umbilical ligament. The anatomic divisions of the mature bladder are the dome, body, and base (fundus). The trigone is a triangular space within the bladder base, located between the ureteral orifices and the internal urethral meatus.

Bladder exstrophy results from failure of mesenchymal cells to properly migrate between the lower abdominal wall ectoderm and the cloaca during the fourth week. There is failure of normal development of the anterior bladder wall and the overlying abdominal wall, resulting in exposure of the bladder lumen. The various urachal anomalies (i.e., sinus, fistula, and cyst) result from persistence of all or a portion of the urachal lumen. Persistence of the inferior aspect of the urachus is particularly common; a small outpouching extending superiorly from the dome of the bladder at the midline (a urachal sinus) is a common clinically insignificant finding on sonography and cystography in neonates and young infants.

Urethra

Male Urethra

The male urethra consists of anterior and posterior portions. The anterior urethra extends from the external meatus to the inferior edge of the urogenital diaphragm, coursing through the corpus spongiosum. The anterior urethra is divided at the penoscrotal junction into penile (pendulous) and bulbous segments. The pendulous portion terminates distally in the penis as the fossa navicularis. In the proximal aspect of the bulbous urethra, there is a slightly enlarged segment that is termed the “sump.” At the bulbomembranous junction (just proximal to the sump), the urethra has a conical shape; this segment is termed the “cone.” The periurethral Littré glands empty into the anterior urethra. The Cowper glands are located on each side of the membranous portion of the posterior urethra; the ducts empty into the bulbous urethral sump.

The anatomic divisions of the posterior urethra are the prostatic and membranous portions. Along the posterior wall of the posterior urethra is the urethral crest, which is a dorsal longitudinal ridge of smooth muscle that extends from the bladder neck to the membranous urethra. The urethral crest is contiguous with the verumontanum, which is an ovoid mound in the posterior wall of the prostatic urethra. The prostatic utricle is located in the center of the verumontanum; this is a small saccular outpouching that is a vestigial remnant of the Müllerian duct. Just distal and lateral to the utricle are the orifices of the ejaculatory ducts. The prostatic glands empty directly into the urethra via multiple small openings adjacent to the verumontanum. The prostatic urethra tapers distally into the membranous urethra, where it meets the bulbous urethra at the inferior margin of the urogenital diaphragm.

The proximal (internal) urethral sphincter extends from the bladder neck to the prostatic urethra above the verumontanum. Although it is similar to the detrusor muscle, it has different innervation. The distal (external) sphincter has both intrinsic and extrinsic components. The intrinsic urethral sphincter is a concentric muscular structure that surrounds the membranous urethra; the intrinsic sphincter is located in the distal third of the prostatic urethra, distal to the verumontanum. Both the internal sphincter and the intrinsic portion of the external sphincter are composed of smooth muscle and function to maintain passive continence. The internal sphincter functions as the primary continence sphincter and the intrinsic sphincter serves as the secondary continence sphincter. The extrinsic sphincter is a paraurethral, striated, voluntary muscle that includes contributions from the levator ani complex. The extrinsic sphincter surrounds the membranous urethra and participates in maintaining active continence.

On a normal voiding cystourethrogram, the verumontanum appears as an oval impression/filling defect along the midportion of the dorsal prostatic urethra. The superior urethral crest is occasionally visible as a Y-shaped fold extending from the bladder neck to the verumontanum. The inferior urethral crest is a midline dorsal fold that extends along the posterior wall of the verumontanum and bifurcates into curved thin membranes that balloon distally with the contrast column; these are the plicae colliculi. Unlike posterior urethral valves, the normal plicae colliculi do not cause obstruction and there is no posterior urethral dilation. The prostatic utricle occasionally fills during voiding cystourethrography; this appears as a small diverticulum extending dorsal to the prostatic urethra. The urogenital diaphragm produces an ill-defined smooth circumferential narrowing of the contrast column at the junction of the posterior and anterior segments of the urethra. There is minimal widening of the urethra at the level of the glans penis; this is the fossa navicularis (Figure 44-1).

The majority of the epithelium of the male urethra derives from the urogenital sinus. Distally, there is contribution from the surface ectoderm of the glandular plate. The connective tissue and smooth muscle of the urethra arise from adjacent splanchnic mesenchyme. In the early embryo, there is a membrane at the site of contact between the cloaca and the perineum. During the fourth week of gestation, proliferating mesenchyme forms a genital tubercle at the cranial margin of this cloacal membrane. Urogenital folds develop on each side of the cloacal membrane and labioscrotal swellings arise just lateral to these folds. Fusion of the urorectal membrane in the sixth week divides the cloacal membrane into anal and urogenital components. The urogenital membrane is in the floor of the midline urogenital groove that is bounded laterally by the urogenital folds. Rupture of the urogenital membrane creates the urogenital orifice.

The genital tubercle elongates to form the phallus and eventually the penis. The urogenital groove elongates to form the urethral groove along the ventral surface of the penis. Endodermal cells that line this groove comprise the urethral plate; these cells will form the majority of the urethral lining. The urogenital folds fuse to form the spongy urethra and the overlying surface ectoderm fuses to form the penile raphe. At the tip of the penis, an ectodermal ingrowth (the glandular plate) meets the spongy urethra, and soon canalizes to complete the urethral lumen; this distal ectodermally derived portion is the fossa navicularis. The corpora cavernosa and corpus spongiosum develop from phallic mesenchyme. The scrotum arises by way of midline fusion of the labioscrotal swellings.

Agenesis of the penis is due to failure of development of the genital tubercle. Development of 2 genital tubercles leads to bifid penis. Failure of canalization of the glandular plate causes distal forms of hypospadias. Hypospadias along the shaft of the penis results from failure of urogenital fold fusion. Failure of labioscrotal fold fusion leads to perineal hypospadias, an unfused scrotum, or cryptorchidism. The embryological basis for epispadias apparently involves an abnormal dorsal location of the genital tubercle, resulting in opening of the urogenital sinus on the dorsal surface of the penis.

In males, the embryonic mesonephric ducts give rise to the epididymi, the ducti deferens, and the ejaculatory ducts. These ducts initially insert into the anterolateral aspects of the cloaca. The orifices migrate posterolaterally as the urorectal septum forms. With urethral closure, the orifices (now the ejaculatory duct orifices) are in the posterior urethra at the distal aspect of the verumontanum. The thin plicae colliculi are vestiges of the mesonephric duct migrations. The most common form of posterior urethral valves is apparently due to abnormally anterior origins of the embryonic mesonephric duct orifices. Consequent alteration in posterolateral migration leads to thick fused “valves” that obstruct the urethral lumen.

Female Urethra

The female urethra extends from the level of the bladder neck at the urethrovesical junction to the vestibule, where it forms the external meatus between the labia minora. Multiple small periurethral glands open into the urethra. Distally, groups of these glands on each side of the urethra (Skene glands) empty through 2 small ducts adjacent to the external meatus. The proximal portion of the urethral wall is made up of 2 layers of smooth muscle that are contiguous with the smooth muscle of the bladder neck. The thicker inner layer of the smooth muscle is longitudinal, and the thinner outer layer is circular. The outer portion of the urethra is composed of striated muscle that is predominantly circular in the upper two-thirds of the urethra; this circular muscle extends proximally to blend with the bladder base. The distal portion of the urethra is located immediately adjacent to the anterior vaginal wall and is enveloped by common musculature (the urethrovaginal sphincter) that extends to the inferior pubic ramus.

Normal Kidney and Bladder Size

Nomograms are available for renal length and volume in children according to age and gender.^4–8 Rosenbaum et al reported the following formulas for normal pediatric renal length: renal length (cm) = 6.79 + 0.22 × age (years). For babies younger than 1 year, the equation is: renal length (cm) = 4.98 + 0.155 × age (months).⁹

A small kidney can be due to idiopathic hypoplasia or result from various acquired insults (Table 44-1). Causes of renal enlargement include hydronephrosis, polycystic kidney disease, Beckwith-Wiedemann syndrome, and renal tumors (Table 44-2). Kidneys with a duplication anomaly are usually longer than normal. Compensatory renal hypertrophy is a common cause of unilateral nephromegaly. Compensatory hypertrophy occurs due to congenital absence, surgical removal, or severe dysfunction of the contralateral kidney. The degree of compensatory hypertrophy relates to the age at which the insult occurred: the effect is more pronounced in the fetus than in the child, and is greater in the young child than in a teenager or adult.

Bladder capacity increases with age. The normal bladder capacity of the newborn is approximately 30 to 50 mL. The capacity is approximately 100 mL at 1 year of age, 200 mL at 5 years, and 400 mL at 10 years and older. Normal bladder capacity for children less than 2 years of age can be estimated with the following formula: bladder capacity (mL) = (age + 2) × 30.

The thickness of the normal bladder undergoes little variation with age. The normal mean thickness is 2.76 mm when nearly empty and 1.55 mm when full. The upper limits of normal are 3 mm when full, and 5 mm when empty.¹⁰

Fetal Lobulations

Deep cortical grooves are present in the fetal kidney overlying the septa between the renal segments. These grooves normally diminish in prominence near the time of birth, and continue to diminish during the first few years of life. These “fetal lobulations” are normally visible with renal sonography and CT in the newborn. Incomplete regression sometimes results in the persistence of this developmental variation in older children and adults. Fetal lobulations occur between calyces and have sharp margins. Focal parenchymal indentation due to scarring, in contradistinction, tends to be ill-defined and oriented over a calyx.

Septa of Bertin

The septa (columns) of Bertin consist of bands of functional cortical tissue that extend between renal segments. A congenitally enlarged septum of Bertin is a developmental variation. The only clinical significance of this “anomaly” is that it not be confused with neoplasm, infection, or other pathology on imaging studies. The most common location of an enlarged septum of Bertin is between the upper and middle infundibula. The finding is bilateral in slightly more than half of patients. Prominent septa of Bertin are typically present in duplicated kidneys, including those with bifid pelvis.

Sonography and contrast-enhanced CT demonstrate an enlarged septum of Bertin as an area of prominent, but otherwise normal, renal parenchyma that extends into the renal sinus. Scintigraphy with a cortical agent shows normal uptake. The calices draining a prominent septum of Bertin frequently appear somewhat distorted on IV urography. The involved calices and infundibula tend to be small and irregular. Adjacent infundibula are sometimes elongated and displaced.

Prenatal Pyelocaliectasis

Dilation of the pelvicaliceal system is a common finding on prenatal ultrasound. Some degree of fetal pelvocaliectasis occurs in between 1 in 100 and 1 in 200 pregnancies. Mild renal pelvic dilation is often transient and of little or no clinical significance. Mild fetal pelvic dilation that resolves spontaneously during infancy is termed “benign transitory renal pelvic dilatation.” However, even mild prominence of the collecting system can be associated with other urinary system pathology, such as vesicoureteral reflux. Obstructive uropathy usually causes substantial collecting system dilation. A third-trimester anteroposterior renal pelvic diameter of less than 10 mm indicates that obstructive uropathy is highly unlikely. Vesicoureteral reflux is present in 10% to 20% of infants with a history of antenatal hydronephrosis. A normal postnatal sonographic examination in an infant with a history of antenatal hydronephrosis does not reliably exclude the diagnosis of vesicoureteral reflux.^11–17 (Please see the section “Congenital Hydronephrosis” for additional discussion of prenatal pyelocaliectasis.)

Ureteral Folds

During the fourth or fifth month of gestation, asymmetric growth of the ureteral muscle wall can occur, resulting in 1 or more kinks in the ureter. These kinks, or mucosal folds, are termed “fetal ureteral folds” (Wolfler-Englisch-Östling folds). These can persist and result in prominent transverse mucosal ureteral folds in the newborn infant. Ureteral folds are demonstrated with urography in up to 20% of otherwise normal newborns. The most common site for a solitary fetal ureteral fold is just below the ureteropelvic junction (UPJ). Any portion of the ureter can be involved, however; multiple folds are present in some infants. Most often, fetal ureteral folds disappear as normal growth occurs during infancy; rarely, a fold may persist and result in a ureteral valve. A contrast-opacified ureter with multiple persistent fetal folds may have a tortuous or corkscrew appearance.

Ectopic Kidney

Ectopic kidney refers to an abnormal renal location due to developmental factors. This is a relatively common congenital anomaly. Renal ectopia is “ipsilateral” or “crossed,” depending on whether or not the orifice of the draining ureter is located on the same side of the body as the kidney. There are cranial and caudal forms of ipsilateral ectopia; the latter is further classified as abdominal (below L2 and above the iliac crest), iliac (at the iliac crest or iliac fossa), and pelvic (in the true pelvis). Ectopia is usually associated with abnormal rotation of the kidney. Various fusion anomalies can also occur.

Caudal Ectopic Kidney

Ipsilateral caudal ectopic kidney is the most common form of ectopia. The abdominal and iliac forms of ectopia occur in approximately 1 in 600 individuals; pelvic kidney is present in approximately 1 in 700.^18,19 Caudal ectopic kidney is due to failure of normal superior migration of the developing fetal kidney. The contralateral kidney is usually in a normal location; bilateral ectopia occurs in approximately 10% of patients with ectopic kidney. Bilateral failure of renal ascent can occur in association with the various types of kidney fusion. Anomalies of the contralateral kidney are common in association with ectopic kidney. The contralateral kidney is absent in 10% of patients with a pelvic kidney. Approximately 15% of males with caudal ectopic kidney have genital anomalies, with hypospadias and undescended testis being the most common. Approximately 75% of females with caudal ectopic kidney have genital anomalies, such as duplication of the vagina, bicornuate uterus, and hypoplasia or agenesis of the uterus and vagina.^20–22 Skeletal anomalies occur in approximately 50% of children with renal ectopy; the most common include plagiocephaly, rib deformities, dysplastic vertebra, and absent bones (radius).²¹ Cardiac septal and valvar defects can occur in these children. Potential GI anomalies include anorectal malformations and malrotation.²³

A caudal ectopic kidney is often small and abnormally rotated. The ureter is short. The renal blood supply is from the lower portion of the aorta (abdominal ectopia) or branches of the iliac artery (iliac or pelvic ectopia); multiple renal arteries are common. The abnormal location and orientation of an ectopic kidney may cause interference with drainage through the collecting system. About half of these children have hydronephrosis, usually due to a primary UPJ obstruction. Alteration in the anatomy of the ureterovesical junction is also common in children with pelvic kidney; vesicoureteral reflux occurs in approximately 70% of these children. Reflux on the side of the normally located kidney is also common. Therefore, voiding cystourethrography is generally indicated for all children with ectopic kidney.²⁴ A hypoplastic ectopic kidney with an ectopic ureter can cause symptoms of continuous dribbling. A pelvic kidney is often palpable, particularly when it is hydronephrotic. The abnormal location makes a pelvic kidney susceptible to blunt trauma. Renal disease develops in 40% of patients with a solitary pelvic kidney.²⁰

Various imaging techniques are useful for establishing the diagnosis of a caudal ectopic kidney. A careful search of the abdomen and pelvis for an ectopic kidney is mandatory for any patient with an apparent congenital single kidney. Sonography is often adequate for the localization and characterization of an ectopic kidney, although overlying bowel gas in the pelvis sometimes obscures the kidney. Also, the ectopic kidney may lack a normal reniform shape and have abnormal parenchymal echogenicity. The normal central renal sinus echo complex is often absent or is eccentric. The parenchyma may be difficult to visualize sonographically in a severely hydronephrotic pelvic kidney. The adrenal gland ipsilateral to a pelvic kidney is in a normal location, but may assume an elliptical shape that can be confused for a hypoplastic kidney on sonography or CT during infancy.^25,26

The most sensitive imaging technique for the detection of an ectopic kidney is scintigraphy with a renal cortical imaging agent such as dimercaptosuccinic acid (DMSA). This also provides accurate quantification of renal function and detection of parenchymal scarring. Catheterization of the bladder may be required to prevent obscuration of a pelvic kidney. Suspected obstruction of the collecting system can be evaluated with diuretic renal scintigraphy. MR is also highly sensitive for demonstration of renal location and morphology (Figure 44-2).²⁷

Excretory urography, MR urography, or CT urography is sometimes helpful to define the anatomy of the collecting system of an ectopic kidney. The renal pelvis is nearly always directed in an abnormal orientation due to anomalous rotation of the kidney (Figure 44-3). An extrarenal configuration of the pelvis is common. Excretory urography is sometimes compromised in patients with caudal ectopia (particularly pelvic kidney) due to poor renal function, anomalous rotation of the kidney, and overlying structures (bowel, bone, and bladder).

Cranial Ectopic Kidney

A cranial, or superior, ectopic kidney accounts for fewer than 5% of instances of renal ectopy. The prevalence is approximately 1 in 13,000 autopsies.²⁸ When located in the chest cavity, this is termed intrathoracic kidney. Cranial ectopic kidney occurs more commonly on the left side, and is more frequent in males.²⁹ The most common mechanism for superior ectopic kidney is excessive developmental migration. This likely involves a lack of normal resistance at the diaphragm, as in patients with a diaphragmatic hernia (most often involving the foramen of Bochdalek). Occasionally, there is a localized diaphragmatic eventration above a superior ectopic kidney, in which case the kidney is not truly intrathoracic. Acquired intrathoracic kidney can occur with traumatic diaphragmatic rupture. Congenital thoracic kidney is usually supplied by a normally arising renal artery, sometimes in association with an accessory vessel from the thoracic aorta. The ipsilateral adrenal gland is usually normal.

Nearly half of children with omphalocele have a cranial ectopic kidney. In these patients, both kidneys are usually in abnormal superior locations; a unilateral cranial ectopic kidney nearly always involves the right kidney. The relationship of the ipsilateral adrenal gland is variable in omphalocele patients.

A congenital thoracic kidney is usually located in the posterior mediastinum, and may appear as a paraspinal soft tissue mass on chest radiographs. The correct diagnosis can be established with CT, excretory urography, sonography, or renal cortical scintigraphy (Figure 44-4). Cross-sectional imaging is helpful to determine if the kidney is located below a diaphragmatic eventration or has passed through a defect in the diaphragm.

Crossed Ectopic Kidney

Crossed renal ectopia refers to anomalous location of a kidney across the midline. Most often, the ectopic kidney rests inferior to the normally located kidney, but other arrangements can occur. The prevalence of crossed renal ectopia is approximately 1 in 7000 autopsies.³⁰ This anomaly is more common in boys. In approximately 90% of instances of crossed renal ectopia, there is fusion between the 2 kidneys. Crossed ectopia of a solitary kidney and bilateral crossed ectopia are rare.³¹ Vesicoureteral reflux is present in about half of children with crossed renal ectopy, usually affecting the ureter draining the ectopic kidney. Other potentially associated lesions include multicystic dysplastic kidney and renal neoplasms.^24,32,33

Sonography is usually sufficient for the diagnosis of crossed renal ectopia. CT and MR provide optimal depiction of the pathological anatomy, however (Figure 44-5). The major findings are lack of a kidney on 1 side and 2 kidneys on the other. Most often, the ectopic kidney is inferior to the orthotopic kidney. There is often abnormal rotation of the ectopic kidney. Ectopia without fusion is suggested if there is separation between the kidneys and if real-time sonography shows independent movement during respiration. When the 2 kidneys are in contact, reliable differentiation between fused and separate kidneys may not be possible with imaging studies, but this information is usually of little clinical importance. Occasionally, a fusion anomaly results in the appearance of a single irregular renal mass. The demonstration of 2 complete pelvicaliceal systems and ureters on excretory urography or CT aids in the diagnosis (Figure 44-6). The ureter of the ectopic kidney nearly always crosses the midline and enters the bladder on the correct side. With the very rare anomaly of bilateral crossed ectopia, each ureter crosses the midline to insert in the contralateral side of the trigone.

Fused Kidneys

Renal fusion anomalies encompass a variety of parenchymal or fibrous connections between the kidneys. A common renal fascia often surrounds the fused kidneys. The developmental event that results in fusion likely occurs early during embryogenesis, with fusion of the 2 nephrogenic blastemas. If fusion occurs deep in the pelvis and ascent proceeds normally, a horseshoe kidney may result. With early fusion of the nephrogenic blastemas and failure of ascent, a pelvic fused kidney (“cake kidney” or “disc kidney”) can occur. Crossed fused renal ectopia refers to extension of 1 kidney across the midline and fusion to the normally located contralateral kidney. Renal fusion anomalies prevent normal rotation of the involved kidney(s), and the renal pelvis is typically directed anteriorly. Anomalous arterial supply of the ectopic kidney is also common. Rarely, multicystic dysplastic kidney involves 1 of the components of a renal fusion anomaly.

Crossed Fused Renal Ectopia

Crossed fused renal ectopia is the second most common renal fusion anomaly, after horseshoe kidney. The estimated prevalence is 1 in 1300 to 1 in 7600 individuals. This anomaly is more common in males. The fused kidneys are located on the right 2 to 3 times more often than on the left. In approximately 90% of patients, there is fusion of the lower pole of the normally located kidney to the upper pole of the ectopic kidney, with the ectopic kidney positioned inferiorly (Figure 44-7). A variety of configurations can occur, however, resulting in an S-, L-, or disc-shaped renal mass. Rarely, the ectopic kidney is positioned above the orthotopic kidney.

Imaging studies of crossed fused ectopia show 2 relatively distinct kidneys, usually with abnormal rotation of the lower (ectopic) kidney (Figure 44-8). In some patients, the orthotopic kidney and the ectopic kidney form a common renal mass that mimics a single enlarged kidney. The correct diagnosis is suggested on sonography by the identification of 2 distinct renal pelves and a small notch or cleft at the fusion site. IV urography or CT urography usually allows a definitive diagnosis by demonstrating 2 ureters, with each emptying into opposite sides of the trigone. The diagnosis of crossed fused renal ectopia is also supported when the 2 renal pelves are oriented in different directions, in contradistinction to the uniplanar orientation of a duplex system. With the rare anomaly of crossed fused renal ectopia and multicystic dysplastic kidney, a multiloculated cystic mass is attached to either an ectopic or a normally located kidney.

Horseshoe Kidney

Horseshoe kidney is the most common type of renal fusion. In 90% of horseshoe kidneys, the fusion is at the lower poles. The prevalence of horseshoe kidney is approximately 1 in 1800 livebirths.³⁴ Individuals with horseshoe kidney are at an increased risk for renal pelvic tumors and nephroblastoma; teratoma has also been reported.³⁵ Wilms tumor is the most common childhood tumor to develop in horseshoe kidneys, with up to a 7-fold increased prevalence compared with the general population (Figure 44-9).³⁶ Urolithiasis develops in 20% of patients with horseshoe kidney. Other urinary tract pathology that occurs with an increased frequency in children with horseshoe kidney includes UPJ obstruction, vesicoureteral reflux, duplication anomalies, megaureter, ectopic ureter, renal dysplasia, and supernumerary kidney. The anomalous position of a horseshoe kidney makes it prone to injury from blunt trauma. Horseshoe kidney can occur in association with Turner syndrome, neural tube defects, trisomy 18, Fanconi anemia, dyskeratosis congenita with pancytopenia, and Laurence-Biedl-Moon syndrome. Anomalies of the cardiovascular, GI, skeletal, and central nervous systems (CNS) occur with an increased frequency in individuals with horseshoe kidney.^37,38

The embryology of horseshoe kidney is incompletely understood. Fusion likely occurs early during embryogenesis when the 2 metanephrogenic blastemas are located adjacent to each other. The isthmus that connects the 2 kidneys is typically located at the lower poles, and can be composed of renal parenchyma or connective tissue (Figure 44-10). Most often, the isthmus is anterior to the aorta and vena cava; rare variants include a location of the isthmus posterior to the aorta and vena cava or posterior to the vena cava alone. Fusion of the upper poles (“upside-down” horseshoe kidney) is quite rare. A typical horseshoe kidney is inferiorly ectopic. There are usually multiple arteries supplying a horseshoe kidney; these may arise from the aorta, the iliac vessels, or the inferior mesenteric artery. The inferior mesenteric artery always courses anterior to the isthmus. The ureters also extend anterior to the isthmus. Bifid renal pelves or completely duplicated ureters are common. Some degree of kidney malrotation is typical, and the renal pelves are usually enlarged.

With sonography, each kidney may have a deceivingly normal appearance despite the presence of a horseshoe kidney anomaly. Visualization of the isthmus provides an unequivocal diagnosis (Figure 44-11), but overlying bowel gas sometimes obscures this structure. In addition, an isthmus that is composed of connective tissue may be difficult to detect. Observation of the abnormal medial and anterior orientations of the lower poles of the kidneys is helpful for establishing the correct diagnosis. Sometimes, each kidney has a pyriform or inverted triangular shape on longitudinal images. The anomaly is readily demonstrated with scintigraphy, CT, or MR (Figure 44-12). When clinically indicated, noninvasive evaluation of the related vascular anatomy is possible with CT angiography or MR angiography. In patients with suspected renovascular hypertension, conventional angiography may be required because of the common occurrence of multiple anomalous supplying arteries.

IV urography of horseshoe kidney typically shows medial deviation of the lower poles of the fused kidneys. With a symmetric midline lower pole fusion, a “hand-holding calyces” pattern occurs. Asymmetric fusions alter this pattern. The lower pole calyces are usually medial to the ipsilateral ureter. Each ureter curves laterally as it crosses the isthmus, sometimes producing a “flower vase” pattern.

The renal pelves of a horseshoe kidney are usually prominent on imaging studies. This may simply be related to malrotation and anomalous development, although mild obstruction of the ureters as they pass over the isthmus is likely a factor as well. Patients with horseshoe kidney are at increased risk for true UPJ obstruction. In this circumstance, imaging studies usually show a greater degree of pelvocaliectasis than the mild dilation that is typical of uncomplicated horseshoe kidney; diuretic renal scintigraphy is confirmatory.

Renal Malrotation

Early in embryonic development of the kidney, the collection system is in an anterolateral position relative to the metanephros. During ascent of the fetal kidney, rotation normally occurs until the pelvis is oriented slightly anteromedially. The normal orientation of the postnatal kidney is such that the collecting system orientation is approximately 30° anterior to the coronal plane. Renal malrotation refers to any substantial variation from this orientation. The prevalence of malrotation is approximately 1 in 390 livebirths.³⁹ Malrotation accounts for approximately 10% of upper urinary tract anomalies. Malrotation is common in association with ectopia kidney and renal fusion anomalies.

The diagnosis of renal malrotation is possible with excretory urography or any of the cross-sectional imaging techniques. There are 5 types of renal malrotation: with nonrotation, the renal pelvis is oriented along the anterior aspect of the kidney, whereas incomplete rotation indicates that the pelvis is directed between 30° and 90° anterior to the horizontal plane; these are the most common rotational anomalies of the kidney. Reverse rotation indicates that the renal pelvis is directed anterolaterally; the renal arteries and veins are draped along the anterior surface of the kidney. Excessive rotation refers to an abnormal posterior orientation of the renal pelvis, with the renal vessels passing along the dorsal surface of the kidney. With transverse rotation, the pelvis is directed superiorly or inferiorly.

Supernumerary Kidney

Supernumerary kidney is a rare anomaly that consists of an extra kidney that is completely separate from the normal kidneys and is surrounded by a capsule that is independent of that of the normal adjacent kidney. This can occur in the form of more than 2 distinct kidneys or as an additional kidney(s) in conjunction with 2 fused kidneys. Up to 5 supernumerary kidneys have been reported. The developmental cause of supernumerary kidney is unknown. A supernumerary kidney has an elevated risk for urolithiasis and hydronephrosis.

Diagnostic imaging studies show a supernumerary kidney as a distinct additional kidney that is usually smaller than the normal kidney. When a completely separate ureter is present, the supernumerary kidney is nearly always located above the resident kidney. The accessory kidney is typically below the normal kidney when the 2 are drained by a bifid ureter (approximately 60% of cases).⁴⁰ Distinction from a duplex kidney requires documentation of separation of the ipsilateral renal masses, usually best visualized on cross-sectional imaging. A supernumerary kidney is sometimes difficult to visualize with IV urography due to its small size and poor function.

Renal Agenesis

Renal agenesis refers to complete failure of development of a kidney. The estimated prevalence of unilateral renal agenesis is between 1 in 1100 and 1 in 1500 individuals.^41,42 There is a slight male predominance. Maternal diabetes is a risk factor for renal agenesis. Between 25% and 40% of patients with unilateral renal agenesis have associated congenital anomalies: cardiovascular in 30%, GI in 25%, and musculoskeletal in 14%.⁴³ Renal agenesis is a potential anomaly in the VACTERL and MURCS (Müllerian ducts, renal, and cervicothoracic) associations. Approximately 20% of male children with unilateral renal agenesis have associated genital anomalies; 70% of affected females have genital anomalies. Complete absence or hypoplasia of the vagina, the Mayer-Rokitansky-Kuster-Hauser syndrome, is frequently accompanied by agenesis of the kidney.²² Other potential genital system anomalies in children with unilateral renal agenesis include prostatic dermoid cyst, seminal vesicle cyst, and ectasia of the rete testis in males, and genital tract duplication and Gartner duct cyst in females. Renal agenesis is associated with Turner syndrome. Up to half of individuals with renal agenesis have an abnormality of the contralateral kidney, such as vesicoureteral reflux, ureterovesical junction (UVJ) obstruction, or UPJ obstruction.^37,44

The pathogenesis of renal agenesis apparently involves the inhibition of growth of either the nephric duct or the ureteral bud. Formation of the mesonephros is dependent on contact with the primary nephric duct. Likewise, appropriate development of the metanephros is dependent on contact with the ureteral bud. The stage at which growth inhibition occurs determines the specific nature of the anomaly. If there is inhibition of nephric duct growth when the duct is at the level of the pronephros, the kidney and ureter fail to develop. Genital anomalies also occur in this situation, with agenesis of the ipsilateral epididymis, vas deferens, seminal vesicle, and (sometimes) testis in males, and agenesis of the vagina, ipsilateral half of the uterus, and (sometimes) ipsilateral ovary in females. If growth inhibition occurs when the nephric duct is approximately half the length of the mesonephros, the ovary and the upper portion of the fallopian tube usually are intact. If inhibition occurs when the nephric duct has developed to a level below the urogenital folds but above the ureteral bud, there is agenesis of the seminal vesicle in the male, and the vagina and ipsilateral half of the uterus in the female, but the gonad develops normally. If inhibition is due to delayed development of the ureteral bud or displacement of the metanephrogenic blastema, the ureter may form despite agenesis of the ipsilateral kidney (approximately 20% of cases).^45,46

Renal scintigraphy is a sensitive technique for confirming a diagnosis of renal agenesis. Scintigraphy accurately excludes the presence of a small, poorly functioning, or ectopic kidney, and provides an accurate technique for quantification of function. MR and CT are also highly sensitive for the confirmation of renal agenesis or hypoplasia. Dynamic contrast-enhanced MR imaging provides information concerning the anatomy and function of the contralateral kidney. Estimation of the glomerular filtration rate (GFR) and parenchymal volume of a single kidney may have long-term prognostic implications. Imaging studies typically show hypertrophy of the contralateral kidney. The ipsilateral adrenal gland is absent in approximately 10% of patients with renal agenesis. When present, the adrenal gland usually lacks a normal V or Y shape. The adrenal gland typically retains a normal shape in cases of renal atrophy or following nephrectomy. Imaging evaluation is also essential for the detection and characterization of the various potential associated genitourinary anomalies, particularly cystic lesions such as hydrometrocolpos. Voiding cystourethrography should be performed to detect vesicoureteral reflux or other pathology in the renal system contralateral to the absent kidney. Rarely, refluxed contrast fills a blind-ending ureter ipsilateral to the absent kidney.^27,47

The prevalence of bilateral renal agenesis is approximately 1 in 4000 births. Forty percent of these fetuses are stillborn. There is a 2% to 5% risk of bilateral renal agenesis in subsequent pregnancies. The male to female ratio is approximately 2.5:1. Bilateral renal agenesis results in maternal oligohydramnios, as fetal urine is the major source of amniotic fluid. Affected newborns have features of Potter syndrome: distinctive facial deformities (broad flat nose, low set ears, and prominent suborbital skin folds) and bilateral pulmonary hypoplasia associated with pneumothoraces. The bladder is hypoplastic or absent in about half of patients with bilateral renal agenesis. Fetal MR plays an important role in supplementing sonography for the evaluation of suspected renal agenesis.^48,49

Renal Hypoplasia

Renal hypoplasia refers to a congenitally small kidney with a subnormal number of nephrons. The pathogenesis may involve contact of the embryonic ureteral bud with only the most caudal portion of the metanephrogenic blastema. Renal hypoplasia is the most common kidney anomaly in individuals with a PAX2 mutation. The pathogenesis may involve reduced branching of the ureteric bud early during embryogenesis of the kidneys. Some patients with PAX2 mutations also have optic nerve colobomas, that is, the renal-coloboma syndrome.

A miniature kidney is a hypoplastic kidney that has a normal reniform shape and drains via a ureter that enters the bladder orthotopically (Figure 44-13). The calyces are morphologically normal, but reduced in number. Overall renal function is usually normal in patients with miniature kidney, even if the anomaly is bilateral. Rarely, miniature kidney is associated with hypertension during infancy.

Oligomeganephronia refers to a bilateral form of renal hypoplasia in which the nephrons are enlarged and diminished in number. It is more common in males. About one-third of cases are associated with a maternal age of 35 years or older at the time of conception. These infants suffer polyuria and polydipsia due to a deficient ability of the kidneys to concentrate urine. Progressive renal failure during infancy is typical. This is followed by a long period of relative stabilization of renal function; around the time of puberty, further deterioration occurs in most patients. Imaging studies show small kidneys bilaterally. The number of renal segments may be reduced; in some patients, only a single renal segment with a solitary papilla is present (unipapillary kidney). The calices and ureters in children with oligomeganephronia are normal in caliber or mildly dilated.

Infants with renal collecting system dilation sometimes have a congenitally hypoplastic kidney, often in association with vesicoureteral reflux. It is uncertain whether the renal parenchymal thinning in these children is due to fetal reflux nephropathy or true developmental renal hypoplasia. In many of these children, there is an ectopic insertion of the ureter; this may cause failure of appropriate interaction between the embryonic ureteral bud and the metanephric blastema. Hypoplastic kidney with a single ureter and lateral ectopia sometimes occurs as a hereditary anomaly, and is most common in females. Hypoplastic kidney in association with an obstructed ectopic ureter is most common in girls. The ureteral insertion is often into the vagina, and causes urine leakage. Scintigraphy or contrast-enhanced CT or MR may be required to detect the small poorly functioning kidney.

Renal Dysplasia

Renal dysplasia is a descriptive term that encompasses various disorders of development and differentiation of the kidney. It is the most common cause of end-stage renal disease in children. The affected kidney is typically hypoplastic, the nephrons are histologically abnormal, and there is fibrosis within the mesenchymal stroma. Multiple cysts within the parenchyma are common. Various developmental mechanisms of renal dysplasia have been proposed. Some cases are apparently related to in utero urinary obstruction or vesicoureteral reflux. Another popular explanation is abnormal interaction between the embryonic ureteral bud and the metanephric blastema (the ureteral bud theory). Renal dysplasia can occur in association with ectopic ureter, posterior urethral valves, and prune belly syndrome.

Imaging studies of renal dysplasia typically show a small kidney that functions poorly. The findings can be unilateral, bilateral, or segmental. Calyceal blunting may be present, and calyceal morphology often is distorted. Hydrodysplasia refers to a small dysplastic kidney with a reduced number of dilated and deformed calyces (the normal kidney has at least 7 calyces). Voiding cystourethrography of children with renal dysplasia frequently demonstrates vesicoureteral reflux; intrarenal reflux of contrast may opacify irregular collecting tubules and cysts. Sonography shows echogenic parenchyma, with deficient corticomedullary differentiation. Cortical cysts are visible in some patients. Findings of renal dysplasia on MR urography include a small (sometimes ectopic) kidney, dysmorphic calyces, parenchymal cysts, deficient corticomedullary differentiation, a weak/patchy nephrogram, and focal scarring.

Renal Tubular Dysgenesis

Renal tubular dysgenesis is a rare developmental disorder of the kidneys that is characterized anatomically by a paucity or absence of proximal tubules. Histological examination shows the tubules to be short, lacking of normal convolutions, and lacking normal connections to the collecting system. The glomeruli are normal.

Identified risk factors for renal tubular dysgenesis include monochorionic twins (twin-to-twin transfusion), neonatal hemochromatosis, and maternal use of angiotensin-converting enzyme or indomethacin during pregnancy. Some cases are familial, with an autosomal recessive pattern.⁵⁰ Renal tubular dysgenesis is characterized clinically by oligohydramnios, pulmonary hypoplasia (Potter sequence), and anuria. With rare exceptions, this results in stillbirth or fatal neonatal respiratory failure.⁵¹

Prenatal sonography shows oligohydramnios. As with other urinary causes, oligohydramnios does not occur until at least the 18th week of gestation, as fetal urine production does not substantially account for amniotic fluid formation until this time. The kidneys are sonographically normal in the fetus. Skull ossification defects may be present. The newborn with renal tubular dysgenesis usually has normal renal morphology on imaging studies. This diagnosis is suggested by a normal renal ultrasound in an anuric newborn, in the absence of perinatal hypoxia. Nonspecific prominence of cortical echogenicity may be observed in some patients, and Doppler examination may show a resistive pattern of renal arterial flow.^52,53

Clinical Presentations

Congenital Hydronephrosis

The prevalence of severe fetal hydronephrosis is approximately 1 in 600 to 1 in 800 fetuses. The most common final diagnosis is UPJ obstruction, which accounts for 35% to 50% of all prenatally detected uropathies. Mild to moderate pyelectasis (renal pelvis diameter ≥7 mm in the third trimester) is present in 3% to 4.5% of fetuses. More than half of these fetuses are found to have urinary tract anomalies as infants, although not all are clinically significant. A fetal renal pelvic diameter of greater than 20 mm indicates at least 90% probability of a significant urinary anomaly.^54–56

The Society for Fetal Urology classification system grades pelvocaliectasis and hydronephrosis based on the sonographic and IV urographic findings: grade 0, normal; grade 1, mild dilation of the pelvis, without calycectasis; grade 2, moderate dilation of the pelvis, with mild calycectasis; grade 3, enlarged pelvis and calyces, with preserved parenchyma; grade 4, markedly enlarged pelvis and calyces, with thinned parenchyma.⁵⁷ As a general rule, a third-trimester anteroposterior renal pelvic diameter of at least 10 mm indicates that postnatal evaluation of the urinary tract should be performed.^11,12

Infants with a history of fetal hydronephrosis should be evaluated initially with sonography (Figure 44-14). If the prenatal findings are confirmed, a voiding cystourethrogram may be indicated, as well as additional studies determined by the imaging findings and clinical presentation. As with prenatal imaging, an anteroposterior renal pelvic diameter of greater than or equal to 10 mm in the newborn suggests the possibility of urinary tract pathology; a diameter of greater than or equal to 15 mm indicates a substantial likelihood of obstructive uropathy. With mild dilation, sequential sonographic examinations to document resolution are usually sufficient. Assessment of the location and severity of obstruction in infants with persistent or moderate-to-marked dilation is possible with diuretic scintigraphy or contrast-enhanced MR urography. Because functional immaturity of the neonatal kidney limits the response to diuretics, diuretic scintigraphy is best delayed until at least 1 month of age.⁵⁸

Obstructive Uropathy

The term obstructive uropathy refers to interference with the flow of urine at any point from the kidney to the bladder. Obstructive uropathy is the most common cause of renal failure in children. Clinically significant obstructive uropathy is a condition that hampers optimal renal development or that causes progressive renal deterioration.

The most common causes of obstructive uropathy in children are UPJ obstruction, UVJ obstruction, primary megaureter, and posterior urethral valves.⁵⁹

The pathogenesis of renal injury in obstructive uropathy includes apoptosis of tubular epithelial cells, tubular atrophy, nephron loss, and interstitial fibrosis. The reduced number of nephrons places stress on the kidney that persists even after correction of the obstructing insult. These patients, therefore, are at increased risk for long-term progression of renal insufficiency, particularly when the contralateral kidney is abnormal. In the fetus and infant, there is some degree of compensatory growth of the kidney contralateral to an obstructed kidney. The clinical manifestations relate to the level and severity of obstruction, the degree of hydration, and the superimposition of complications such as trauma or infection. There is little alteration in renal function with ureteral obstruction of up to two-thirds the diameter. Prompt and accurate diagnosis is essential to plan an optimal strategy for the treatment of the child with obstructive uropathy and to limit long-term effects from nephron loss. The degree of injury from obstructive uropathy in the developing kidney depends on the severity and duration of obstruction.^59,60

The most important imaging manifestation of urinary system obstruction is dilation of the renal collecting system, that is, pyelocaliectasis. Some conditions associated with pyelocaliectasis are not obstructive (Table 44-3). The term hydronephrosis is usually reserved for instances of dilation that are due to obstruction. Ureterectasis is dilation of the ureter. Obstruction of the distal aspect of the ureter causes hydroureteronephrosis.

Sonography is the initial imaging technique employed for most patients with obstructive uropathy. Many congenital forms of urinary tract obstruction are detected prenatally with this technique. Sonography provides information concerning the severity of dilation, the thickness of the renal cortex, and the presence of additional anomalies such as duplication or ureterocele. Sonography, however, does not provide specific information about renal function and there is poor correlation between the degree of collecting system dilation and the severity of obstruction. Renal function can be measured with scintigraphy or MR renography. Quantification of urinary tract obstruction is possible with diuretic scintigraphy, diuretic MR urography, or a pressure perfusion study (Whitaker test).

Diuretic scintigraphy is a noninvasive diagnostic imaging technique that assesses the dynamics of urine transit through the collecting systems. This is performed with an agent that is rapidly excreted in the urine, such as diethylene triamine pentacetic acid (DTPA) or mercaptoacetyltriglycine (MAG-3). IV diuretic (furosemide) is administered either at a standardized time after tracer injection (usually 20 minutes; “F + 20”) or at the point that the collecting system is judged on the basis of the imaging appearance to be distended with radiolabeled urine. Additional image acquisition is carried out for 30 minutes. Time–activity plots of the pelvicaliceal systems depict the dynamics of diuretic-induced washout of tracer; washout half-times are calculated from the data. To diminish interference in the results by bladder activity, the study is best performed with continuous catheter drainage of the bladder; in older children, emptying of the bladder immediately prior to the examination is usually sufficient. The child should be well hydrated at the time of the examination.

Guidelines for interpretation of diuretic renal scintigraphy are as follows: normal, T_1/2 less than 10 minutes; equivocal, T_1/2 greater than or equal to 10 minutes and less than or equal to 20 minutes; obstructed, T_1/2 greater than 20 minutes (Figure 44-15). Other diuretic scintigraphy techniques utilize diuretic injection at the time of radiopharmaceutical injection (F + 0), or 15 minutes prior (F – 15). Factors that diminish the reliability of diuretic scintigraphy for the characterization of urinary obstruction include dilution of tracer in a dilated urine-filled renal pelvis, distensibility of the renal pelvis, and limited capacity for response to diuretics by the neonatal or damaged kidney. Normalization of washout half-time after technically successful surgical correction of an obstructed kidney may not occur for several months.^61–63

Dynamic contrast-enhanced MR urography of patients with suspected obstructive uropathy provides anatomic information, an assessment of renal function, as well as information concerning the temporal course of urine drainage through the pelvicaliceal systems and ureters. The renal transit time on MR urography is the time between the appearance of contrast material in the cortex and its appearance in the upper aspect of the ureter. A renal transit time of less than or equal to 4 minutes is normal, between 4 and 8 minutes is indeterminate, and greater than 8 minutes is highly suggestive of obstruction in the pelvicaliceal system or upper ureter.^64,65

Obstruction, particularly when acute, also can lead to changes in the calyceal transit time on MR evaluation. This is the time for contrast to pass from the cortex into the calyces. The contralateral kidney, when normal, serves as an internal standard for this parameter. Severe obstruction often leads to a delayed calyceal transit time. After successful surgical relief of the obstruction, the transit time usually normalizes or becomes more rapid than in the normal kidney. An abnormally rapid calyceal transit time implies glomerular hyperfiltration. Persistent glomerular hyperfiltration in the ipsilateral and/or contralateral kidney may be an indicator of an increased long-term (i.e., years or decades) risk for eventual decompensation and renal failure.

With MR urography, there are 2 methods for estimating differential renal function. Measurement of the volume of enhancing renal parenchyma in each kidney provides similar data as renal cortical scintigraphy. The other technique assesses contrast flow from the renal vascular compartment into the nephrons; this is the Rutland-Patlak technique. The “Patlak” analysis relates to individual kidney GFR. With an acute urinary tract obstruction, the differential renal function calculation based on the Patlak plot is often greater than the volume-based differential function.

The Whitaker test is a minimally invasive technique that assesses pelvicaliceal system pressure at controlled volumes of urine flow. This study is occasionally useful in selected patients to quantify the degree of urinary tract obstruction or to confirm lack of clinically significant obstruction despite pelvicaliceal dilation. This test can be useful in patients with persistent symptoms after surgical repair of an obstruction, as some degree of collecting system dilation after repair is common despite appropriate relief of the obstruction. Classically, the Whitaker test is performed with percutaneous needle introduction into the pelvicaliceal system utilizing either a double-lumen needle or 2 single-lumen needles. One needle/lumen is used to constantly monitor the pressure within the collection system and the other is used to infuse a saline/contrast solution. Pressures are also monitored in the bladder through a catheter. Normally, the differential pressure between the kidney and bladder during an infusion of 10 mL/min is less than 13 cm H₂O. A differential of 14 to 22 cm H₂O suggests mild obstruction, and a differential of greater than 22 cm H₂O indicates moderate-to-severe obstruction.⁶⁶

Urinoma

A urinoma is an encapsulated collection of extravasated urine in the perirenal space. The formation of a urinoma requires the presence of a functioning kidney, a ruptured collecting system, and a distal obstruction. Renal trauma, surgery, and obstructive uropathy constitute the most common causes of urinoma. In the fetus, a urinoma can be caused by posterior urethral valves, UPJ stenosis, or other high-grade obstruction. In most instances, leakage of urine occurs by way of a microperforation of the renal pelvic wall, allowing urine to accumulate beneath the renal fascia or retroperitoneally. Imaging studies show a large clear fluid collection adjacent to the kidney. Accumulation of IV contrast or excreted radiopharmaceutical in the cyst confirms the diagnosis.^67,68

Ureteral Stenosis

Ureteropelvic Junction Obstruction

UPJ obstruction is the most common type of urinary tract obstruction in infants and children. The prevalence is approximately 3 in 1000 livebirths. There is a 65% male predominance. Bilateral obstructions are present in up to 20% of patients.⁶⁹ Of unilateral cases, there is a higher prevalence on the left side, with a ratio of 3:2. Descriptive forms of UPJ obstruction include acute, chronic, total, partial, congenital, and acquired. Most common is chronic partial obstruction due to a congenital stenosis.

Abnormal development of the proximal ureteral smooth muscle is the most important pathological abnormality in many children with UPJ obstruction. There is often lack of normal peristalsis in the abnormal segment. Obstruction in some children is related to a ureteral valve that is composed of mucosa and muscle. In other patients, extrinsic compression from an aberrant vessel (a factor in about one-third of UPJ obstructions), adhesion, or band crossing the upper ureter is implicated. Infundibular or infundibulopelvic stenoses are uncommon; these are part of a spectrum of obstructive renal disease referred to as infundibulopelvic dysgenesis.⁷⁰

There are various theories of the ontogeny of congenital ureteral stenosis. Primary intrinsic mechanisms of focal ureteral maldevelopment have been proposed. An extrinsic cause that could also lead to ureteral obstruction is compression of the developing ureter by transient fetal vessels that produce localized developmental arrest, focal ureteral narrowing, and diminished ureteral muscle. In some children with congenital ureteropelvic obstruction, there is a lower pole hilar vessel at a site of ureteral angulation and narrowing. However, this does not prove cause and effect, as UPJ obstruction from another cause could lead to draping of the dilated pelvis over a normal vessel.^71,72

UPJ obstruction is the most common cause of an abdominal mass in a neonate. However, the condition is clinically silent in many children, and the diagnosis is made serendipitously or due to a late complication, such as hematuria, renal injury due to minor trauma, or hypertension. Prenatal diagnosis with sonography is common; in developed countries with widespread utilization of prenatal ultrasound, this is the most frequent presentation of UPJ obstruction. As a general guideline, a renal pelvis diameter of less than 10 mm during the third trimester or in the newborn is unlikely to indicate UPJ obstruction, and a diameter of greater than 15 to 20 mm is highly suspicious for clinically significant urinary tract obstruction.

In the neonate with UPJ obstruction, sonography shows dilation of the pelvicaliceal system, without ureteral dilation. The renal function and the hydration status of the infant influence the degree of dilation proximal to a ureteral obstruction. During the first few days of life, imaging studies may underestimate the severity of a UPJ obstruction because of the oliguric state of even healthy newborns. Also, obstruction can occur intermittently due to kinking of the ureter during episodes of high urine production. When there is marked pelvicaliceal dilation due to UPJ obstruction, generalized renal parenchymal thinning is common. Other potential findings include focal areas of scarring and signs of renal dysplasia. Contralateral urinary system abnormalities are present in one-fourth to one-third of children with a congenital UPJ obstruction. Common contralateral abnormalities include vesicoureteral reflux and UPJ obstruction.⁷³

Although usually a congenital lesion, ureteral stenosis can present at any age. Older children may suffer various nonspecific clinical manifestations. Most common in this age group is periumbilical abdominal pain, sometimes accompanied by nausea and vomiting. Recurrent episodes of colicky abdominal pain can occur. Microscopic or gross hematuria is common. Hematuria and/or flank pain after blunt trauma are fairly common presenting findings in older children and adults with UPJ obstruction.

As in neonates, the typical sonographic appearance of a UPJ obstruction in the older child is that of a substantially dilated pelvocaliceal system, and a normal or nonvisualized ureter. Doppler imaging can be utilized to search for an anomalous crossing vessel at the UPJ. With acute obstruction, Doppler studies show an increased resistive index in renal arteries and a diminished venous impedance index due to reduced intraparenchymal renal compliance.⁷⁴ Doppler evaluation is of limited value in patients with long-term obstruction, however, as pelvicaliceal compliance normalizes the collecting system pressure.

In children with nonspecific abdominal symptoms, the initial diagnostic imaging finding of hydronephrosis due to UPJ obstruction sometimes consists of soft tissue density fullness in the flank on standard radiographs. These children are usually further evaluated with sonography or CT because of a suspected mass. CT demonstrates pelvicaliceal system dilation. Although renal function is usually preserved, there may be diminished intensity of contrast opacification in the dilated collecting system due to dilution with unopacified urine. There is often delayed contrast opacification of the ipsilateral normal-sized ureter. Helical contrast-enhanced CT depicts the vascular anatomy in the area of the UPJ; this information is particularly helpful for avoiding vascular complications with a subsequent endopyelotomy procedure.⁷⁵

IV urography is generally not part of the evaluation of suspected UPJ obstruction in modern practice. As with CT, this study shows weak, and sometimes delayed, contrast opacification of the dilated pelvicaliceal system. UPJ obstruction is always associated with pyelocaliectasis. The identification of a normal- or small-sized ureter is required in order to suggest the diagnosis of UPJ obstruction. Delayed images are frequently necessary for adequate visualization of the ureter in these patients. In the past, radiography after IV administration of a diuretic was utilized to gauge the presence or absence of obstruction in the presence of hydronephrosis.

In patients with proximal ureteral obstruction due to a crossing vessel, urography (by IV, CT, MR, antegrade, or retrograde techniques) often shows the UPJ to be patent, with slight dilation of a short segment of the ureter immediately below the renal pelvis; this segment of the ureter is posteriorly angulated and there is an anterior indentation in the contrast column. Correlation of contrast-enhanced MR angiographic images with volume-rendered T2-weighted images of the collecting system often allows a precise diagnosis.^76,77

The presence of hydronephrosis alone does not necessarily indicate that surgical repair of a UPJ obstruction will lead to long-term clinical benefit. Many infants undergo spontaneous improvement. Therefore, functional evaluation with scintigraphy or MR urography plays an important role in selecting those patients who will be best served by intervention versus observation (see the Obstructive Uropathy section above). The typical finding of substantial UPJ obstruction on diuretic scintigraphy is delayed washout (e.g., >20 minutes) from a dilated pelvicaliceal system. Dynamic contrast-enhanced MR urography similarly demonstrates a prolonged (e.g., >8 minutes) renal transit time. Prolongation of the calyceal transit time is often present with high-grade or acute obstructions. Other MR findings of substantial obstruction (i.e., decompensated UPJ) include parenchymal hyperintensity on T2-weighted images (due to edema), an intense and delayed nephrogram, and a delayed peak with poor washout on the signal intensity versus time data plot. An additional finding suggestive of decompensation is a greater decrease in renal function of the obstructed kidney as measured with the Patlak plot method than on the renal parenchymal volume method (i.e., >4% difference in differential renal function calculations by the volumetric and Patlak techniques).

The function of a hydronephrotic kidney with very thin parenchyma may not improve significantly following repair of a UPJ obstruction. Elective nephrectomy is sometimes preferable for these patients. Scintigraphy or MR urography is often useful in this situation to quantify renal function. An obstructed kidney that contributes less than 8% to 10% of renal function is unlikely to respond to pyeloplasty. However, prediction of outcome is imprecise, and some of these kidneys do have improved function after relief of obstruction. For selected patients, a temporary percutaneous nephrostomy allows direct assessment of renal function after relief of the obstruction. The split renal function is accurately measured by determining urine output from the bladder and the nephrostomy tube. Those kidneys that undergo substantial improvement in function after nephrostomy are likely to respond to pyeloplasty; nephrectomy is usually indicated if the function remains poor despite decompression.⁷⁸

The standard surgical treatment for UPJ obstruction is the dismembered pyeloplasty. Optimally, the relocated ureter is positioned in a dependent location along the medial-inferior aspect of the pelvis. Ureteral kinks and any extrinsic compressions are addressed. Anastomosis of a lower pole calyx to the ureter (ureterocalicostomy) can be used to treat a failed pyeloplasty, and is occasionally selected as a primary repair procedure. A variety of endoscopic techniques (endopyelotomy) have been developed to treat UPJ obstruction as well; these are most useful in adults and older children. There is some evidence that the presence of crossing vessels in the region of a UPJ obstruction may lower the success rate and increase the complication rate for endopyelotomy procedures; therefore, preoperative vascular imaging (e.g., helical CT or MR urography) can be helpful.⁷⁹

Imaging studies play an important role in the diagnosis of acute or delayed failure of a surgical UPJ obstruction repair. Some degree of pyelocaliectasis often persists after successful pyeloplasty. The demonstration of substantial decrease in pelvicaliceal system size on postoperative imaging usually indicates relief of obstruction; the converse is not necessarily true, however. Diuretic renography and MR urography are much more accurate than sonography for determining the effectiveness of pyeloplasty. Features on MR urography that suggest a successful pyeloplasty include shortening of the renal transit time and calyceal transit time, diminished hydronephrosis, resolution of cortical edema, and normalization of the caliber and course of the proximal ureter. Preoperative findings of uropathy or a weak heterogeneous nephrogram may portend poor long-term functional outcome for the kidney despite successful relief of obstruction.^73,80

Ureterovesical Junction Obstruction

Ureteral stenosis at sites inferior to the UPJ is uncommon. The next most frequent location after the UPJ is at the UVJ. An ectopic orifice and/or a ureterocele often accompany congenital UVJ obstruction. A secondary form of UVJ obstruction is common in patients with bladder wall thickening, for example, neurogenic bladder. Imaging studies in patients with UVJ obstruction show dilation of the ureter in addition to a prominent pelvicaliceal system. Assessment of the dynamics of ureteral emptying is possible with diuretic scintigraphy or MR urography. A voiding cystourethrogram is usually indicated to determine if the dilation is due to vesicoureteral reflux.

Acquired Ureteral Obstruction

Congenital stenosis of the midureter is rare. Acquired forms of ureteral stenosis can occur anywhere along its course. Potential causes include trauma, stone-related fibrosis, infection, ischemia, retroperitoneal fibrosis, surgery (iatrogenic), congenital ureteral valve, fibroepithelial polyp, and extrinsic or intrinsic neoplasm.^81,82 The diagnosis is best established with contrast opacification of the ureter, via IV injection/renal excretion, retrograde introduction, or percutaneous antegrade introduction. In patients with a possible extrinsic cause, imaging with CT or MR is generally indicated.

Duplication

Renal duplication anomalies range from bifid renal pelvis to complete ureteral duplication. The term duplex kidney refers to a single encapsulated kidney that is drained by 2 pelvicaliceal systems. A duplicated, or duplex, pelvocaliceal system can be partial or complete. There are 2 types of partial duplex systems (bifid system). A bifid pelvis refers to the presence of 2 pelvicaliceal systems that join at the UPJ. The term bifid ureters indicates that the kidney is drained by 2 ureters that join prior to reaching the bladder. A complete duplex system is a single kidney that is drained by 2 pelvicaliceal systems and 2 independent ureters (“double ureters”) that do not connect distally.^2,83

Duplication is the most common congenital anomaly of the urinary tract. Some form of duplication anomaly is present in approximately 10% of the population.⁸⁴ The prevalence of complete duplication is approximately 1% of the population.⁸⁵ Females are affected 2 to 4 times more frequently than males. The anomaly is bilateral in 17% to 33% of affected individuals. Duplication anomaly has an autosomal dominant pattern of inheritance, with variable penetrance.⁸⁶ The parents or siblings of a child with a duplication anomaly have as much as a 1 in 8 chance of having a similarly affected child.⁸⁷ In some individuals, duplex kidney with congenital UPJ obstruction is inherited as an autosomal dominant trait with variable penetrance.⁸⁸

The embryological basis of complete duplication (double ureters) involves the development of 2 ureteral buds from 1 mesonephric duct. The more superior of these buds contacts the upper aspect of the metanephric blastema and therefore drains the upper pole of the mature kidney, whereas the inferior bud supplies the lower pole. The developing ureter (metanephric duct) destined to drain the lower pole of the kidney separates earlier from the mesonephric duct than the other ureter, and is absorbed into the wall of the bladder in a relatively lateral position. The other developing ureter remains attached to the mesonephric duct for a longer period, and is carried medially and inferiorly by migration of the duct prior to separation. This inverse orientation of ureteral insertion is termed the “Weigert-Meyer rule.” Because of this developmental sequence, the lower pole ureter tends to have a short intramural segment and is prone to vesicoureteral reflux. The upper pole ureter is prone to obstruction and an ectopic insertion. Bifid pelvis and bifid ureter (2 ureters that join prior to insertion into the bladder) are due to premature branching of a single metanephric duct.

Many individuals with a duplication anomaly are completely asymptomatic and never suffer an identifiable consequence throughout their lifetimes. A variety of complications can occur, however. As described above, complete duplication is associated with an elevated risk for vesicoureteral reflux due to shortened intramural segments of the distal aspects of the ureters; reflux is most common in the lower pole system. As with reflux in non-duplicated systems, the major clinical manifestations are recurrent urinary tract infections and reflux nephropathy. There is a propensity for congenital obstruction of the upper pole ureter. When obstruction is severe, the upper pole parenchyma is dysplastic and thin, and segmental renal function is poor. The lower pole moiety of a duplex kidney is at elevated risk for congenital UPJ obstruction. The upper pole ureter most often inserts in a more inferior aspect of the bladder than normal, but more severe ectopia can also occur; potential insertion sites include the bladder neck, urethra, and vagina. In girls, an ectopic ureteral insertion beyond the urethral sphincter leads to the clinical manifestation of persistent dribbling of urine. Clinical sequelae are much less likely with the incomplete forms of duplication than with a complete duplication anomaly. Rarely, clinically significant ureteroureteral reflux occurs in a Y-shaped ureter, leading to stasis and dilation in the upper tracts.

Collecting system anatomy in children with a duplication anomaly is best demonstrated with an IV urogram, CT urogram, or MR urogram. The involved kidney is nearly always somewhat longer than the contralateral kidney if the duplication is unilateral. Oblique views and/or fluoroscopy may be required to determine if the duplicated ureters join prior to insertion into the bladder, and to characterize an ectopic orifice. Most duplications are discovered with sonography or voiding cystourethrography performed for other indications (e.g., urinary tract infection). Imaging studies in addition to sonography and cystourethrography are not generally indicated unless there are findings suggestive of a complication of the duplication, such as hydronephrosis.

A key feature of a duplication anomaly on sonography is the presence of a parenchymal “band” that courses through the midportion of the kidney, bisecting the renal sinus and resulting in 2 central echo complexes. Separate renal pelves can often be identified with careful sonographic evaluation. These findings, however, do not allow distinction between complete and incomplete forms of duplication. The bladder wall and pelvis should be examined for evidence of more than 1 ureter, an ectopic insertion, or ureterocele. If there is obstruction or reflux, pelvicaliceal dilation may be identified in 1 or both renal poles (Figure 44-16). Severe obstruction of the upper pole ureter may lead to marked ureteral and pelvicaliceal dilation and dysplastic parenchymal thinning (Figure 44-17). In some children, the upper pole is thinned to the point that it appears as a suprarenal cyst on sonographic evaluation. Scintigraphy or MR urography can be utilized to confirm the diagnosis (Figure 44-18). These studies are also useful for selected patients to determine the degree of segmental renal function and to evaluate the severity of obstruction (Figure 44-19). Functional evaluation is helpful in planning for surgical repair versus a heminephrectomy.