Simple renal ectopia is a condition in which a kidney is located in an abnormal position but remains on its own side of the midline. Often, associated incomplete rotation of the renal unit occurs. The most common position is in the true pelvis; less common locations are the iliac fossa or the thorax. Ectopia occurs in 1 in 500 to 1,200 live births; is more common on the left side; and most often is discovered as part of an evaluation for other abnormalities, the most common of which are other urologic, musculoskeletal, cardiovascular, gastrointestinal, and otolaryngologic anomalies. Treatment, if any, should address pathologic factors (primarily ureteral obstruction or vesicoureteral reflux) but not the position of the renal unit.

During embryogenesis, the kidney rotates from a position in which the renal pelvis faces anterior to its usual postnatal position, which is with the renal pelvis facing medially. Disorders of renal ascent often are associated with persistence of the original orientation. A better term would be incomplete rotation rather than malrotation. Unless problems associated with this abnormality of rotation exist, such as ureteropelvic obstruction, no treatment is needed.

In patients with fusion anomalies, the two renal units may remain in the true pelvis and be connected by a large mass of parenchyma (cake kidney) in the midline, or they may be partially ascended and connected by a thin isthmus of tissue below the inferior mesenteric artery connecting the lower poles (horseshoe kidney). In crossed fused ectopia, one renal unit crosses the midline and fuses to the normally positioned contralateral kidney; the ureters arise from the appropriate sides of the bladder, but one crosses the midline to enter the lower segment of the fused renal units. The anomaly is seen slightly more commonly in boys than in girls, and the kidneys reside on the right side twice as often as on the left. No treatment is needed, although an increased incidence of reflux and ureteral obstruction that might require surgery is associated with this entity.

Horseshoe kidney is the most common fusion anomaly, accounting for approximately 90% of these abnormalities. It often is discovered incidentally during an evaluation of associated anomalies or urinary infection, or at autopsy. The kidneys are positioned lower than normal, with their lower poles joined by an isthmus of tissue. They are incompletely rotated, and their axes are more vertical than normal, a consistent and characteristic sign on intravenous urography (Fig. 320.1). The isthmus usually crosses the midline anterior to the great vessels below the inferior mesenteric artery, which blocked further ascent of the kidneys during fetal development. The ureters may be somewhat dilated superior to the isthmus, which they cross; however, the need for surgical repair is unusual. Associated anomalies are common findings but usually are less serious than are those related to crossed renal ectopia. Urologic evaluation should include voiding cystourethrography because reflux is a common finding. Treatment decisions are based on the same criteria as those for a normal kidney: infection, pain, and deterioration of renal function. The abnormal location of the kidneys, as well as the isthmus, which crosses the spinal column, increases the possibility of renal injury occurring in these patients. Thus, parental counseling concerning the risks of contact sports is appropriate.

Unilateral renal agenesis is present in 1 in 450 to 1,800 live births. Its etiology is related to maldevelopment of the metanephric duct (primitive ureter) and renal blastema. Often, the ipsilateral ureter and vas deferens are absent because their embryologic origins are similar. Compensatory hypertrophy of the solitary kidney is a common finding but not specific for this lesion. Diagnosis often is made during an investigation of other anomalies (cardiovascular, gastrointestinal, musculoskeletal). No treatment is needed, but the child and parents should be cautioned that activities that put the single kidney at undue risk of injury (e.g., organized football, rugby, riding motorcycles) should be avoided.

Bilateral renal agenesis is a rare entity that is incompatible with life. A high incidence of associated anomalies and developmental abnormalities of the bladder, urethra, and ureters occurs. Pulmonary hypoplasia is a common occurrence and generally is the immediate cause of demise. The infants have typical features, described by Potter, that include increased distance between the eyes, flattening or broadening of the nose, a prominent inner canthal fold, spade-like hands, and amnion nodosum. These findings, characteristic of Potter syndrome, are the result of severe oligohydramnios, which is caused by the absence of intrauterine urine production. This syndrome is not specific for bilateral renal agenesis, however, because any condition in which markedly decreased amniotic fluid volume occurs would produce similar findings.

Renal hypoplasia is an unusual condition in which the number of renal lobules is reduced, thereby producing a kidney that is small but with normal nephron differentiation. The number of calyces is decreased, and the renal weight is diminished. If the condition is bilateral, the total nephron mass is deficient, and the result is progressive renal insufficiency with its typical complications of growth arrest and developmental delay. Patients may require renal dialysis or transplantation at any time from shortly after birth until early adulthood. Unilateral hypoplasia, on the other hand, does not cause any problems or require intervention.

Other, more common causes of small kidneys must be considered before hypoplasia is diagnosed. They include atrophy secondary to reflux nephropathy, atrophic pyelonephritis, vascular ischemia, renal vein thrombosis, and dysplasia.

Renal dysplasia is caused by abnormal metanephric differentiation and is a histologic rather than a clinical diagnosis. Dysplastic kidneys may involve both cystic and hypoplastic changes, although both elements not always are present. Histologically, primitive glomerular and tubular elements, cartilage, smooth muscle, and cysts are seen in the parenchyma. The dysplasia may be segmental or involve the entire renal unit. Affected parts of the kidney generally do not function. The kidney may be reniform in shape, or the dysplasia may be so severe that the unit has little resemblance to a normal kidney. Urinary tract obstruction may be associated with dysplasia and contribute to its formation.

The most common and well-known dysplastic disorder is the multicystic kidney, which consists of numerous fluid-filled cysts that do not communicate (Fig. 320.2). It is one of the two most common renal masses in the newborn (the other is ureteropelvic junction obstruction), and its diagnosis usually is suggested by ultrasound, performed either prenatally or later for evaluation of an abdominal mass. The presence of a multicystic kidney generally is confirmed by the absence of any function on a renal scan. Voiding cystourethrography should be obtained because reflux is associated in 10% of these cases. Rarely, complications occur and include urinary tract infection (UTI), rupture of renal cysts, and hypertension. The question of malignant degeneration has been raised, but the incidence of cancers is very small. Whether these lesions require nephrectomy remains controversial. Most urologists follow these children with ultrasound because, with time, almost all multicystic kidneys involute and do not seem to cause problems. Fewer physicians remove these kidneys to avoid the long-term, limited risk of development of complications and the necessity of following the children for an extended time with repeated studies.

Polycystic kidney disease is an inherited disorder, either autosomal recessive (infantile polycystic disease) or autosomal dominant (adult polycystic disease). The two entities are distinct and should not be confused. The recessive form is found in homozygotes, the dominant form in heterozygotes. Other organs, especially the liver, are involved; in the dominant form, cerebral aneurysms are common findings.

In the recessive form, the kidneys retain their reniform configuration but are enlarged. The parenchyma is filled with dilated renal collecting tubules that appear as small radial cysts. The collecting system (renal pelvis and ureter) is normal, as is the renal pedicle. All children with recessive polycystic kidney disease have involvement of the liver consisting of dilation and proliferation of the bile duct, with varying amounts of periportal fibrosis. Areas of uninvolved parenchyma are interspersed among these involved segments. The degree of renal and hepatic involvement appears to be inversely related, with younger children having more renal but less hepatic involvement. Children who are older when they present usually have more severe hepatic impairment, with marked periportal fibrosis. Prognosis is related to age at diagnosis; those children diagnosed at birth have the worst outcome. Those in whom the disease is found late in childhood do better, but most die before reaching adulthood, often of hepatic complications. Renal and liver transplantations have offered these patients a chance for longer survival.

The recessive disease in infants can be identified prenatally by ultrasound and suspected in a family with a history of polycystic kidney disease or early childhood death from renal or unknown causes. In the neonate, the diagnosis usually is made as part of an evaluation for palpable renal masses noted on routine examination. An ultrasound shows enlarged kidneys with increased, diffuse echogenicity. Intravenous urography shows typical radial streaking of the dilated collecting tubules. Although the kidneys function, they do so poorly; without delayed films, visualization of dye in the renal pelvis, ureter, or bladder is unusual (Fig. 320.3).

The dominant form usually is noted in adults with a positive family history for renal cystic disease and is a completely different problem. It, too, is slowly progressive and ultimately results in renal insufficiency in most cases. The cysts are of various sizes and may become quite large. The kidneys may be huge and fill almost the entire abdomen. Treatment usually is limited to controlling hypertension and any infections that occur and intervening with dialysis or transplantation when necessary. A complete and thorough family history for the past several generations may identify other affected family members and assists in establishing the diagnosis. Appropriate genetic counseling should be done. Although this disorder rarely causes symptoms during childhood, small renal cysts often can be identified by renal ultrasound in affected children.

Ureteral duplication is the most common congenital urologic anomaly; it affects approximately 1 in 150 individuals. Duplication results when two ureteral buds arise from a single Wolffian duct. Both buds reach the developing metanephros and stimulate renal differentiation. The ureter to the lower segment is absorbed into the developing bladder earlier and, therefore, travels further along the trigone, finally resting lateral and cephalad to the upper pole ureter, which lies medial and caudal. This relationship is known as the Weigert-Meyer law. The lower-pole ureter is prone to reflux, whereas the upper-pole ureter (medial and inferior) is associated more often with obstruction from either ectopia or an ectopic ureterocele.

A full spectrum of renal involvement has been observed, ranging from the child with severe bilateral lower-pole reflux and bilateral obstructing ureteroceles to the asymptomatic adult in whom duplication anomalies are discovered serendipitously. In the severe case, the diagnosis can be made after evaluation has been undertaken as a result of an abnormal prenatal ultrasound, but often it is made during the workup of a UTI or symptoms of an ectopic upper-pole ureter (see following discussion). Intravenous pyelography may show a nonfunctioning upper-pole moiety depressing the functioning lower-pole collecting system and pushing it laterally (the “drooping lily” deformity) (Fig. 320.6). A renal ultrasound shows similar findings: a hydronephrotic upper-pole cap of tissue depressing a normal lower-pole segment. The bladder may demonstrate a negative filling defect caused by nonopacification of the ureterocele. A renal scan can estimate the functional capacity of each segment, and voiding cystourethrography is required for assessment of possible lower-pole reflux.

Treatment is as varied as is the presentation. Often, when the upper pole has no function, an upper-pole partial nephroureterectomy is performed. If, on the other hand, good function exists in that segment, a transurethral incision of the ureterocele can be performed. This procedure usually relieves the obstruction, but it may cause the development of upper-pole reflux. At other times, a ureteropyelostomy connecting the upper-pole
ureter to the lower-pole renal pelvis, in conjunction with partial resection of the distal upper-pole ureter, is preferred. If lower-pole reflux is present with an upper-pole ureterocele, some surgeons reimplant the ureters at the same time the upper systems are addressed. Other surgeons delay performing any bladder surgery for several months or years, in the hope that the reflux may resolve once the ureterocele, which has been distorting the bladder, has been decompressed. When reflux to the lower pole is present without upper-pole obstruction, a common sheath ureteroneocystostomy usually is all that is required.

The prognosis depends on the degree of renal damage present at the time of intervention. However, renal function generally is adequate, and further problems seldom occur.

Division of the ureteral bud after it originates from the Wolffian duct causes incomplete ureteral duplication. Two ureteral buds thus promote adjacent renal differentiation but arise from a single ureteral orifice. This condition generally is asymptomatic and does not need attention (Fig. 320.7).