Overview, Etiology, and Clinical Presentation: A multicystic dysplastic kidney (MCDK) is the most common form of cystic renal dysplasia. With high-grade obstruction or atresia of the upper urinary tract (renal pelvis and/or ureter) during early renogenesis, disordered parenchymal development results in an MCDK.¹ Two types of MCDK exist: the pelvoinfundibular type (common) and the hydronephrotic type (uncommon).^2,3 MCDK can occur in half of a duplex kidney (usually the upper moiety) or in renal fusion anomalies, such as a horseshoe kidney and crossed fused renal ectopia. MCDK is sporadic in most cases, although familial cases have been described.^1,4 Vesicoureteral reflux (VUR) (both ipsilateral and contralateral) and contralateral ureteropelvic junction obstruction are common.^1,5 Long-term sequelae (e.g., infection and hypertension) are rare, because most affected kidneys either partially or completely involute on their own over time.^6–9

Imaging: On ultrasound, computed tomography (CT), and magnetic resonance imaging (MRI), a classic MCDK is characterized by multiple cysts of various sizes (ranging from 1 mm to several cm) that do not communicate, with no normal intervening renal parenchyma (Fig. 114-1). The affected kidney may be small, normal, or enlarged.¹ Compensatory hypertrophy of the contralateral kidney often is present.¹⁰ The hydronephrotic form has cysts that communicate, mimicking pelvocaliectasis, but no normal functioning renal tissue.² The diagnosis of both classic and hydronephrotic MCDK can be confirmed by diuretic renal scintigraphy (e-Fig. 114-2) or MR urography (MRU) (e-Fig. 114-3). A voiding cystourethrogram (VCUG) may reveal VUR into a blind-ending ureter on the side of the MCDK.^11,12

Treatment: Follow-up ultrasound typically demonstrates involution of the cysts over time, eventually resulting in remnant dysplastic renal tissue that may be impossible to visualize.13,14 Immediate resection of an MCDK is rarely indicated¹⁵ and is reserved for cases in which the size of the kidney is a problem. In the past, routine nephrectomies were performed because of a fear of malignant degeneration. Current belief discounts this possibility, and as long as no other complication exists, surgery is not indicated.

Overview and Etiology: Autosomal-dominant polycystic kidney disease (ADPKD), the most common form of inherited renal cystic disease, is seen in 1 in 800 live births.16,17 Two types of ADPKD exist, each with clearly defined chromosomal mutations. The more severe form (PKD1) accounts for about 85% to 90% of cases, whereas the milder phenotype (PKD2) presents later in life, results in less morbidity, and is less common.^16,18 Cysts arise from both the renal cortex and the medulla.^16,19

Clinical Presentation: Systemic hypertension, hematuria, and slowly progressive renal insufficiency are the typical clinical manifestations.1,20 ADPKD is very rarely detected prenatally because fewer than 5% of nephrons are cystic before birth.¹ Diagnosis of ADPKD early in life portends a poor prognosis; 43% of affected persons die in the first year of life, hypertension develops in 67%, and some affected children progress rapidly to end-stage renal disease.²⁰ Because it can be difficult to delineate between autosomal-recessive and autosomal-dominant forms of polycystic kidney disease in a neonate, screening of siblings and parents may be helpful.

Imaging: The diagnosis of ADPKD usually is made on the basis of ultrasound (Fig. 114-4), although CT or MRI also can suggest it. Although the kidneys may be normal during the first decade of life, they also may contain one or more simple cysts (Fig. 114-5).^16,21 The kidneys may be abnormally enlarged and echogenic, mimicking autosomal-recessive polycystic kidney disease (ARPKD) in early life (e-Fig. 114-6).^16,22 Although it is typically a bilateral process, imaging findings may be asymmetric, and in young children they can manifest as a unilateral abnormality.^16,21,23 Microscopic cysts, as well as larger cysts that can be detected by cross-sectional imaging techniques, are common in the liver and less frequent in the pancreas, spleen, lungs, thyroid, ovaries, and testes.¹⁹ The remainder of the urinary tract is usually normal.

Treatment: Although renal function is generally maintained during childhood, ADPKD is responsible for the need for chronic dialysis in 10% to 12% of adult patients.¹ Hypertension should be managed medically to prevent long-term complications. About 12% to 15% of patients with ADPKD have an intracranial aneurysm, most often arising from the circle of Willis.^19,23–25 Screening MR angiography usually is not performed in childhood, however, because aneurysm rupture is rare in this period.

Overview: ARPKD is a rare disorder (occurring in 1 : 20,000 live births) that affects both kidneys.26,27 Genetic studies have localized the inheritance of ARPKD to chromosome 6p.^27,28 The PKHD1 gene is expressed in fetal and adult kidneys and livers, the two major sites affected by the disease.²⁸ Approximately 1 in 70 persons are a carrier for this condition.^26,27

Clinical Presentation: ARPKD has been subdivided into categories based on clinical presentation. The perinatal and neonatal forms, which have the poorest prognosis, are associated with severe renal dysfunction and minimal hepatic portal tract fibrosis. Affected children often die of the disease early in life.27,29 Infantile and juvenile forms are associated with less severe renal dysfunction and more significant hepatic fibrosis.²⁷ The degrees of renal and hepatic abnormalities tend to be inversely proportional.^17,27 A number of children with ARPKD also have Caroli disease (see Chapter 88).²⁷

Pathology: Histologically, the kidneys have a spongelike appearance, replaced by myriad minute 1- to 2-mm cysts of relatively uniform size, representing markedly dilated and elongated collecting ducts.1,27 These cysts radiate from the hilum to the surface of the kidney without a clear demarcation between the medulla and cortex. Fibrosis is present in the renal interstitium. Children with significant hepatic involvement are at increased risk for complications related to portal hypertension, and ascending cholangitis can lead to sepsis and death.²⁷

Imaging: Abdominal radiography in the neonatal period commonly reveals bilateral abdominal or flank masses with centralized gas-filled bowel loops. The lungs may appear small because of a combination of pulmonary hypoplasia and mass effect from marked nephromegaly (e-Fig. 114-7).²⁷

On ultrasound, affected neonatal kidneys tend to be smoothly enlarged (frequently more than four standard deviations above the mean expected length for age), are hyperechoic as a result of the multiple acoustic interfaces at the walls of the dilated ducts, and have poor corticomedullary differentiation (e-Fig. 114-8).^1,27,29,30 On occasion, discrete cysts (solitary or multiple) of variable size (most commonly <1 cm) may be seen (Fig. 114-9).^26,30 A sonolucent rim may be seen at the periphery of the kidney, especially in older infants, as a result of cortical compression and relative sparing.^26,27 Contrast-enhanced CT (Fig. 114-10) or MRI show delayed opacification of the renal parenchyma with a prolonged and striated nephrogram. When studied with MRI, the dilated tubules in affected kidneys appear linear and hyperintense on T2-weighted imaging, radiating from the medulla toward the cortex (Fig. 114-11).³¹

Treatment: Eighty-six percent of patients who survive beyond 1 month of age are alive at 1 year, and 67% are alive at 15 years.³² In infants who have initially adequate (although not normal) renal function, the disease may not be detected until later in the first decade, when ultrasound is being performed for unrelated purposes. Systemic renovascular hypertension may occur and require medical management.^17,27 Persons with ARPKD who have severely impaired renal function eventually will require renal replacement therapy, either dialysis or renal transplantation.

Overview, Etiology, and Clinical Presentation: A calyceal diverticulum represents a focal outpouching of the renal collecting system that is located within the renal parenchyma and that contains urine.³³ This condition has an incidence of 3.3-4.5 : 1000 in children, based on excretory urography.^34–36 Calyceal diverticula are likely developmental in origin; they are lined with urothelium and surrounded by muscularis mucosa.^33,35 Although these structures most commonly are an incidental finding, they can be associated with both infection and calculus formation as a result of urinary stasis.^33,35,37

Imaging: Abdominal radiographs may be normal or may demonstrate one or more calculi (or milk of calcium) within a calyceal diverticulum.³⁵ Upon ultrasound, such diverticula may mimic a solitary renal cyst, appearing as a round, thin-walled anechoic structure (Fig. 114-12).³⁴ They can vary in size ranging from a few millimeters to several centimeters (with a mean size around 11 mm).^35,37 They most commonly occur within the upper pole of the kidney.³³ Upon excretory phase imaging (including excretory urography, MRI, and CT), calyceal diverticula typically fill with contrast material (Fig. 114-13). A thin connection, or neck, to an adjacent calyceal fornix may or may not be identified.³⁵

Treatment: A symptomatic calyceal diverticulum may require surgical ablation. Although both lithotripsy of calculi within the diverticulum and percutaneous ablation of the cavity have decreased the invasive treatment in adults,37,38 such techniques have not yet been applied widely to children.

Overview, Etiology, and Clinical Presentation: Congenital infundibulopelvic stenosis is a very rare congenital anomaly of the renal collecting system that presents as unilateral or bilateral caliectasis. Although the exact etiology of this condition is uncertain, Lucaya et al.³⁹ hypothesized that this abnormality is a milder manifestation of the process that results in multicystic dysplastic kidney. Affected children may be predisposed to urinary tract infections, and this condition may be associated with other renal and urinary tract anomalies.³⁹

Imaging: Upon contrast-enhanced excretory phase imaging studies (including excretory urography, MRU, and CT), this condition has a specific appearance: the renal collecting system infundibula are abnormally narrowed but not entirely obliterated, and the calyces appear abnormally rounded and dilated (Fig. 114-14). The renal pelvis also appears narrowed as a result of stenosis or hypoplasia. This condition may be unilateral or bilateral. Despite the cystlike appearance of the calyces on such contrast-enhanced studies, the ultrasound appearance of the kidneys can be normal. Renal function may be normal to severely impaired.³⁹

Treatment: Most affected individuals demonstrate normal or stable renal function and do not require treatment. Surgical approaches to manage this condition have been described, including ureterocalicostomy and renal transplantation.³⁹

Overview, Etiology, and Clinical Presentation: Congenital megacalyces is a very rare anomaly of the renal collecting system that may be easily confused for hydronephrosis.⁴⁰ Calyceal involvement may be unilateral or bilateral. This abnormal calyceal dilatation is not due to urinary tract obstruction but instead is likely a result of underdevelopment/hypoplasia of the renal medullary pyramids.^41,42 Boys are more commonly affected than are girls. Associated urinary stasis may predispose affected children to urinary tract infection and urolithiasis.⁴¹

Imaging: Congenital megacalyces may be misidentified as obstructive caliectasis by ultrasound. With excretory urography, however, this condition is easily identifiable because of the presence of abnormally dilated calyces that lack normal papillary impressions, have a polygonal or faceted shape, and appear to be increased in number (20 or more calyces may be seen) (Fig. 114-15).⁴² The renal parenchyma overlying affected calyces may appear thin. The renal pelvis can be normal in caliber or mildly dilated,⁴¹ and classically, no related urinary tract obstruction should be present. This appearance of the calyces on MRU and excretory-phase CT (CT urography) also should suggest the diagnosis.

Treatment: Congenital megacalyces usually are an incidental finding that requires no specific treatment.

Overview: Ureteropelvic junction (UPJ) obstruction is the most common type of congenital urinary tract obstruction, occurring in 3 per 1000 live births. Boys are more frequently affected than are girls, and the left kidney is more commonly involved than the right kidney. About 30% of cases are bilateral, although severity of obstruction and pelvocaliectasis may be asymmetric.¹

Pathophysiology: An intrinsic narrowing (e.g., smooth muscle deficiency, increased fibrosis and collagen, stricture, valve, kinking, and altered peristalsis) occurs at the level of the UPJ, leading to dilatation of the pelvicalyceal system.1,43 On occasion, extrinsic narrowing can be seen from a crossing renal vessel in older children and adults.⁴⁴ UPJ obstruction often is seen in association with other urinary tract anomalies, such as contralateral multicystic dysplastic kidney, upper urinary tract duplication,^45,46 VUR,⁴⁷ and ureterovesical junction obstruction.⁴⁸

Clinical Presentation: When UPJ obstruction is not detected prenatally, affected children commonly present with a palpable abdominal mass in the neonatal period. Older children may present with intermittent hydronephrosis and flank pain, often associated with crossing vessels. UPJ obstruction can be complicated by calculi, infection (pyonephrosis), and hemorrhage, especially when diagnosis is delayed.²⁹

Imaging: On ultrasound, abnormal dilatation of the pelvicalyceal system of varying degrees is seen, whereas the ureter is normal in caliber.1,29 Ultrasound findings suggestive of renal dysplasia may be present in cases of severe obstruction (described later). Debris within the collecting system can represent infection or hemorrhage (Fig. 114-16). Careful interrogation of the UPJ region with Doppler ultrasound may identify a crossing vessel, when present.⁴⁹ Frequently, separating dilated nonobstructed from dilated obstructed renal collecting systems is difficult by ultrasound alone.