Renal hypoplasia is a rare entity that results in congenitally small kidneys with normal architecture but reduced number of nephrons. Renal hypoplasia may be unilateral with contralateral compensatory hypertrophy. Bilateral renal hypodysplasia can be part of genetic syndromes such as branchio-oto-renal syndrome and renal-coloboma syndrome [12, 13]. When bilateral, renal hypoplasia often results in end-stage renal disease (ESRD) with pathology consistent with focal segmental glomerular sclerosis and interstitial fibrosis presumably from hyperfiltration injury of the reduced number of nephrons [14].

The diagnosis of renal hypoplasia is a clinical diagnosis when an imaging study, typically renal ultrasonography, demonstrates a kidney smaller than 2 standard deviations of the mean kidney size by age (Fig. 9.12) [15]. Absence of renal scarring on a 99mTc-dimercaptosuccinic acid (DMSA) radionuclide scan is expected (Fig. 9.13).

Renal dysplasia is more common than renal hypoplasia and is characterized by a kidney that is often smaller than normal depending on the size of associated cysts. Renal dysplasia occurs in about 3 of every 1,000 births and can be bilateral or unilateral with males affected 30–90 % more frequently than females [16]. Collecting system anomalies such as VUR, megaureter, ureteral atresia, and duplicated ureters occur at an increased incidence in kidneys with dysplasia [17–19]. Therefore, a voiding cystourethrogram (VCUG) is often obtained along with a renal ultrasound. As with renal hypoplasia, unilateral renal dysplasia is often associated with contralateral renal hypertrophy, and bilateral renal dysplasia is often associated with ESRD.

In children, ultrasonography is often the first imaging modality to evaluate renal dysplasia. Cystic areas, echogenic renal parenchyma, small kidneys, and hydronephrosis with thin renal parenchyma are examples of ultrasound findings with dysplasia but findings can be quite variable (Fig. 9.14). Findings on CT scan for renal dysplasia can also be quite variable (Fig. 9.15). A static renal scan such as a DMSA scan will show heterogenous, decreased uptake of radionuclide and possibly a small kidney representing poor renal function compared to a normal contralateral kidney (Fig. 9.16). As stated above, a VCUG is often obtained to evaluate for VUR which is often associated with renal dysplasia.

A multicystic dysplastic kidney (MCDK) is a very severe form of renal dysplasia characterized by a kidney that is non-reniform in shape, composed of noncommunicating cysts, lacking functional renal tissue, and has an absent or atretic ureter [20]. Most cases of MCDK are detected antenatally by ultrasound and are found between 0.3 and 1 in 1,000 live births [21]. As with other forms of renal dysplasia, if the contralateral kidney is normal, it often shows compensatory hypertrophy. Children with unilateral MCDK have a low risk of any significant sequelae such as hypertension, proteinuria, significant renal impairment, or contralateral abnormalities such as VUR or ureteropelvic junction obstruction (UPJO) that require surgical correction [22, 23]. Historical concerns about an increased risk of malignancy in children with unilateral MCDK are not supported by current literature [24, 25].

On ultrasound, a MCDK generally appears as a collection of noncommunicating cystic structures of variable size. MCDK can be quite large in neonates and may even be palpable, but the majority of children with unilateral MCDK show involution of the MCDK over the first few years of childhood (Fig. 9.17). Cross-sectional imaging with CT scan will demonstrate similar findings as ultrasound as well as contralateral compensatory hypertrophy (Fig. 9.18). DMSA radionuclide scan demonstrates no significant update to the MCDK (Fig. 9.19). Grossly, a MCDK does not have the typical reniform shape, contains multiple cysts, and the ureter may be atretic (Fig. 9.20).

Autosomal recessive polycystic kidney disease (ARPKD), previously called infantile polycystic kidney disease, has an estimated incidence of 1 per 10,000–40,000 births [26, 27]. Mutations in the PKHD1 gene on chromosome 6p12 that encodes for the protein fibrocystin are responsible for ARPKD with over 200 mutations identified [28]. The kidneys are increased in size and are filled with small cysts (<3 mm) arising from the collecting ducts that appear to radiate from the medulla to the cortex. The liver is also affected in ARPKD with evidence of congenital fibrosis due to malformation of the biliary system and may show hepatomegaly, increased echogenicity, and dilation of bile ducts on ultrasonography [29]. The clinical spectrum of ARPKD varies widely from death in utero or in infancy due to complications of oligohydramnios, to bilateral nephrectomies at birth with dialysis, and to delayed diagnosis into early adulthood with only signs of hepatic fibrosis [30]. In general, the earlier the diagnosis is discovered, the more severe the renal impairment will be. Most severe cases of ARPKD are detected by prenatal ultrasound.

On ultrasound, the kidneys are large most commonly have a reniform shape but will be filled with multiple small cysts causing the echotexture to be hyperechoic (Fig. 9.21). Cross-sectional imaging can demonstrate the cysts that arise in the cortex and radiate from the medulla to the cortex (Fig. 9.22). A newborn with severe ARPKD will have oligohydramnios, and the kidneys can be quite large and may need to be removed for any potential for survival with dialysis (Fig. 9.23).

Autosomal dominant polycystic kidney disease (ADPKD), previously adult polycystic kidney disease, may present in childhood. ADPKD is much more common than ARPKD with 1 case occurring every 400–1,000 live births. The mutation for ADPKD occurs in the PKD1 or PKD2 gene with PKD1 mutations accounting for 85 % of the cases [31]. The PKD1 and PKD2 genes encode the polycystin-1 and polycystin-2 proteins, respectively. Pathologically, the kidneys develop cysts of variable size in all parts of the nephron. Since the number of cysts generally increases with age, children may only have a few cysts. The natural history of ADPKD is a progressive increase in the number and size of cysts with corresponding deterioration of renal function resulting in renal replacement in middle age to late adulthood. Most children with ADPKD are asymptomatic, but occasionally complications associated with the later stages of ADPKD such as ESRD, hypertension, pain, hematuria, or cyst infection can occur [32]. Other significant organ involvement occurs including hepatic, pancreatic, or splenic cysts; cerebral aneurysms in up to 10 % of patients; and increased left ventricular size [33, 34].

Renal ultrasound in children with ADPKD can be normal, have a small number of cysts, or demonstrate kidneys that are filled with cysts (Fig. 9.24). The cysts on cross-sectional imaging often have varying signal intensity due to hemorrhage or infection history (Fig. 9.24).

Isolated renal cysts are common incidental findings with the prevalence increasing with increasing age. Given how common isolated renal cysts are and the concern about risk of malignancy with complex renal cysts, the Bosniak classification system was created in which cysts are classified into one of five categories based upon morphologic and enhancement characteristics with CT scanning [35].

The Bosniak classification system has been evaluated on a limited scale in children with complex renal cysts. In one study, 5 of 39 children with complex renal cysts underwent resection; three had a benign cyst (Category IIF in 2, Category III in 1), and two had renal cell carcinoma (Category III in 1 and Category IV in 1) [37]. In another study of 22 children, all lesions classified as Category I or II were benign, and all malignant lesions were classified as Category III or IV [38].

The most common type of isolated renal cyst seen in children and adults is the simple renal cyst. Among adults, the prevalence of simple renal cysts increases from approximately 5 % in the fourth decade of life to over 30 % in the eighth decade of life [39]. The prevalence among people age 15–30 years old is less than 1 % [40]. One study evaluated over 16,000 renal ultrasounds of children from birth to age 18 found that the prevalence of simple renal cysts was 0.22 %, which was relatively stable across all age groups [41]. The average cyst size was 1 cm (range 0.3–7 cm), and of the 23 cysts followed, only 2 were slightly larger and only 1 cyst required treatment with percutaneous drainage [41].

Ultrasound is used most commonly in children to evaluate simple renal cysts, and a CT scan is not needed if a cyst meets ultrasound criteria for a simple cyst. On ultrasound a simple renal cyst is defined as being round and sharply demarcated with smooth walls, no internal echoes within the cyst, and there is a posterior wall echo with enhanced transmission beyond the cyst (Fig. 9.27) [42]. Rarely, a large simple cyst will cause pain and will require treatment such as laparoscopic cyst decortication or percutaneous drainage (Fig. 9.28).

The congenital absence of renal parenchymal tissue, renal agenesis, results from an early disruption of metanephric development and occurs approximately every 1 per 3,000–5,000 births [1, 43]. Renal agenesis is thought to be due to a failure of ureteral bud induction or development of the mesonephric duct [1]. Most patients are asymptomatic and are diagnosed during screening antenatal ultrasound or for imaging performed for other reasons showing an absent renal fossa and contralateral compensatory renal hypertrophy. A postnatal ultrasound and radionuclide scan confirm the diagnosis. Other congenital anomalies such as VUR, UPJO, and ureterovesical junction obstruction (UVJO) are more common in the remaining kidney [44, 45]. Unilateral renal agenesis in males can be associated with abnormalities or absence of structures derived from the mesonephric duct such as the seminal vesicles, vas deferens, and epididymis [46]. Unilateral renal agenesis in females is associated with an increased risk of Mullerian abnormalities such as uterine didelphys, duplicated vagina, and obstructed hemivagina [47].

The most common position for an ectopic kidney is below the ipsilateral pelvic brim. These pelvic kidneys are found in 1 in 500–1,200 births [46]. Pelvic kidneys also fail to rotate properly and tend to have an anteriorly directed blood supply and renal pelvis. The blood supply is variable and can come from a one or a combination of the internal iliac, external iliac, common iliac, or aorta. Rarely, an ectopic kidney can be located in the thoracic region [49]. There is an increased incidence of other urologic abnormalities in the ectopic kidney as well as the contralateral kidney with vesicoureteral reflux to the ectopic kidney being the most common [48]. In females, Mullerian abnormalities have been associated with ectopic kidneys such as Mayer-Rokitansky-Kuster-Hauser syndrome or ipsilateral uterine agenesis [50, 51].

Simple renal ectopia is often evaluated with ultrasonography or cross-sectional imaging (Fig. 9.29). A DMSA scan can be obtained to evaluate function of the ectopic kidney (Fig. 9.30). A VCUG is commonly obtained to evaluate for VUR in the ectopic or contralateral kidney.

Crossed renal ectopia occurs in 1 in 1,000–2,000 births and is associated with some fusion to the other kidney 90–95 % of the time [46, 52]. Most commonly, the fusion is between the upper pole of the crossed ectopic kidney and the lower pole of the kidney on the appropriate side. The ureter of the crossed ectopic kidney generally crosses back to the other side and inserts into the bladder trigone. Other rare forms of crossed renal ectopic include crossed unfused renal ectopia and bilateral crossed renal ectopia.

Crossed renal ectopia can also be detected with ultrasonography and cross-sectional imaging, and an excretory phase can be obtained to further characterize the anatomy if needed (Fig. 9.31). Radionuclide scans are also helpful in determining the anatomy and relative function of the kidneys (Fig. 9.32).

Horseshoe kidneys are the most common renal fusion abnormality occurring in 1 in 600 individuals [53]. The horseshoe kidney occurs when the lower poles of the developing kidneys fuse prior to renal ascent. The part of the horseshoe kidney that connects the two sides is called the isthmus and can contain functional renal tissue or just be a fibrous band depending on how early the fusion event occurred. The fused kidneys are not able to rotate properly, and ascent is limited by the isthmus coming into contact with the inferior mesenteric artery. As a result, a horseshoe kidney has its distinct horseshoe shape, is located more caudal than normal kidneys, has an anteriorly directly renal pelvis, and has a highly variable blood supply from the aorta and iliac vessels [54, 55]. The presence of a horseshoe kidney is often asymptomatic but is associated with an increased risk of VUR, hydronephrosis, UPJO, kidney stones, and UTIs [21].

Ultrasonography can usually detect the isthmus of a horseshoe kidney (Fig. 9.33), but sometimes cross-sectional imaging is needed (Fig. 9.34). In addition, studies such as a VCUG or MAG3 furosemide renal scan are obtained if there is the presence of hydronephrosis and/or urinary tract infections (Fig. 9.35).

Antenatal hydronephrosis is found in approximately 1–2 % of all fetuses screened by prenatal ultrasound and seems to be more common in male fetuses [56, 57]. Clinical outcomes are related to severity of hydronephrosis, and two main grading systems are used. The Society for Fetal Urology (SFU) criteria grade antenatal hydronephrosis and postnatal hydronephrosis based upon the degree and location of hydronephrosis as well as presence of thinned renal parenchyma [58].