Imaging

US: A kidney which is smaller by 2 Sd as compared to the mean kidney size expected at that age.

Radionuclide scan (DMSA): Rules out scarring.

Supernumerary kidneys

Less than 100 cases of this anomaly have been reported in literature. This is an extremely rare anomaly. The accessory kidneys are usually seen on the left, caudal to the left kidney. Accessory kidneys are usually smaller in size and show suboptimal function. They can be detected on ultrasound and confirmed with cross-sectional modalities like MR urography/CT urography. Function can be evaluated on postcontrast CT/MR urography or with dimercaptosuccinic acid (DMSA) and diethylene triamine penta-acetic acid (DTPA) scans. Associated urogenital or other anomalies maybe be seen.

Persistent foetal lobulations

Incomplete fusion of the developing renal lobules causes a lobulated appearance of the kidneys. This has to be differentiated from renal scarring.

Persistent foetal lobulations are seen as a smooth indentation of the renal outline. This is seen between the renal pyramids on ultrasound whereas the indentation in scarring overlies the renal pyramids and is not smooth and symmetrical.

This can be appreciated on US, CT and MRI.

Hypertrophied column of bertini

This occurs due to prominent renal cortical tissue between the pyramids extending towards the parenchyma. It can mimic a renal mass on ultrasound. The interpolar region of the left kidney is a common site. It can be differentiated from a mass as it is in continuity with normal renal parenchyma and shows the same appearance as the rest of the renal parenchyma on all imaging modalities.

Dromedary hump

This is a normal variation in the contour of the left kidney. The lower pole of the spleen causes an impression on the superolateral aspect of the kidney, which can mimic a renal mass. It shows the same echogenicity/density and signal intensity as the rest of the kidney. Postcontrast images on CT and MR also show enhancement akin to the rest of the kidney. Also, the calyces extend further laterally into the hump as compared to the other calyces.

Renal hilar lip

Infolding of the cortex at the level of the renal sinus can appear as a pseudomass, known as the renal hilar lip. As in the case of the dromedary hump the imaging characteristics of this lesion being akin to the rest of the renal tissue help reaching a diagnosis.

Junctional parenchymal defect

This occurs due to embryonic fusion of renunculi. It is a normal variant which is located in the interpolar region. It occurs due to extension of sinus fat into the cortex and is seen as a triangular echogenic area on ultrasound which is in contiguity with the renal sinus fat. Differential diagnosis can include a renal angiolipoma which on ultrasound is well defined, echogenic, round, and is not in contiguity with the renal sinus fat.

Simple cyst

Simple renal cysts are rarely seen in children. An underlying genetic cystic disease or rarely a malignancy needs exclusion. The diagnosis of a simple cyst should be one of exclusion. At least one follow-up ultrasound should be done to evaluate change in size and imaging appearance as well as to rule out development of additional cysts. Further cross-sectional imaging evaluation with CT and MRI and contrast studies are not required unless atypical findings are seen initially or on follow-up.

Multicystic dysplastic kidney

This is a severe form of renal dysplasia which may be unilateral or bilateral. Renal dysplasia occurs due to abnormal metanephric differentiation. It has an incidence of 0.3 to 1 in 1000 live births. When bilateral it is incompatible with life. The kidney is not reniform and is composed of multiple cysts of varying sizes which are noncommunicating. There is no visible functioning renal parenchyma appreciable. The collecting system and ureter are both not seen/atretic.

The condition is now being detected more often on antenatal ultrasound foetal MRI (Fig. 10.7.1). The contralateral kidney, if normal, shows compensatory hypertrophy.

Sequelae such as hypertension, proteinuria and renal impairment are uncommon as is contralateral VUR, PUJO. There is no increased incidence of malignancy reported.

Imaging findings (Fig. 10.7.2)

US: Multiple cysts of varying sizes which are noncommunicating are seen. The kidney may be enlarged and even palpable in neonates. Subsequently, the kidney may involute. The contralateral kidney shows compensatory hypertrophy.

CT/MR: It shows similar findings. The kidney is nonfunctioning on postcontrast CT/MR or a DMSA scan. The ureter may be atretic.

Autosomal recessive polycystic kidney disease (ARPKD)

ARPKD is a paediatric cystic renal disease with an incidence of 1 in 10,000 to 40,000 live births.

There is no gender predilection. This condition results from mutations in the PKHD1 gene on chromosome 6p12 that encodes for the protein fibrocystin.

The earlier the diagnosis is made the condition will be more severe.

Imaging findings (Fig. 10.7.3)

• US: These neonates may have severe oligohydramnios on antenatal US.
  
  
  

  
  
  • The kidneys are enlarged, more than 4 standard deviations more than expected for age. They are reniform in shape. They show increased echogenicity due to the innumerable ectatic renal tubules or microcysts. The microcysts are better seen with high-frequency transducers. Multiple nonshadowing punctate foci may also be seen showing a ring-down artefact.
    
  • An enlarged liver is seen showing increased echogenicity suggestive of CHF. Findings suggestive of Caroli disease may be seen which may be localized or diffuse. There may be intrahepatic bile duct ectasia and cystic biliary duct dilatation. The central dot sign is seen which is due to a small branch of the portal vein being circumferentially surrounded by the abnormally dilated bile duct. Findings suggestive of portal hypertension, that is, splenomegaly and portosystemic varices may be seen.
  
  
• On CT/MR, the kidneys are enlarged. The cysts are seen to arise in the cortex and radiate from the medulla to the cortex. The kidneys show a striated appearance because of dilatation of the collecting ducts. The cysts are hypointense on T1 and hyperintense on T2-weighted images. The cysts in ARPKD are less commonly complicated by haemorrhage or infection as compared to the cysts in ADPKD.