Congenital anomalies of the kidney and urinary tract (CAKUT) are frequently detected in utero and in up to 5% of the neonates. If CAKUT occurs unilaterally, the prognosis is usually good. Bilateral renal disease (Fig. 6.2) with oligohydramnios in utero indicates global renal dysfunction and potentially pulmonary hypoplasia with respiratory distress after birth and increased risk of pneumothorax (Fig. 6.2a). Typically, posterior urethral valves (PUV) with marked urinary tract dilatation and severe renal dysplasia, bilateral renal dysplasia associated with massive vesicoureteric reflux will potentially be associated with neonatal RF (that would evolve towards chronic renal failure) (Fig. 6.2b, c). Similarly, anomalies (obstruction, or massive reflux) occurring on a single kidney will also induce neonatal RF (Fig. 6.3).

Any significant antenatal data and any clinical suspicion of complex CAKUT should lead to rapid postnatal ultrasound examination in order to confirm and characterize the anomaly. The examination should start by an evaluation of the bladder and urethra (including using the perineal access): enlarged thickened bladder in a male neonate would suggest PUV whereas enlarged thinned bladder would suggest VUR. In case of VUR and PUV, the urinary tract may or may not be dilated uni- or bilaterally. Small kidneys, echogenic renal parenchyma, absent CMD, cortical or medullary cysts (Fig. 6.2b) are all features indicating renal (hypo)dysplasia. Perirenal urinoma appearing as a fluid collection (Fig. 6.2c) around the kidney may be present as well. To be noted, the kidneys in PUV may appear normal and conversely highly dysplastic kidneys do not necessarily imply RF.

A voiding cystourethrogram should be performed whenever the US examination raises a suspicion of PUV or massive VUR in order to confirm the anomaly. Functional assessment by scintigraphy should be delayed till the age of 6 months, waiting for the kidneys physiological maturation [5–10].

When obstetrical and/or perinatal ultrasound demonstrate small kidneys (<2SD to the mean), a suspicion of hypodysplastic kidneys should be raised. The etiologies of hypodysplastic kidneys are numerous; familial and maternal history and genetic studies are essential in order to precise the diagnosis. While hypodysplasia is a histologic diagnosis (hypoplasia = reduction of the number of nephrons; dysplasia = architectural disorganization of the kidney), in selected cases, the sonographic characteristics of the kidneys may orient the diagnosis [1–4, 11, 12].

First, and mentioned above, bladder outlet obstruction (as in PUV) and VUR may be associated with renal hypodysplasia in fetuses and newborns (Fig. 6.3). Fetal vesical sphincter hypertonicity induces hyperpression uphill inducing tubular dysfunction. Glomerular and renal development is negatively affected.

On US, the kidneys will usually appear small, hyperechoic, and without CMD [13].

Genetic mutations may induce hypodysplastic kidneys. Major progresses have been made for understanding the implication of various genes in the development of the kidney, especially in the interaction between the ureteral bud and the metanephric blastema. It has been demonstrated that any event interfering with this interaction in relation with genetic mutations would induce perturbations of the renal development and typically hypodysplasia. Several specific genetic mutations have been characterized. Some will present an RF already at birth. Mutations of transcription factor TCF2 and gene HNF1β have been described several years ago. These mutations determine among others a spectrum of renal anomalies, partial pancreatic agenesis, and genital anomalies. Hypodysplastic kidneys are among the phenotypic expressions that can be encountered in association with such mutations. On ultrasound, the kidneys of HNF1β mutation may appear small (or asymmetric in size), hyperechoic, without CMD; small subcapsular cysts may be visible as well (Fig. 6.4) [14–16]. Other mutations (such as genes PAX 2, SALL1 or EYA 1) may be involved as etiology for hypodysplastic kidneys as well; the US characteristics seem less specific.

Congenital tubular dysgenesis (CTD) related to a dysfunction of the renin-angiotensin system (RAS) is another cause of renal hypodysplasia. On histology, CTD is characterized by the absence of the proximal renal tubules. Oligohydramnios is observed in utero due to decreased urine output; death may occur already in utero. If the patient survives, respiratory distress and massive pneumothorax develop at birth due to pulmonary hypoplasia. The newborns will remain oligoanuric and consequently their mortality is high. CTD can be genetically transmitted but also acquired. The genetic disease is transmitted as autosomal recessive trait. Acquired CTD appears secondary to twin-twin-transfusion syndrome (TTTS), to congenital hemochromatosis and secondary to several maternal medications that are susceptible to block the RAS. Clinically, the symptoms are similar to those observed in the hereditary type of CTD. In all these entities, the kidneys may appear small and highly echogenic on US (Fig. 6.5a); other patterns can be observed as well (Fig. 6.6). Furthermore, the kidneys may appear normal. The most typical cases will show marked delay in ossification of the cranial vault (Fig. 6.5b) [17–22].

Hereditary renal cystic diseases include various entities classified as ciliopathies (and the so-called hepato-renal fibro-cystic diseases). Therefore, it should be stressed again that the US examination should include not only the urinary tract but also the liver, the spleen, the pancreas and the internal genital organs in order to look for associated malformations. A few among the renal cystic diseases will display RF already at birth progressing into CRF. Still, for the majority of patients, RF will develop later in infancy or childhood.

Autosomal recessive polycystic kidney disease (ARPKD) is related to a mutation of the gene PKHD1 (cases with truncated mutations have a poorer perinatal outcome). This mutation causes fusiform dilatation of the renal collecting ducts to a variable extent. There are variable phenotypes in relation with the extent of the tubular anomalies. Cases with extensive tubular anomalies will usually be detected in utero a very large kidneys (8–10 cm) with hyperechoic kidney, poor or reversed CMD; oligohydramnios will develop. Large medullary cysts may already be visible (Fig. 6.7). At birth, these patients may present spontaneous pneumothorax and oligoanuria. Mortality is high in such early presentation [5, 23].

Autosomal dominant polycystic kidney disease (ADPKD) is the most common genetically transmitted renal cystic disease. Mutations of the gene PKD1, PKD2, and PKD3 have been reported. In most cases, the disease is clinically silent up to adolescence or adulthood; in some instances, the disease will be symptomatic at birth with hypertension and ARF. In such cases, the kidneys will appear enlarged (6–7 cm), hyperechoic, without CMD but with subcapsular cysts (Fig. 6.8) (the so-called glomerulocystic type of ADPKD) [5, 23].

Glomerulocystic kidneys, or kidneys with glomerular cysts, represent a third group of hereditary kidneys diseases. On histology, the peri-glomerular Bowman space will be distended and appear cystic. On US, the cysts will be typically located in the periphery of the cortex and in the subcapsular area. The rest of the kidneys will appear hyperechoic, and without CMD. Glomerulocystic kidneys are typically encountered in the autosomal dominant familial glomerulocystic diseases, in HNF1β mutation, in various syndromes (e.g., Melas syndrome). Furthermore, glomerular cysts can be found in association with obstructive dysplasia. AS mentioned above, HNF1β mutation encoding the TCF2 factor has been shown to correspond to the most common cause of hyperechoic kidneys in the fetus. Various phenotypes may appear in relation with this mutation. Most typically, on US the kidneys will appear normal sized, with a CMD and with subcapsular cysts (Figs. 6.4 and 6.8). Other patterns can be encountered: small kidneys, absent CMD, asymmetric kidneys, renal agenesis, multicystic kidney, urinary tract dilatation, etc. The cysts may also develop postnatally or within the medulla. Infrequently, a RF may be present at birth already. Noteworthy, a pancreatic hypoplasia may also be a neonatal feature of the disease [5, 15, 23].