Development of the urogenital system in humans is a complex process; consequently, renal anomalies are among the most common congenital anomalies. The fetal urinary tract can be visualised ultrasonically from 11 weeks onwards, allowing recognition of megacystis at 11–14 weeks, which warrants comprehensive risk assessment of possible underlying chromosomal aneuploidy or obstructive uropathy. A mid-trimester anomaly scan enables detection of most renal anomalies with higher sensitivity. Bilateral renal agenesis can be confirmed ultrasonically, with empty renal fossae and absent bladder filling, along with severe oligohydramnios or anhydramnios. Dysplastic kidneys are recognised as they appear large, hyperechoic, and with or without cystic spaces, which occurs within the renal cortex. Presence of dilated ureters without obvious dilatation of the collecting system needs careful examination of the upper urinary tract to exclude duplex kidney system. Sonographically, it is also possible to differentiate between infantile type and adult type of polycystic kidney diseases, which are usually single gene disorders. Upper urinary tract dilatation is one of the most common abnormalities diagnosed prenatally. It is usually caused by transient urine flow impairment at the level of the pelvi-ureteric junction and vesico-ureteric junction, which improves with time in most cases. Fetal lower urinary tract obstruction is mainly caused by posterior urethral valves and urethral atresia. Thick bladder walls and a dilated posterior urethra (keyhole sign) are suggestive of posterior urethral valves. Prenatal ultrasounds cannot be used confidently to assess renal function. Liquor volume and echogenicity of renal parenchyma, however, can be used as a guide to indirectly assess the underlying renal reserve. Renal tract anomalies may be isolated but can also be associated with other congenital anomalies. Therefore, a thorough examination of the other systems is mandatory to exclude possible genetic disorders.
Introduction
Assessment of fetal anatomy during the second trimester using ultrasound scanning has now become standard practice in most antenatal care set-ups, thus permitting the diagnosis of most structural abnormalities in the fetus. Renal anomalies constitute about 20% of all congenital abnormalities . As a result, prenatal identification of renal anomalies provides options for prospective parents as they significantly affect perinatal morbidity and mortality. Additionally, it also functions as a key mechanism in promoting early detection of conditions, which may otherwise present itself later on in life, conceivably with more advanced sequelae. In this chapter, we concentrate on ultrasound diagnosis of renal tract abnormalities.
Embryology of the human kidney development
The urogenital system develops from the mesodermal ridge (intermediate mesoderm) in the posterior wall of the abdominal cavity. During the stages of intrauterine life, three renal systems develop; namely the pronephros, mesonephros, and metanephros. The pronephros and mesonephros are transient excretory systems, and disappear without contributing to the permanent renal system. The mesonephric duct gives rise to some reproductive organs in male fetuses while degenerating in female fetuses.
The metanephros appears in the 5th week of development and contributes to the metanephric mesoderm, forming the nephron units of the kidney. The ureteric bud arises from the mesonephric bud and forms the collecting system of the kidney, including the ureter, the renal pelvis, the major and minor calyces, and about 1–3 million collecting tubules. The cranial end of the ureteric bud comes into contact with the metanephric cell mass, and this induces the development of the metanephric mesoderm into the future nephron and the ureteric bud into the collecting system. The definitive kidney develops in the sacral region and ascends up into the renal fossa in the lumbar region as the embryo matures, and demonstrates differential growth of the abdominal wall.
During weeks 4–7, the cloaca develops into the urogenital sinus anteriorly and the anal canal posteriorly. The developing mesonephric ducts drain into the upper part of the urogenital sinus, laying the foundation for the future bladder. This developing bladder is initially in communication with the allantois, but with progressive development of the anterior abdominal wall, the allantois disappears, leaving behind an obliterated thick fibrous tissue called the urachus, which connects the bladder apex to the umbilicus. It forms the medial umbilicus ligament in the adult. The developing bladder incorporates the mesonephric duct into its trigone, and posteriorly spaces out the two ureteric orifices appropriately.
Subsequently the lower part of the urogenital sinus develops into the prostatic and membranous urethra. The external genital development differs significantly between the two sexes; hence, elaboration is beyond the scope of this review.
The definitive kidney becomes functional by week 12, although they do not have any major excretory function as the placenta works as an excretory organ until birth. Urine production starts around 10 weeks of gestation, and is the major contributor to amniotic fluid from about 14 weeks of gestation. Nephrons continue to be formed up until the time of birth when they are about 1 million in number. The number of nephrons remains static while they grow in size during infancy.
Embryology of the human kidney development
The urogenital system develops from the mesodermal ridge (intermediate mesoderm) in the posterior wall of the abdominal cavity. During the stages of intrauterine life, three renal systems develop; namely the pronephros, mesonephros, and metanephros. The pronephros and mesonephros are transient excretory systems, and disappear without contributing to the permanent renal system. The mesonephric duct gives rise to some reproductive organs in male fetuses while degenerating in female fetuses.
The metanephros appears in the 5th week of development and contributes to the metanephric mesoderm, forming the nephron units of the kidney. The ureteric bud arises from the mesonephric bud and forms the collecting system of the kidney, including the ureter, the renal pelvis, the major and minor calyces, and about 1–3 million collecting tubules. The cranial end of the ureteric bud comes into contact with the metanephric cell mass, and this induces the development of the metanephric mesoderm into the future nephron and the ureteric bud into the collecting system. The definitive kidney develops in the sacral region and ascends up into the renal fossa in the lumbar region as the embryo matures, and demonstrates differential growth of the abdominal wall.
During weeks 4–7, the cloaca develops into the urogenital sinus anteriorly and the anal canal posteriorly. The developing mesonephric ducts drain into the upper part of the urogenital sinus, laying the foundation for the future bladder. This developing bladder is initially in communication with the allantois, but with progressive development of the anterior abdominal wall, the allantois disappears, leaving behind an obliterated thick fibrous tissue called the urachus, which connects the bladder apex to the umbilicus. It forms the medial umbilicus ligament in the adult. The developing bladder incorporates the mesonephric duct into its trigone, and posteriorly spaces out the two ureteric orifices appropriately.
Subsequently the lower part of the urogenital sinus develops into the prostatic and membranous urethra. The external genital development differs significantly between the two sexes; hence, elaboration is beyond the scope of this review.
The definitive kidney becomes functional by week 12, although they do not have any major excretory function as the placenta works as an excretory organ until birth. Urine production starts around 10 weeks of gestation, and is the major contributor to amniotic fluid from about 14 weeks of gestation. Nephrons continue to be formed up until the time of birth when they are about 1 million in number. The number of nephrons remains static while they grow in size during infancy.
Normal ultrasonic imaging of fetal kidney
Congenital abnormalities of the genitourinary tract, especially of the kidney and bladder, affect 3–4% of the population . The fetal kidneys contribute to the amniotic fluid volume from about 14 weeks, and are essential for maintaining liquor volume throughout pregnancy. The presence of a structural, functional renal anomaly, or both, may result in oligohydramnios or anhydramnios, which may in turn affect pulmonary development. It is therefore necessary to establish the normalcy of the renal system as early as possible.
The primary imaging modality used to visualise the fetal urogenital tract antenatally is ultrasound. Normal kidneys along with the adrenal glands may be visible in a scan from as early as 9 weeks. They are seen on either side of the fetal spine just below the level of the fetal stomach. The kidneys appear echogenic in the early weeks, and gradually become hypoechoic compared with the adjacent bowel and liver. It is recommended that the kidneys are seen in axial, sagittal and coronal planes ( Fig. 1 ). The renal cortex appears echogenic compared with the medulla, and the renal pelvises are seen as anechoic spaces in the medial aspect in the transverse sections. In the third trimester, the pyramids can be differentiated from the cortex as it appears more hypoechoic. The kidneys grow as long as the pregnancy continues, and the size is directly proportional to the gestational age. Adrenal glands can be seen on the superior pole of the kidney. In the absence of kidneys in their normal positions in the renal fossa, the adrenals can occupy the renal fossa and mimic the renal structure. The fetal kidney should be seen in all fetuses in the anomaly scan, whereas it tends to be seen in 80% of the cases at 11 weeks and in 92% of cases at 13 weeks of gestation . Many centres routinely visualise the renal arteries using colour Doppler as a part of the scan ( Fig. 1 ). These can be seen as direct branches of the abdominal aorta in a posterior coronal view, just inferior to the origin of the superior mesenteric artery.
Fetal ureters are not usually visible antenatally unless they are dilated. The fetal bladder can be visualised in the pelvis from 11–12 weeks of gestation, and persistent absence of the bladder should be considered as abnormal from 15 weeks . Fetal urine production begins between 8–10 weeks of gestation. Oligohydramnios, however, cannot be found before 10 weeks of age, as amniotic fluid at this point of the gestation period is mainly formed by the secretions of the placenta, fetal membranes and skin.
Abnormal renal development
Renal agenesis
Renal agenesis is the congenital absence of kidneys, and can be bilateral or unilateral. Bilateral renal agenesis is not compatible with life, and occurs in 0.1–0.3 per 1000 births. Isolated unilateral agenesis accounts for 1 in 1000 births, and is three times more common in males . Renal agenesis can be an isolated finding, but is more commonly part of a syndrome and warrants a detailed anomaly scan to look for associated anomalies . Various inheritance patterns exist in families with renal agenesis. The risk of bilateral renal agenesis in the fetus is thought to be around 1% if one parent has unilateral agenesis . If renal agenesis is detected in the fetus, it warrants renal imaging in parents.
The most common presentation in bilateral renal agenesis is anhydramnios, generally identified at the time of a routine mid-trimester anomaly scan. The fetal kidneys are not visualised in the renal fossa, and the bladder does not fill during the scan, with noticeably reduced or absent liquor from 16 weeks (produced by placenta in the first trimester). In general, if the fetal bladder is not visualised during the scan, it is recommended to repeat the scan after 30 mins to allow for an empty bladder to refill. Persistence of an empty bladder in the absence of demonstrable renal tissue and anhydramnios are all strong pointers to bilateral renal agenesis. The lack of recognisable renal arteries on colour Doppler would strongly support the suspicion of bilateral renal agenesis .
Additionally, the adrenal gland will show a linear appearance but can sometimes appear ovoid in certain scans, and can be mistaken for the kidney . Proper visualisation of the renal fossa is restricted by factors such as severe oligohydramnios, maternal obesity, and the positioning of the bowel can also obscure the view. Therefore, it is essential to carefully evaluate the renal fossa for an accurate conclusion.
Unilateral agenesis is three to four times more common than bilateral agenesis, and carries a better prognosis. On an ultrasound examination, the fetal bladder is usually seen to fill and empty normally with normal liquor volume. The contralateral kidney may appear hypertrophied, and usually functions normally. Cho et al. showed that the ratio of antero-posterior to transverse renal diameter gives a better guide to the size of the contralateral kidney . If the ratio is greater than 0.9, it is highly suggestive of absence of the other kidney, and has a sensitivity, specificity and accuracy of 100%. The identification or suspicion of unilateral or bilateral renal agenesis should always prompt a thorough search for other anomalies in the fetus, as the risk of associated anomalies or genetic syndrome may be as high as 30% ( Table 1 ) .
| Syndrome (Gene) | Inheritance | Other anomalies |
|---|---|---|
| Acro–renal–ocular syndrome ( SALL 4) | Autosomal dominant | Eye: unilateral/bilateral coloboma; Duane anomaly |
| Branchio–oto–renal syndrome (EYA1, SIX1) | Autosomal dominant | Ear anomalies: ear pits, microtia; anotia; auditory canal atresia; absent or hypoplastic ear ossicles; branchial cysts |
| Ectrodactyly–ectodermal dysplasia–cleft syndrome (P63) | Autosomal dominant | Ectrodactyly; ectodermal dysplasia; cleft palate |
| Pallistere Hall syndrome (GLI3) | Autosomal dominant | Imperforate anus; mesoaxial polydactyly; hypothalamic hamartoma |
| Renal–coloboma syndrome (PAX2) | Autosomal dominant | Myopia; nystagmus; optic nerve coloboma |
| Townes–Brocks syndrome (SALL1) | Autosomal dominant | Imperforate anus; external ear anomalies; deafness; thumb anomalies |
| Antley–Bixler syndrome (FGFR2) | Autosomal recessive | Craniosynostosis; bowing of ulna and femur; vertebral anomalies; thin ribs; ambiguous genitalia |
| Fraser syndrome (FRAS1) | Autosomal recessive | Cryptophthalmos; cutaneous syndactyly; ambiguous genitalia; malformed ears |
| Smith–Lemli-Opitz syndrome (DHCR7) | Autosomal recessive | Toe syndactyly; growth retardation; cardiac and cerebral malformations; genital anomalies |
| Goltz–Gorlin syndrome (PORCN) | X-linked | Focal dermal hypoplasia; variable limb defects; cardiac anomalies |
| Kallman syndrome (KAL1) | X-linked | Hypogonadotropic hypogonadism; anosmia |
| Lenz microphthalmia (BCOR) | X-linked | Microphthalmia; genital anomalies |
Dysplastic kidneys
A dysplastic kidney is a kidney that has abnormal development of the glomeruli and nephrons along with a disproportionately increased stroma . Dysplastic kidneys are often of abnormal size, structure, or both, and are identified prenatally with or without cystic changes. The dysplasia is thought to be caused by altered interactions between the ureteric bud and the metanephric mesenchyme . Nephrons are present in reduced numbers in dysplastic kidneys, leading to reduced renal function and abnormal connections with the collecting system, resulting in cyst formation. Although renal dysplasia has many underlying causes, such as genetic defects, lower urinary tract obstruction, and teratogens, most cases are sporadic, with a multifactorial etio-pathogenesis. Multicystic dysplastic kidneys often follow ureteric obstructions owing to improper canalisation at 8 weeks of gestation. Several genes have also been identified, but their precise roles in the development of the kidneys have not been well defined. Dysplastic kidneys have a characteristic appearance at the routine 20-week anomaly scan; they appear large and bright, usually with cystic spaces lining the cortex. Typically, the cysts are multiple, thin walled with no connections, randomly placed in the renal parenchyma, and form an irregular outlined kidney ( Fig. 2 ). In the presence of large cysts, the entire renal parenchyma seems to be filled with cysts, and the kidneys lose all signs of cortico-medullary differentiation. Rarely, dysplastic kidneys appear uniformly echogenic on ultrasound without cysts, and can be difficult to differentiate from normal kidneys. The renal pelvis and ureters are not seen, and the renal artery flow on Doppler cannot always be recorded. The degree of abnormality can be judged by assessing the size of the affected kidney with a normal kidney, the echogenicity compared with the surrounding organs, the liquor volume (even though this is primarily dependent on the function of the contra lateral kidney), and the presence and number of cysts. Furthermore to establish a diagnosis, family history, ultrasound of parents’ kidneys, genetic referral, and karyotyping, for example, should be considered. The presence of any associated anomaly in the fetus should also be noted.
Dysplastic kidneys are often associated with other syndromes such as VACTERL association, Meckel–Gruber syndrome, Bardet–Biedl syndrome, Fraser syndrome and CHARGE syndrome . The outcome would obviously be dictated by the associated findings in these conditions. Most babies with isolated unilateral multicystic renal disease tend to have a good outcome . The size and number of cysts in unilateral multicystic disease generally do not influence the outcome, although the main prognostic factor is the contralateral kidney size and presence or absence of other anomalies . Bilateral involvement is generally associated with severe oligohydramnios, and has a poor prognosis owing to the resultant pulmonary hypoplasia. The presence of significant associated anomalies or bilateral renal disease would create situations when the option to terminate the pregnancy would need to be discussed. In the case of perinatal death or if the parents elect to terminate the pregnancy, an expert postmortem should be carried out. If postmortem is declined, at least a needle biopsy of the kidney should be considered.
Duplex kidney
Duplex kidney system is one of the common renal tract anomalies often diagnosed postnatally . It is characterised by duplication of collecting systems (duplex collecting systems) in which two pyelocaliceal systems are present in a single kidney with single or double ureters. Usually duplex system is unilateral and more common among females . Prenatal diagnosis is possible with the presence of the following ; length of the kidney in the sagittal view greater than 95° percentile; cystic-like structure surrounded by a rim of renal parenchyma in the upper pole of the kidney; kidney with two separate non-communicating renal pelvises; and dilated ureter usually draining the upper pole and echogenic cystic structure in the bladder (ureterocele). As most of the operators are not familiar with the duplex system, prenatal diagnosis is infrequent . Therefore, presence of dilated ureters without obvious dilatation of the collecting system needs careful examination of the upper urinary tract to exclude duplex system.
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