THE FETAL GENITOURINARY TRACT

16 THE FETAL GENITOURINARY TRACT





Embryology


Structurally and functionally, the urogenital system can be divided into two entirely different components—the urinary system and the genital system. Both of these develop from a common mesodermal ridge of tissue, the intermediate mesoderm, along the posterior wall of the abdominal cavity. Initially, the excretory ducts of both systems enter a common cavity, the cloaca.1


Human embryos develop three sets of excretory organs or kidney systems during intrauterine life.1,3 The embryonic kidneys, which are formed in a cranial to caudal sequence, in order of appearance are the pronephros, the mesonephros, and the metanephros. The pronephros, analogous to the kidneys in primitive fishes and the mesonephros, which are well developed and analogous to the kidneys of amphibians, both regress in utero. The third set of kidneys, the metanephroi, become the permanent kidneys (Fig. 16-1).2,3



The pronephros is seen in the 3rd week of development and completely degenerates by the start of the 5th week. The second kidney, the mesonephros, also regresses by the 4th month; however, it serves as an excretory organ for the embryo while the definitive kidney begins its development. Certain elements of the mesonephros are retained in the mature urogenital system as part of the reproductive tract.3


The metanephros, or definitive kidney, forms in the sacral region as a pair of new structures called the metanephric diverticulum or ureteric bud. It emanates from the distal portion of the mesonephric duct and comes in contact with and penetrates the metanephric mesenchymal blastema (metanephric mesoderm) at approximately the 28th day.2,3


The ureteric bud and metanephric mesoderm exert reciprocal inductive effects toward each other, and the proper differentiation of these structures depends on these inductive signals. The metanephric mesoderm induces the ureteric bud to branch, and in turn, the ureteric bud induces the metanephric mesoderm to condense and undergo mesenchymal-epithelial conversion. The nephron, which consists of the glomerulus, proximal tubule, loop of Henle, and distal tubule, is thought to derive from the metanephric mesoderm, whereas the collecting system, consisting of collecting ducts, calyces, pelvis, and ureter, is formed from the ureteric bud (see Fig. 16-1).3


The division of the ureteric bud results in the eventual pelvicalyceal patterns and their corresponding renal lobules. The first few divisions of the ureteric bud give rise to the renal pelvis, major and minor calyces, and collecting tubules. Thereafter, the first generations of collecting ducts are formed. When the ureteric bud first invades the metanephric mesoderm, its expanded end forms an ampulla that will eventually give rise to the renal pelvis. By the 6th week, the ureteric bud has bifurcated at least four times, yielding 16 branches. These branches then coalesce to form two to four major calyces extending from the renal pelvis. By the 7th week, the next four generations of branches also fuse, forming the minor calyces. By the 32nd week, approximately 11 additional generations of bifurcation have resulted in approximately 1 to 3 million branches, which will become the collecting duct tubules. In humans, although renal maturation continues to take place postnatally, nephrogenesis is completed before birth.3




The Normal Urinary Tract




Kidneys


With the use of transvaginal probes, fetal anatomic structures can be visualized earlier than with transabdominal ultrasound (US). Therefore, the fetal kidneys can be demonstrated at approximately 11 weeks transvaginally and at 12 weeks using transabdominal probes. During the first trimester, the kidneys appear as hyperechoic oval structures at both sides of the spine (their hyperechogenicity can be compared to that of the liver or spleen) (Fig. 16-5). This echogenicity will progressively decrease, and during the third trimester, the cortical echogenicity will always be less than that of the liver or spleen. Simultaneous to the decreased echogenicity, corticomedullary (CMD) differentiation will appear at approximately 14 to 15 weeks. This should always be demonstrated in fetuses older than 18 weeks. Prominent pyramids should not be misinterpreted as calyceal dilatation (Fig. 16-6). The fetal kidneys’ growth can be evaluated throughout pregnancy by measuring renal length and comparing it to normal charts. (As a simple rule, renal growth is 1.1 mm/gestational week.) During the second and third trimesters, the kidneys are easily identified by imaging the dorsolumbar spine and scanning on either side in parasagittal and transverse axial sections. Urine distending the renal pelvis may help in their identification. Under normal conditions, the fetal ureters are not visible.46






The Abnormal Urinary Tract


The overall prevalence of renal tract anomalies is estimated at 5/1000 births. The percentage is likely higher when considering transient anomalies. Anomalies involving the UT are numerous and variable. They can be isolated or appear in association with another organ system’s anomalies. Therefore, the sonographic examination should be as meticulous as possible in order to visualize possible associated features. These additional findings will determine the prognosis.814



Abnormal Renal Number


Bilateral renal agenesis is incompatible with extrauterine life. This condition, at times referred to as Potter’s syndrome, results in pulmonary hypoplasia and musculoskeletal abnormalities. The diagnosis is based initially on anhydramnios after 15 weeks and on the nonvisualization of normal renal structures (Fig. 16-7). The bladder is empty (very rarely, a small fluid-filled structure can be observed in the fetal pelvis. It might correspond to retrograde filling of the bladder). Whenever there is a question as to whether the kidneys are present, fetal magnetic resonance imaging (MRI) may help with the diagnosis. Enlarged, globular adrenals should not be misinterpreted as dysplastic kidneys.1518



Unilateral agenesis occurs in 1/500 pregnancies. At the time of sonography, a normal kidney will not be identified in either lumbar area. When no other complication or malformation is present, the prognosis for postnatal life is excellent (Fig. 16-8). The ipsilateral adrenal gland is usually present, and appears globular and should not be mistaken for the kidney. Without the adjacent kidney, the adrenal gland may be elongated in appearance in what has been called “the lying down adrenal sign” (see Fig. 16-8B). In case of left renal agenesis, the left colonic flexure occupies the empty lumbar fossa and should not be confused with a cystic kidney or a dilated ureter. Whenever one or both lumbar fossas are empty, the kidneys should be searched for in an ectopic location (see later) (noteworthy, renal agenesis can theoretically result from the in utero regression of multicystic renal dysplasia).1921








Definition.


Various criteria are used in order to objectively evaluate renal dilatation. In our experience, the best criterion is the measurement of the anteroposterior diameter of the renal pelvis on a transverse scan of the fetal abdomen (Fig. 16-13). Using this measurement, several threshold values have been applied in order to be able to predict the postnatal outcome, especially in cases with moderate dilatation (less than 15 mm). Many authors agree that the upper limit should be 4 mm during the second and 7 mm during the third trimester of the pregnancy (see Figs. 16-13 to 16-15). These limits are set in order to detect not only patients that will need corrective surgery (in case of obstructive dilatation) but also the majority of fetuses and neonates presenting with vesicoureteric reflux. These patients are at risk for developing complications and eventually worsening renal function.2735





Pyelectasis refers to a visible renal pelvis below the significant threshold. During the second trimester, there are reports of this feature as a minor sign of chromosomal anomaly.32 Other sonographic evidence of an abnormality of the UT includes the visibility of the fetal ureter during pregnancy (Figs. 16-16 and 16-17) and the demonstration of an enlarged bladder (more than 3 cm length during the second and 5 cm during the third trimester) (Fig. 16-18).34,36






Findings During Obstetric Ultrasound.


Abnormalities resulting in dilatation of the UT can be found at any time during pregnancy. The degree of dilatation may increase or decrease during each trimester. Therefore, in order to screen all potentially abnormal cases, some authors advocate performing one sonographic examination during each trimester.33 This is controversial, because most abnormalities, particularly mild UT dilatation, do not change significantly during pregnancy and, therefore, are unlikely to alter pregnancy and fetal management.


Once dilatation of the collecting system has been detected in utero, the subsequent evaluation should answer three major questions: the origin of the dilatation, the coexistence of associated anomalies, and finally, the prognosis of the malformation. The most common cause for UT dilatation is ureteropelvic obstruction (UPJ). Other causes include ureterovesical junction obstruction (UVJ), vesicoureteric reflux (VUR), complicated duplex kidneys, and bladder outlet obstruction (BOO) (Table 16-1).


Table 16-1 Causes of Urinary Tract Dilatation
















UPJ, ureteropelvic obstruction; UVJ, ureterovesical junction obstruction; VUR, vesicoureteric reflux.


In cases of UPJ obstruction, the renal pelvis is dilated. As mentioned earlier, the threshold measurement on a transverse scan of the kidney is 7 mm for mild dilatation, between 7 and 15 mm for moderate dilatation, and more than 15 mm for marked dilatation. The more dilated the system, the more likely there is to be a decrease in renal function after birth. Furthermore, thinned, hyperechogenic renal cortices with cysts often correspond to obstructive dysplasia and impaired function (Fig. 16-19). Yet, unfortunately, there is often no direct correlation between the renal appearance and postnatal function (see Figs. 16-14 and 16-15).3438



Noteworthy, obstruction may lead to leakage (rupture of a renal calyx or even bladder), and to urinary extravasation either as a perirenal urinoma or as ascites. The functional significance of the leakage is not straightforward. In some instances, it may protect the renal parenchyma, whereas in others renal growth is impaired (Fig. 16-20).24,39,40



The main differential diagnosis of UPJ obstruction is nonobstructive dilatation (which is a postnatal diagnosis), multicystic dysplastic kidney (MDK) (part of cystic renal diseases, see below) and UVJ obstruction. Rarer differential diagnoses of UPJ obstruction are megacalycosis due to medullary hypoplasia (the calyces are more dilated than the renal pelvis) and infundibular stenosis (no medullary hypoplasia, but still calyceal dilatation).


The diagnosis of UVJ obstruction is based on the demonstration of a dilated ureter. Peristaltic waves modify the caliber of the ureter (see Fig. 16-16).1214,36 The dilatation may increase in utero, but it usually decreases after birth. In most instances, it is not possible to differentiate between dilatation secondary to UVJ obstruction from that secondary to high-grade vesicoureteric reflux (see Fig. 16-17). A hint for a differential diagnosis is the variability of the diameter of the renal pelvis during one single examination. This would favor VUR.


Renal duplication is usually easy to demonstrate once dilatation has developed. Various complications may occur at the level of renal duplications; for instance, obstruction at the upper or lower moiety or MDK at the upper or lower moiety. Both can be associated with distal insertion of the ureter ending either as an ureterocele or into an ectopic extravesical insertion. The ureterocele is seen as a septum within the bladder. It may prolapse into the urethra and result in acute in utero obstruction. The parenchyma related to obstruction may be thinned and dysplastic. The ectopic extravesical insertion may be difficult to diagnose in utero (Figs. 16-21 and 16-22).4144




Finally, the level of UT obstruction can be located below the bladder. The most frequent cause in male fetuses is posterior urethral valves (PUVs). The condition may or not induce dilatation of the upper UT. The dilatation can be unilateral or bilateral, and may be related to obstruction or to VUR.45,46 The degree of associated dysplasia is also variable. There seems to be a correlation between cortical echogenicity and the degree of obstructive dysplasia (Figs. 16-19 and 16-23).37



In cases of obstruction, the bladder “reacts” in different ways: it can enlarge, its wall can thicken, or the bladder may rupture and urinary ascites appears; other protective decompressive mechanisms may develop through VUR, urinomas, extravasation, or recanalization of the urachus (Fig. 16-24).39



Bladder enlargement due to obstruction secondary to PUV must be differentiated from other causes of BOO and from other causes of large bladder without obstruction (Table 16-2). Urethral atresia may result in megacystis detected during the first trimester and oligohydramnios (see Fig. 16-18); the condition often has a poor prognosis. A dilated penile urethra may be seen. Megacystis-megaureter association is related to massive VUR (Fig. 16-25). In such a case, the amniotic fluid volume is normal. Megacystis-microcolon-hypoperistalsis syndrome is a rare condition occurring in female fetuses with a poor postnatal prognosis (see later).4649


Table 16-2 Causes of Large Bladder
















Prognosis and In Utero Treatment.


Whenever a renal anomaly (mainly dilatation) is detected, a complete survey of the fetal anatomy should be performed in order to detect associated malformations that would indicate the need for chromosomal analysis or the possibility of polymalformative syndromes; both worsen the prognosis.10,50,51


In cases of dilatation, the prognosis will depend upon the type and extent of anomalies. Features of poor prognosis include early diagnosis, bilateral marked dilatation, persistently obstructed bladder, oligohydramnios, and secondary lung hypoplasia. Bilateral renal dilatation and BOO have an increased risk of associated chromosomal anomalies, and therefore, in such cases, an evaluation of fetal chromosomes may be warranted. The finding of associated hyperechogenic and cystic renal parenchyma is frequently but only partially informative (positive predictive value [PPV] = 59%, negative predictive value [NPV] = 56%) about renal function. Conversely, normal cortical echogenicity does not exclude dysplasia.


The role of measuring urinary electrolytes in the fetal urine obtained through transabdominal puncture is controversial. There are discrepancies in the predictive values of urine biochemistry owing to small sample size, variations in cutoff values, gestational age, and sampling frequency. Fetuses with renal damage show decreased urinary concentrating especially of sodium and calcium without clear convincing confirmation. Measurements of B2-immunoglobulin on C cystatin in the fetal urines allow a better accuracy (Table 16-3). The outcome of vesicoamniotic shunting is also controversial. Although technically relatively easy, the long-term results have not been convincing.5256


Table 16-3 Fetal Urinary Biochemistry—Good Prognosis



























Analyte Value
Sodium* < 100 mg/dl
Chloride* < 90 mg/dl
Osmolality* < 200 mg/dl
Calcium < 8 mg/dl
β2-Microglobulin < 4.0 mg/L
Total Protein§ < 20 mg/dl
Cystatin C < 1 mg/L

* Glick PL, Harisson MR, Golbus MS, et al: Management of the fetus with congenital hydronephrosis. II. Prognostic criteria and selection for treatment. J Pediatr Surg 20:376, 1985.


Nicolaides KH, Cheng HH, Snijders RJM, et al: Fetal urine biochemistry in the assessment of obstructive uropathy. Am J Obstet Gynecol 166:932, 1992.


Mandelbrot L, Dumez Y, Muller F, et al: Prenatal prediction of renal function in fetal obstructive uropathies. J Perinat Med 19:283, 1991.


§ Johnson MP, Bukowski TP, Kithier K, et al: Fetal urine albumin/globulin ratio in the in utero evaluation of obstructive uropathies [Abstract]. Am J Hum Genet 51:259, 1992.


Muller F, Bernard M-A, Benkirane A, et al: Fetal urine cystatin C as a predictor of postnatal renal function in bilateral uropathies. Clinical Chemistry 45:2292, 1999.



Management of Fetal Renal Pelvic Dilatation After Birth.


It is of utmost importance that any information relevant for the proper postnatal management is correctly transmitted to the postnatal team that will be in charge of the newborn. A variety of findings may be seen in the neonate after birth that suggest an abnormality of the UT (Table 16-4). After birth, some conditions require an immediate confirmation and therapeutic maneuvers. For instance, obstructive PUVs or prolapsed ectopic ureterocele into the urethra leading to oligoanuria necessitate immediate treatment. In case of such suspicions, US and a voiding cystourethrogram (VCUG) should be performed immediately after birth in order to confirm the anomaly.57


Table 16-4 Characteristics Suggesting an Abnormality of the Urinary Tract on Neonatal Ultrasound















In all other cases, the work-up should be planned without emergency. There is controversy regarding the respective role of US and VCUG in the postnatal work-up. Some authors advocate the systematic use of a VCUG in every case of antenatal detection of UT dilatation; for others, only patients with persistent dilatation should undergo a VCUG.58,59 Whatever the choice, all patients are put under prophylactic antibiotic therapy until a final diagnosis and a final therapeutic decision is made.


Practically, we have applied an algorithm based on US examinations (Fig. 16-26). An US including the kidneys, bladder, and ureters should be performed during the first week of life in order to verify the UT abnormality. The sonographic analysis should be as detailed as possible, and any significant anomaly should lead one to perform a VCUG. If the examination is negative, a repeat sonographic examination should be performed at the age of 1 month. Again, if any abnormality is found, a VCUG should be performed, but if no anomaly is demonstrated, no further evaluation is needed. At this stage, any prophylactic antibiotic therapy should be stopped.60



The role of VCUG is clearly to detect vesicoureteric reflux that will render long-term follow-up and persistent prophylaxis necessary. This attitude is intended to reduce unnecessary complications that are associated with high grades of VUR. It will also demonstrate anomalies of the urethra and potentially abnormal ureteral endings. Follow-up studies include US (every 6 months for a period of 2 years) in order to verify renal growth, VCUG every year to monitor the reflux, and MAG3 isotopic studies in order to verify renal function.6163


If no reflux is present, complementary imaging is necessary in order to determine the origin of the dilatation. Renal function is assessed through isotopic studies; the morphology of complex or complicated UT malformations are best evaluated by MRI. The technique is particularly helpful for the assessment of the severely dilated UT and complicated duplex systems with dilatation of the upper and lower pole moieties. Also, the technique is able to provide information on the functional status of the kidney. It will help the surgeon in optimizing treatment.64 After this evaluation and if a conservative attitude is elected, a sonographic follow-up is advised in order to follow renal growth and dilatation. It has been shown that a large proportion of UT dilatation resolves spontaneously.63

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Mar 10, 2016 | Posted by in ULTRASONOGRAPHY | Comments Off on THE FETAL GENITOURINARY TRACT
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