Ultrasound Evaluation Fetal Genitourinary System.

Objectives Understand the sonographic evaluation the fetal kidneys and bladder

Learn how to differentiate renal anatomical changes with gestational age

Know how to measure renal length correlated with menstrual age

Learn the assessment of amniotic fluid volume

Differentiate obstructive and nonobstructive abnormalities of the urinary tract

Know how to evaluate pylectasis and calyectasis

Learn the causes of hydronephrosis

Learn the sonographic findings in ureterovesical junction obstruction and posterior urethral valves

Introduction Prenatal ultrasound has been integrated into the obstetrical examination to provide information to the clinician regarding the development and overall well-being of the fetus. Specifically, ultrasound has the capability of imaging the genitourinary system as early as the first trimester with the endovaginal examination; however improved anatomical detail is obtained with transabdominal ultrasound after eighteen gestational weeks. The sonographer has the opportunity to assess the presence and amount of amniotic fluid, to clearly visualize the fetal kidneys and bladder in the early second trimester, as well as the opportunity to demonstrate many genitourinary system anomalies.

A complete fetal genitourinary examination would include evaluation and measurement of both kidneys, documentation of the urinary bladder, and assessment of amniotic fluid. Amniotic fluid volume reflects the prognosis of renal function; early detection of amniotic fluid variations may influence the management of the obstetrical patient. Abnormal amounts of amniotic fluid should lead the sonographer to suspect problems with the genitourinary, gastrointestinal, or central nervous system, premature rupture of membranes, or intrauterine growth restriction. In the presence of poly- or oligohydramnios the sonographer should carefully image the renal areas and bladder with ultrasound and to determine if obstruction is present that may be the cause of the abnormal production of amniotic fluid.

The genitourinary system is the most common site (30%) of all antenatally detected anomalies. Abnormalities of the genitourinary system may also be discovered incidentally or secondarily if another anomaly has been detected during the obstetrical ultrasound evaluation.

Development of the Urogenital System

The fetal urinary system begins to develop in the first trimester. The genitourinary system consists of two kidneys that excrete urine, the ureters which transport urine to the urinary bladder, and the urethra through which the urine is discharged to the exterior.

Development of the Kidneys. The kidneys begin to develop early in the fifth week of life although they do not begin to function and produce urine until around the end of the first trimester to the beginning of the second trimester.

The assessment of amniotic fluid is correlated with gestational age. Prior to approximately sixteen weeks, the placenta contributes the fluid waste to form the amniotic fluid volume. After the second half of gestation, the kidneys are the primary contribution of urine waste formation that continues to fill the amniotic cavity throughout fetal life. Therefore, early in gestation the amniotic fluid is present in the absence of renal function. After sixteen weeks, normal amniotic fluid implies that at least one functioning kidney is present.

The fetal kidneys are subdivided into lobes that may be separated by grooves. This lobulation usually diminishes by the end of the fetal period, but in some cases the lobes may still be noticible by the end of the neonatal period. In the adolescent and the adult patient, persistent of the fetal lobulation and groove may be seen on ultrasound as an echogenic "triangular" notch along the anterior wall of the right kidney.

The arterial vascular supply to the kidneys is supplied by arteries that arise from the aorta. Usually these vessels disappear as the kidneys ascend from the pelvis into the retroperitoneal cavity, but some of them may persist to account for the variations in number that the sonographer may find in the renal arteries. At least 25% of adult kidneys have two to four renal arteries.

Development of the Urinary Bladder. The fetal urinary bladder is derived from the hindgut derivative known as the urogenital sinus. The caudal ends of the mesonephric ducts open into the cloaca and parts of them are gradually absorbed into the wall of the urinary bladder. This development causes the ureters (derived from the ureteric buds) and the mesonephric ducts to enter the bladder separately.

Development of the Urethra. The epithelium of the female urethra and most of the epithelium of the male urethra is derived from the endoderm of the urogenital sinus.

The Urachus. Early in development the urinary bladder is continuous with the allantois. The allantois regresses to become a fibrous cord known as the urachus. This cord, or ligament, extends from the apex of the bladder to the umbilicus. If the lumen of the allantois persists as the urachus forms, a urachal fistula develops which causes urine to drain from the bladder to the umbilicus.1 If only a small part of the lumen of the allantois persists it is called a urachal cyst. If a larger portion of the lumen persists, it may cause a urachal sinus to develop that may open at the umbilicus or into the urinary bladder.

Sonographic Evaluation of the Genitourinary Tract

The kidneys should be evaluated by assessing their anatomy, texture, and size. Normal anatomic structures of the kidney include the relatively homogeneous renal cortex and parenchyma, hypoechoic pyramids and echogenic calyces, and anechoic renal pelvis. Kidney texture that appears significantly enhanced or echogenic should be a cause for concern.

Demonstration of the genitourinary tract is best imaged with transabdominal ultrasound after sixteen weeks gestation. By twenty weeks gestation sonography may clearly define over 90% of fetal kidneys.2 Transvaginal ultrasound has occasionally demonstrated the renal area as slightly hyperechoic as compared to the hypoechoic adrenal glands near the end of the first trimester. However with transabdominal ultrasound the renal area is hypoechoic at fourteen to sixteen weeks. As the gestation progresses, the retroperitoneal fat increases surrounding the renal area

and thus makes an echogenic border to differentiate the kidneys from the abdominal structures. By the third trimester, the corticomedullary junction may be distinguished from the renal cortex and the hypoechoic renal pyramids are well delineated. The renal borders may appear lobulated on the longitudinal image. These fetal lobulations gradually develop into a smoother border as the gestation progresses.

The kidneys may be demonstrated on the ultrasound image in the transverse and longitudinal plane. Transversely, the kidneys are demonstrated as a circular area adjacent to the fetal spine. (Fig. 1) The sonographer may locate the kidneys in the transverse plane by slowing "sweeping" the transducer in a cephalic to caudal direction. Locate the fetal heart or fluid-filled stomach and then slightly angle the transducer inferior toward the area of the fetal kidneys; if the fluid-filled bladder is seen the sweep was angled too inferior (caudal). (Fig. 2) Analysis of the central renal sinus and pelvis should be assessed for the presence of fluid.

Fig.1

Fig.2

If the kidneys are difficult to locate, color Doppler may be utilized to locate the fetal vascular structures that flow in and out of the central renal area. (Fig. 3) The pulse rate frequency should be reduced and the transducer held still over the renal area to detect vascular flow. Color Doppler mapping may be especially useful in the presence of oligohydramnios.

Fig.3

Fig.4

Normally, the fetal ureters are too small to image (measuring 1-2 mm). If "cystic" type structures are seen separate from the kidney, the sonographer should evaluate for the presence of hydronephrosis, dilated ureters, prominent fluid-filled bowel, or other abnormality. An abdominal

structure (separate from the psoas muscle) or mass that touches the fetal spine most likely originates within the retroperitoneal urinary tract.2

Once the kidneys have been located on the transverse plane, rotate the transducer 90 degrees to the longitudinal plane. (Fig. 4) Hold the transducer still over the renal area to watch for respiratory movement of the kidney and diaphragm. The prominent adrenal gland that caps the superior pole of the kidney may be demonstrated.

Fetal Urinary Bladder. The fetal urinary bladder is well seen on ultrasound if fluid is present within. The bladder has a thin wall and may be seen as a triangular sac in the central anterior position on the transverse image within the fetal pelvis. The bladder becomes more elliptical in shape with fluid distention. With color Doppler, the umbilical (superior vesical) arteries may be seen on the transverse plane just lateral to the bladder as they flow towards the umbilicus.

The production of fetal urine increases with gestational age from a mean value of 5 mL/hr at 20 weeks to 56 mL/hr at 41 weeks. The bladder volume at 20 weeks is approximately 1 mL, by term gestation at 41 weeks the volume has increased to 36 mL.3 The normal bladder fills and empties (partially or completely) on average every 25 minutes. Therefore, it is a good idea to image the genitourinary system early in the ultrasound examination. If the bladder is not full, it may be reassessed at the end of the exam.

Failure to observe the bladder may indicate a severe renal abnormality when accompanied by oligohydramnios. The bladder wall should be thin in a normal fetus. When obstruction occurs at the level of the urethra, the bladder wall becomes hypertrophied. The presence of ureteral jets may be assessed in the fetus to rule out obstruction. Color Doppler is focused over the area of the bladder, near the base, and the presence of ureteral jets streaming into the bladder over a period of time indicates the ureter is not obstructed.

The urethra, like the ureters, is usually unidentifiable in the normal fetus. Dilation of the posterior urethra is highly suspicious for an obstructive process such as posterior urethral valve syndrome.

Assessment of Renal Growth. Normal fetal renal measurements for thickness, width, length, and volume have been reported as a function of menstrual age. As expected, these measurements increase with age and correlate with measurements in the postnatal period. The easiest measurements to obtain are the anterior-posterior and the longitudinal dimensions. A "rule of thumb" measurement is that the menstrual age in weeks approximates the normal fetal kidney length in millimeters or twice the anteroposterior diameter in millimeters.2

Normal Fetal Renal Dimensions (mm) Correlated with Menstrual Age*

Gest. Age Thickness Width Length Volume

16 2/6/10 6/10/13 7/13/18 0/0.4/2.6

18 4/8/12 6/10/14 12/17/22 0/0.7/2.9

20 6/10/13 7/11/15 15/21/26 0/1.1/3.3

24 9/13/17 10/14/18 22/28/33 0.3/2.5/4.7

28 12/16/20 13/17/21 28/33/38 2.5/4.7/6.9

32 15/19/23 17/20/24 33/38/43 5.4/7.5/9.7

36 17/21/25 19/23/27 36/41/47 8.1/10.2/12.4

*5th/50th /95th percentile

Adapted from Romero R. Pilu G. Jeanty P. et.al: Prenatal Diagnosis of Congenital Anomalies. East Norwalk, CT. Appleton & Lange, 1989.

Measurement of the renal length may be difficult to assess the upper pole border because of rib shadowing or adrenal tissue. The sonographer should observe the kidney movement as the fetus breathes and the complete renal border may be visually assessed.

Assessment of Amniotic Fluid Volume. The amniotic fluid volume (AFV) provides information about renal and placental function. It also is an important biophysical assessment tool. The assessment of amniotic fluid may be may by the single vertical pocket method or the four quadrant summation method. The single vertical pocket measurement may be made after the uterine surface has been evaluated for the greatest single vertical pocket of amniotic fluid (without cord or fetal parts) has been demonstrated.

Single Vertical Pocket Measurement

<2 cm suggests oligohydramnios

2-8 cm is normal fluid

>8 cm suggests polyhydramnios. The amniotic fluid index (AFI) method divides the uterine cavity into four quadrants. Each quadrant is measured for the greatest pocket of fluid minus umbilical cord or fetal parts (color Doppler is useful to assure the cord is not included in the measurement). The four quadrants are added together to provide one measurement known as the AFI. This method varies with gestational age and is objective and reproducible. An AFI of less than 5 cm presents oligohydramnios, 5-20 is normal, 20-25 is upper normal, and >25 cm polyhydramnios.

Obstructive and Nonobstructive Abnormalities of the Urinary Tract

Systematic Approach to Evaluate Genitourinary Anomalies. The sonographer should determine if the urinary bladder is present and fluid filled during the examination. The kidneys should be imaged in their normal position and the size and texture evaluated. If the renal pelvis is dilated, the sonographer should try to assess the level of the obstruction and whether it is intrinsic or extrinsic. Assessment of the fetal gender will also help to determine the gender specific anomaly.

The recognition of urinary tract anomalies is of significant clinical concern, since several fetal conditions are incompatible with life. In unilateral obstructions of the urinary tract, early delivery of the fetus is often warranted to salvage the normal kidney. Intrauterine decompression of an obstructed urinary tract (posterior urethral valve syndrome) has been performed to relieve the obstruction and allow expansion of the lungs to prevent pulmonary hypoplasia.4 Recognition of lethal or treatable renal anomalies is necessary to ensure appropriate clinical and therapeutic management. When bilateral, decide if the condition is asymmetric (dissimilar abnormal patterns for each kidney) or symmetric (same abnormal pattern for both kidneys; this may imply a genetic condition, such as autosomal-recessive infantile polycystic kidney or multicystic dysplastic kidney disease).

If the fetus has a known chromosomal defect or if other abnormalities are found, the risk is greater for the fetus to have a renal anomaly as well. Additional malformations indicate a higher risk for chromosome anomalies (x30 for multiple defects and x3 for isolated defects).

As discussed earlier, assessment of the amniotic fluid volume is important to evaluate renal function. If an abnormality of the genitourinary tract is found, normal AFV after twenty weeks implies at least one kidney is functioning and the fetus has a good prognosis. If oligohydramnios

is seen early in the second trimester, the prognosis is poor because of the development of pulmonary hypoplasia.

Renal obstructive malformations may be divided into two categories: (1) obstructive and (2) those resulting from a non- obstructive process. The consequences of renal malformations vary, depending on the type of lesion and extent of obstruction.

Dilatation of the Urinary Tract. Marked deviations of anatomy should alert the sonographer to investigate the urinary tract more extensively. It is important to remember that a small amount of urine may be seen in the renal pelvis in the normal fetus after twenty weeks gestation, measuring in its anterior-posterior diameter less than 5 mm. Prior to twenty weeks, this measurement should not exceed 4 mm. This slight dilatation should not be misinterpreted as an abnormal collection of urine within the renal pelvis, pelviectasis, (5-9mm); or caliectasis, rounded calyces with renal pelvis dilation (greater than10 mm) which may lead to severe hydronephrosis.

Fig. 5

Fig.6

A renal pelvis diameter, measured in an anteroposterior direction in a transverse plane that exceeds 10 to 15 mm is considered to be abnormal. (Fig. 5, 6) Follow-up ultrasound evaluation in six to eight weeks should continue throughout the pregnancy and postnatally to evaluate the degree of obstruction.

Pylectasis is found in two percent of normal fetuses. An increased number (17-25%) of fetuses with Trisomy 21 have been reported to have pylectasis. If renal dilatation is the only ultrasound finding, the risk of having a Trisomy 21 fetus increases only slightly and therefore should be considered a soft finding for a chromosomal anomaly.

Obstructive Urinary Tract Abnormalities

Obstruction of the urinary system may originate anywhere along the urinary tract. The consequences of an obstruction depend on the origin of the blockage. For example, in fetuses with complete posterior urethral valve obstruction, urine is unable to pass through the urethra and into the amniotic fluid. Consequently, urine backs up in the posterior urethra, bladder, ureters, and often extends to the kidneys (hydronephrosis).

The urinary tract may be obstructed at the junction of the ureter entering the renal pelvis

(ureteropelvic junction) or at the junction of the ureter as it enters the bladder (ureterovesical junction) or at the level of the urethra (megacystis).

Hydronephrosis. Fetal hydronephrosis is the most common fetal anomaly. The sonographic appearance of urinary tract obstruction varies depending on the site and extent of blockage. Dilation of the renal pelvis (hydronephrosis) occurs in response to a blockage of urine at some junction in the urinary system. This blocked urine is unable to pass the obstruction, and urine is continually produced, it will back up into the kidney. Hydronephrosis commonly occurs when there is an obstruction in the ureter, bladder, or urethra. Hydronephrosis is generally the end result of an obstruction at a lower level in the urinary tract.

Cause of Hydronephrosis

44% Ureteropelvic Junction Defect

21% Ureterovesical Junction Defect

14% Vesicoureteric Reflux

12% Duplex Collecting System

9% Posterior Urethral Valves

The ultrasound appearance of hydronephrosis varies according to the severity of the underlying obstruction. The dilated renal pelvis is centrally located and distended with urine, which often communicates with the calyces (caliectasis). The remaining renal tissue may be identified in all but the most severe cases of hydronephrosis. Renal dysplasia often occurs and represents cystic changes within the renal tissue. Several obstructive patterns may be observed. The sonographic team should attempt to define the severity of the cystic changes affecting the kidney.

Ureteropelvic Junction Obstruction. Fetal hydronephrosis may occur as a unilateral or bilateral process. Unilateral renal hydronephrosis commonly results from an obstruction at the junction of the renal pelvis and the ureter. This is called a ureteropelvic junction obstruction (UPJ). The prognosis of the fetus is good with unilateral UPJ obstruction.

Sonographically, the renal pelvic is dilated, pylectasis. As the dilatation increases in size, a collection of urine located medially within the renal pelvis communicates with the calyces, caliectasis. The ureter and bladder are usually normal in size; the amniotic fluid volume may be normal to slightly increased. When unilateral hydronephrosis is accompanied by oligohydramnios the contralateral renal pathology should be investigated. With severe chronic obstruction, there may be marked dilation of the renal pelvis with sonographic findings of a thin renal cortex and a large unilocular "cystic" mass.

Bilateral UPJ obstruction occurs in 10 to 30% of fetuses. The prognosis is poor with bilateral obstruction if the hydronephrosis is severe. The sonographer should image the fetal bladder and amniotic fluid volume to assess renal function. Serial examinations are necessary to monitor the degree of obstruction and to assess renal function.

Ureteropelvic junction obstruction is the most common reason for hydronephrosis in the neonate. Only half of these disorders are found during early childhood; therefore early prenatal detection may improve long-term renal function. The causes of UPJ obstructions include abnormal bends or kinks in the ureter, adhesions, abnormal valves in the ureter, abnormal outlet shape at the ureteropelvic junction, or absence of the longitudinal muscle that is imperative to the normal excretion of urine from the kidney.5

Ureterovesical Junction Obstruction. The second most common cause for hydronephrosis is the junction where the ureter enters the posterior wall of the bladder, ureterovesical junction obstruction (UVJ). Ureterovesical junction obstruction commonly presents with dilation of the ureter, megaureter. Megaureter may result from a primary ureteral defect (stenotic ureteral valves or fibrosis) or occur secondary to obstruction at another level (causing reflux or backward flow of urine). UVJ obstruction is usually unilateral.

Sonographic findings include hydroureter and hydronephrosis. The dilated ureter should be distinguished from bowel. The ureter lies closer to the spine; both bowel and the dilated ureter may show peristalsis. Bowel may present with more echogenic texture internally than the ureter.

Coexisting anomalies such as vesicoureteral reflux, UPJ obstruction, and multicystic dysplasia are frequently present.

Duplication Anomalies. Other defects, such as duplication of the renal collecting system, are common and may be diagnosed prenatally. This anomaly is more common in females When a dilated upper renal pole is observed with a normal lower pole an obstructed duplicated collecting system may be indicated. This may result from an ectopic ureterocele (15% have bilateral ureteroceles) within the bladder causing bladder outlet obstruction and hydronephrosis of the upper pole of the kidney. The ectopic ureter may be enlarged and tortuous to mimic a septated, cystic mass.

Lower Urinary Tract Obstruction

Urethral Obstruction. Obstruction at the bladder outlet would cause dilatation of the bladder that may reflux into the ureters and renal pelvis. The time of onset and severity of the obstruction would be influential in the amount of obstruction present.

Posterior Urethral Valves. This condition is the most common cause of lower urinary tract obstruction. Posterior urethral valve obstruction results in hydronephrosis, hydroureters, or dilation of the bladder and posterior urethra. This entity occurs only in male fetuses and is manifested by the presence of a valve(s) in the posterior urethra. As a result, urine is unable to pass through the urethra and into the amniotic fluid. This causes a back-up of urine in the bladder, ureter, and, in the most severe cases, the kidneys. Severe oligohydramnios is a classic finding in the complete obstruction form.

Sonographic findings include the classic "keyhole" bladder sign. This occurs secondary to the outlet obstruction, causing the bladder wall to thicken greater than 2 mm. The ureters are both dilated and tortuous. Severe dilatation of the bladder may lead to spontaneous rupture of the bladder to cause urinary ascites or peri-renal urinoma. Hydronephrosis is often present. If the obstruction is severe, renal fibrosis and dysplasia develop which leads to decreased renal function. Oligohydramnios occurs and causes pulmonary hypoplasia and compression deformities of the fetus. When these sonographic signs occur in the female fetus, abnormalities of the sacrum (caudal regression anomalies) and megacystis-microcolon intestinal hypoperistalsis syndrome should be considered.

Fetal renal function may be assessed by aspirating urine from an obstructed bladder. Cystic dysplasia and poor renal function are suggested when sodium, chloride, and osmolality are unusually elevated.5

Intermittent posterior urethral valve obstruction may occur with a normal amount of amniotic fluid. Diminishing fluid volume and increased hydronephrosis may prompt early delivery.

The prognosis of posterior urethral valve syndrome is invariably fatal, but in selected fetuses with documented normal renal function, placement of an in-dwelling bladder shunt to relieve the obstruction has improved chances for survival in some cases.6 This shunt drains the blocked

urine into the amniotic fluid, allowing the fetal lungs to develop. When the urinary tract is completely blocked, severe oligohydramnios and the Potter sequence occur.7

Urethral Atresia. Rare disorders, such as urethral atresia, may cause a massively dilated bladder without evidence of amniotic fluid, (anhydramnios). This is the most severe form of obstructive uropathy in the lower urinary system. This condition is fatal without treatment as renal dysplasia develops along with pulmonary hypoplasia.

Fetal Megacystis. Dilation of the ureters may occur as isolated lesions (primary megaureters), resulting from atresia of the distal ureter.8 The disorders are generally associated with adequate to increased amounts of amniotic fluid and a normal bladder. Infrequently, renal hydronephrosis may occur.

Cloacal Malformation. The failure of the urorectal septum to reach the perineum results in cloacal malformation. This condition occurs in 1/50,000 births and affects phenotypic females. Other lower urinary tract malformations are common.

Sonographic findings include decreased amniotic fluid volume, normal-to enlarged- to nonvisulization of the fetal bladder, a retrovesical cystic mass, ascites, hydronephrosis, ambiguous genitalia, and vertebral anomalies.

Prune-Belly Syndrome. This syndrome may be called the urethral obstruction malformation complex. The condition consists of: cryptorchidism, agenesis of abdominal wall muscle, megaureters, bladder outlet obstruction due to urethral anomalies such as atresia, stenosis valves, or diverticulum.

Sonographic findings in prune-belly syndrome include oligohydramnios, mild to severe bilateral hydronephrosis, fetal ascites, and hypoplastic lungs. The abdomen is extremely distended as compared to the small thoracic cavity. The dilated ureters and bladder appear as numerous cystic lesions within the distended abominal cavity.

Bibliography Moore, KL: Before we are born, ed 3, Toronto, W.B. Saunders Co., 1989. Callen DW, editor: Ultrasonography in obstetrics and gynecology, ed 3, Philadelphia,

2000, WB Saunders. Rabinowitz R. Peters MT, Vya S, et al: Measurement of fetal urine production in normal

pregnancy by real-time ultrasonography. Am J Obstet Gynecol 161:1264, 1989. Golbus MS, Harrison MR, Filly RA, et al: In utero treatment of urinary tract obstruction,

Am J Obstet Gynecol 142:383, 1982. Nyberg DA, Mahony BS, and Pretorius DH, editors: Diagnostic ultrasound of fetal

anomalies: text and atlas. St. Louis, 1990, Mosby. Harrison MR, Golbus MS, and Filly RA: Congenital hydronephrosis. In Harrison MR,

Golbus MS, and Filly RA, editors: The unborn patient: prenatal diagnosis and management. Orlando, Fla., 1984, Grune & Stratton.

Berkowitz RL, Glickman MG, Walker-Smith GJ, et al: Fetal urinary tract obstruction: what is the role of surgical intervention in utero? Am J Obstet Gynecol 144:367, 1982.

Dunn V and Glasier CM: Ultrasonographic antenatal demonstration of primary megaureters, J Ultrasound Med 4:101, 1985.