Kidney

There are critical elements to an antenatal ultrasound examination that may help identify urologic pathology. A constellation of aberrations may indicate pathology, particularly when placed in context with other clinical findings. Specific details of the examination need to be reported to assist antenatal counseling. The ultrasound evalution of the kidney should comment on number, location, size, duplication, renal parenchyma (echogenicity), pelvic dilation, calyceal dilation, urothelial thickening, and cystic disease.

Kidneys should be appropriate size for gestational age and relatively symmetrical (Chitty and Altman, 2003). Large differences in size may indicate contralateral compensatory growth. Absence of the kidney in the normal location may represent ectopia or agenesis/dysplasia. The normal kidney should be elliptical and have distinctive internal echolucency, representative of normal medullary pyramids (Fig. 114–1). The appearance of the medullary pyramids should not be confused with renal calyceal dilation. The echogenicity of the kidney should be slightly less than the corresponding spleen or liver. Abnormalities of echogenicity with or without hydronephrosis may indicate renal disease. Isolated increased echogenicity has been associated with renal parenchymal disorders, but it has also been shown to be of no clinical significance (Carr et al, 1995). When seen with hydronephrosis it may indicate renal dysplasia, particularly if there is accompanying decreased amniotic fluid (Kaefer et al, 1997b).

Figure 114–1 Ultrasound appearance of normal fetal kidney with echolucent medullary pyramids distinguishable from the more echogenic cortical parenchyma. The cortical parenchyma should be of lower echogenicity than adjacent liver or spleen.

Renal cystic disease can be seen in utero. Autosomal-recessive polycystic kidney disease (ARKPD) may appear as large, bright or echogenic kidneys, because the numerous small renal cysts cannot be resolved by ultrasonography. In contrast, a multicystic dysplastic kidney (MCDK) typically presents as a large noncommunicating macrocyst (Fig. 114–2). A single cyst may represent a dilated renal calyx or diverticulum, an atypical MCDK, severe hydronephrosis, or a nonrenal structure.

Figure 114–2 Multicystic dysplastic kidney with large, multiple, variable-sized cysts without a central large cystic area. As here, most cases show virtually no parenchyma.

Ureter, Bladder, and Urinoma

In addition to kidney-specific findings, ureteral dilation, bladder filling and emptying, bladder wall thickness, intravesical cystic structures, dilation of the posterior urethra (keyhole sign), urinoma, amount of amniotic fluid, intra-abdominal/pelvic mass, and external genitalia should be noted. Hydroureter is best seen in cross section of a full bladder but is often difficult to detect (Fig. 114–3). Duplication and ureteral ectopia may also make this a difficult diagnosis.

Figure 114–3 Fetal ultrasonography showing a dilated, tortuous ureter (arrow). These may be associated with reflux, valves, ectopic ureters, ureteroceles, and ureterovesical junction obstruction. In this case the ureter is associated with a dilated upper pole indicating an ectopic ureter or ureterocele.

Although the bladder is sometimes difficult to image well, visualization of the bladder can be very informative, because a full bladder implies renal function. Inability to identify the bladder on repeat studies should raise the question of bladder exstrophy. Increased bladder wall thickness may indicate outlet obstruction, and dilation of the posterior urethra (keyhole sign) is strongly suggestive of posterior urethral valves (Fig. 114–4). In duplex systems, ureteroceles may present as an intravesical cystic structure and a dilated upper pole.

Figure 114–4 Fetal ultrasonography at 22 weeks’ gestation of a male with posterior urethral valves. The bladder is thick walled and has a dilated posterior urethra (keyhole sign). There was also bilateral hydronephrosis, echogenic renal parenchyma, and a perinephric urinoma.

Perirenal urinoma may indicate an obstructive condition (Yerkes et al, 2001) (Fig. 114–5). Typically it will have the appearance of an anechoic structure around the kidney or be in a subcapsular location. When identified, a urinoma or urinary ascites is often associated with cases of severe bladder obstruction, or posterior urethral valves, in which the urinoma may indicate a popoff mechanism. The popoff mechanism may be renal protective (Adzick et al, 1985). A urinoma also may be associated with a unilateral hydronephrotic/obstructed kidney (Mandell et al, 1994).

Figure 114–5 Appearance of a fetal perinephric urinoma associated with posterior urethral valves.

Amniotic Fluid

Critical to the evaluation of the urinary tract is the assessment of the amniotic fluid level and changes during pregnancy. After 16 weeks, amniotic fluid production shifts from placental transudate to fetal urine; and by 20 to 22 weeks the vast majority of amniotic fluid is fetal urine (Takeuchi et al, 1994). Consequently, reduced amniotic fluid or oligohydramnios identified after 18 to 20 weeks’ gestation may be a result of urinary tract obstruction or poor renal development (Stiller et al, 1988).

External Genitalia

Proper identification of the external genitalia also can be very valuable in cases of gender-specific diagnoses, such as posterior urethral valves. In cases of virilization (e.g., congenital adrenal hyperplasia), a clitoris may appear as a small phallus; therefore the presence of scrotal testes is critical for male gender assignment (Benacerraf et al, 1989; Bromley et al, 1994; Mandell et al, 1995). Megalourethra, or a dilated, elongated penile urethra, may be an isolated anomaly or associated with prune-belly syndrome (Dillon et al, 1994) (Fig. 114–6). A female fetus with bilateral renal obstruction and bladder outlet obstruction would suggest a cloacal anomaly (Cilento et al, 1994; Ohno et al, 2000; Taipale et al, 2004) (Fig. 114–7).

Figure 114–6 Fetal ultrasound appearance of a male with a dilated and patulous urethra typical of a megalourethra. This may be seen in prune-belly syndrome as well as in isolation. This child also had marked vesicoureteral reflux.

Figure 114–7 Fetal MR image illustrating severe bladder dilation (black arrow) and a separate dilated pelvic structure behind the bladder (white arrow) in a female fetus at 22 weeks’ gestation. The kidneys were bilaterally and symmetrically dilated with ureteral distention. This pattern can be seen with oligohydramnios and represents a urogenital sinus malformation with bladder outlet obstruction caused by vaginal distention from urine flowing into the urogenital sinus.

Hydronephrosis

Hydronephrosis, or dilation of the renal pelvis, is the most common urologic abnormality found on ultrasound evaluation. Numerous grading systems have been developed; however, there is no consensus on the best and most consistent method of reporting ANH (Fig. 114–8) (Fernbach et al, 1993). Measurement of the APD has been used widely, but there have been no formal studies to determine the interobserver and intraobserver reproducibility of ANH measurement. One of the disadvantages of the APD can be the failure to describe pelvic configuration, calyceal dilation, and the laterality of findings, which should be included. The APD can be affected by gestational age, hydration status of the mother, bladder hypertonicity, and degree of bladder distention. Because the dimensions of the renal pelvis may normally increase with gestational age, most investigators have adjusted threshold APD values for early and later gestational age. Unfortunately, a simple threshold APD value that separates normal from abnormal does not exist, because even severe cases of ANH have the potential to resolve without incident, whereas mild degrees of ANH have the potential to progress (Pates and Dashe, 2006).

Figure 114–8 Severe fetal hydronephrosis with diffuse calyceal dilation arrayed around the markedly dilated renal pelvis. The renal parenchyma is stretched over the dilated collecting system, but this does not mean loss of functional potential. Corticomedullary differentiation is difficult to see in this configuration.

Varying the minimal APD threshold can significantly alter the specificity and sensitivity of APD as a measure of ANH and postnatal pathology. To date there is no consensus on the optimal APD threshold for determining the need for postnatal follow-up. An APD cutoff of 15 mm for determining obstruction yielded a postnatal sensitivity of 73% and specificity of 82% (Coplen et al, 2006). A late gestational age APD cutoff of 10 mm would detect aproximately 23% of abnormal kidneys, whereas a cutoff of 7 mm detected 68% (Ismaili et al, 2003). One large systematic review estimated that only 11.9% of total pathology presented with late gestational age APD less than 9 mm, whereas 39% of total pathology was noted at APD levels less than 15 mm (Lee et al, 2006). Nearly identical results have been demonstrated by other investigators (Wollenberg et al, 2005). What appears certain is that lower cutoffs will be more sensitive in detecting postnatal pathology but will incur a higher false-positive rate.

Categorizing ANH by APD

There is near agreement that APD greater than 15 mm represents severe or significant hydronephrosis, and some would also agree that a lower threshold of 4 to 5 mm is an appropriate value for considering APD to be abnormal (Feldman et al, 2001; Ahmad and Green, 2005; Wollenberg et al, 2005; Lee et al, 2006; Coelho et al, 2007, 2008). Taking these limitations into consideration, we define ANH in the second and third trimesters using APD thresholds for which the best available evidence provides prognostic information; along with these definitions the estimated distribution of the ANH based on the previous defined definition of APD is outlined in Table 114–2.

Table 114-2 Definition of Antenatal Hydronephrosis by Anterior Posterior Diameter and Estimated Severity Percentage Range

Diagnostic Accuracy

As both ultrasound and MRI technology improve, more accurate radiographic information is obtainable (Laifer-Narin et al, 2007). However, determining accurate postnatal diagnosis and prognosis remains challenging. Regardless of the diagnosis, early intervention is uncommon except in the potential case of severe obstruction or posterior urethral valves. In the vast majority of other cases, early detection of ANH may be an impetus for future postnatal evaluation.

The ability to determine definitive postnatal pathology based on antenatal findings is difficult. As an example, a recent systematic review of the ANH literature attempted to determine the risk of a pathologic diagnosis for patients with varying severity of ANH (Lee et al, 2006). The review included 1308 patients who were identified with ANH and sufficient postnatal radiographic follow-up. The degree of ANH was defined by APD identified in a particular trimester. Approximately 36% of the patients had a postnatal pathologic diagnosis, and the overall risk for any pathologic process increased with increasing degree of ANH (Table 114–3). However, the risk of vesicoureteral reflux (VUR) remained consistent regardless of the degree of ANH, thereby implying that ANH is not an appropriate indicator for VUR.

Table 114-3 Risk of Pathology by Degree of Antenatal Hydronephrosis

Similarly, previous literature suggests that obstruction location can be determined prenatally in 88% of the cases; many others have reported a high false-positive rate (9% to 22%) (Hobbins et al, 1984; Scott and Renwick, 1993). The majority of false-positive findings in these studies were nonobstructive causes of hydronephrosis, such as high-grade reflux, large, nonobstructed extrarenal pelves, or transient hydronephrosis.

Early and accurate diagnosis of posterior urethral valves is critical; however, it can be difficult. The hallmark signs of an in-utero diagnosis of posterior urethral valves have been described (e.g., oligohydramnios, dilated posterior urethra, thickened bladder, and hydroureteronephrosis). Other findings such as increased renal echogenicity and decreased amniotic fluid have also been suggested to be indicative of obstructive conditions (Kaefer et al, 1997b). Regardless, there are very few studies that have prospectively examined the clinical urologic implications of these findings alone or in combination (Lee et al, 2006). In one series of 22 fetuses, the false-positive rate was as high as 58% (Abbott et al, 1998), and in a population-based series the sensitivity in detecting valves was as low as 23% (Scott and Renwick, 1993).

Regardless of the degree or severity of the finding, with any antenatal detection of a urinary tract anomaly a thorough fetal survey must be conducted. Amniocentesis and karyotype should be considered if intervention or a major anomaly is suspected, because the incidence of concurrent chromosomal anomalies is relatively high in fetuses with concomitant urologic anomalies (Callan et al, 1990; Nicolaides et al, 1992a, 1992b; Snijders et al, 1995).