Prenatal Imaging and Intervention

Chapter 113


Prenatal Imaging and Intervention




Overview


Fetal genitourinary system abnormalities encompass a wide spectrum of disorders varying in degrees of severity and include developmental anomalies, obstructive lesions in the urinary tract, and renal parenchymal diseases. The prenatal evaluation of the genitourinary system includes assessment of the amniotic fluid, the kidneys, the bladder, and associated anomalies. Amniotic fluid is important for normal fetal development, particularly the fetal lungs. In the first trimester, amniotic fluid forms as a dialysate of maternal serum. Fetal urine production begins by 12 weeks of gestation and becomes the major source of amniotic fluid by 16 weeks. Fetal urine production is typically 120 mL/day at 20 weeks and increases gradually to 1200 mL/day by term. The fetus voids every 30 to 60 minutes. The bladder should be visualized at least once during an examination.1,2


Identification of the fetal urinary bladder is attempted in the first trimester ultrasound, and more detailed evaluation of the kidneys and bladder is a fundamental part of the second trimester fetal anatomic assessment. Complex cases such as exstrophy, confusing duplex collecting systems, and obstructive masses may be further evaluated by fetal magnetic resonance imaging (MRI).3–5 Sonographic fetal anatomic assessment may be limited in the setting of oligohydramnios, and MRI may be particularly useful. Prenatal findings may influence the further pursuit of genetic and laboratory testing or surgical intervention and may guide the delivery plan.



Normal Fetal Genitourinary Tract






Overview and Imaging: Fetal kidneys can be identified at 13 weeks of gestation. Normal renal sizes at various gestations (mean kidney length in millimeters is slightly longer than weeks of gestation) are shown in Table 113-1.6,7 Corticomedullary differentiation is apparent by 20 weeks of gestation (Fig. 113-1). Renal calyces and ureters typically are not seen on ultrasound unless they are pathologically dilated,8 and the renal pelvis is only readily visualized once the anterior-posterior diameter exceeds 2 mm.9 The fetal bladder can be visualized at 10 weeks of gestation. The bladder wall should not measure more than 3 mm. Bladder wall thickening may be a sign of outlet obstruction.10,11




As mentioned earlier, the amniotic fluid volume is a reflection of renal health. Oligohydramnios (defined by an amniotic fluid index less than 8 cm)12 may be seen in the setting of placental insufficiency, intrauterine growth retardation, chromosomal abnormality, premature rupture of membranes, post-dates gestation, or urologic pathology (Box 113-1).13




Renal Parenchymal Disorders






Overview: Fetal renal echogenicity may be abnormally increased as a normal variant but more often reflects an underlying abnormality. The fetal kidney is considered echogenic if sonographically it is brighter than the fetal liver. Echogenic kidneys are observed in approximately 1 to 2 per 1000 fetal ultrasounds, and approximately 10% of cases of renal malformations include echogenic kidneys.14,15 Etiologic considerations for the echogenic kidney are summarized in Box 113-2.1624 If echogenic kidneys are identified and the amniotic fluid volume is normal, the findings suggest nonlethal renal disease. Echogenic kidneys with oligohydramnios portend a very poor prognosis.25




Cystic Renal Disease






Overview: Congenital cystic renal diseases occur in about 2 to 4 in 1000 live births and include autosomal-dominant and autosomal-recessive polycystic kidney disease (Fig, 113-2 and Video 113-1), multicystic dysplastic kidney (Fig. 113-3), cystic glomerulopathies, and other cystic renal dysplasias. The complex pathogenesis of these disorders involves an abnormal orchestration of transcription and growth factor expression (Table 113-2), as well as ciliopathies affecting the renal tubule primary cilium (Table 113-3).2629 Structural and functional abnormalities in primary cilia contribute to various cystic phenotypes that involve not only the kidney but the liver and pancreas as well.27







Imaging: These types of cystic renal dysplasias may be demonstrated in the fetus either by ultrasound or MRI.27,3037 Fetal sonographic evaluation of the kidneys should include renal length, overall echogenicity, preservation of corticomedullary differentiation, and presence of macroscopic cysts. Renal size may be considered abnormally increased if it is ≥2 standard deviations (moderately enlarged) or ≥4 standard deviations (markedly enlarged) greater than expected for gestational age.6 Renal echogenicity greater than that of the liver or spleen is abnormal (Fig. 113-4), and corticomedullary differentiation may be lost or reversed if renal parenchymal disease is present.27 MRI also readily shows abnormalities in renal size and detects renal cysts. Dysplastic parenchyma typically increases the T2-weighted signal (e-Fig. 113-5).






Approach to Diagnosis: A differential diagnostic approach to fetal echogenic renal cystic diseases is summarized in Table 113-4. If macroscopic cysts are observed without abnormal renal echogenicity, possible etiologies will depend on the number of cysts observed. Visualization of a single renal cyst should prompt consideration of a cystic tumor, a duplex collecting system with cystic dysplasia of the upper pole, urinoma, asymmetric presentation of autosomal-dominant polycystic kidney disease (ADPKD), or an isolated cyst in an otherwise normal kidney. Multiple cysts may be seen with multicystic dysplastic kidney, ADPKD, tuberous sclerosis complex, and TCF2 gene mutation–associated nephropathy.




Renal Tumors






Overview and Imaging: A solid, rounded echogenic mass in the fetal kidney (Fig. 113-6) is most likely to be a congenital mesoblastic nephroma (CMN), also known as a leiomyomatous hamartoma or a fetal renal hamartoma, although a Wilms tumor may present in the fetus as well.38 Both tumor types can replace the kidney or be localized. Tumors may be vascular with cystic areas as a result of hemorrhage and cystic degeneration. Polyhydramnios may occur (it is seen in 40% of renal tumor cases) and may lead to premature labor. Although the CMN is benign, a low-grade malignancy cannot be excluded, and resection is recommended. In one review of 28 prenatally diagnosed renal masses, 26 were CMNs and two were Wilms tumors (stage 1). The two children with Wilms tumors were followed up and remained disease free at ages 4 and 5 years.39




Antenatal Hydronephrosis






Overview: A commonly seen fetal renal abnormality is pyelectasis (dilatation of the fetal renal collecting system), which is seen in 1% to 5% of screened fetuses.40–46 Numerous factors influence renal pelvis dilation, including maternal hydration, fetal bladder filling, or the presence of an extrarenal pelvis (Fig. 113-7). Fetuses with trisomy 21 have an increased incidence of mild pyelectasis (>4 mm in the second trimester).47 Other anomalies or risk factors should be present before recommending an amniocentesis if mild pyelectasis is observed, because the specificity of aneuploidy with isolated pyelectasis is low.4850 Most fetuses with mild pyelectasis are normal (approximately 90% or more).43,46,51,52 Regardless of the risk for chromosomal abnormalities, pyelectasis in the fetus may signify either obstruction (Fig. 113-8 and e-Fig.113-9) or vesicoureteral reflux, and therefore follow-up later in gestation and postnatally must be considered. Causes of bilateral hydronephrosis are presented in Box 113-3.








Imaging: The degree of collecting system dilatation is based on the anterior to posterior diameter (APD) of the renal pelvis as imaged in transverse plane (Fig. 113-10). Other imaging characteristics that may be pertinent include the morphology of the collecting system (for instance, disproportionately dilated renal pelvis in a configuration suggesting a ureteropelvic junction obstruction), visualization of the ureter, and features of the renal parenchyma, such as cortical echogenicity and the presence of cortical thinning.


Dec 20, 2015 | Posted by in PEDIATRIC IMAGING | Comments Off on Prenatal Imaging and Intervention
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