Fetal Urogenital Imaging

Definition

Pyelectasis (or pelviectasis) is a fluid collection causing dilation of the fetal renal pelvis, indicating a risk for persistent postnatal renal impairment and/or need for surgical intervention.

Incidence/Epidemiology

• Renal pyelectasis is seen in 2%–5% of all pregnancies.

Etiology/Pathogenesis

• Isolated renal pelviectasis is due to

  • Ureteropelvic junction (UPJ) obstruction, secondary to incomplete canalization and/or maturation of the ureter(s)

  • Vesicoureteral reflux (VUR), particularly in male fetuses

• The incidence of postnatal VUR in prenatally detected pyelectasis is 12%–38%.

Diagnostic Features

• The fetal kidneys should be imaged in three planes if renal dilation is seen.

  • Axial—measure the anterior-posterior diameter of the renal pelves ( Fig. 25.1 ; Video 25.1 , Video 25.2 )

    

  • Sagittal—evaluate the surrounding renal parenchyma and calyces for dilation and/or cysts ( Fig. 25.2 ; Video 25.3 , Video 25.4 )

    

  • Coronal—may reveal dilated ureters, which are typically nonvisualized unless dilated ( Fig. 25.3 )

    

• There is poor consensus on systems for scoring the severity of renal dilation to define clinically significant pyelectasis (i.e., likely to need postpartum treatment).

  • Renal pelvic diameter (RPD) , measured in the anteroposterior plane on axial imaging

    • Ouzounian et al., 1996

      • ≥5 mm at any gestational age

    • Anderson et al., 1995

      • >4 mm at 16–23 weeks

      • >6 mm at 23–30 weeks

      • >8 mm at 30–40 weeks

    • Nguyen et al., 2014 (multidisciplinary consensus statement)

      • ≥4 mm at 16–27 weeks

      • ≥7 mm at 28–40 weeks

  • Society for Fetal Urology (SFU)

    • Grade 0: Normal renal pelvis without dilation (renal hilum and calyceal walls completely opposed)

    • Grade I: Mild dilation of the renal pelvis without calyceal dilation

    • Grade II: Mild dilation of the renal pelvis and a few calyces visible

    • Grade III: Moderate dilation of the renal pelvis with uniformly dilated calyces

    • Grade IV: Severely dilated renal pelvis and calyces with cortical thinning

  • Urinary tract dilation (UTD) classification system (multidisciplinary consensus statement, 2014), improves inter-observer reliability over SFU grading ^,

    • Anteroposterior renal pelvic diameter (≥4 mm at 16–27 weeks, ≥7 mm at 28–40 weeks)

    • Calyceal dilation

    • Renal parenchymal thickness

    • Renal parenchymal appearance (echogenicity, corticomedullary differentiation, cysts)

    • Ureteral dilation (ureters are normally not typically visualized prenatally)

    • Bladder abnormalities (wall thickening, ureterocele, dilated posterior urethra)

    • Unexplained oligohydramnios, suspected to be of genitourinary cause

Differential Diagnosis

• Transient physiologic urinary tract dilation (50%–70%)

  • Maternal hydration status may affect renal dilation.

  • A small amount of fluid (≤3 mm) seen in the second trimester is likely physiologic, but may warrant further imaging in the third trimester, depending on the appearance of the calyces.

• UPJ obstruction (10%–30%)

• Vesicoureteral reflux (10%–40%)

• Ureterovesical junction (UVJ) obstruction—ureter(s) dilated (5%–15%)

• Bladder outlet obstruction (posterior urethral valves, PUV)—bladder dilated (1%–5%)

• Multicystic dysplastic kidney (2%–5%)

• Duplicated renal collecting system with dilated upper or lower pole moiety with reflux and/or ureterocele (0.5%–1%)

• Isolated renal cyst (rare)

Associated Anomalies

• Caliectasis (distended renal calyces) implies a greater degree of renal dilation, with hydronephrosis being the combination of pyelectasis plus caliectasis ( Fig. 25.4 ; Video 25.5 ), and increases the likelihood of need for postnatal surgery and/or long-term renal compromise.

  

• Ureteromegaly and/or distended bladder with UVJ obstruction and/or bladder outlet obstruction.

• Oligohydramnios if severe bilateral ureteral or bladder outlet obstruction (e.g., posterior urethral valves).

• Aneuploidy (trisomy 13, 18, 21; triploidy)

  • As an isolated finding, pelviectasis does not increase the risk of aneuploidy; however, the finding of pyelectasis should prompt careful sonographic evaluation of the fetus for other minor markers for aneuploidy.

Prognosis

• Long-term prognosis depends on the presence of coexisting anomalies or aneuploidy, and whether unilateral or bilateral kidneys are affected.

• Approximately 90% of pelviectasis seen in midtrimester ultrasound resolves during the antenatal or early neonatal period.

• Persistent renal dilation is associated with postnatal renal pathology (UPJ, VUR, PUV)

  • ≤7 mm, 11.9%

  • 7-10 mm, 45.1%

  • >10 mm, 88.3%

• Approximately one-third of fetuses with persistent moderate to severe hydronephrosis (≥7 mm or with associated caliectasis) require postnatal urologic surgery. ^,

• Untreated vesicoureteral reflux can lead to recurrent renal infections, parenchymal scarring, and ultimate renal failure. ^,

Antenatal Management

• Consider amniocentesis or noninvasive prenatal testing for karyotype only if additional ultrasound abnormalities are seen.

• Repeat ultrasound assessment of the fetal kidneys in the third trimester to monitor progression/resolution.

  • Among fetuses with >7 mm renal dilation, approximately 40% will resolve or improve over the course of the pregnancy, 50% will remain unchanged, and 10% will worsen.

  • With severe renal dilation (>15 mm), in utero resolution is unlikely ( Fig. 25.5 ).

    

• Consider referral to a pediatric urologist for prenatal consultation if

  • Renal pelvis >10 mm

  • Associated caliectasis

  • Additional renal findings

• If there is associated oligohydramnios, fetal nonstress and/or biophysical profile testing, twice weekly, beginning at 32–34 weeks.

• Delivery in a tertiary care facility with pediatric urology availability required only if severe obstruction suspected prenatally.

• Cesarean delivery is reserved for usual obstetric indicators.

• Notify the pediatricians of the suspected renal dilation to ensure adequate postnatal evaluation and treatment as appropriate.

Neonatal Management

• Renal ultrasound should be performed ≥48 hours following birth, earlier if prenatal oligohydramnios, high-grade renal dilation, or suspected lower urinary tract obstruction.

• If ≥7 mm renal pelvic dilation, or other renal anomalies, further investigation may be warranted

  • Voiding cystourethrogram ( Figs. 25.6 and 25.7 )

    
    

  • Intravenous pyelography

  • MRI

  • Nuclear medicine renography

• Antibiotic prophylaxis, particularly if reflux is suspected.

• Follow-up renal ultrasound at 3-month intervals.

• Pediatric urology consultation.

Key Points

• The degree of renal dilation necessary to diagnose fetal pyelectasis depends on the gestational age at which the renal pelvis is measured.

• The most common causes for fetal renal pyelectasis are ureteropelvic junction obstruction and vesicoureteral reflux.

• Although pelviectasis or hydronephrosis is a “minor marker” for aneuploidy, as an isolated finding, it does not increase the risk of aneuploidy.

• Most fetal pyelectasis resolves in the antenatal or early neonatal period.

Definition

Echogenic kidneys are abnormally bright-appearing on ultrasound, indicative of abnormal renal parenchyma and suggestive of abnormal function.

Incidence/Epidemiology

• Autosomal dominant polycystic kidney disease (ADPKD), also known as “adult” polycystic kidney disease (PKD): frequency is 1:1000 live births. ^,

• Autosomal recessive polycystic kidney disease (ARPKD), also known as “infantile” PKD: frequency is 1:20,000 live births. ^,

• Obstructive cystic dysplasia: frequency is 1:2500 live births.

Etiology/Pathogenesis

• Primary cilia extending from the surface of renal tubular epithelial cells regulate cell proliferation and differentiation in the developing kidneys. Structural or functional defects of primary cilia can lead to cystic phenotypes ( Table 25.1 ). ^, Abnormal ciliary function in other organs in the body (e.g., brain, liver, lungs) potentially produce cystic changes and other malformations in various syndromes (e.g., hepatorenal fibrocystic disease).

  Table 25.1

  Inherited Ciliary Fibrocystic Renal Diseases Detectable on Prenatal Ultrasound

  Modified from Talati AN et al. Prenatal genetic considerations of congenital anomalies of the kidney and urinary tract (CAKUT). Prenat Diagn. 2019;39:679; and Avni FE et al. Imaging and classification of congenital cystic renal diseases. Am J Roentgenol. 2012;198:1004.

  Disorder	Gene(s)	Inheritance	Prenatal Ultrasound Findings
  Autosomal dominant polycystic kidney disease (ADPKD)	PKD1, PKD2	Autosomal dominant	Bilateral echogenic kidneys, increased corticomedullary differentiation
  Autosomal recessive polycystic kidney disease (ARPKD)	PHKD1	Autosomal recessive	Bilateral enlarged cystic kidneys, decreased corticomedullary differentiation
  Meckel-Gruber syndrome	MKS1, TMEM216, TMEM67, RPGRIP1L, CEP290, CC2D2A	Autosomal recessive	Multicystic enlarged kidneys, occipital encephalocele, polydactyly, possible oligohydramnios
  Bardet-Biedl syndrome	BBS1→12, MSK1, TMEM67, CEP290	Autosomal recessive	Large, hyperechogenic kidneys, polydactyly, possible genital anomalies
  Oro-facial-digital syndrome	OFD1	X-linked dominant	Polycystic kidneys, brachydactyly

• ADPKD is caused by mutations in the PKD1 (85%) or PKD2 (15%) genes, which encode an abnormal protein, polycystin, that upregulates cellular proliferation, with large cysts forming throughout the nephron and concurrent interstitial fibrosis. ^,

• ARPKD is due to mutations in the PKHD1 gene, which encodes another abnormal protein, fibrocystin, that produces innumerable corticomedullary microcysts in the collecting tubules, with subsequent interstitial fibrosis. ^,

  • Over 300 mutations of the PKHD1 gene have been identified.

  • Carrier rate of recessive PKHD1 gene mutation is 1:70.

  • Most cases of ARPKD result from two heterogeneous PKHD1 gene mutations.

• Obstructive cystic dysplasia results from pressure with chronic first- or second-trimester urinary tract obstruction (e.g., posterior urethral valves) or persistent vesicoureteral reflux.

  • Pressure produces cystic dilation of primitive collecting duct system, damaging the nephron, developing nodules of metaplastic cartilage and renal dysplasia.

  • Eventually, the damaged nephric unit involutes.

Diagnostic Features

• The fetal kidneys should be measured in the midsagittal and axial planes, and compared with standard reference tables; renal volume can be calculated using 2D and 3D ultrasound.

  • Kidney size may be normal at <20 weeks’ gestation.

  • Assess sonographic appearance of bilateral kidneys including the renal pelves for dilation, and surrounding renal parenchyma for echogenicity, cysts, and corticomedullary differentiation.

  • Fetal MRI using T2- and T1-weighted sequences and/or MR urography may be warranted to better delineate renal anatomy if ultrasound imaging limited by large body habitus and/or severe oligohydramnios ( Video 25.6 ). ^,

• To be considered echogenic, the kidneys must appear brighter than the liver.

  • Increased echogenicity is due to multiple microscopic cysts ( Fig. 25.8 ), dysplasia, and/or multiple interfaces of the dilated tubules. ^,

    

  • See Fig. 25.9 for diagnostic algorithm.

    

• ADPKD ^,

  • Moderately enlarged echogenic kidneys with increased corticomedullary differentiation ( Fig. 25.10 ; Video 25.7 ); discrete cysts atypical

    

  • Normal bladder

  • Normal amniotic fluid

  • Parent(s) with renal cysts and/or echogenic kidneys

• ARPKD

  • Massively enlarged (>5 SD above mean) echogenic kidneys with loss of corticomedullary differentiation ( Fig. 25.11 ; Video 25.8 )

    

  • Small or absent bladder

  • Hepatomegaly possible

  • Oligohydramnios or anhydramnios (80% of cases)

  • Affected sibling(s) possible

• Obstructive cystic dysplasia

  • Small echogenic kidneys with thin renal cortex and poor corticomedullary differentiation ( Fig. 25.12 ; Video 25.9 )

    

  • Oligohydramnios if bilateral

  • Early ultrasound may show dilated bladder, ureter, kidney

Differential Diagnosis

• Multicystic dysplastic kidney (usually unilateral with multiple noncommunicating cysts of varying size)

• Glomerulosclerosis

• Tuberous sclerosis (often associated with cardiac rhabdomyomas)

• Infection (e.g., cytomegalovirus [CMV])

• Renal vein thrombosis

• Adrenal nephroblastoma or hematoma

• Normal variant

Associated Anomalies

• Meticulous assessment of all fetal organs is needed to rule out other congenital malformations and genetic syndromes; ADPKD and ARPKD are typically isolated anomalies.

• Syndromes associated with enlarged echogenic kidneys

  • Beckwith-Wiedemann syndrome

  • Meckel-Gruber syndrome ( Fig. 25.13 ; Video 25.10 )

    

  • Finnish-type nephrotic syndrome (high alpha fetoprotein)

  • Perlman syndrome

• VACTERL association ( v ertebral abnormality, a nal atresia, c ardiac defect, t racheo e sophageal fistula, r enal agenesis, and radial l imb abnormality)

• Aneuploidy (trisomy 13, 18, 21)

• Oligohydramnios

• Potter syndrome (oligohydramnios sequence) with pulmonary hypoplasia with ARPKD

Prognosis

• Hyperechoic renal parenchyma is associated with abnormal renal function in 80% of cases.

• Short-term prognosis depends most on amniotic fluid.

  • Oligo- or anhydramnios with large echogenic kidneys (>4 SD) in second trimester is associated with a poor prognosis, with neonatal mortality due to pulmonary hypoplasia and/or renal failure.

    • ARPKD is associated with 30%–40% neonatal mortality due to pulmonary hypoplasia; however, 1-year survival rate for those surviving the neonatal period is approximately 95%.

  • Normal or increased amniotic fluid is associated with a good prognosis for neonatal survival.

• Nonisolated hyperechoic kidney(s) with extrarenal structural abnormalities carry a poor prognosis for survival (75% perinatal mortality).

• Long-term survivors will likely develop hypertension, recurrent urinary tract infections, and end-stage renal failure requiring dialysis and/or transplantation (in childhood with ARPKD, in later adulthood with ADPKD). ^,

  • ADPKD

    • May not be discovered until adulthood with hypertension and end-stage renal disease (ESRD).

    • With ADPKD, 85% have normal blood pressure and renal function at birth; approximately two-thirds will remain without hypertension, proteinuria, or renal insufficiency by 5 years of age.

    • Earlier onset of hypertension and progression to ESRD is likely with PKD1 mutation compared with PKD2 mutation.

  • ARPKD

    • 60% require dialysis and/or renal transplantation by 10 years of age.

    • 7% need liver transplantation due to concomitant hepatic fibrosis and portal hypertension (Caroli disease).

Antenatal Management

• Amniocentesis with microarray testing or noninvasive prenatal testing should be considered for karyotype, particularly if there are other associated anomalies and/or to test for genetic markers ( PKD1, PKD2, PKHD1 ).

• T oxoplasmosis, o ther agents, r ubella, c ytomegalovirus, and h erpes simplex (TORCH) titers if other findings suspicious for intrauterine infection.

  • If amniocentesis is performed, polymerase chain reaction for CMV.

• Fetal echocardiogram to rule out associated congenital heart defects.

• Referral to genetic counselor or geneticist to obtain detailed family history of inherited renal disease.

• Parental renal ultrasound to assess possible inherited renal disease.

• Consider termination if ARPKD and/or anhydramnios is suspected.

• Serial ultrasound examinations every 2–3 weeks to monitor fetal growth, amniotic fluid, and general fetal condition.

• Fetal nonstress and/or biophysical profile testing, twice weekly, beginning at 32–34 weeks.

• Prenatal neonatology and pediatric nephrology consultation to discuss postnatal management and prognosis.

• Delivery at a tertiary care facility is recommended.

• Cesarean delivery may be necessary to prevent abdominal dystocia from massively enlarged kidneys.

Neonatal Management

• Initial respiratory support may necessitate mechanical ventilation.

  • Milder forms of pulmonary hypoplasia may benefit from high-frequency ventilation.

  • Associated pulmonary hypertension may require inhaled nitric oxide and/or extracorporeal membrane oxygenation.

• Assess kidneys for function and reflux

  • Renal ultrasound

  • Voiding cystourethrogram

  • Nuclear medicine renography

• Monitor serum creatinine, blood urea nitrogen, and electrolytes.

• Consider genetic testing (PKD1, PKD2, PKHD1).

• Pediatric nephrology consultation.

• Uni- or bilateral nephrectomy of massive kidney(s) may be warranted in ARPKD to improve ventilation and nutrition and to facilitate peritoneal dialysis.

• Decision to offer or withhold resuscitation and/or dialysis depends on degree of renal dysfunction, other associated abnormalities/fetal condition, and parental desires.

Key Points

• Echogenic kidneys appear brighter than the fetal liver.

• Family history is an important component in evaluating the patient carrying a fetus with echogenic kidneys.

• Prognosis depends on fetal kidney function, as evidenced by amniotic fluid volume; low or absent amniotic fluid portends a poor prognosis.

• Long-term survivors are likely to have end-stage renal disease, necessitating dialysis and/or transplantation.

Definition

Multicystic dysplastic kidney (MCDK) is a nonhereditary cystic renal disease in which normal renal tissue is replaced by variably sized cysts surrounded by abnormally functioning dysplastic renal parenchyma.

Incidence/Epidemiology

• Unilateral MCDK is seen in 1:4000 live births; bilateral MCDK is seen in 1:12,000 live births.

• Male to female ratio approximately 2:1, ^, but female fetuses more likely to have bilateral MCDK and other nonrenal anomalies.

Etiology/Pathogenesis

• Pathogenesis of MCDK disease is likely the result of obstruction in the metanephric stage of embryogenesis, causing atresia of the ureteral bud with subsequent enlarged and noncommunicating collecting tubules forming cysts. ^,

• Gene mutations associated with MCDK include CHD1L, GATA3, HNF1B, PAX2, ROBO2, and SALL1.

Diagnostic Features

• Multiple irregular noncommunicating echolucent cysts seen within the kidney ( Fig. 25.14 ; Video 25.11 , Video 25.12 , Video 25.13 ), with the intervening parenchyma mildly echogenic due to dysplasia from compression by the cysts. ^,

  

• Mildly enlarged renal length due to parenchymal cysts ( Fig. 25.15 ).

  

• Normal reniform outline is lost.

• Normal-appearing bladder and ureters with unilateral MCDK (i.e., no findings to suggest obstructive uropathy); the bladder may not be visible if bilateral MCDK.

• Sonographic appearance can change over time.

Differential Diagnosis

• Autosomal dominant (“adult”) or autosomal recessive (“infantile”) polycystic kidney disease

• Obstructive cystic dysplasia

• Duplicated renal system ( Fig. 25.16 )

  

• Tuberous sclerosis (often associated with cardiac rhabdomyomas)

• Simple isolated renal cyst(s)

• Pyelectasis/hydronephrosis

• Ureteromegaly

• Adrenal hemorrhage

Associated Anomalies

• Approximately 25%–35% of contralateral kidneys have other urologic defects. ^,

  • Vesicoureteral reflux

  • Ureteropelvic junction obstruction

  • Ureterovesical junction obstruction

  • Ectopic ureter

  • Renal agenesis

  • Pelvic kidney ( Fig. 25.17 )

    

• Compensatory hypertrophy of the normal contralateral kidney is typical with the mean renal length being >2 SD above the mean by age 3; lack of compensatory hypertrophy may indicate risk of renal insufficiency. ^,

• Oligohydramnios or anhydramnios may be present if bilateral disease.

• Approximately 25%–35% of cases will have associated nonrenal abnormalities, including cardiac, gastrointestinal, spine, extremity, central nervous system, and facial anomalies; single umbilical artery (two-vessel cord) is common. ^,

• Bilateral disease increases the risk of extrarenal anomalies (50% vs. 16%), genetic syndromes (12% vs. 1%), and major anorectal anomalies (7% vs. 1%).

• MCDK can be seen in over 80 syndromes, including VATER syndrome ( v ertebral defects, imperforate a nus, t racheo e sophageal fistula, r enal defects), VACTERL association, and Meckel-Gruber syndrome.

• Aneuploidy possible, but typically not with isolated unilateral MCDK ^; bilateral disease and/or concomitant nonrenal abnormalities increase risk of aneuploidy.

• On microarray analysis, approximately 15% of MCDK cases exhibit pathogenic copy number variants. ^,

Prognosis

• Isolated unilateral MCDK ( Fig. 25.18 ) has good prognosis, though the cystic kidney is expected to be nonfunctional.

  

• Involved kidney involutes during childhood

  • 35% by 2 years old

  • 50% by 5 years old

  • 60% by 10 years old

• Bilateral MCDK or unilateral MCDK with contralateral renal agenesis ( Fig. 25.19 ) has a poor prognosis, and is likely lethal. ^,

  

• Oligo- or anhydramnios is strongly associated with low likelihood of survival >7 days.

• Long-term complications include vesicoureteral reflux, recurrent infection, and hypertension; however, the risk of hypertension with MCDK is low (<5%), particularly if unilateral disease (hypertension rate 5:1000; 95% CI, 1.9–11.7:1000).

• Malignant degeneration (e.g., Wilms tumor) unlikely.

Antenatal Management

• Amniocentesis with microarray testing or noninvasive prenatal testing should be considered for karyotype, particularly if bilateral disease or other nonrenal anomalies are present.

• Consider fetal echocardiogram to rule out heart defect.

• Consider termination if bilateral MCDK and/or anhydramnios is suspected.

• Serial ultrasound examinations to monitor fetal growth, amniotic fluid, and renal appearance.

• Fetal nonstress and/or biophysical profile testing, twice weekly, beginning at 32–34 weeks.

• Prenatal neonatology and pediatric urology consultation to discuss postnatal management and prognosis.

• Delivery in a tertiary care facility if bilateral disease, oligohydramnios, other nonrenal anomaly, or uncertain diagnosis.

• Cesarean delivery is reserved for usual obstetric indications.

Neonatal Management

• Assess kidneys for function and reflux

  • Renal ultrasound ( Video 25.14 )

  • Voiding cystourethrogram

  • Nuclear medicine renography

• Antibiotic prophylaxis if vesicoureteral reflux is present.

• Pediatric urology consultation.

• Follow-up renal ultrasound at 3- to 6-month intervals.

• Surgical excision of nonfunctional kidney to reduce the incidence of hypertension is controversial; an individualized approach is warranted with a severely dysplastic kidney. ^,

• With confirmed bilateral MCDK and anhydramnios, withdrawal of life support and comfort care for the neonate is appropriate.

Key Points

• Multicystic dysplastic kidney (MCDK) is identified by multiple noncommunicating cysts within the kidney.

• Bilateral disease increases the risk of extrarenal abnormalities, genetic syndromes, and oligohydramnios.

• Unilateral MCDK offers a good long-term prognosis; bilateral disease with low or absent amniotic fluid predicts a poor and possibly lethal prognosis.

Definition

Posterior urethral valves (PUV) are membranes within the posterior urethra that cause bladder outlet obstruction with subsequent hydronephrosis and pressure-induced renal dysplasia.

Incidence/Epidemiology

• Lower urinary tract obstruction (LUTO) is seen in approximately 3:10,000 live births, two-thirds of which are caused by PUV. ^,

• PUV occurs almost exclusively in males; in females, urethral atresia may present similarly with complete bladder outlet obstruction.