Antenatal Hydronephrosis

Before the mid-1980s, diagnosing hydronephrosis was a postnatal occurrence, and pediatricians were forced to be reactive in our evaluations and therapy. With the advent of improved ultrasonography (US) equipment prenatally, pediatricians working alongside radiologists, obstetricians, and urologists were tasked with prenatal abnormalities and determining what was pathologic and what was developmentally typical for a fetus. One such abnormality is antenatal hydronephrosis (AHN), which is the dilatation of the renal collecting system. Routine maternal US detects AHN after the first trimester in 1%–5% of all pregnancies, rendering AHN as the leading cause of abnormal prenatal US.Although the condition is most frequently transient or physiologic (50%–75% cases), it can also be representative of obstructive or nonobstructive abnormalities, including ureteropelvic junction obstruction (UPJO), ureterovesical junction obstruction, vesicoureteral reflux, posterior urethral valves (PUVs), and multicystic dysplastic kidney as well as syndromic and/or genetic conditions. These causes, which may be associated with significant morbidity, are typically confirmed postnatally with examination and additional imaging including renal-bladder US, voiding cysto-urethrography, and diuretic renography, with additional investigations offered for specific causes. Many cases that persist postnatally will be low grade, of these some will resolve during infancy and others will require intervention to mitigate complications of AHN like urinary tract infections, urinary stones, and significant renal dysfunction. The grading systems we will discuss along with appropriate communication among the obstetrician, pediatrician, and potentially a urologist will aid to differentiate between physiologic and pathologic AHN, as this early identification is critical in ensuring rapid identification and intervention.The simplest grading system uses anteroposterior renal diameter (APD) along with gestational age to risk stratify urinary tract dilatation (UTD) as mild, moderate, or severe. The APD, measured in the transverse plane, determines degrees of renal pelvis dilatation. Abnormal values indicative of prenatal HN (PHN) are generally accepted as at least 4 mm in the second trimester and at least 7 mm in the third (Table 1).Another classification system was developed at a multidisciplinary consensus meeting, held in March 2014 resulting in the UTD classification system. For prenatally detected hydronephrosis, UTD classification uses US findings including APD, caliectasis, parenchymal thickness/appearance, ureterectasis, bladder abnormalities, and unexplained oligohydramnios to stratify fetuses into low or increased risk categories based on the presence of the most concerning finding (Table 2).The UTD classification is divided into low risk (UTD A1) and increased risk (UTD A2–A3). According to UTD A1, for those diagnosed before 32 weeks, follow-up prenatal US can be done after 32 weeks to monitor progression. However, this raises the question, “If resolution of PHN is demonstrated, is cessation of further follow-up appropriate?” There are mixed recommendations for postnatal follow-up in the transient group, with some suggesting that as the risk of postnatal pathology is low for prenatally resolved HN cessation of follow-up is reasonable, and others citing that, despite this, all patients should be provided with postnatal US.1 Ultimately, there is yet to be an understanding of how to distinguish those that will have progression or late worsening and those that will not. Hence, until further study is accomplished to answer this question, it is imperative to provide postnatal follow-up. This does highlight the importance of collaboration and communication between the obstetrician and pediatrician to provide the details regarding the timing and results of prenatal US.For those in UTD A2-3, it is recommended to have a repeat US during the prenatal period and follow-up postnatally. Regardless of whether the initial postnatal US is normal, it is imperative to perform a second US as an abnormal second US following normal initial US was found in 45% of children, and the risk of recurrence or worsening of HN was noted in 15% of children. It goes without saying that risk of uropathy is worse in those with persisting postnatal HN. The initial postnatal US should be performed after 48 hours, as the degree of UTD can be falsely reduced by neonatal oliguria and dehydration of infants. The second postnatal US should be completed within 1–6 months after the initial US.Frequency of prenatal US surveillance has had limited study; however, it is generally appreciated that those with higher-severity HN, bilateral disease, or evidence of lower urinary tract obstruction (LUTO) should have more frequent US (every 4–6 weeks depending on the severity of the findings) as opposed to those without, who may be satisfactorily monitored with a second US in the third trimester.There are several proposed exceptions to the delayed timeframe of postnatal surveillance, requiring earlier initial evaluation (<48 h) and more frequent follow-up for those with suspected obstructive uropathies or bilateral disease.Concerns for those with the renal pathology that may progress and develop obstruction requiring surgery and renal dysplasia, as well as UTIs requiring antibiotic treatment, drive the need for surveillance and further examination. Considering that high-grade HN has a high likelihood of progression requiring surgical intervention, that childhood UPJO typically presents by 18 months, and that most low-grade HN will resolve by 2 years of age, follow-up may be reasonably provided until this age. Deterioration past this age is estimated to be 2%; however, it is still not clear how to identify those that will progress to obstruction, hence the need to educate patient caregivers regarding obstruction and UTI presentation on discharge.Treatment options for PHN depend on etiology, timing, and risk of postnatal pathology.In utero interventions, such as vesicoamniotic shunting, can be considered in the mid–second trimester for fetuses with suspected LUTO. This intervention does not come without risk but does offer improvement of perinatal survival for those with severe obstruction. As such, involvement of tertiary centers and pediatric urologist counseling is necessary in these cases.Alongside classification systems, the clinician must have a thorough understanding of risk factors for postnatal pathology. The main risk factors for postnatal pathology are those who are diagnosed in the second trimester and persist into the third trimester; demonstrate progression; present with bilateral disease and/or evidence of LUTO, such as oligohydramnios and thick-walled or dilated bladder. The presence of those risk factors should influence the frequency of prenatal and postnatal care and the need for involvement of a pediatric urologist.The presence of antenatal abnormalities of any kind can cause significant anxiety for the families of these infants. Therefore, we must strive to provide them with an understanding of what signs they should look out for to seek urgent medical attention and to develop a plan for evaluation and treatment. This highlights the need for a widely adopted grading system as well as the ongoing analysis of grading systems to ensure appropriate application and to reduce clinical practice variation with respect to appropriate prenatal and postnatal care.Comments: This “In Brief” article highlights the benefits and complexities that prenatal US brings in the variety of renal abnormalities that may be identified and the challenges in determining as soon as possible which abnormal findings may resolve and lead to eventual normal functioning and those that lead to long-term morbidity and renal dysfunction. I once cared for a patient in my practice who was diagnosed with PUVs postnatally, leading to renal dysfunction and his need to have 2 renal transplants during his first 20 years of life. I wish the US technology had been available then to help prevent the degree of renal dysfunction he experienced. Yet, future research is needed to better inform the predictive properties of these grading systems. Consider the importance of acknowledging the uncertainty of future diagnosis and balancing important factual information while supporting families through the inevitable anxiety that may result from identification of these abnormalities prenatally. While focusing on the continuum from the prenatal care, provided by the obstetrician, to the postnatal care, that is the responsibility of the pediatrician, the urologist, when involved, bridges the life continuum and is essential in the communication loop. I have always been an advocate of prenatal pediatric visits, and this is an example of when they can be beneficial. The opportunity for a pediatrician to meet with parents during their pregnancy can alert the pediatrician to potential issues that are relevant at the time of birth and the early postnatal period and allow the pediatrician and parents to establish an earlier trusting relationship and “hit the ground running” in collaboration with urologists should more urgent treatment be needed.Janet Serwint, MDAssociate Editor, In Brief