Perinatal urinary tract dilation (UTD), or hydronephrosis, occurs in approximately 1% of all pregnancies and may be attributed to transient dilation, vesicoureteral reflux (VUR) or an obstructive uropathy. There is a lack of evidence regarding the use of continuous antibiotic prophylaxis (CAP), postnatal imaging and the optimal interval for followup for these patients. The Urinary Tract Dilation classification system was developed in 2014 by specialists in multiple fields in an effort to standardize nomenclature for the grading of UTD across the prenatal and postnatal continuum. Additionally, a risk stratification system was developed to guide evidence-based recommendations for further evaluation and management.¹

UTD Classification System

The UTD classification system combines the objective and subjective characteristics of the commonly used Society for Fetal Urology (SFU) grading system and the anteroposterior renal pelvic diameter (APD) system. Measured ultrasonographic parameters include APD, central calyceal dilation, peripheral calyceal dilation, appearance of kidney parenchyma, appearance of ureters and bladder, and unexplained oligohydramnios (see figure). Patients are stratified into three levels of risk: low (UTD P1), intermediate (UTD P2) and high (UTD P3).

Figure 1. Postnatal Urinary Tract Dilation Classification System. P1, low risk. P2, moderate risk. P3, high risk. AP, anteroposterior. C, cysts. Ec, abnormal echogenicity. NL, normal. Th, thinning. ©2021, Children’s Hospital of Philadelphia, All Rights Reserved.

Postnatal recommendations are based on this assigned level of risk and guide the use of CAP, need for subspecialist referral, appropriate interval of surveillance renal ultrasonography and the need for additional imaging (table 1). A shared decision-making model is endorsed, with guidance based on individualized patient risk and benefit assessment.

Postnatal Ultrasonographic Evaluation of Antenatal UTD

Renal bladder ultrasonography (RBUS) is the initial imaging of choice in a child with prenatally diagnosed UTD because of its availability, low cost, non-invasiveness and lack of radiation. For optimal assessment of UTD, RBUS should be performed after 48 hours of life, as studies during the first 48 hours might underestimate the degree of dilation. RBUS lacks diagnostic specificity although increased dilation does correlate with increased need for surgery. The timing of subsequent RBUS after discharge should be based on the degree of dilation of the initial postnatal RBUS.

Risk of Urinary Tract Infection (UTI) and CAP: Assessment of Individual Risk

Historically, patients with antenatal UTD were empirically prescribed CAP to prevent UTI, a practice that was not evidence based. Rates of UTI in patients with antenatal UTD range from 8%–22% with identifiable risk factors (table 2) that include high-grade hydronephrosis,² distal ureteral dilation >7 mm,³ female gender,⁴ intact foreskin,⁵ presence of obstructive uropathy and renal scarring on renal scintigraphy.⁶

Historical data on the benefits of CAP in prevention of UTI in patients with UTD are conflicting and derived from retrospective or single-center reviews. Recent data from the multicenter prospective SFU hydronephrosis registry demonstrated two important findings which include a significant benefit of CAP in those patients with ureteral dilation >7 mm³ and no benefit for isolated ureteropelvic junction-like hydronephrosis.² No studies to date have addressed the benefit of CAP on renal scarring in patients with UTD and those that have assessed benefits in subpopulations (eg VUR) are underpowered.⁷ Potential risks of long-term antibiotic prophylaxis include bacterial resistance and effects on the gut and urinary microbiome.

Some studies have shown circumcision to be an equally preventive alternative to CAP in prevention of UTI in boys with UTD.⁸ Amoxicillin is the primary antibiotic prescribed for CAP in the newborn, and after 2 months of life the most commonly prescribed antibiotic is trimethoprim-sulfamethoxazole. In general, the overall patient risk for UTI should be considered when deciding whether to initiate CAP.

Adjunct Imaging for UTD: Assessment of Individual Risk

Voiding cystourethrogram (VCUG) is the gold standard for diagnosis and grading of VUR. VUR is a known risk factor for recurrent pyelonephritis, renal scarring, hypertension and end-stage renal disease. Screening for VUR has traditionally been recommended in patients with prenatally diagnosed UTD because VUR has been reported in up to 31%; however, prenatal UTD correlates poorly with severity of VUR.

Frequent use of VCUG has been argued against given that VUR associated with prenatally diagnosed UTD has a high incidence of spontaneous resolution, and the clinical relevance of reflux in the absence of UTI with normal bladder function is unclear. Studies that assess whether or not patients with both UTD and VUR are at increased risk for UTI show conflicting results. It is largely acknowledged that diagnosis of VUR in this setting is to identify those patients at highest risk of recurrent pyelonephritis and its sequelae; UTD in isolation does not mandate performance of a VCUG in asymptomatic infants. Similar to CAP, a shared-decision making model that includes the risk of UTI should be used when determining the need for lower urinary tract imaging. A paradigm shift has taken place away from the indiscriminate use of VCUG.

Contrast-enhanced voiding urosonography is also used to detect VUR; this limits radiation and does have high sensitivity but does not allow for assessment of bladder and urethral morphology and is not available at most institutions.

Renal scintigraphy (RS) allows for the assessment of differential renal function and to differentiate between non-obstructive and obstructive UTD. A recent large review identified an APD cutoff of 15 mm to be predictive of the need for intervention.⁹ If warranted, RS should be deferred until after 6 weeks of age to allow for constitution of adequate renal blood flow. Its use should be considered in those in whom there is a high risk for decreased differential renal function (abnormal or thin parenchyma on ultrasonography). Indications for intervention include: deterioration of renal unit function to <40%, a decrease in differential renal function of >5% over time, symptoms (eg febrile UTI or feeding intolerance) or obstructive (flattened) drainage curve. Fortunately, a majority of patients with UTD will not need surgical correction and can be safely observed with serial RBUS imaging.¹⁰

In summary, the development of the UTD risk stratification system combined with recent prospective data from the SFU multicenter Hydronephrosis Registry has allowed providers to make evidence-based management decisions for patients. Recommendations for the use of CAP, timing for postnatal renal ultrasonography and use of lower urinary tract imaging for patients with antenatal UTD should be made in a shared decision-making fashion with parents. Future work for antenatal UTD is directed at earlier identification of those patients who warrant RS and ultimate surgical intervention for ureteropelvic junction obstruction.