Postnatal evaluation and management of hydronephrosis

INTRODUCTION — Hydronephrosis (dilation of the renal pelvis) is a common, readily diagnosed finding on antenatal ultrasound examination, which may be a transient benign condition or be associated with significant congenital anomalies of the kidney and urinary tract (CAKUT). The goal of postnatal management of infants with prenatally diagnosed hydronephrosis is to identify those with clinically significant CAKUT while avoiding unnecessary testing in patients with physiologic or clinically insignificant hydronephrosis.

Postnatal evaluation and management of infants prenatally diagnosed with hydronephrosis are reviewed here. Other related topics include:

●(See "Fetal hydronephrosis: Etiology and prenatal management".)

●(See "Overview of congenital anomalies of the kidney and urinary tract (CAKUT)" and "Evaluation of congenital anomalies of the kidney and urinary tract (CAKUT)".)

●(See "Clinical presentation, diagnosis, and course of primary vesicoureteral reflux" and "Management of vesicoureteral reflux".)

●(See "Congenital ureteropelvic junction obstruction".)

●(See "Primary megaureter in infants and children".)

●(See "Ectopic ureter".)

GENERAL PRINCIPLES

●Goal – The goal of the postnatal evaluation of infants with prenatally diagnosed hydronephrosis is to identify patients with congenital anomalies of the kidney and urinary tract (CAKUT) while avoiding unnecessary testing in patients with physiologic or clinically insignificant fetal hydronephrosis. The evaluation includes physical examination and imaging studies to detect abnormalities requiring postnatal intervention.

●General approach – In our practice, we use an approach that entails confirming the persistence of hydronephrosis after birth and assessing the severity (algorithm 1A-B) [1-3]. The need for additional testing (eg, voiding cystourethrogram [VCUG]) is based on results of the initial postnatal ultrasound and the presence of other risk factors such as family history of clinically significant vesicoureteral reflux or CAKUT and findings of concern for neurogenic bladder (eg, imperforate anus, spinal abnormalities, myelomeningocele). This approach limits unnecessary testing and minimizes parental/caregiver's distress. (See "Fetal hydronephrosis: Etiology and prenatal management", section on 'Congenital anomalies of the kidney and urinary tract (CAKUT)'.)

●Urgency of evaluation – It is important to recognize that neonates with an obstructed kidney rarely require surgical intervention in the first few days or weeks after birth unless it is a solitary kidney. The fetal kidneys start to make urine between the fifth and ninth week of gestation and therefore, an additional few days and/or weeks of observation after birth awaiting an accurate diagnosis rarely affects long-term outcome. In addition, the added anesthetic risk in the neonatal period needs to be taken into account. An important exception are patients with conditions that affect both kidneys, such as posterior urethral valves (PUV). These infants generally require intervention to relieve post-bladder obstruction as soon as possible after birth to preserve kidney and bladder function. (See "Management of posterior urethral valves", section on 'Postnatal management'.)

INITIAL POSTNATAL EVALUATION

Physical examination — The neonatal physical examination can identify abnormalities that are suggestive of congenital anomalies of the kidney and urinary tract (CAKUT), including the following:

●The presence of an abdominal mass that could represent an enlarged kidney due to obstructive uropathy or multicystic dysplastic kidney. (See "Congenital ureteropelvic junction obstruction" and "Kidney cystic diseases in children", section on 'Multicystic dysplastic kidney'.)

●A palpable bladder in a male infant, especially after voiding, may suggest posterior urethral valves (PUV). (See "Clinical presentation and diagnosis of posterior urethral valves".)

●A male infant with prune-belly syndrome will have deficient abdominal wall musculature and undescended testes. (See "Prune-belly syndrome".)

●The presence of outer ear abnormalities is associated with an increased risk of CAKUT. (See "Congenital anomalies of the ear", section on 'Association with kidney anomalies'.)

●A single umbilical artery is associated with an increased risk of CAKUT, particularly vesicoureteral reflux (VUR). (See "Assessment of the newborn infant", section on 'Umbilical cord'.)

●Spinal and/or lower extremity abnormalities suggesting a neurogenic bladder, which may result in hydronephrosis and dilated ureters. (See "Myelomeningocele (spina bifida): Urinary tract complications".)

Postnatal ultrasound — The preferred initial postnatal imaging study for infants with clinically significant fetal hydronephrosis is an ultrasound examination of the kidneys and bladder. Ultrasonography can detect most CAKUT anomalies without exposing the infant to radiation [4,5].

Indications — Postnatal ultrasound evaluation is warranted if the prenatal ultrasound (performed in the third trimester) had a renal pelvic diameter (RPD) of ≥10 mm. RPDs below this level are unlikely to be associated with CAKUT. (See "Fetal hydronephrosis: Etiology and prenatal management", section on 'Congenital anomalies of the kidney and urinary tract (CAKUT)'.)

Timing — In general, the postnatal ultrasound examination should be avoided in the first two or three days after birth because hydronephrosis may not be detected, due to extracellular fluid shifts that will underestimate the degree of hydronephrosis. Important exceptions to this include infants with bilateral hydronephrosis associated with ureteral dilation and/or a dilated bladder and those with a hydronephrotic solitary kidney; these infants require urgent evaluation within 48 hours of birth because of they are likely to have more clinically significant CAKUT (eg, PUV) and may require early intervention (algorithm 1A). (See "Management of posterior urethral valves".)

●Bilateral hydronephrosis associated with ureteral dilation or a dilated bladder on prenatal ultrasound – Infants with bilateral hydronephrosis associated with ureteral dilation or/or a dilated bladder on prenatal ultrasound and those with concerning findings on examination (eg, a palpable bladder in a male infant) should have the postnatal ultrasound performed within 48 hours after birth (algorithm 1A). This is because these findings suggest possible post-bladder obstruction (eg, PUV) that is best managed with prompt drainage and potential surgical correction. (See "Management of posterior urethral valves".)

If the postnatal ultrasound demonstrates persistent hydronephrosis, hydroureteronephrosis, or a dilated bladder a voiding cystourethrogram (VCUG) should be performed, as discussed below. (See 'Subsequent evaluation and management' below.)

●Hydronephrotic solitary kidney on prenatal ultrasound – If the prenatal ultrasound showed a hydronephrotic solitary kidney (RPD ≥10 mm on third trimester ultrasound), postnatal ultrasound is performed within 48 hours after birth. Significant hydronephrosis in a solitary kidney may be due to ureteral ectopy, ureterocele, and/or severe reflux. Prompt attention is required as these infants may require early intervention with a focus on preserving kidney function. (See "Ectopic ureter" and "Ureterocele" and "Management of vesicoureteral reflux".)

●Unilateral hydronephrosis on prenatal ultrasound – If the prenatal ultrasound showed unilateral hydronephrosis (RPD ≥10 mm on third trimester ultrasound) with an unaffected normal-appearing contralateral kidney, the postnatal ultrasound should be delayed until the infant has fully regained birth weight, indicating a normal volume status (usually two to four weeks of age). In our practice, we typically wait until two to four weeks of age (algorithm 1A). The rationale for delaying the ultrasound is that it allows the infant to return to a normal volume status as hypovolemia may decrease urinary flow and hydronephrosis may be missed or significantly reduced.

Severity of hydronephrosis — Severity of hydronephrosis is categorized according to the findings on postnatal ultrasound. Various grading systems are used in clinical practice, including classification according to the anterior-posterior RPD, the Society of Fetal Urology (SFU) grading system (image 1), and the urinary tract dilation (UTD) grading system [6-8]. (See "Fetal hydronephrosis: Etiology and prenatal management", section on 'Definition and grading'.)

The severity according to RPD and SFU is as follows (image 1):

●Mild hydronephrosis – RPD <10 mm, SFU grades 1 and 2

●Moderate hydronephrosis – RPD 10 to 15 mm, SFU grade 3

●Severe hydronephrosis – RPD >15 mm, SFU grade 4

Infants with SFU grades 3 and 4 hydronephrosis (image 1) are at greatest risk for significant kidney injury and may require surgical correction [4].

The UTD grading system incorporates other parameters into the classification (eg, ureteral abnormalities, bladder abnormalities, and kidney parenchymal abnormalities) which may increase the accuracy for identifying infants who are at risk of developing kidney injury [9-11]. However, it also has higher inter-rater variability as compared with the SFU system [12]. The SFU and UTD grading systems generally are more informative than the RPD measurement alone.

Regardless of the grading system used, radiologists should include descriptors that define ureteropelvic junction (UPJ)-type hydronephrosis as opposed to a megaureter or findings consistent with posterior urethral valves.

SUBSEQUENT EVALUATION AND MANAGEMENT — Decisions regarding the need for further evaluation and antibiotic prophylaxis are based upon the severity of hydronephrosis, as assessed with postnatal ultrasonography. (See 'Postnatal ultrasound' above.)

Normal ultrasound or mild hydronephrosis — Infants with a normal postnatal ultrasound examination without any evidence of any other genitourinary abnormality (ie, no calyceal or ureteric dilation, no signs of renal dysplasia or anomalies) require no further evaluation.

For infants with mild unilateral hydronephrosis on postnatal ultrasound (renal pelvic diameter [RPD] 5 to <10 mm), the need for follow-up imaging depends on when the postnatal ultrasound was performed:

●Early postnatal ultrasound – If the postnatal ultrasound was performed in the early neonatal period (within the first three weeks after birth), we suggest repeating the ultrasound at four to six months of age. This is because clinically significant hydronephrosis may not be seen on early postnatal imaging, due to relative dehydration in the early newborn period.

Prophylactic antibiotic therapy is not necessary while awaiting the follow-up ultrasound, as discussed below. (See 'Antibiotic prophylaxis' below.)

However, if the infant develops a urinary tract infection (UTI) during the observation period, they should be treated with antibiotics and should undergo further evaluation with voiding cystourethrogram (VCUG).

If the infant has no UTIs and the ultrasound findings at four to six months are improved or stable, this confirms the diagnosis of transitional hydronephrosis and further urologic follow-up is not necessary.

If the follow-up ultrasound shows worsening hydronephrosis (RPD ≥10 mm), the infant should undergo additional evaluation, as discussed below.

●Later postnatal ultrasound – If the initial postnatal ultrasound was performed after three to four weeks of age and showed only mild hydronephrosis, no further evaluation is necessary.

This approach is supported by follow-up data at two years of age and at 12 and 15 years of age that reported excellent outcomes for infants with resolved or mild hydronephrosis on postnatal ultrasound [13,14].

Persistent moderate to severe hydronephrosis — Infants with a persistent moderate to severe hydronephrosis (RPD ≥10 mm) on postnatal ultrasound generally require additional evaluation. The choice of imaging test depends on whether the ultrasound findings are concerning for possible posterior urethral valves (PUV) or vesicoureteral reflux (VUR) or whether they suggest ureteropelvic junction (UPJ) obstruction.

Findings concerning for PUV or VUR — For infants with either of the following findings on postnatal ultrasound, the next step in the evaluation is a VCUG (algorithm 1B):

●Bilateral hydronephrosis of any severity

●Hydronephrosis associated with ureteral or bladder dilation

The VCUG is done to identify patients with bladder outlet obstruction (most commonly PUV) and to detect VUR. The latter accounts for approximately 10 percent of cases of neonatal hydronephrosis [15]. (See "Clinical presentation, diagnosis, and course of primary vesicoureteral reflux".)

The VCUG is performed under fluoroscopy. A urinary catheter is inserted into the bladder, contrast material is instilled, and images are obtained while the bladder fills and during voiding. Infants usually tolerate this procedure well. Although the duration of fluoroscopy is minimized, the gonads, especially the ovaries, are exposed to radiation [16]. Newer techniques with radiopaque contrast bubbles detected by sonography thereby avoiding radiation are used in some centers [17]. (See "Clinical presentation, diagnosis, and course of primary vesicoureteral reflux", section on 'Imaging'.)

Findings suggestive of ureteropelvic junction obstruction — For patients in whom the postnatal ultrasound suggests classic UPJ-type obstruction (ie, unilateral moderate to severe hydronephrosis with nondilated ureter, normal bladder, and normal contralateral kidney), the diagnosis is confirmed with diuretic renal scan, typically performed after six weeks of age. Magnetic resonance imaging can also be used to make the diagnosis. (See 'Specialized imaging studies' below and "Congenital ureteropelvic junction obstruction", section on 'Diagnosis'.)

Congenital UPJ obstruction is the most common pathologic cause of prenatally diagnosed hydronephrosis. In our practice, we do not routinely perform VCUGs in these patients. Other experts routinely perform VCUG to definitively exclude VUR since a small minority of patients with UPJ obstruction may also have VUR. Historically, VCUG was performed in all patients with findings suggestive of UPJ obstruction; however, accumulating evidence suggests that infants with classic UPJ-type obstruction have a low risk of significant VUR. Therefore, we feel VCUG is not warranted in this group of patients [18,19].

The diagnosis of UPJ obstruction is discussed in greater detail separately.(See "Congenital ureteropelvic junction obstruction", section on 'Diagnosis'.)

Follow-up after voiding cystourethrogram — For infants who undergo VCUG, the need for further evaluation is determined as follows:

●If bladder outlet obstruction (eg, PUV) is detected – The hallmark finding on VCUG that suggests PUV is a dilated and elongated posterior urethra during the voiding phase (in the absence of a urethral catheter) (image 2). The next step is to perform cystoscopy to confirm the diagnosis and ablate the PUV. This is discussed separately. (See "Clinical presentation and diagnosis of posterior urethral valves", section on 'Diagnosis' and "Management of posterior urethral valves", section on 'Cystoscopy'.)

●If reflux is detected – If the VCUG detects VUR, therapeutic options are discussed with the family/caregivers and include observation, medical management including antibiotic prophylaxis, or surgical correction for more severe grades of reflux. Management of VUR is discussed separately. (See "Management of vesicoureteral reflux", section on 'Therapeutic options'.)

●If there is no reflux on VCUG – If the VCUG does not show VUR, further evaluation depends upon the degree of hydronephrosis (algorithm 1B).

•Severe hydronephrosis – In our center, infants with persistent postnatal severe hydronephrosis (RPD >15 mm) undergo further testing with a diuretic renography to detect possible obstruction. (See 'Diuretic renography' below.)

At this stage of evaluation, referral to a center with expertise in pediatric urologic care is warranted because the renal scan is an invasive procedure requiring placement of an intravenous line and bladder catheter, and surgical interventions may be required if the kidney is obstructed. In general, diuretic renography can be performed after six weeks of life because surgical intervention is rarely required prior to this time (eg, severe hydronephrosis without VUR).

The role of antibiotic prophylaxis in patients with severe hydronephrosis without VUR is discussed below. (See 'Antibiotic prophylaxis' below.)

•Moderate hydronephrosis – In our center, infants with moderate postnatal hydronephrosis (RPD 10 to 15 mm) have a repeat ultrasound when they reach four to six months of age. Further management decisions are made based on the subsequent ultrasound (algorithm 1B).

-Resolution – Most cases of mild to moderate postnatal hydronephrosis resolve by 18 months of age [20-22].

-No change – If there is no change on the follow-up ultrasound, we continue to monitor the degree of hydronephrosis with an ultrasound performed at one year of age, and if needed, between three and five years of age. If the patient develops symptoms (eg, abdominal pain, hematuria), or if the RPD increases, diuretic renography may be performed to determine if there is an obstructive process. (See 'Diuretic renography' below.)

-Worsening hydronephrosis – If the degree of hydronephrosis increases, diuretic renography is performed to determine if there is an obstructive process. (See 'Diuretic renography' below.)

This scenario is uncommon. In a retrospective study of 344 patients with prenatally diagnosed hydronephrosis that persisted on prenatal ultrasound and whose evaluation did not reveal a condition requiring intervention, only four patients (1 percent) subsequently developed worsening hydronephrosis [23]. All four patients presented with clinical symptoms of intermittent UPJ obstruction (eg, abdominal pain, gross hematuria, and vomiting).

Prevention of urinary tract infection — Higher rates of urinary tract infections (UTIs) have been reported in infants with prenatally diagnosed hydronephrosis compared with the general pediatric population [24-28]. The risk of infection rises if there is underlying VUR and is greater in girls compared with boys [26,28,29]. The risk also rises with the severity of hydronephrosis [30].

In our practice, interventions used to reduce the risk of UTI include judicious use of antibiotic prophylaxis and treatment of physiologic phimosis with local steroid ointment in male infants with hydronephrosis.

Antibiotic prophylaxis — The role of prophylactic antibiotics in infants undergoing diagnostic evaluation for prenatally diagnosed hydronephrosis remains uncertain. It had been postulated that hydronephrosis was a risk factor for UTI due to urinary stasis; however, accumulating data suggest that antibiotic prophylaxis may not be beneficial for patients with low-grade VUR and those with UPJ obstruction, as discussed separately. (See "Management of vesicoureteral reflux", section on 'Antibiotic prophylaxis' and "Congenital ureteropelvic junction obstruction", section on 'Antibiotic prophylaxis'.)

●Indications – In our practice, we do not administer antibiotic prophylaxis to infants undergoing evaluation for prenatally diagnosed hydronephrosis if the hydronephrosis is low- to moderate-grade (ie, RPD ≤12 mm, SFU [Society of Fetal Urology] grades 1, 2, or 3 (image 1)) or if the findings suggest classic unilateral UPJ obstruction (ie, unilateral hydronephrosis with nondilated ureter, normal bladder, and normal contralateral kidney) [13,31]. Other centers provide prophylactic antibiotics for moderate hydronephrosis (ie, RPD between 10 to 15 mm).

We reserve antibiotic prophylaxis for patients with any of the following:

•Dilated ureters (ectopic or refluxing)

•An enlarged bladder (suggestive of PUV)

•Bilateral UPJ obstruction

•Giant hydronephrosis (massively enlarged kidney with RPD >30 mm)

This approach is supported by observational studies. In a systematic review and meta-analysis that included 21 observational studies, the following findings were noted [31]:

•In patients with low-grade hydronephrosis (SFU grades 1 and 2; n = 2181), rates of UTI were similar between patients treated with or without antibiotic prophylaxis (2.2 versus 2.8 percent).

•In contrast, patients with high-grade hydronephrosis (SFU grades 3 and 4; n = 507) who received antibiotic prophylaxis had a lower rate of UTI compared with those who were not treated with antibiotics (15 versus 29 percent).

In our practice, patients with SFU grade 3 hydronephrosis are typically not given antibiotic prophylaxis unless there is a family history of UTI, VUR, or other uropathy [32]. However, practice varies and other centers routinely provide prophylactic antibiotics for patients with SFU grade 3 hydronephrosis.

The role of antibiotic prophylaxis in infants with confirmed VUR or UPJ obstruction is discussed separately. (See "Management of vesicoureteral reflux", section on 'Antibiotic prophylaxis' and "Congenital ureteropelvic junction obstruction", section on 'Antibiotic prophylaxis'.)

●Antibiotic regimen – When prophylactic antibiotic therapy is warranted, the choice of antibiotic depends on postnatal age:

•Age <2 months – For this age group, we use amoxicillin unless the infant has a penicillin allergy. The dose for amoxicillin prophylaxis is 10 to 15 mg/kg orally once daily. Cephalexin (10 mg/kg orally once daily) is a reasonable alternative. We use these antibiotics because alternative antibiotics (trimethoprim-sulfamethoxazole or nitrofurantoin) are associated with serious adverse effects (eg, hyperbilirubinemia) in this age group and should be avoided.

•Age ≥2 months – After two months of age, we switch to either trimethoprim-sulfamethoxazole (2 to 3 mg/kg trimethoprim component orally once daily, maximum 80 mg/dose) or nitrofurantoin (1 to 2 mg/kg orally once daily, maximum 100 mg/dose). These antibiotics are preferred over amoxicillin, ampicillin, or cephalosporins because they have lower rates of antibiotic resistance.

Uncircumcised males — The risk of UTI is higher in males who are not circumcised. In our practice, we review the care of the uncircumcised penis with parents/caregivers and, if appropriate, treatment of physiologic phimosis with topical steroid ointment. (See "Care and complications of the uncircumcised penis in infants and children", section on 'Management of pathologic phimosis'.)

We no longer routinely recommend circumcision to parents/caregivers of uncircumcised neonatal males with hydronephrosis. We review the data with the parents/caregivers regarding the risk of recurrent UTI in uncircumcised males with hydronephrosis [27] and offer the option of circumcision as a potential intervention to decrease the risk for future UTI. (See "Neonatal circumcision: Risks and benefits", section on 'Reduction in urinary tract infection'.)

SPECIALIZED IMAGING STUDIES

Diuretic renography — Diuretic renography (renal scan and the administration of a diuretic, typically furosemide) is used to diagnose urinary tract obstruction in infants with severe persistent hydronephrosis after a voiding cystourethrogram (VCUG) has demonstrated no vesicoureteral reflux (VUR) [33]. It measures the drainage time from the renal pelvis and assesses the relative contribution of each kidney to overall kidney function.

The test requires insertion of a bladder catheter to relieve any pressure that can be transmitted to the ureters and kidneys. Intravenous access is needed for hydration and the administration of the radioisotope and diuretic. The preferred radioisotope is technetium-99m-mercaptoacetyltriglycine (Tc99mMAG3), which is taken up by the renal cortex, filtered across the glomerular basement membrane (GBM) to the renal tubules, and excreted into the renal pelvis and urinary tract [34].

The study includes two phases:

●Initial phase – Radioisotope is injected intravenously, and kidney parenchymal (cortical) uptake is measured during the first two to three minutes. The relative contribution of each kidney to overall kidney function, referred to as "split kidney function," is assessed quantitatively. Split kidney function is the most useful measure to detect differences in kidney function between the two kidneys. As a rule of thumb, spilt kidney function of less than 5 percent difference is unlikely to be clinically significant.

In patients with unilateral hydronephrosis (which is the most common clinical scenario), if the normal nonhydronephrotic kidney and hydronephrotic kidney both have similar function (ie, difference in split kidney function <5 percent), conservative management without surgery is a safe option. Subsequent studies can be compared with the initial baseline scan to determine whether kidney function remains stable or whether increasing differences in split kidney function develop that indicate a decrease in the function of the hydronephrotic kidney, most likely due to significant obstruction, which may require intervention [35].

●Second phase – In the second phase, at peak kidney uptake, intravenous furosemide is administered and the excretion of isotope from the kidney is measured (referred to as the "washout curve"). This phase indicates the extent of obstruction, if present. In the normal kidney, the administration of furosemide results in a prompt washout. In a dilated system, if washout occurs rapidly after diuretic administration (<15 minutes), the system is not obstructed. If washout is delayed beyond 20 minutes, the pattern is consistent with obstructive uropathy. However, a delayed washout must be interpreted with caution [36,37]. As an example, in a series of 39 infants with antenatal unilateral hydronephrosis followed without surgery, diuretic renography indicated obstruction in 24 patients whose kidney function never decreased and thus could not have been obstructed [37]. These results may partly be due to the normally low neonatal glomerular filtration rate that can be refractory to diuretic therapy. If washout is between 15 and 20 minutes, the study is considered indeterminate.

A number of factors can affect the accuracy of the diuretic renogram. This includes the state of hydration of the infant, the functionality of the bladder catheter, the timing of diuretic administration, the accuracy of physically outlining the kidney tissue in the presence of severe hydronephrosis, and the background effect from the liver and spleen.

Magnetic resonance urography — Magnetic resonance urography (MRU) in children is becoming more commonly used in the diagnosis and management of congenital uropathies, such as ureteropelvic junction (UPJ) obstruction [38,39]. MRU is especially useful in the management of obstructed kidneys that have rotation or ascent anomalies, or are single. MRU can more clearly define the anatomy and delineate the proper surgical approach (ie, retroperitoneal versus transperitoneal). The disadvantage of MRU is that the study often requires general anesthesia or heavy conscious sedation in infants and children. However, for patients in the newborn period, many centers are now performing "swaddle MRI" after the infant falls asleep after feeding, thus avoiding general anesthesia and conscious sedation. Another disadvantage is the use of the contrast agent gadolinium, which can only be used if the kidney function is normal (requiring a preprocedure serum creatinine test) because of reports of irreversible renal fibrosis in patients with kidney insufficiency. Newer MRU technology may even define obstruction, eliminating the need for diuretic renal scans.

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Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.)

●Basics topic (see "Patient education: Hydronephrosis in babies (The Basics)")

SUMMARY AND RECOMMENDATIONS

●Background – Hydronephrosis (dilation of the renal pelvis) is a common finding on antenatal ultrasound examination. It may be a transient benign condition, or it may indicate a significant congenital anomaly of the kidney and urinary tract (CAKUT). The goal of the postnatal evaluation is to identify patients with clinically significant CAKUT while avoiding unnecessary testing in patients with physiologic or clinically insignificant fetal hydronephrosis. (See 'General principles' above and "Fetal hydronephrosis: Etiology and prenatal management".)

●Initial evaluation

•Indications for postnatal ultrasound – Postnatal ultrasound evaluation is warranted if the prenatal ultrasound (performed in the third trimester) had a renal pelvic diameter (RPD) of ≥10 mm. (See 'Indications' above and "Fetal hydronephrosis: Etiology and prenatal management", section on 'Congenital anomalies of the kidney and urinary tract (CAKUT)'.)

•Timing of postnatal ultrasound – For most infants, the postnatal ultrasound should be deferred until two to four weeks after birth to allow time for early neonatal fluid shifts to resolve (algorithm 1A). Important exceptions to this include infants with bilateral hydronephrosis associated with ureteral dilation or a dilated bladder and those with a hydronephrotic solitary kidney. Infants with these findings require urgent evaluation within 48 hours of birth because of they are likely to have more clinically significant CAKUT (eg, posterior urethral valves [PUV]) and may require early intervention. (See 'Timing' above.)

•Severity of hydronephrosis – The severity of hydronephrosis is categorized according to the findings on postnatal ultrasound (image 1) (see 'Severity of hydronephrosis' above):

-Mild hydronephrosis (grades 1 and 2) – RPD <10 mm

-Moderate hydronephrosis (grade 3) – RPD 10 to 15 mm

-Severe hydronephrosis (grade 4) – RPD >15 mm

●Subsequent evaluation – The need for additional testing depends on the severity of hydronephrosis on postnatal ultrasound (algorithm 1B). (See 'Subsequent evaluation and management' above.)

•Normal postnatal ultrasound – No further evaluation is required. (See 'Normal ultrasound or mild hydronephrosis' above.)

•Mild hydronephrosis (RPD 5 to <10 mm) – The need for repeat imaging depends on the timing of the postnatal ultrasound. If it was performed in the early neonatal period (within the first three weeks after birth), we suggest repeating the ultrasound at four to six months of age. If the postnatal ultrasound was performed after three to four weeks of age and showed only mild hydronephrosis, no additional evaluation is necessary. (See 'Normal ultrasound or mild hydronephrosis' above.)

•Moderate to severe hydronephrosis – Infants with persistent moderate to severe hydronephrosis (RPD ≥10 mm) on postnatal ultrasound generally require additional evaluation. The choice of imaging test is as follows (algorithm 1B):

-If the postnatal ultrasound shows ureteral or bladder dilation or if there is bilateral hydronephrosis, a voiding cystourethrogram (VCUG) is performed to detect the presence of vesicoureteral reflux (VUR) or PUVs. (See 'Persistent moderate to severe hydronephrosis' above.)

-For patients in whom the postnatal ultrasound suggests classic ureteropelvic junction (UPJ)-type obstruction (ie, unilateral moderate to severe hydronephrosis with nondilated ureter, normal bladder, and normal contralateral kidney), the diagnosis is confirmed with diuretic renal scan and VCUG is generally not necessary. (See "Congenital ureteropelvic junction obstruction", section on 'Diagnosis'.)

•Follow-up after VCUG – Evaluation and management based on the VCUG findings is as follows:

-If VUR or PUV are detected – Management options for VUR include observation, medical management (including antibiotic prophylaxis), and/or surgical correction for severe VUR. PUVs are managed surgically. These conditions are discussed separately. (See "Management of vesicoureteral reflux" and "Management of posterior urethral valves".)

-No reflux and severe hydronephrosis – Diuretic renography is performed to detect possible obstruction for infants with severe hydronephrosis (RPD >15 mm) and no VUR. At this stage of evaluation, referral to a center with expertise in pediatric urologic care is warranted. (See 'Diuretic renography' above.)

-No reflux and moderate hydronephrosis – Ultrasound examination is repeated at four to six months of age. In most cases, hydronephrosis will have resolved by this age. For infants with increased hydronephrosis at six months, a diuretic renal scan is performed. (See 'Diuretic renography' above.)

●Prevention of urinary tract infections

•Prophylactic antibiotics – The role of antibiotic prophylaxis in infants undergoing diagnostic evaluation for prenatally diagnosed hydronephroses is uncertain. We suggest antibiotic prophylaxis for patients with any of the following (Grade 2C) (algorithm 1A) (see 'Antibiotic prophylaxis' above):

-Dilated ureters

-Enlarged bladder

-Bilateral UPJ obstruction

-Giant hydronephrosis (massively enlarged kidney with RPD >30 mm)

We do not routinely prescribe prophylactic antibiotics for infants with milder degrees of hydronephrosis (ie, RPD ≤12 mm; grades 1, 2, 3 (image 1)) and those with classic unilateral UPJ obstruction. Other centers provide prophylactic antibiotics for moderate hydronephrosis (ie, RPD between 10 to 15 mm).

The role of antibiotic prophylaxis in infants with confirmed VUR or UPJ obstruction is discussed separately. (See "Management of vesicoureteral reflux", section on 'Antibiotic prophylaxis' and "Congenital ureteropelvic junction obstruction", section on 'Antibiotic prophylaxis'.)

•Uncircumcised males – Measures to reduce the risk of urinary tract infection (UTI) in uncircumcised males are discussed in greater detail separately. (See "Care and complications of the uncircumcised penis in infants and children", section on 'Management of pathologic phimosis' and "Neonatal circumcision: Risks and benefits", section on 'Infants with congenital uropathy'.)