# Pediatric UPJ Obstruction and Multicystic Dysplastic Kidney

## Prenatal Hydronephrosis: Overview

Prenatal hydronephrosis is the most common abnormality detected on prenatal ultrasound, occurring in approximately 1-5% of pregnancies. The majority of these cases resolve spontaneously, representing transient or physiologic dilation. The differential diagnosis for prenatal hydronephrosis includes ureteropelvic junction (UPJ) obstruction, which is the most common surgical cause, vesicoureteral reflux (VUR), multicystic dysplastic kidney (MCDK), posterior urethral valves (which typically cause bilateral hydronephrosis), ureterovesical junction obstruction resulting in megaureter, ureterocele or duplex collecting systems, and physiologic or transient dilation.

### SFU Grading of Prenatal Hydronephrosis

The Society for Fetal Urology (SFU) grading system classifies hydronephrosis based on ultrasound findings. Grade 1 indicates a renal pelvis that is barely visible. Grade 2 shows a visible renal pelvis with a few calyces. Grade 3 involves dilation of the pelvis with all calyces visible. Grade 4 is characterized by dilation of both the pelvis and calyces along with thinning of the renal parenchyma.

### UTD Classification (Urinary Tract Dilation)

The Urinary Tract Dilation (UTD) classification system is replacing the SFU grading to provide standardized prenatal and postnatal criteria. It integrates measurements of the anterior-posterior diameter (APD) of the renal pelvis, calyceal dilation, parenchymal changes, ureteral dilation, and bladder abnormalities. UTD P1, considered low risk, includes an APD of 4 to less than 7 mm between 16 and 27 weeks of gestation or 7 to less than 10 mm after 28 weeks. UTD P2, indicating increased risk, is defined by an APD of 7 mm or greater (16-27 weeks) or 10 mm or greater (>28 weeks) accompanied by peripheral calyceal dilation. UTD P3, representing the highest risk, includes an APD meeting these thresholds along with parenchymal or ureteral abnormalities.

## UPJ Obstruction in Children

### Pathophysiology

UPJ obstruction primarily results from intrinsic narrowing at the ureteropelvic junction. This narrowing is often due to an aperistaltic segment characterized by abnormal smooth muscle and collagen composition, along with disrupted pacemaker cell activity that impairs coordinated peristalsis. Less commonly, extrinsic compression by a crossing lower-pole renal vessel can cause obstruction, although this is more frequent in adults than children. Another cause includes a high ureteral insertion on a capacious renal pelvis. These abnormalities lead to impaired drainage of urine from the renal pelvis into the ureter, causing hydronephrosis and potential renal damage.

### Clinical Presentation

Today, most cases of UPJ obstruction are identified prenatally through routine ultrasound demonstrating hydronephrosis. In the neonatal period, presentation may include a palpable abdominal or flank mass or urinary tract infection (UTI). Older children often present with intermittent flank pain, especially after hydration, hematuria following trauma, or recurrent UTIs. A classic presentation is Dietl crisis, characterized by episodic severe flank pain accompanied by nausea and vomiting after fluid bolus administration, reflecting intermittent obstruction.

### Postnatal Evaluation

Postnatal evaluation begins with a renal ultrasound performed at 7 to 14 days of life, avoiding the first 48 hours when physiologic oliguria may underestimate dilation. Ultrasound assesses the anterior-posterior diameter (APD), calyceal morphology, cortical thickness, and the contralateral kidney. Voiding cystourethrogram (VCUG) is indicated to rule out vesicoureteral reflux, especially in cases of bilateral hydronephrosis, visible ureteral dilation, UTD P2-P3 classification, or in male infants to exclude posterior urethral valves. The MAG3 diuretic renogram is the functional study of choice, evaluating differential renal function (DRF) and drainage. A DRF less than 40% is concerning for significant obstruction. Drainage half-time (T1/2) greater than 20 minutes indicates an obstructive pattern, less than 10 minutes is non-obstructive, and 10-20 minutes is indeterminate. This study is optimally performed after 4 to 6 weeks of age due to renal immaturity affecting earlier scans.

<image>Renal ultrasound showing unilateral hydronephrosis with dilated renal pelvis and thinned cortex (SFU grade 4), and corresponding MAG3 renogram curves showing obstructive drainage pattern</image>

### Management

#### Observation (Conservative)

Conservative management is appropriate for asymptomatic UPJ obstruction with preserved differential renal function greater than 40%, mild to moderate hydronephrosis without cortical thinning, and stable or improving dilation on serial ultrasounds. The observation protocol includes renal ultrasound every 3 to 6 months during the first two years of life, then annually thereafter. Repeat MAG3 renograms are performed if hydronephrosis worsens or symptoms develop. The use of antibiotic prophylaxis remains controversial but may be considered in cases of recurrent UTIs. Notably, 50-70% of prenatally detected UPJ obstructions do not require surgical intervention.

#### Indications for Pyeloplasty

Surgical intervention via pyeloplasty is indicated when differential renal function falls below 40% or declines on serial MAG3 studies, when symptomatic obstruction occurs (such as pain or recurrent UTIs), when hydronephrosis worsens on serial ultrasounds despite observation, when impaired drainage is demonstrated by a T1/2 greater than 20 minutes with declining function, or in cases of recurrent febrile UTIs with documented UPJ obstruction.

#### Pyeloplasty Technique

The dismembered (Anderson-Hynes) pyeloplasty remains the gold standard surgical approach. It can be performed via open, laparoscopic, or robotic methods. The open flank approach is most common in infants, while robotic surgery is increasingly utilized in older children over one to two years of age. The procedure involves excision of the narrowed UPJ segment, reduction of a redundant renal pelvis, spatulation of the proximal ureter, and a watertight, tension-free anastomosis to the dependent pelvis. Internal stents such as JJ stents or external stents/nephrostomy tubes are often placed. The success rate is high, ranging from 95 to 98%. Potential complications include prolonged urinary drainage due to urine leak, stent-related issues, strictures occurring in less than 3% of cases, and UTIs.

#### Endopyelotomy

Endopyelotomy is rarely used in the pediatric population but may be considered for failed pyeloplasty or secondary obstruction. It has a lower success rate compared to pyeloplasty.

## Multicystic Dysplastic Kidney (MCDK)

### Pathophysiology

Multicystic dysplastic kidney represents severe renal maldevelopment, where the affected kidney is non-functioning and replaced by multiple non-communicating cysts. This condition results from early complete ureteral obstruction or failed induction of the metanephric mesenchyme during development. The ureter is typically atretic and does not connect to the bladder in most cases. There are no functioning nephrons, and the affected kidney contributes no measurable renal function.

### Clinical Features

MCDK is almost always unilateral, as bilateral involvement is incompatible with life. The left side is slightly more commonly affected. It is often diagnosed prenatally via ultrasound. In neonates, it may present as a palpable abdominal mass. The contralateral kidney usually undergoes compensatory hypertrophy. Associated anomalies include contralateral vesicoureteral reflux, which occurs in 15-28% of cases and is the most common association, contralateral UPJ obstruction in 3-12%, as well as cardiac, gastrointestinal, and other genitourinary anomalies.

### Imaging Characteristics

Ultrasound reveals multiple non-communicating cysts of varying sizes without an identifiable renal pelvis, which helps distinguish MCDK from hydronephrosis. The renal parenchyma is absent. Functional imaging with MAG3 or DMSA scans shows no function in the affected kidney, confirming a differential renal function of zero percent. Differentiation from severe hydronephrosis is critical: in MCDK, cysts do not communicate and there is no identifiable pelvis, whereas in hydronephrosis, the dilated pelvis and calyces communicate.

<image>Renal ultrasound comparing multicystic dysplastic kidney (multiple non-communicating cysts of varying size with no renal pelvis) versus severe hydronephrosis (dilated pelvis with communicating calyces)</image>

### Natural History

The majority of MCDKs involute over time, shrinking and often becoming undetectable on ultrasound by 5 to 10 years of age in 30-50% of cases. Involution is the typical course, while growth or persistence is unusual. The risk of malignancy, particularly Wilms tumor, has historically been cited but is extremely rare, occurring in less than 0.5% of cases. The risk of hypertension is also very low, under 2%.

### Management

Conservative management is the standard of care, as endorsed by the American Urological Association. Observation involves serial renal ultrasounds every 6 to 12 months initially, then annually until involution or until the child reaches 8 to 10 years of age. Monitoring of the contralateral kidney is essential to assess compensatory hypertrophy and screen for vesicoureteral reflux. Blood pressure should be regularly monitored. Many clinicians recommend voiding cystourethrogram to evaluate for contralateral VUR given its prevalence. Nephrectomy is rarely indicated and reserved for cases of persistent hypertension attributed to the MCDK, failure to involute with concern for malignancy (which is extremely rare), recurrent infections, or parental anxiety, although the latter is not a standard indication.

## UVJ Obstruction (Primary Obstructive Megaureter)

### Overview

Primary obstructive megaureter results from obstruction at the ureterovesical junction (UVJ), causing dilation of the ureter with or without hydronephrosis. The distal ureteral segment is adynamic and aperistaltic, similar in mechanism to UPJ obstruction at the UPJ. It is important to differentiate this condition from refluxing and non-refluxing non-obstructive megaureters.

### Classification of Megaureter (King/Pfister)

Megaureters are classified based on their mechanism. Primary obstructive megaureter involves intrinsic obstruction at the UVJ. Primary refluxing megaureter is caused by vesicoureteral reflux. Primary non-refluxing, non-obstructive megaureter is idiopathic dilation without obstruction or reflux. Secondary megaureters arise from conditions such as neurogenic bladder or posterior urethral valves.

### Evaluation

Evaluation includes renal ultrasound demonstrating a dilated ureter greater than 7 mm with or without hydronephrosis. VCUG is performed to rule out vesicoureteral reflux. MAG3 renogram assesses renal function and drainage. Most cases, particularly non-refluxing non-obstructive megaureters, resolve spontaneously.

### Management

Observation is the mainstay for most cases, with spontaneous resolution expected by 2 to 3 years of age. Surgical intervention, typically ureteral reimplantation with tapering, is indicated if there is declining renal function, recurrent febrile UTIs, or increasing dilation. Tapering techniques include excisional methods such as the Hendren technique or plication methods like Starr or Kalicinski. Temporary stenting or diversion may be necessary in neonates with severe obstruction and infection.

## Clinical Pearls

Prenatal hydronephrosis is a common finding, occurring in 1-5% of pregnancies, and most cases resolve without requiring surgery, so families should not be alarmed prematurely. Postnatal renal ultrasound should be performed at 7 to 14 days of life rather than within the first 48 hours, as physiologic oliguria during that period can mask dilation. The MAG3 renogram is the key functional study for UPJ obstruction; a differential renal function below 40% or declining function on serial studies is the primary indication for pyeloplasty. The dismembered pyeloplasty technique has a success rate of 95-98% and remains the gold standard. Multicystic dysplastic kidney is a non-functioning kidney that almost always involutes over time, making nephrectomy rarely necessary. It is essential to evaluate the contralateral kidney in MCDK cases, as 15-28% have contralateral vesicoureteral reflux. Finally, the UTD classification system is increasingly replacing the SFU grading system for standardized reporting of prenatal and postnatal hydronephrosis.

## References
- AUA/SFU Guidelines on Prenatal Hydronephrosis and UPJO
- EAU/ESPU Guidelines on Pediatric Urology, 2024 Update
- Nguyen HT, et al. "Multidisciplinary consensus on classification of prenatal and postnatal urinary tract dilation" (UTD Classification). *J Pediatr Urol*. 2014;10(6):982-998.
- Onen A. "An alternative grading system for reflux." *J Urol*. 2007.
- Chertin B, et al. "Conservative management of MCDK." *World J Urol*. 2009.
- Campbell-Walsh-Wein Urology, 12th Edition, Chapters on UPJO and MCDK