Hydronephrosis in Children: Causes, Symptoms & Treatment

What Is Hydronephrosis in Children?

Hydronephrosis is a condition where the kidney pelvis becomes swollen or dilated because urine cannot drain properly. It can affect one kidney (unilateral) or both kidneys (bilateral). In children, it is most commonly detected during prenatal ultrasound and often resolves on its own as the urinary system matures.

Hydronephrosis literally means "water inside the kidney" and refers to dilation of the renal pelvis and calyces. The kidney pelvis is the funnel-shaped part of the kidney that collects urine before it flows down the ureter to the bladder. When there is an obstruction or backflow of urine, this collecting system becomes overfilled and enlarged.

The condition ranges from mild (minimal dilation) to severe (significant kidney swelling with potential functional impairment). Understanding the degree of hydronephrosis is crucial because it determines whether watchful waiting, medical management, or surgical intervention is appropriate. The Society for Fetal Urology (SFU) grading system classifies hydronephrosis into grades 0-4 based on ultrasound findings.

In the context of pediatric urology, hydronephrosis is particularly significant because it represents the most common abnormality detected on prenatal ultrasound. This early detection allows healthcare providers to plan appropriate postnatal evaluation and management, potentially preventing complications such as urinary tract infections and kidney damage.

Understanding the Urinary System in Children

To understand hydronephrosis, it helps to know how the urinary system works. The kidneys filter blood and produce urine, which collects in the renal pelvis. From there, urine flows through the ureter (a thin tube connecting the kidney to the bladder), into the bladder for storage, and eventually out through the urethra during urination.

In newborns and infants, the urinary system is still developing. The tissues are more pliable, and the coordination of the muscles that move urine is not yet fully mature. This is one reason why mild hydronephrosis often resolves on its own as the child grows – the urinary system simply needs time to develop properly.

How Common Is Hydronephrosis?

Prenatal hydronephrosis is detected in 1-5% of all pregnancies during routine ultrasound examinations. It accounts for approximately 50% of all urologic abnormalities found prenatally. Boys are affected about twice as often as girls, though the reasons for this gender difference are not entirely understood.

While these numbers may seem concerning, it's important to note that most cases are mild and resolve without any intervention. Only a small percentage of affected children will require surgery or experience long-term kidney problems.

What Causes Hydronephrosis in Babies?

The most common causes of hydronephrosis in children are ureteropelvic junction (UPJ) obstruction (blockage where kidney meets ureter), vesicoureteral reflux (urine flowing backward from bladder to kidney), and ureterovesical junction obstruction. In many cases, especially mild ones, no specific cause is identified.

Understanding the underlying cause of hydronephrosis is essential for determining the appropriate treatment approach. The causes can be broadly categorized into obstructive and non-obstructive types, with some cases being transient or physiological.

Ureteropelvic Junction (UPJ) Obstruction

UPJ obstruction is the most common cause of significant hydronephrosis in children, accounting for approximately 35-50% of cases. This occurs when there is a blockage or narrowing at the junction where the renal pelvis connects to the ureter. The obstruction prevents urine from flowing freely from the kidney to the bladder.

UPJ obstruction can be caused by intrinsic factors (problems with the ureter wall itself) or extrinsic factors (external compression, such as from crossing blood vessels). The narrowed segment may have abnormal smooth muscle architecture or inadequate nerve supply, preventing the normal peristaltic waves that move urine along.

Vesicoureteral Reflux (VUR)

Vesicoureteral reflux occurs when urine flows backward from the bladder into the ureters and potentially up to the kidneys. Normally, a one-way valve mechanism at the ureterovesical junction (where the ureter enters the bladder) prevents this backflow. When this mechanism is defective, urine can reflux upward, causing hydronephrosis.

VUR is graded from I to V based on severity, with higher grades indicating more significant reflux reaching the kidneys and causing dilation. Many cases of low-grade reflux resolve spontaneously as the child grows and the ureterovesical junction matures.

Other Causes

• Ureterovesical junction (UVJ) obstruction: Blockage where the ureter enters the bladder, less common than UPJ obstruction
• Posterior urethral valves (PUV): Abnormal tissue flaps in the urethra that obstruct urine flow, occurring exclusively in boys
• Ureterocele: A balloon-like swelling of the ureter near its opening into the bladder
• Ectopic ureter: When the ureter inserts in an abnormal location
• Megaureter: Abnormally dilated ureter that may or may not be obstructive
• Transient or physiological: Temporary dilation without underlying pathology, common in the third trimester

What Are the Symptoms of Hydronephrosis in Children?

Most children with hydronephrosis, especially infants, have no symptoms – the condition is typically discovered on prenatal or postnatal ultrasound. When symptoms do occur, they may include recurrent urinary tract infections, abdominal or flank pain, a palpable abdominal mass, or failure to thrive in severe cases.

One of the challenging aspects of hydronephrosis in children is that it often presents with no outward signs. This "silent" nature underscores the importance of prenatal ultrasound screening, which can detect the condition before any symptoms develop. Early detection allows for monitoring and intervention before kidney damage occurs.

Symptoms in Infants

Newborns and young infants with hydronephrosis rarely show specific symptoms related to the kidney dilation itself. Instead, symptoms typically arise from complications such as urinary tract infections. Parents and caregivers should watch for signs of UTI in infants, which can be subtle:

• Fever without an obvious source: Sometimes the only sign of UTI in infants
• Poor feeding or refusing to eat: General signs of illness
• Irritability or fussiness: The baby may seem uncomfortable or in pain
• Vomiting or diarrhea: Can accompany UTI in infants
• Foul-smelling urine: May indicate infection
• Failure to gain weight: In severe, prolonged cases
• Palpable abdominal mass: In cases of significant kidney enlargement

Symptoms in Older Children

As children grow older and can communicate their symptoms, they may describe more specific complaints related to hydronephrosis:

• Abdominal pain: Often localized to one side (flank) where the affected kidney is located
• Back pain: Pain in the lower back on the affected side
• Recurrent urinary tract infections: With typical symptoms like burning during urination, frequent urination, or urgency
• Blood in urine (hematuria): Visible or microscopic blood
• Nausea or vomiting: Especially with severe obstruction or infection
• Pain that worsens with fluid intake: Because more urine production increases pressure

How Is Hydronephrosis Diagnosed?

Hydronephrosis is typically first detected during prenatal ultrasound (usually in the second trimester). After birth, the diagnosis is confirmed with postnatal ultrasound at 1-2 weeks of age. Additional tests may include voiding cystourethrogram (VCUG) to check for reflux and MAG3 renal scan to assess kidney function and drainage.

The diagnostic workup for hydronephrosis follows a systematic approach, starting with non-invasive imaging and progressing to more detailed studies based on findings. The goal is to determine the severity of the condition, identify the underlying cause, assess kidney function, and guide treatment decisions.

Prenatal Ultrasound Detection

Most cases of hydronephrosis are first identified during routine prenatal ultrasound examinations, typically during the second trimester (around 18-20 weeks of pregnancy). The fetal kidneys and urinary tract are evaluated as part of the standard anatomical survey.

When hydronephrosis is detected prenatally, the obstetrician may recommend additional ultrasounds to monitor the condition throughout the pregnancy. The measurements of the renal pelvis (anterior-posterior diameter, or APD) help classify the severity:

• Mild: APD 4-7 mm in second trimester, 7-10 mm in third trimester
• Moderate: APD 7-10 mm in second trimester, 10-15 mm in third trimester
• Severe: APD >10 mm in second trimester, >15 mm in third trimester

Postnatal Ultrasound

After birth, an ultrasound is performed to confirm the prenatal findings and assess the current status. This is typically done 1-2 weeks after delivery rather than immediately after birth. The delay is important because in the first few days of life, relative dehydration can mask hydronephrosis or underestimate its severity.

The postnatal ultrasound evaluates the size of the renal pelvis, the thickness of the kidney tissue (parenchyma), and whether one or both kidneys are affected. The findings help determine what additional testing may be needed.

Voiding Cystourethrogram (VCUG)

A VCUG is an X-ray study used to evaluate for vesicoureteral reflux (VUR) and to assess the bladder and urethra. During this test, a small catheter is inserted into the bladder through the urethra, contrast dye is instilled, and X-rays are taken as the bladder fills and as the child urinates.

VCUG is typically recommended for children with:

• Moderate to severe hydronephrosis
• Bilateral hydronephrosis
• Hydronephrosis with dilated ureter (hydroureteronephrosis)
• History of urinary tract infection
• Suspicion of posterior urethral valves (in boys)

Renal Scintigraphy (MAG3 or DMSA Scan)

Nuclear medicine scans provide information about kidney function that ultrasound cannot provide. The two main types used in evaluating hydronephrosis are:

MAG3 (Mercaptoacetyltriglycine) scan: This dynamic scan evaluates both kidney function and drainage. A small amount of radioactive tracer is injected, and a camera records how quickly each kidney takes up and excretes the tracer. This test is particularly useful for:

• Determining if there is significant obstruction
• Measuring differential function (how much each kidney contributes to total kidney function)
• Monitoring changes over time

DMSA (Dimercaptosuccinic acid) scan: This static scan provides detailed images of kidney tissue and can detect scarring or damage. It's often used after urinary tract infections to assess for kidney damage.

How Is Hydronephrosis Treated?

Treatment depends on the severity and cause. Mild hydronephrosis usually requires only observation with periodic ultrasounds. Prophylactic antibiotics may be prescribed to prevent urinary tract infections. Surgery (most commonly pyeloplasty) is reserved for cases with significant obstruction, worsening dilation, recurrent infections, or declining kidney function.

The management of hydronephrosis in children is individualized based on several factors: the degree of dilation, underlying cause, kidney function, presence of symptoms, and the trend over time. The overarching principle is to preserve kidney function while avoiding unnecessary interventions.

Watchful Waiting (Observation)

For many children with mild to moderate hydronephrosis, particularly those detected prenatally, the initial approach is observation with serial ultrasound monitoring. This strategy is appropriate when:

• Hydronephrosis is mild (SFU grade 1-2)
• Kidney function is preserved
• The child is asymptomatic
• There is no associated urinary tract infection

Studies have shown that 50-70% of prenatal hydronephrosis cases resolve spontaneously by age 2 without any intervention. Regular ultrasound monitoring (every 3-6 months initially, then yearly) tracks the condition to ensure it is improving or stable rather than worsening.

Prophylactic Antibiotics

Antibiotic prophylaxis involves giving a low daily dose of antibiotics to prevent urinary tract infections rather than treat them. This approach is commonly used in children with:

• Moderate to severe hydronephrosis
• Vesicoureteral reflux
• History of urinary tract infections
• Bilateral hydronephrosis

Common prophylactic antibiotics include trimethoprim, nitrofurantoin, or amoxicillin. The duration of prophylaxis depends on the underlying condition and is typically continued until the hydronephrosis resolves or definitive treatment is provided.

Surgical Treatment

Surgery is considered when there is evidence that hydronephrosis is causing or will likely cause kidney damage. Indications for surgical intervention include:

• Significant obstruction: Confirmed by MAG3 scan showing impaired drainage
• Worsening dilation: Progressive enlargement despite observation
• Decreased kidney function: Differential function less than 40% on nuclear scan
• Recurrent UTIs: Despite antibiotic prophylaxis
• Symptoms: Pain or other symptoms attributable to the hydronephrosis

Pyeloplasty: The Most Common Surgery

Pyeloplasty is the surgical reconstruction of the ureteropelvic junction (UPJ) and is the gold standard treatment for UPJ obstruction. During this procedure, the obstructed or narrowed segment is removed, and the remaining healthy ureter is reconnected to the renal pelvis.

The surgery can be performed through several approaches:

• Open pyeloplasty: Traditional approach through a small flank incision
• Laparoscopic pyeloplasty: Minimally invasive using small incisions and a camera
• Robotic-assisted pyeloplasty: Uses robotic instruments for precise repair

Regardless of the approach, pyeloplasty has an excellent success rate, exceeding 95% in most centers. Children typically stay in the hospital for 1-3 days after surgery and recover quickly.

After Surgery: What to Expect

Recovery after pyeloplasty involves several phases:

Immediate recovery (hospital stay): Your child may have a drainage tube (stent) placed temporarily to keep the ureter open while healing. Pain is managed with medications, and most children can eat and drink the day after surgery.

At home (first few weeks): Activity is gradually increased. Your child should avoid strenuous activities, contact sports, and heavy lifting for about 4-6 weeks. Follow-up appointments ensure proper healing.

Long-term follow-up: Ultrasounds are performed at regular intervals (typically 6 weeks, 3 months, 1 year post-surgery) to confirm resolution of hydronephrosis and ensure good kidney function. Most children have normal kidney function and no long-term issues after successful surgery.

Does Hydronephrosis Go Away on Its Own?

Yes, many cases of hydronephrosis resolve spontaneously, especially mild cases detected prenatally. Studies show that approximately 50-70% of prenatal hydronephrosis cases resolve by age 2 without intervention. However, moderate to severe cases and those with underlying obstruction or reflux are less likely to resolve and may require treatment.

The prognosis for hydronephrosis in children is generally excellent, particularly when the condition is detected early and managed appropriately. The outcome depends on several factors including the severity, underlying cause, and whether there is any damage to kidney function.

Factors Affecting Outcome

Several factors influence whether hydronephrosis will resolve on its own or require intervention:

• Degree of dilation: Mild hydronephrosis (SFU grade 1-2) is much more likely to resolve than severe cases (SFU grade 3-4)
• Unilateral vs. bilateral: Hydronephrosis affecting one kidney has a better prognosis than bilateral involvement
• Underlying cause: Cases without identifiable obstruction ("transient" hydronephrosis) resolve most often
• Kidney function: Preserved kidney function at diagnosis indicates better long-term outcomes
• Trend over time: Improving or stable findings on follow-up imaging are reassuring

Long-Term Outlook

For children whose hydronephrosis resolves spontaneously or after successful surgery:

• Normal kidney function is typically preserved
• No increased risk of kidney problems in adulthood
• Normal urinary tract function
• No special dietary restrictions or activity limitations

Even children who require surgery generally have excellent outcomes with normal kidney function. The key is appropriate monitoring and timely intervention when indicated.

How Long Will My Child Need Follow-Up?

Children with hydronephrosis require regular follow-up appointments, with the duration depending on severity and response to treatment. Mild cases may only need monitoring for 1-2 years, while more significant cases may require follow-up into adolescence. Some children are monitored until adulthood to ensure long-term kidney health.

Regular follow-up is an essential component of hydronephrosis management. Even when the condition appears to have resolved, continued monitoring ensures that any changes are detected early and addressed promptly.

Typical Follow-Up Schedule

The frequency and duration of follow-up varies based on the individual situation:

For mild hydronephrosis that is improving:

• Ultrasound every 3-6 months initially
• Can extend to yearly once stable improvement is documented
• May be discontinued after 1-2 years if completely resolved

For moderate to severe hydronephrosis or post-surgery:

• More frequent ultrasounds initially (every 3 months)
• Additional imaging studies as needed (MAG3 scans)
• Continued monitoring for several years
• Some centers follow patients until adulthood

Follow-up appointments may be conducted in person or, increasingly, through telemedicine visits when imaging is obtained locally. Your child's urologist or nephrologist will determine the appropriate follow-up schedule based on their specific situation.