Hydronephrosis is a medical condition in which one or both kidneys become swollen due to the buildup of urine. Normally, urine flows from the kidneys through the ureters to the bladder and is then eliminated from the body. When this flow is blocked or impaired, urine accumulates in the kidney, causing the renal pelvis and calyces to dilate.
Hydronephrosis can affect people of all ages, including newborns, children, adults, and older individuals. The condition may develop suddenly (acute hydronephrosis) or gradually over time (chronic hydronephrosis). If left untreated, severe hydronephrosis can damage kidney tissue and reduce kidney function.
Ultrasound is the most commonly used imaging method for diagnosing hydronephrosis because it is safe, non-invasive, and highly effective at detecting dilation of the kidney’s collecting system.
This guide explains everything you need to know about hydronephrosis, including its causes, symptoms, diagnosis, grading, ultrasound findings, treatment options, and prevention strategies.
What Is Hydronephrosis?
Hydronephrosis refers to the dilation or swelling of the kidney caused by an obstruction or impairment of urine flow.
When urine cannot drain properly:
- Pressure builds inside the kidney.
- The renal pelvis enlarges.
- The calyces become dilated.
- Kidney tissue may become compressed.
The condition may involve:
- One kidney (unilateral hydronephrosis)
- Both kidneys (bilateral hydronephrosis)
The severity can range from mild dilation to severe kidney enlargement with loss of kidney tissue.
How Do the Kidneys Normally Work?
The kidneys continuously filter waste products and excess fluid from the blood.
Normal urine flow follows this pathway:
- Kidneys produce urine.
- Urine collects in the renal pelvis.
- Urine travels through the ureters.
- Urine enters the bladder.
- Urine leaves the body through the urethra.
Any blockage along this pathway can lead to hydronephrosis.
What Causes Hydronephrosis?
Hydronephrosis occurs when urine drainage is obstructed or when urine flows backward toward the kidneys.
Common causes include:
Kidney Stones
Kidney stones are one of the most common causes.
A stone can block:
- Renal pelvis
- Ureter
- Ureterovesical junction
This blockage prevents urine from draining normally.
Ureteral Obstruction
The ureter may become blocked by:
- Scar tissue
- Blood clots
- Tumors
- Inflammation
Enlarged Prostate
In men, an enlarged prostate gland may obstruct urine flow from the bladder.
This can eventually affect both kidneys.
Pregnancy
During pregnancy, the enlarging uterus may compress the ureters.
This often causes temporary hydronephrosis, especially on the right side.
Tumors
Tumors affecting the urinary tract may obstruct urine drainage.
Examples include:
- Bladder cancer
- Kidney tumors
- Cervical cancer
- Prostate cancer
Congenital Abnormalities
Some individuals are born with structural abnormalities that interfere with urine flow.
Examples include:
- Ureteropelvic junction obstruction (UPJ)
- Ureterovesical junction obstruction (UVJ)
Vesicoureteral Reflux (VUR)
Urine flows backward from the bladder into the ureters and kidneys.
This condition commonly affects children.
Neurogenic Bladder
Nerve disorders may impair bladder emptying.
Examples include:
- Spinal cord injuries
- Multiple sclerosis
- Diabetes-related nerve damage
Types of Hydronephrosis
Hydronephrosis can be classified in several ways.
Unilateral Hydronephrosis
Only one kidney is affected.
This is commonly caused by:
- Kidney stones
- Ureter obstruction
- Localized abnormalities
Bilateral Hydronephrosis
Both kidneys are affected.
Common causes include:
- Enlarged prostate
- Bladder outlet obstruction
- Severe urinary retention
Acute Hydronephrosis
Develops suddenly.
Usually caused by:
- Kidney stones
- Sudden ureteral blockage
Chronic Hydronephrosis
Develops gradually over time.
Long-term obstruction may eventually damage kidney tissue.
Symptoms of Hydronephrosis
Symptoms vary depending on severity and the underlying cause.
Flank Pain
Pain may occur in the:
- Back
- Side
- Lower abdomen
The pain can be mild or severe.
Nausea and Vomiting
These symptoms commonly occur with kidney stone-related hydronephrosis.
Urinary Symptoms
Patients may experience:
- Frequent urination
- Painful urination
- Urgency
- Difficulty urinating
Blood in Urine
Hematuria may occur when kidney stones or urinary tract abnormalities are present.
Fever
Fever may indicate a urinary tract infection associated with hydronephrosis.
No Symptoms
Mild hydronephrosis is sometimes discovered incidentally during imaging studies.
Hydronephrosis in Pregnancy
Hydronephrosis is relatively common during pregnancy.
Causes include:
- Hormonal changes
- Compression of the ureters by the growing uterus
Features include:
- Usually affects the right kidney
- Often mild
- Usually resolves after delivery
Ultrasound is the preferred imaging modality during pregnancy.
Hydronephrosis in Infants and Children
Hydronephrosis is one of the most frequently detected prenatal abnormalities.
Prenatal ultrasound may reveal:
- Dilated renal pelvis
- Enlarged collecting system
Possible causes include:
- UPJ obstruction
- VUR
- Congenital urinary tract abnormalities
Many mild cases resolve naturally after birth.
How Is Hydronephrosis Diagnosed?
Several diagnostic methods are available.
Medical History and Physical Examination
Doctors assess:
- Symptoms
- Pain location
- Urinary complaints
- Medical history
Laboratory Tests
Tests may include:
- Urinalysis
- Kidney function tests
- Blood tests
- Urine culture
Ultrasound
Ultrasound is the primary imaging tool for diagnosing hydronephrosis.
Ultrasound Findings in Hydronephrosis
Ultrasound provides excellent visualization of the kidney collecting system.
Typical findings include:
Renal Pelvis Dilation
The renal pelvis appears enlarged and fluid-filled.
Calyceal Dilation
The calyces become visible and enlarged.
Anechoic Areas
Urine appears black (anechoic) on ultrasound.
Kidney Enlargement
The affected kidney may appear enlarged.
Cortical Thinning
Severe chronic hydronephrosis may cause thinning of the renal cortex.
Ultrasound is often sufficient for diagnosis and follow-up.
Grading of Hydronephrosis
Hydronephrosis is commonly graded according to severity.
Grade 1 (Mild)
Features:
- Slight renal pelvis dilation
- Normal calyces
- No cortical thinning
Grade 2 (Mild to Moderate)
Features:
- Renal pelvis enlargement
- Mild calyceal dilation
Grade 3 (Moderate)
Features:
- Significant calyceal dilation
- Enlarged collecting system
Grade 4 (Severe)
Features:
- Marked dilation
- Cortical thinning
- Reduced kidney tissue
Higher grades indicate more severe obstruction.
Additional Imaging Studies
Sometimes additional imaging is required.
CT Scan
Useful for:
- Kidney stones
- Tumors
- Obstruction evaluation
MRI
Provides detailed soft tissue imaging.
Nuclear Medicine Renal Scan
Assesses:
- Kidney function
- Urine drainage
Complications of Hydronephrosis
Untreated hydronephrosis may lead to serious complications.
Kidney Damage
Persistent pressure can damage kidney tissue.
Kidney Failure
Severe bilateral hydronephrosis may cause renal failure.
Urinary Tract Infection
Urine stagnation increases infection risk.
Sepsis
Severe infections may spread throughout the body.
Permanent Loss of Kidney Function
Long-standing obstruction may cause irreversible damage.
Treatment of Hydronephrosis
Treatment focuses on removing the underlying cause.
Kidney Stone Removal
Treatment options include:
- Natural passage
- Lithotripsy
- Ureteroscopy
Ureteral Stent Placement
A stent helps restore urine flow.
Nephrostomy Tube
A drainage tube may be placed directly into the kidney.
Surgery
Surgical correction may be necessary for:
- Congenital abnormalities
- Severe obstruction
- Tumors
Treatment of Infection
Antibiotics are prescribed when infection is present.
Prevention of Hydronephrosis
Some cases cannot be prevented, but risk can be reduced by:
- Staying hydrated
- Treating urinary tract infections promptly
- Managing kidney stones
- Following up on urinary symptoms
- Regular medical evaluations
Early detection helps prevent complications.
Role of Ultrasound in Monitoring Hydronephrosis
Ultrasound is frequently used for follow-up examinations.
Benefits include:
- No radiation exposure
- Safe for children and pregnant patients
- Cost-effective
- Real-time imaging
Sonographers monitor:
- Degree of dilation
- Kidney size
- Cortical thickness
- Treatment response
Conclusion
Hydronephrosis is a condition characterized by swelling of the kidney due to impaired urine drainage. It may result from kidney stones, urinary tract obstruction, congenital abnormalities, tumors, pregnancy, or reflux disorders. Symptoms can range from severe flank pain and urinary problems to no symptoms at all. Ultrasound is the primary diagnostic tool because it clearly demonstrates dilation of the renal pelvis and calyces while avoiding radiation exposure. Early diagnosis and treatment are essential to prevent kidney damage and preserve normal renal function. Understanding hydronephrosis helps healthcare professionals, sonographers, and patients recognize the condition and seek timely medical care.
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Zak is a dedicated medical and career writer specializing in sonography, healthcare education, and professional development. Through SonographerSalary.com, he shares in-depth insights on sonographer salaries, education pathways, and career tips to help readers build successful futures in medical imaging. His content combines accuracy with practical, easy-to-understand guidance, empowering students and professionals to make confident, informed career decisions.