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Hydronephrosis: Kidney Swelling Explained

Hydronephrosis: Kidney Swelling Explained

📖 12 min read Written/reviewed by Dr. Alhad Naragude, MBBS, MS, DrNB Urology Last updated: September 6, 2026

Hydronephrosis: The short answer

Hydronephrosis means swelling of the kidney because urine is not draining normally. It is usually seen on ultrasound or CT scan. It is not a final diagnosis by itself — it is a sign that urine may be blocked, flowing backward or collecting under pressure. Mild hydronephrosis may sometimes be monitored, but moderate or severe hydronephrosis, fever, severe pain, blood in urine, reduced urine output, raised creatinine, swelling in both kidneys or swelling in a single functioning kidney needs timely urology evaluation.

The most important question is: why is the kidney swollen?

What is hydronephrosis?

Hydronephrosis is swelling or dilatation of the kidney’s urine-collecting system. Normally, urine forms in the kidney, passes through the ureter, collects in the bladder and then comes out through the urethra.

If urine cannot pass freely, pressure builds up behind the obstruction. This pressure stretches the drainage system inside the kidney. Hydronephrosis can affect one kidney or both kidneys. When the ureter is also swollen, the report may say hydroureteronephrosis.

A simple way to understand it: Kidney → ureter → bladder → urine passage

A blockage or back-pressure anywhere along this pathway can cause kidney swelling.

Hydronephrosis does not always mean kidney stone. Stones are common, but hydronephrosis can also happen due to prostate blockage, ureter narrowing, tumours, pregnancy-related pressure, urine reflux or poor bladder emptying.

Hydronephrosis meaning in ultrasound report

Many patients first discover hydronephrosis after an ultrasound KUB. Reports may use different words for the same broad problem.

Report wording Patient-friendly meaning
Mild hydronephrosis Slight swelling of the kidney drainage system
Moderate hydronephrosis More visible swelling; needs evaluation
Severe hydronephrosis Marked swelling; may risk kidney function
PCS dilatation / pelvicalyceal system dilatation The urine-collecting area inside the kidney is enlarged
Prominent renal pelvis The central collecting area of the kidney looks enlarged
Hydroureteronephrosis / HDUN Kidney and ureter are both swollen
Back-pressure changes Urine pressure is affecting the kidney
Cortical thinning Kidney tissue may be thinned, often suggesting long-standing pressure

The wording alone does not decide treatment. A urologist looks at symptoms, creatinine, urine infection, stone suspicion, prostate status, CT findings and whether one or both kidneys are involved.

Is hydronephrosis serious?

Hydronephrosis can be mild, moderate or severe.

Mild hydronephrosis without pain, fever, infection or raised creatinine may sometimes be followed with repeat testing. But hydronephrosis becomes serious when it is due to obstruction, infection or kidney function changes. Over time, kidney function can reduce and kidney damage may occur.

Hydronephrosis is more concerning if there is:

  • Fever with chills.
  • Severe flank pain.
  • Vomiting.
  • Blood in urine.
  • Reduced urine output.
  • Raised creatinine.
  • Swelling of both kidneys.
  • Swelling in a single functioning kidney.
  • Diabetes with urinary infection.
  • Cortical thinning on ultrasound.

Common causes of hydronephrosis

Hydronephrosis has many causes. The common causes vary by age, sex and symptoms.

Kidney stone or ureteric stone

A stone stuck in the ureter is one of the common causes in adults. It can cause sudden flank pain, vomiting, blood in urine and hydronephrosis. Pain may move from the back to the lower abdomen, groin or testicle.

Enlarged prostate

In older men, prostate enlargement can block urine flow at the bladder outlet. Severe obstruction can cause high residual urine, bladder distension and sometimes swelling of both kidneys.

Ureteric stricture

A ureteric stricture is narrowing of the ureter. It may happen after previous stone surgery, infection, tuberculosis, radiation, injury or abdominal/pelvic surgery.

PUJ obstruction

PUJ obstruction means narrowing at the pelvi-ureteric junction, where the kidney pelvis joins the ureter. Some people are born with it, but symptoms may appear later in life.

Tumours or cancer

Tumours of the kidney, ureter, bladder, prostate, cervix, uterus, colon or retroperitoneum can block or compress the urinary tract. This may cause painless hydronephrosis in some patients.

Pregnancy

Pregnancy can cause temporary hydronephrosis due to pressure from the uterus and hormonal changes. But pain, fever, vomiting or urine infection during pregnancy should be checked urgently.

Urine reflux

Vesicoureteric reflux means urine flows backward from the bladder toward the kidney. It is more common in children but may be relevant in selected adults.

Neurogenic bladder or poor bladder emptying

Diabetes, spinal disease, neurological conditions or previous pelvic surgery can affect bladder emptying. A poorly emptying bladder can create back-pressure and hydronephrosis.

Symptoms of hydronephrosis

Hydronephrosis may be silent. Some patients have no pain and discover it during routine ultrasound.

Possible symptoms include:

  • Pain in the side, back or flank.
  • Pain spreading to groin or testicle.
  • Nausea or vomiting.
  • Burning urination.
  • Fever or chills.
  • Blood in urine.
  • Frequent urination.
  • Weak urine stream.
  • Feeling of incomplete bladder emptying.
  • Recurrent urinary infection.
  • Reduced urine output.
  • Tiredness or swelling if kidney function is affected.

Sudden obstruction, such as a ureteric stone, often causes severe pain. Slow obstruction, such as a stricture or tumour compression, may be painless for a long time.

When should you see a urologist for hydronephrosis?

You should consult a urologist if your ultrasound, CT scan or report mentions:

  • Hydronephrosis.
  • Hydroureteronephrosis.
  • PCS dilatation.
  • Pelvicalyceal system dilatation.
  • Obstructive uropathy.
  • PUJ obstruction.
  • Ureteric obstruction.
  • Back-pressure changes.
  • Cortical thinning.
  • Bilateral hydronephrosis.
  • Hydronephrosis with raised creatinine.

Even if there is no pain, the cause should be identified. A painless swollen kidney can still lose function if the obstruction is chronic.

Emergency warning signs

Seek urgent medical care if hydronephrosis is associated with:

  • Fever with chills.
  • Severe flank pain.
  • Persistent vomiting.
  • Very low urine output.
  • Confusion, weakness or low blood pressure.
  • A single kidney with swelling.
  • Swelling of both kidneys.
  • Raised creatinine.
  • Pregnancy with pain, fever or urinary infection.
  • Diabetes with fever and urinary symptoms.

Important:
An obstructed kidney with infection or no urine output is a urological emergency. Urgent drainage by DJ stent or nephrostomy may be needed in selected patients.

How is hydronephrosis diagnosed?

Ultrasound KUB

Ultrasound is usually the first test. It can show kidney swelling, stones, bladder distension, prostate size and post-void residual urine. But ultrasound may not always show the exact cause. A ureteric stone, stricture or tumour may need CT or further testing.

Urine routine and urine culture

These tests check for infection, pus cells, blood, crystals and bacteria. If fever is present, urine culture helps guide antibiotic treatment.

Serum creatinine and kidney function tests

Creatinine helps assess kidney function. Raised creatinine with hydronephrosis is more concerning, especially if both kidneys are affected or the patient has one functioning kidney.

CT KUB or CT urography

CT KUB is commonly used when a ureteric stone is suspected. CT urography may be advised when the doctor needs to assess the ureter, bladder, tumours or unclear obstruction.

Uroflowmetry and post-void residual urine

In men with weak stream, straining or incomplete emptying, uroflowmetry and residual urine testing help assess prostate-related obstruction.

Cystoscopy

Cystoscopy is a telescope test to inspect the urinary passage and bladder. It may be needed if there is blood in urine, suspected bladder tumour, ureteric opening problem or lower urinary tract obstruction.

Functional kidney scan

A DTPA or EC renal scan may be advised when PUJ obstruction or long-standing obstruction is suspected. It helps assess drainage and split function of each kidney.

Mild, moderate and severe hydronephrosis

Hydronephrosis is commonly graded as mild, moderate or severe.

Mild hydronephrosis: slight swelling. It may need observation, repeat ultrasound or further testing depending on symptoms.

Moderate hydronephrosis: more definite swelling. The cause should usually be identified.

Severe hydronephrosis: marked swelling. If associated with cortical thinning, infection, pain or kidney function changes, it needs timely urology evaluation.

The grade is important, but it is not the only factor. Mild hydronephrosis with fever may be urgent. Severe hydronephrosis without pain may still be important if it is chronic.

Hydronephrosis treatment

Treatment depends on the cause, severity, symptoms and kidney function.

Observation and follow-up

Some mild cases may only need repeat ultrasound, urine tests and creatinine monitoring if there is no pain, infection or kidney function problem.

Medicines

Medicines may help with pain, infection, prostate symptoms or selected small stones. But medicines cannot correct every obstruction. Repeated painkillers without checking kidney function can be unsafe, especially in patients with kidney disease, dehydration, diabetes or high blood pressure.

Urgent drainage

Urgent drainage may be needed if hydronephrosis is associated with infection, sepsis, single kidney obstruction, bilateral obstruction, uncontrolled pain, vomiting, anuria or acute kidney injury. Drainage is usually done by DJ stent or nephrostomy tube.

Stone treatment

If a stone is causing hydronephrosis, treatment may include observation, medical expulsive treatment, URSL, RIRS, PCNL or shockwave lithotripsy depending on stone size, location, infection, kidney function and anatomy.

Prostate treatment

If prostate enlargement is causing back-pressure, treatment may include medicines, catheter drainage or prostate surgery such as TURP, HoLEP or laser prostate surgery depending on severity.

Ureteric stricture treatment

Treatment may include endoscopic dilatation, internal incision, stenting, ureteric reimplantation or reconstructive surgery depending on the site and length of narrowing.

PUJ obstruction surgery

If PUJ obstruction is significant, pyeloplasty may be advised. This operation removes the narrowed segment and reconnects the drainage pathway.

Cancer-related obstruction

If a tumour is causing hydronephrosis, treatment depends on cancer type and stage. Sometimes the kidney needs drainage first so that kidney function improves before cancer treatment.

What should you avoid if you have hydronephrosis?

Avoid these common mistakes:

  • Do not ignore hydronephrosis just because pain has reduced.
  • Do not repeatedly take painkillers without checking creatinine.
  • Do not delay care if fever or chills are present.
  • Do not assume mild hydronephrosis is always harmless.
  • Do not miss follow-up after a DJ stent or nephrostomy.
  • Do not delay evaluation if hydronephrosis is seen in both kidneys.
  • Do not delay if you have one kidney and the report shows swelling.
  • Do not self-treat urinary infection if there is flank pain or fever.

Can hydronephrosis damage the kidney permanently?

Yes, it can. The risk is higher when obstruction is severe, infected, bilateral or long-standing. Kidney recovery depends on how long the obstruction has been present, how severe it is, baseline kidney function and whether infection is present. Prolonged obstruction can lead to irreversible kidney damage.

Early diagnosis and timely treatment improve the chance of kidney recovery.

What to bring for consultation

  • Ultrasound KUB report and images if available.
  • CT KUB or CT urography films/CD if done.
  • Urine routine report.
  • Urine culture report.
  • Serum creatinine and eGFR.
  • CBC and blood sugar reports if available.
  • Previous stone surgery records.
  • Previous discharge summaries.
  • Current medicines.
  • Diabetes, BP and kidney disease reports.
  • Catheter, DJ stent or nephrostomy details if already placed.
  • Old scans for comparison.

Questions to ask your urologist

  • Is my hydronephrosis mild, moderate or severe?
  • Is one kidney affected or both?
  • What is the likely cause?
  • Is my creatinine normal?
  • Do I need CT scan or renal scan?
  • Is there infection behind the blockage?
  • Do I need urgent drainage?
  • Can this be treated with medicines?
  • Do I need a stent, nephrostomy or surgery?
  • Is my kidney function likely to recover?
  • How often should I repeat imaging?

FAQs

Is hydronephrosis the same as kidney failure?

No. Hydronephrosis means kidney swelling due to urine back-pressure. Kidney failure means kidney function is reduced. But untreated severe hydronephrosis can lead to kidney damage.

Can hydronephrosis happen without pain?

Yes. Slow obstruction can be painless. This is why hydronephrosis on ultrasound should not be ignored even if you feel well.

Does mild hydronephrosis need CT scan?

Not always. It depends on symptoms, urine report, creatinine, stone suspicion and whether the finding persists. Your urologist may advise repeat ultrasound, CT KUB, CT urography or observation.

Can hydronephrosis reduce after passing a stone?

Yes. If the stone passes and urine flow returns to normal, hydronephrosis may reduce. Follow-up imaging is often needed to confirm this.

What is hydroureteronephrosis?

Hydroureteronephrosis means both the kidney and ureter are swollen. It often suggests obstruction lower down in the ureter, bladder outlet or prostate region.

Can prostate enlargement cause hydronephrosis?

Yes. Severe prostate obstruction can cause bladder back-pressure and swelling of both kidneys. This needs evaluation, especially if creatinine is high or residual urine is significant.

Is hydronephrosis dangerous in pregnancy?

Mild hydronephrosis can occur during pregnancy. But fever, severe pain, vomiting, urine infection or reduced urine output during pregnancy needs urgent medical care.

Can children have hydronephrosis?

Yes. Hydronephrosis may be detected before birth or during childhood. Causes include PUJ obstruction, reflux, posterior urethral valves and other structural problems. Children need paediatric urology evaluation when advised.

Related reading

References

Note: This information is for educational purposes only and is not a substitute for medical advice. Please consult your doctor for any symptoms.