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Health condition · Clinically reviewed

Hydronephrosis, an obstructed kidney - what it means and how it is treated.

Sometimes it is a quiet finding on a scan. Sometimes it is a urological emergency. This guide walks through both - and the imaging and treatment pathway in between.

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A radiographer guides a patient onto the bed of an advanced 3 Tesla MRI scanner in a London imaging suite

Why trust this guide

  • 01

    Clinically reviewed

    Written by our editorial team and reviewed by a registered UK clinician before publication.

  • 02

    Sourced from guidance

    Checked against NICE, BAUS and peer-reviewed urology sources you can see at the end.

  • 03

    Current for 2026

    Reflects modern UK urology practice including nephrostomy, JJ stenting and pyeloplasty pathways.

Key facts

Hydronephrosis at a glance.

The essentials in plain English - what causes it, how it is graded, and when a swollen kidney becomes urgent.

  • What it is

    Dilatation of the renal pelvis and calyces caused by obstruction to urinary flow anywhere between the kidney and the urethra.

  • Grading

    Society for Fetal Urology grades 1 to 4, or measured by antero-posterior pelvic diameter on ultrasound.

  • Common causes

    Kidney stones, ureteric stricture, pelvi-ureteric junction (PUJ) obstruction, tumours, pregnancy and neurogenic bladder.

  • Often silent

    Many cases are incidental on imaging - symptoms typically mean stones, infection or acute obstruction.

  • When it is urgent

    An infected obstructed system (pyonephrosis) or acute kidney injury needs same-day drainage - nephrostomy or JJ stent.

  • Definitive care

    Depends on the cause - lithotripsy for stones, pyeloplasty for PUJ obstruction, stenting for strictures, MDT for tumour.

Why this guide matters

A cause-first, kidney-preserving plan.

Hydronephrosis is a sign, not a diagnosis. Sorting the cause quickly - and draining an obstructed kidney when needed - is what protects long-term function.

  • Fever with obstruction is an emergency

    An infected obstructed system (pyonephrosis) needs IV antibiotics and same-day drainage - a nephrostomy or JJ stent.

  • Imaging tells you the cause

    Ultrasound confirms dilatation, CT KUB maps stones and lesions, and a MAG3 renogram proves true obstruction from a baggy but flowing system.

  • Definitive care is cause-specific

    Stones, PUJ obstruction, stricture, tumour, reflux, retroperitoneal fibrosis and pregnancy each have their own pathway - guided by a consultant urologist.

How the diagnosis is made

From first scan to a clear plan.

The pathway a UK urology team will normally follow, in order - so you know what each test adds and why.

  1. 01

    Assessing

    History and red flags

    Loin pain, fever, haematuria, urinary symptoms, reduced urine output and any known stones, cancer or prior surgery.

  2. 02

    Assessing

    Bloods and urine

    U&Es and creatinine to check kidney function, urinalysis and culture to look for infection.

  3. 03

    Assessing

    Renal ultrasound

    The first imaging test - confirms dilatation, grades severity and looks for stones or bladder distension. See our renal ultrasound scan guide.

  4. 04

    Confirming

    CT KUB for stones

    The gold-standard scan for suspected renal or ureteric stones and to map any obstructing lesion. See our private CT scan guide.

  5. 05

    Confirming

    MAG3 or DTPA renogram

    A dynamic nuclear medicine scan that separates true obstruction from a baggy but draining system, and measures split kidney function.

  6. 06

    Planning

    MRI and cystoscopy

    MRI urogram for soft-tissue causes or pregnancy; flexible cystoscopy where a bladder or ureteric lesion is suspected.

  7. 07

    Planning

    Specialist urology review

    A consultant urologist ties the findings together and plans definitive treatment - stones, stricture, PUJ, tumour or reflux.

Typical timeline: urgent imaging in hours, functional studies and a plan within days.

Symptoms

What hydronephrosis feels like.

Sometimes nothing at all, sometimes textbook loin colic. The features below help sort mild dilatation from a kidney that needs draining today.

  • Often no symptoms

    Mild hydronephrosis is frequently picked up on imaging done for another reason - not everyone feels it.

  • Loin or flank pain

    A dull ache or - with an acute stone - severe colicky pain radiating from loin to groin.

  • Fever and rigors

    Fever with an obstructed kidney suggests infection above the blockage - a urological emergency needing urgent drainage.

  • Visible or dipstick haematuria

    Blood in the urine can point to stones, a ureteric tumour or bladder cancer as the cause.

  • Urinary symptoms

    Frequency, urgency, hesitancy or a poor stream may indicate outflow obstruction driving bilateral hydronephrosis.

  • Reduced urine output

    Anuria or oliguria with bilateral obstruction (or obstruction in a solitary kidney) is a medical emergency.

  • Antenatal or paediatric finding

    Often detected on antenatal scans - most mild cases settle, but PUJ obstruction, VUR and posterior urethral valves need specialist follow-up.

  • Red flag - infected obstructed kidney

    Loin pain plus fever plus known hydronephrosis is pyonephrosis until proven otherwise - go to A&E.

Treatment

How hydronephrosis is treated in the UK.

Drain first if infected or acutely obstructed - then fix the cause. The specific procedure depends on whether the blockage is a stone, a stricture, a PUJ, a tumour or something else.

  • Emergency drainage

    For an infected obstructed system or bilateral acute obstruction - percutaneous nephrostomy or retrograde JJ ureteric stent under urology.

  • Percutaneous nephrostomy

    A tube placed through the skin into the kidney to drain urine when a stent is not feasible. See our nephrostomy clinic guide.

  • JJ ureteric stent

    A soft internal tube keeping the ureter open - the usual first step for stones, strictures and post-operative drainage. See our JJ ureteric stent guide.

  • Shockwave lithotripsy

    External shockwaves used to fragment smaller kidney and upper-ureteric stones without an incision.

  • Ureteroscopy and laser

    A flexible telescope up the ureter to break and remove obstructing stones - now the workhorse for most ureteric stones.

  • Percutaneous nephrolithotomy

    PCNL - keyhole surgery through the flank for large or complex kidney stones causing hydronephrosis.

  • Pyeloplasty for PUJ obstruction

    Anderson-Hynes pyeloplasty - laparoscopic or robotic reconstruction of the pelvi-ureteric junction, the definitive fix.

  • Cause-specific care

    Steroids for retroperitoneal fibrosis, MDT-led surgery for tumour, reflux management for VUR and conservative care in pregnancy.

What this guide is based on

The sources behind every claim on this page.

UK and European guidance, current at the time of last review.

Key references

Guidelines and standards we relied on.

A quiet reminder

This guide is for information, not medical advice.

Your GP or consultant urologist knows your history and imaging. If you are worried, get seen the same day.

  • NICE. Renal and ureteric stones: assessment and management (NG118).

  • British Association of Urological Surgeons (BAUS). Patient information and standards on nephrostomy, JJ stents and pyeloplasty.

  • European Association of Urology (EAU). Guidelines on urolithiasis and urological infections.

  • Royal College of Radiologists. Guidance on imaging the urinary tract - ultrasound, CT KUB and renography.

Red flags

When hydronephrosis needs urgent attention.

Most cases are safely investigated in outpatients. These are the situations that are not - and where the kidney needs draining or an urgent scan today.

  • Infected obstructed kidney

    Loin pain with fever in a dilated system is pyonephrosis - a urological emergency needing IV antibiotics and urgent drainage.

  • Bilateral or solitary-kidney anuria

    No urine output with obstruction is acute kidney injury waiting to happen - go to A&E without delay.

  • Acute kidney injury on bloods

    A rising creatinine with hydronephrosis needs same-day urology and often same-day drainage.

  • Visible haematuria over 45

    Painless visible blood in the urine warrants a two-week-wait referral to exclude bladder or ureteric cancer.

  • Rapidly progressive hydronephrosis

    A dilating system on serial scans, or new bilateral disease, points to obstruction that needs urgent workup.

  • Paediatric red flags

    Antenatally detected hydronephrosis with worsening dilatation, febrile urinary infections or reduced split function needs paediatric urology.

  • Sepsis features

    Confusion, breathlessness, low blood pressure or a fast heart rate in a urinary infection is sepsis - dial 999.

  • Post-operative deterioration

    Any new loin pain, fever or reduced output after urological or gynaecological surgery deserves an urgent scan.

  • Encrusted or blocked stent

    A JJ stent that has been in for months without exchange can encrust and block - book the change on time.

Living with it

A treatable problem, with a clear plan.

Four things that make the biggest difference day to day - hydration, follow-up scans, keeping on top of stent exchanges, and knowing exactly when to escalate.

A quiet reminder

Kidneys forgive early, not late.

An obstructed kidney recovers well if drained quickly. Weeks of unrecognised obstruction can cause lasting loss of function - so never sit on new loin pain and fever.

  1. 01 Hydration

    Drink to a pale straw colour

    Two to three litres of fluid a day keeps urine dilute, helps flush small stones and reduces infection risk.

  2. 02 Follow-up

    Keep the scan diary

    Serial ultrasound or renography is how the team knows a kidney is safe - don't miss the appointments.

  3. 03 Stents

    Diary the exchange date

    JJ stents are exchanged every 3 to 6 months (or sooner in stone disease) - overdue stents block and cause harm.

  4. 04 Escalate

    Know your red flags

    Loin pain with fever, a shivering rigor or no urine output means A&E, not a phone call to the GP.

Frequently asked

Everything we get asked about hydronephrosis.

Quick answers on causes, imaging, when it is urgent and what definitive treatment looks like.

  • What is hydronephrosis?

    Hydronephrosis is swelling of the collecting system inside the kidney - the renal pelvis and calyces - caused by obstruction to the flow of urine. It ranges from mild dilatation seen only on a scan to severe back-pressure that damages the kidney if left untreated.

  • Is hydronephrosis painful?

    Not always. Chronic slow-onset hydronephrosis can be completely painless and picked up incidentally on imaging. Acute obstruction - typically from a stone - causes severe colicky loin-to-groin pain, and infection on top of obstruction adds fever and feeling very unwell.

  • When is hydronephrosis an emergency?

    Fever combined with a known obstructed kidney (pyonephrosis), bilateral obstruction, obstruction in a solitary kidney, or a rapidly rising creatinine all need same-day urology assessment and urgent drainage with a nephrostomy or JJ stent.

  • How is the cause found?

    The pathway is usually renal ultrasound first, then a CT KUB if stones are suspected, then a MAG3 or DTPA renogram to prove whether the system is truly obstructed. MRI, cystoscopy and specialist urology review fill in the picture where needed.

  • Does mild hydronephrosis in pregnancy need treatment?

    Usually not. Physiological hydronephrosis of pregnancy - typically right-sided and worst in the second and third trimesters - is common and settles after delivery. Treatment is reserved for pain, infection or evidence of true pathological obstruction under joint obstetric and urology care.

  • What is the definitive treatment?

    It depends on the cause. Stones are treated with shockwave lithotripsy, ureteroscopy or PCNL; PUJ obstruction is repaired with an Anderson-Hynes pyeloplasty (usually robotic or laparoscopic); strictures are managed endoscopically; tumours through an MDT; and retroperitoneal fibrosis with steroids and specialist urology follow-up.

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