Urine builds up in the kidney’s collecting system when a stone, narrowing or other blockage in the urinary tract prevents drainage from the kidney. As the pressure within the kidney increases, it swells progressively. This condition is hydronephrosis, literally, ‘water in the kidney.’ This is one of the most important urological findings because without prompt relief of the obstruction the kidney progressively and permanently loses function.
Hydronephrosis is not a diagnosis per se; it is a consequence. It tells you that something is obstructing the flow of urine, and the question is always: what is obstructing it, how much and for how long? A small stone that causes partial obstruction may be found early and treated before there is any significant kidney damage. If not identified and surgically corrected, a congenital narrowing at the outlet where the kidney drains can slowly destroy a kidney’s function over years in a child. An infected obstructed kidney is a life threatening emergency and requires drainage within hours.
Hydronephrosis is the dilation and distension of the collecting system inside the kidney, the renal pelvis and calyces, caused by obstruction to the outflow of urine from the kidney into the ureter and ultimately the bladder. When urine cannot drain freely, it backs up behind the obstruction, slowly distending the collecting system and increasing pressure within the kidney.
This is the increased pressure in the kidney which causes kidney damage. Under normal conditions the kidney produces urine at a pressure low enough to allow free flow to the bladder. When the outflow is blocked, back pressure builds up and is transmitted to the nephrons themselves, the kidney’s tiny filtering units. Back pressure impairs glomerular filtration, decreases tubular function and over time causes tubular cell death and glomerular scarring. The longer an obstruction lasts, and the more pressure there is, the greater and more permanent the damage will be.
May be unilateral (single kidney) or bilateral (both kidneys). Bilateral hydronephrosis is more serious as it poses a simultaneous threat to both kidneys and acute bilateral obstruction can lead to rapidly progressive renal failure. Unilateral hydronephrosis does not pose an immediate threat to overall kidney function but causes progressive damage to the affected kidney, and can eventually destroy it if the cause is not treated. Recovery of kidney function after relief of obstruction depends on the severity and duration of the obstruction . A severely obstructed kidney for many months may have suffered irreversible damage even after relief of the obstruction.
Grading systems are used to grade the severity of hydronephrosis based on the degree of dilation of the collecting system and the extent of thinning of the kidney tissue. Mild grades should be monitored, whereas severe grades require urgent intervention to prevent loss of kidney function.
Grade 1 Hydronephrosis – Mild Renal Pelvis Dilatation
The renal pelvis is mildly dilated only, the calyces appear normal. The kidney cortex is totally preserved in thickness. Hydronephrosis grade 1 is often normal variation, intermittent partial obstruction or physiological dilatation. Most cases in this grade resolve spontaneously or remain stable without causing kidney damage. Surveillance with periodic ultrasound is the standard approach.
The renal pelvis and calyces are dilated but the kidney cortex is of normal thickness and echogenicity. At this grade obstruction is more definite and should be investigated for the underlying cause. Nuclear medicine scan (MAG3/DTPA) To quantify differential renal function and drainage. Many will need treatment once the cause is identified.
Ultrasound showed marked dilation of the renal pelvis and calyces with beginning thinning of the kidney cortex. This grade indicates considerable, sustained obstruction with early parenchymal loss in the kidney. Impaired drainage is usual with differential renal function on nuclear scan. Prompt treatment of the underlying cause is indicated to prevent further irreversible loss of cortex.
Kidney with gross hydronephrosis and severely thinned paper-thin cortex. Already parenchyma loss has been extensive. The differential renal function is markedly decreased. If the function is still more than 10 to 15 percent, relief of the obstruction may preserve residual function. If below this threshold nephrectomy may be considered. This grade means serious damage to the kidney and needs urgent referral to a specialist.
The Society for Fetal Urology (SFU) grading system (0 to 4) is used specifically for antenatal and postnatal hydronephrosis in children, similar to the adult grading scale. SFU Grade 0 is normal; Grade 1 and 2 is mild to moderate dilation of the pelvis with normal calyces; Grade 3 is calyceal dilation with preserved cortex; Grade 4 is severe dilation with cortical thinning. SFU grades 3 and 4 generally require nuclear medicine functional scanning, close monitoring and often surgery.
Hydronephrosis can be asymptomatic, incidentally found on imaging or can present with a spectrum of signs and symptoms depending on the etiology, severity and presence of infection . Seek medical attention if you have any of the following symptoms.
Dull Aching Pain in the Flank or Back (Loin Pain)
The urgency of specialist assessment depends on the severity and associated symptoms of the hydronephrosis. Here is a clear guide:
Severe Flank Pain With Fever – Emergency Situation
Hydronephrosis is one of the most common foetal abnormalities detected, and is found in about one to two per cent of all pregnancies on routine anomaly scanning. Most cases are mild and resolve spontaneously but some require investigation and surgical intervention to prevent permanent kidney damage in the developing child.
Antenatal Hydronephrosis – Detected Before Birth on Foetal Scan
Antenatal hydronephrosis is often picked up on the routine fetal anomaly scan at 18 to 20 weeks gestation, or on a subsequent third trimester growth scan. The most important parameter to record is the anterior posterior renal pelvis diameter (APD), the width of the renal pelvis measured on ultrasound. An abnormal APD was defined as an APD > 7 mm in the second trimester and > 10 mm in the third trimester and required post-natal follow-up. The cause is usually not identified antenatally, with possible underlying diagnoses such as PUJO, VUR, multicystic dysplastic kidney, posterior urethral valves (in boys) and duplex collecting systems being clarified by post-natal investigation.
All babies with antenatal hydronephrosis should have a post-natal renal ultrasound. Timing depends on severity. If there is significant bilateral hydronephrosis or you suspect posterior urethral valves (a condition that affects boys and can cause bladder outlet obstruction), then ultrasound on day one or two of life is essential. Ultrasound in the first week of life for moderate to severe unilateral hydronephrosis. For mild hydronephrosis (APD under seven millimetres at the anomaly scan), ultrasound can be arranged at four to six weeks. Further investigation is guided by post-natal ultrasound findings VCUG (voiding cystourethrogram) if VUR is suspected; MAG3 nuclear scan if PUJO or significant obstruction is suspected; further imaging as required.
Most cases of mild antenatal hydronephrosis , SFU Grade 1 to 2 , or postnatal APD of less than ten millimetres , resolve spontaneously in the first two to three years of life without intervention . As the child grows, the developing urinary tract matures and the functional narrowing at the PUJ improves. Resolution is monitored by serial ultrasound at three-monthly intervals during the first year and six-monthly thereafter. Resolution was defined as normalization of renal pelvis diameter on ultrasound with no deterioration in kidney function. Children with hydronephrosis not resolving or progressing , with worsening APD on serial ultrasound, impaired differential function on MAG3, or breakthrough urinary infections need specialist review for surgical intervention.
Children with hydronephrosis who are considered to be at significant risk of urinary tract infection are prescribed low-dose prophylactic antibiotics (trimethoprim, cefalexin or nitrofurantoin depending on age) , mainly those with concurrent VUR, those with bilateral significant hydronephrosis and boys with suspected posterior urethral valves awaiting further assessment. Antibiotic prophylaxis decreases the risk of febrile urinary tract infections resulting in renal scarring in the obstructed or refluxing kidney. The need for prophylaxis in all children with antenatal hydronephrosis remains controversial and is grade- and aetiology-dependent; Dr Vikas Singh individualizes this decision based on the child’s unique anatomy and risk profile.
Surgical correction is recommended if the hydronephrosis in a child is due to PUJO, the most common cause of significant childhood hydronephrosis, and the obstruction causes impaired kidney function (differential function below 40 percent), worsening hydronephrosis on serial imaging, or breakthrough infections. The gold standard treatment for PUJO in children, laparoscopic pyeloplasty, the Anderson-Hynes dismembered pyeloplasty performed through tiny keyhole incisions, has success rates above 95 percent and a dramatically faster recovery than open surgery. Children generally go home in two to three days and are back to normal activity in one to two weeks. Three to six months postoperative MAG3 scan will confirm successful drainage. Dr. Vikas Singh performs laparoscopic pyeloplasty in children of all ages with age appropriate anaesthetic management and paediatric sized instrumentation.
Hydronephrosis is a frequent finding in pregnancy, occurring in up to 90% of pregnant women by the third trimester. Correct management depends on differentiating physiologic hydronephrosis of pregnancy (normal and benign) from pathologic hydronephrosis that needs intervention.
Why Pregnancy Causes Physiological Hydronephrosis
Physiologic hydronephrosis of pregnancy is mediated by two mechanisms. Initially, an enlarging uterus, especially in the second trimester and beyond, mechanically compresses the ureters at the pelvic brim, resulting in partial obstruction of urine flow. Secondly, the rise in circulating progesterone levels in pregnancy leads to relaxation of smooth muscle throughout the body, including the ureteric smooth muscle, resulting in decreased ureteric peristalsis and passive dilatation of the ureter and renal pelvis. In most pregnant women, these two mechanisms combine to produce mild to moderate bilateral hydronephrosis, usually more pronounced on the right side (the left-sided sigmoid colon partially shields the ureter from direct uterine compression). This physiologic hydronephrosis is normal and harmless, requires no treatment and resolves completely after delivery.
When Pregnancy-Related Hydronephrosis Needs Treatment
Most hydronephrosis in pregnancy is physiological and benign. Intervention is needed when: the hydronephrosis is significantly asymmetric (suggesting a pathological cause on one side rather than bilateral physiological change); there is significant flank pain unresponsive to position changes and paracetamol; a kidney stone is identified as the cause of the obstruction; there is fever suggesting infected hydronephrosis (a urological emergency in pregnancy); or kidney function is deteriorating. The threshold for intervention during pregnancy is lower than outside it as a deteriorating kidney function situation has implications for both the mother and the foetus .
Safe Treatment Options – DJ Stenting During Pregnancy
The main treatment for significant symptomatic or complicated hydronephrosis in pregnancy is DJ ureteric stenting (cystoscopic insertion of a double-J stent through the urethra under local or short general anaesthesia). With appropriate anaesthetic precautions, it is safe at any stage of pregnancy, requires no radiation exposure and gives immediate relief of obstruction and pain. The stent bypasses the obstructing lesion (stone, compression or stricture) and lets the urine drain freely from the kidney. Stent exchange is often required every 4 to 6 weeks in pregnant women due to increased rates of encrustation related to hypercalciuria and bacteriuria in pregnancy. If cystoscopic stenting is unsuccessful or not possible, percutaneous nephrostomy (ultrasound-guided drainage tube into the kidney) is an alternative.
Monitoring Kidney Function During Pregnancy With Hydronephrosis
If a significant hydronephrosis is found in a pregnant woman, it is important to monitor the kidney function to ensure that the obstruction is not causing any progressive damage. In significant hydronephrosis, ultrasound is repeated every four to six weeks to assess the severity. Serum creatinine and eGFR monitored, rising creatinine indicates need for intervention ( stenting or nephrostomy ). Pregnant women with hydronephrosis have a much higher rate of ascending urinary tract infection and pyelonephritis which can lead to premature labour, and urine cultures are therefore taken regularly. Any symptomatic UTI during pregnancy is promptly treated with pregnancy-safe antibiotics.
When Does Hydronephrosis Resolve After Delivery
Physiologic hydronephrosis of pregnancy usually resolves completely within 4 to 6 weeks after delivery as the mechanical compression of the uterus is removed and progesterone levels normalize. Women with significant hydronephrosis during pregnancy should have a post-natal ultrasound at six weeks after delivery to confirm resolution and to exclude any underlying pathological cause (such as silent stone or pre-existing structural abnormality) which may have been masked by or confused with the physiological pregnancy related changes. If hydronephrosis persists more than six weeks after delivery further investigation is needed.
Hydronephrosis is the response of the kidney to obstruction and the risk of this is entirely dependent on the degree, duration and presence of infection. Partial obstruction with mild hydronephrosis that regresses rapidly may not cause permanent damage. Complete obstruction untreated for weeks causes severe hydronephrosis which causes permanent kidney damage. Pyonephrosis (infected hydronephrosis) is a life-threatening condition that needs emergency treatment within hours. The take home message is that hydronephrosis should never be ignored , it is always a sign of something obstructing the urinary tract and needs to be identified and dealt with.
Whether hydronephrosis resolves spontaneously depends on the cause. Most common scenario is mild antenatal hydronephrosis in babies . Majority of them resolve spontaneously as the urinary tract matures without any treatment. Physiologic hydronephrosis of pregnancy resolves completely after delivery. If a stone has passed, hydronephrosis due to a small ureteric stone may resolve. However, hydronephrosis from structural causes, PUJO, ureteric stricture, bladder outlet obstruction from BPH or tumour, does not resolve without treatment of underlying cause. ‘It is not appropriate to ‘watch and wait’ without establishing the cause and following up kidney function with significant hydronephrosis.
Treatment of hydronephrosis is treatment of the underlying cause. The stone causing obstruction is treated by laser ureteroscopy, PCNL or ESWL. Obstruction due to PUJO is treated by pyeloplasty (surgical reconstruction of the narrowed PUJ). Treatment of bilateral hydronephrosis due to BPH is urethral catheterisation followed by medical or surgical treatment specific for BPH. Endoscopic dilation or open/laparoscopic ureteric reconstruction is done for a ureteric stricture. Emergency situations (infected obstructed kidney) require immediate drainage by DJ stenting or percutaneous nephrostomy followed by definitive treatment after control of infection . Hydronephrosis is a mechanical problem with a mechanical solution . There is no drug treatment for it.
Pyeloplasty is the surgical reconstruction of the pelviureteric junction (PUJ). PUJO is the narrowing of the connection between the collecting system of the kidney and the ureter . This results in hydronephrosis . The gold standard technique is the Anderson-Hynes dismembered pyeloplasty, which removes the narrowed segment and reattaches the spatulated ureter to the renal pelvis with a wide funnel-shaped anastomosis. It is done laparoscopically through three to four small keyhole incisions and has achieved success rates (resolution of hydronephrosis and improved drainage on nuclear scan) of 92-98 percent in published series. It’s safe for all ages, including young infants and adults. Hospitalization: 2-3 days. Total recovery: 2-3 weeks. It is one of the most consistently successful reconstructive procedures in all of urology.
Ultrasound is the first investigation of choice in hydronephrosis. It clearly demonstrates the dilated renal pelvis and calyces, estimates the degree of swelling and assesses cortical thickness. This is a radiation free, outpatient test. CT urogram (contrast-enhanced CT) provides information on the cause of obstruction (stone, stricture, tumour or extrinsic compression) and upper urinary tract anatomy not available with ultrasound alone. DTP or MAG3Nuclear medicine scan is used to evaluate the differential renal function (percentage contribution of each kidney to total kidney function) and drainage kinetics, which are essential before planning surgery for PUJO or other obstructive conditions. In children and pregnant women, MRI urography is used as a radiation-free alternative to CT when ultrasound is not sufficient.
Yes, and this is the most important clinical sequela of untreated hydronephrosis. Chronic obstruction with sustained elevation of intrarenal pressure causes progressive destruction of nephron function by tubular atrophy, glomerular scarring, and interstitial fibrosis. The extent of the blockage (complete vs. partial) The duration of the blockage (hours vs. months) Presence of infection (infection accelerates damage significantly) The level of permanent damage is influenced by these factors. If the kidney has been obstructed severely and completely for more than a few weeks, the damage may be significant and irreversible even after relief of the obstruction . The hydronephrosis will resolve but the nephrons are lost . That is why the prompt identification and treatment of significant hydronephrosis is so important for long-term kidney function preservation.
Antenatal hydronephrosis is one of the commonest findings on foetal anomaly scans and occurs in approximately one to two percent of all pregnancies. Most cases are mild and go away on their own after the baby is born and do not have long-term effects on the baby. Parents need not be alarmed, but proper post-natal follow-up should be arranged prior to delivery. The important questions are: how severe is the hydronephrosis (APD measurement), is it bilateral or unilateral and is there any suggestion of a posterior urethral valve (which affects boys, and needs urgent post-natal assessment). Dr. Vikas Singh provides antenatal counselling consultations to help families understand their specific finding and prepare an individualised post-natal assessment plan before your baby is born.
A nephrostomy is a thin drainage tube that is inserted directly through the skin of the back into the collecting system of the kidney under the guidance of ultrasound and X-ray. This allows urine to drain out externally into a collection bag. It is used when the obstructed kidney cannot be drained by the internal route (retrograde ureteric stenting via the bladder) e.g. when a stone is too large for a stent to pass alongside; when the ureter is completely obstructed and cannot be cannulated; when general anaesthesia for cystoscopic stenting carries too high a risk. Nephrostomy offers rapid clinical improvement of infected obstructed kidney with local anaesthesia alone and immediate and reliable kidney drainage. The stone or obstruction is then definitively treated and the nephrostomy removed once the infection is cleared and the patient is stable.
The extent of hydronephrosis regression after successful treatment of its cause depends upon the degree of previous dilatation and the duration of obstruction. Following successful pyeloplasty for PUJO, hydronephrosis usually shows marked improvement on ultrasound within 3-6 months, with further gradual normalization over 1-2 years. Even after a successful operation the renal pelvis can be a little dilated and this does not mean the operation has failed if the MAG3 scan shows improved drainage. Mild hydronephrosis usually resolves within days to weeks after removal of the stone obstruction. Chronic obstruction leads to markedly dilated kidneys that may have some permanent degree of collecting system dilatation even after relief of the obstruction.
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