Hydronephrosis and urinary tract obstruction

• Hydronephrosis is the swelling of the kidney's collecting system from urine that cannot drain; urinary tract obstruction is the blockage that causes it, anywhere from the renal tubules to the external opening of the urethra.
• The kidney is the casualty, not the villain: reduced filtration, stasis and infection are what do damage, and much of that damage is preventable if the blockage is found and relieved in time.
• Pain maps to the level of obstruction: flank pain for the renal pelvis and ureter, suprapubic or urethral pain for the bladder and below; complete blockage low in the tract causes anuria.
• The site of obstruction is found by a sequence of simple tests: bladder volume after voiding, then renal ultrasound, then CT or a diuretic renogram depending on what the first tests show.
• Relief comes first and diagnosis second in danger: when infection accompanies obstruction, or a single functioning kidney is blocked, drain it now, by catheter, nephrostomy or stent, before completing investigations.

By the end of this page you should be able to:

1. Define hydronephrosis and obstructive uropathy, and name the causes at each level of the urinary tract from tubules to urethra.
2. Explain why obstruction damages kidney function, and why unilateral obstruction does not reduce urine output while complete low obstruction causes anuria.
3. Recognise the presentations that are emergencies: infected obstructed system, obstruction in a solitary or single functioning kidney, anuria, rising creatinine, and antenatal bladder outlet obstruction.
4. Choose and sequence the right tests (bladder volume, ultrasound, CT, diuretic renogram) for the suspected level of obstruction.
5. Plan immediate and definitive management: drainage by catheter, nephrostomy or stent, treatment of the cause, and safe fluid and electrolyte management after relief.
6. Describe complications, prognosis, and how the approach changes in pregnancy, children, older adults and in low-resource settings.

Hydronephrosis and obstructive uropathy are usually discussed together because hydronephrosis is the visible consequence: the kidney swells above the blockage while the urine below it may be scant. A related but distinct point is that not every dilated collecting system is obstructed. Vesicoureteral reflux, a non-obstructive megaureter, pregnancy and a congenital intrarenal pelvis can all mimic obstruction on imaging. Where the cause is genuinely unclear, a diuretic renogram is used to show whether washout is delayed.

Obstructive uropathy does not have a single global incidence figure, because it is a mechanism with many causes. Its age distribution is bimodal. In childhood, congenital anomalies of the urinary tract dominate, including posterior urethral valves in boys and obstruction at the ureterovesical or ureteropelvic junction. After a quiet middle adult life, incidence rises again after age 60 years, especially in men, where benign prostatic hyperplasia and prostate cancer become common, and where urethral stricture, calculi and neurogenic bladder add to the list. Obstructive uropathy is an uncommon cause of end-stage kidney disease overall, but it is a common and usually reversible cause of acute kidney injury when it is caught. ^[1]

Among newborns, antenatal ultrasound detects hydronephrosis in 0.6 to 5.4 percent of pregnancies, depending on the criteria used, and it is transient and resolves by the third trimester in roughly half of those cases. Most of the remainder need no intervention, but the minority that are significant, particularly lower tract obstruction, can threaten kidney development if missed. ^[4]

Obstruction may occur at any level, from the renal tubules to the external urethral meatus. The table groups the causes the way a clinician meets them: by where the blockage sits and what usually produces it.

The clinical shorthand by age is useful at the bedside: children usually have congenital anomalies, young adults usually have calculi, and older adults, especially men over 60, usually have benign prostatic hyperplasia, prostate cancer, calculi, urethral stricture and neurogenic bladder working alone or together. Obstruction is much more common in men, largely because of the prostate, but acquired and congenital urethral strictures and meatal stenosis occur in both sexes, and women can be obstructed by a pelvic mass or a post-treatment stricture.

A special note on pregnancy. Hydronephrosis in late pregnancy is often physiological, more pronounced on the right, produced by ureteric compression by the gravid uterus and by progesterone-induced ureteric relaxation; it can affect up to 80 percent of pregnant women and is usually asymptomatic. The distinction that matters is between this benign dilation and a true obstruction with features of the causes above, infection or falling renal function. Vesicoureteral reflux is not an obstruction, but it predisposes to recurrent infection and must be thought of when a dilated system is found in child or adult.

Vesicoureteral reflux deserves its own line. In antenatal screening, reflux is found in 8 to 38 percent of fetuses with hydronephrosis, against a general-population figure under 1 percent, and the severity of the antenatal dilation does not predict the grade of reflux. Reflux does not obstruct, but it adds infection risk and, when combined with infection, scarring. Posterior urethral valves are the classic lower-tract obstruction of boys, presenting with bilateral hydroureteronephrosis, a dilated thick-walled bladder that does not empty and a visible dilated posterior urethra. ^[4]

The sequence is the same wherever the level.

1. Behind the obstruction, urine accumulates and the pressure inside the collecting system rises. Proximal to a blockage the effects include increased intraluminal pressure, urinary stasis, infection and sometimes new calculus formation, which can itself worsen obstruction.
2. Rising pressure is transmitted back through the ureter to the renal pelvis and then to the nephron. The pressure in the urinary space of the glomerulus climbs, glomerular filtration falls, and nitrogenous waste accumulates.
3. Obstructive nephropathy is kidney dysfunction, acute kidney injury, chronic kidney disease, failure or tubulointerstitial damage, produced by obstruction. The mechanism includes increased intratubular pressure, local ischaemia and often infection. If obstruction is bilateral, kidney dysfunction follows; with unilateral obstruction in a patient with two functioning kidneys, function is largely preserved, although rene regulation in the unobstructed kidney can occasionally be disturbed by reflex spasm.
4. Dilation of the collecting system takes about three days to develop. Before that, the system is relatively non-compliant, so early obstruction can exist with little or no hydronephrosis on imaging, particularly in acute illness, encased ureters (retroperitoneal fibrosis or tumour) or hypovolaemia. This is a genuine diagnostic trap. ^[1]
5. Microscopically, prolonged obstruction produces dilation of collecting ducts and distal tubules, chronic tubular atrophy and little glomerular damage, the pattern of a slow physical constriction rather than a chemical poisoning.
6. Recovery after relief depends on duration. Short obstruction is usually reversible, with adequate return of function; obstruction persisting for weeks to months may leave permanent tubulointerstitial fibrosis and a fixed loss of glomerular filtration rate. Non-dilated obstruction adds further uncertainty to timing. ^[2]

The key clinical implication is that the kidney's tolerance is measured in days to weeks, not months. Obstruction should be diagnosed and treated as promptly as possible^[1].

The symptoms vary with site, degree and speed of onset.

• Renal pelvis or proximal ureter (upper tract). Flank pain, often severe and colicky when a calculus acutely blocks the ureter; nausea and vomiting commonly accompany it. A large fluid load from alcohol, caffeine or an osmotic contrast load can unmask a partial obstruction by driving urine production above the narrowed segment's capacity. Pain is typically minimal or absent when obstruction is congenital and slow, as in congenital ureteropelvic junction obstruction or a pelvic tumour.
• Mid or lower ureter. Pain that may radiate to the ipsilateral testis or labium, in the T11 to T12 dermatomal distribution.
• Bladder outlet (lower tract). Suprapubic discomfort, poor stream, hesitancy, straining, terminal dribbling, nocturia and frequency. A palpable, percussable or ultrasound-distended bladder may be present.
• Urethra. Difficulty voiding, spraying, weak stream, sprouting or a post-void leak. Absolute anuria occurs with complete obstruction at the bladder or urethra.

Urine volume does not diminish in unilateral obstruction unless the other kidney is missing or non-functioning. It is often the combination of anuria with known single-kidney anatomy, or with a palpable bladder, that makes the diagnosis locally obvious.

• Infection complicating obstruction. Dysuria, urgency and frequency, referred kidney and ureteral pain, costovertebral angle tenderness, fever and, occasionally, septicemia. In infants and young children the presentation can be subtler: vomiting, poor feeding and growth or weight faltering.
• Long-standing obstruction. Increased urine output from polyuria and nocturia, when nephropathy has impaired concentrating capacity and sodium reabsorption; long-standing nephropathy may also result in hypertension. An oliguria-polyuria alternating pattern should raise suspicion.
• Mass. Hydronephrosis may occasionally produce a palpable flank mass, particularly massive hydronephrosis in infancy and childhood.
• Incidental finding. A patient with slow partial obstruction may have no symptoms at all, or only dull, aching flank discomfort, and the hydronephrosis is found on imaging done for another reason.

Prenatal ultrasound finds hydronephrosis in a rising minority of pregnancies. The presentation in utero is an imaging finding in a mother who feels well; the clinically relevant signs are ultrasound measurements of the renal pelvis and the features that suggest lower-tract obstruction: oligohydramnios and a thick-walled or dilated bladder. Postnatally, mild isolated hydronephrosis is common and usually resolves without intervention, while posterior urethral valves present as a serious disease of the newborn with hydronephrosis, sepsis risk and progressive kidney injury if missed.

When obstruction is bilateral or affects a single functioning kidney, the biochemical picture is acute kidney injury. Hyperkalaemia may result from type 1 renal tubular acidosis due to decreased hydrogen ion and potassium secretion by distal segments of the nephron. Elderly men with prostatic obstruction can present with a creatinine that has crept up unnoticed, a normal or increased urine output and hyperkalaemia, with no dramatic pain at all.

These are the situations in which drainage must not be deferred:

• Infected obstructed system (pyonephrosis). Obstruction with infection produces high fever, sepsis, occasionally septic shock and rapid loss of kidney. NICE defines pyonephrosis as an infected and obstructed kidney, and such a system must be decompressed promptly. ^[3]
• Obstruction in a solitary or single functioning kidney. Any obstruction where the other kidney is absent or non-functioning, because there is no reserve.
• Bilateral upper urinary tract obstruction. Both sides blocked, producing acute kidney injury.
• Complete low obstruction (bladder or urethra) with anuria. A full bladder with no urine can lead quickly to acute kidney injury.
• Complications of acute kidney injury from obstruction. Hyperkalaemia, acidosis, fluid overload, pulmonary oedema.
• Suspected lower urinary tract obstruction in a newborn, particularly posterior urethral valves, where urgent bladder decompression is needed.
• A ureteric calculus in a single kidney or with a solitary functioning kidney.

Red flags are summarised as a callout for bedside use.

Hydronephrosis on imaging raises a structured differential, and the first branch is obstructed versus non-obstructed dilation.

The clinically important mimic to remember is retroperitoneal fibrosis or tumour, where ultrasound may be falsely negative because the ureter is encased rather than dilated, despite real obstruction.

Bladder catheterisation and bedside post-void volume. If urine output is diminished, or there is a distended bladder or suprapubic pain, catheterise. If urine flows normally, or the catheter is difficult to pass, a urethral obstruction (prostatic enlargement, urethral valve, urethral stricture) is suspected. In the absence of palpable bladder distention, the volume remaining after voiding can be estimated by bedside ultrasound, and a post-void residual of more than 50 mL (slightly higher in older adults) suggests inadequate emptying. That finding should lead to cystourethroscopy, and children usually undergo voiding cystourethrography. Both threasholds and the local anatomy matter only if the clinician remembers to scan before declaring the findings. ^[1]

Urinalysis and serum chemistries. Urinalysis and serum electrolytes, urea and creatinine are performed in all suspected cases. In an asymptomatic patient with long-standing obstructive uropathy, the urinalysis may be normal or show only a few casts, white or red cells. In a patient with acute kidney injury and a normal urinalysis, bilateral obstructive nephropathy should be considered. Infection with urinary obstruction requires immediate evaluation and treatment. If chemistries show kidney dysfunction, obstruction is probably bilateral and severe or complete, and hyperkalaemia may be present.

Post-void residual in AKI advice. A post-void residual above 100 mL already suggests inadequate emptying when acute kidney injury is present.

Abdominal ultrasound is the initial imaging test of choice in most patients without urethral abnormalities. It avoids contrast, it is safe in pregnancy and children, and it shows dilation of the collecting system. It also estimates bladder volume after voiding and the amount of residual urine, and it looks at renal parenchymal thickness, which hints at chronicity, and kidney size, which hints at reversibility. It is often the imaging modality of choice after ultrasound in children for the next steps.

The limits are worth knowing rather than learning by mistake:

• Ultrasound may be falsely negative in the first few days of obstruction, before dilation has had time to develop.
• It can be falsely normal when the ureter is encased by retroperitoneal fibrosis or tumour, which prevents dilation.
• Hydration state and volume depletion blunt dilation; a dehydrated neonate's ultrasound can look normal when the system is under real pressure.
• Point-of-care ultrasound has a modest positive predictive value for an obstructive cause (61 percent in one prospective study), so a normal study does not settle the question when clinical suspicion is high; non-contrast CT is the fallback. ^[1]

A non-contrast CT is the modality of choice for obstruction caused by ureteral calculi, because it shows even non-radiopaque stones, the site of obstruction and the degree of dilation. Contrast CT is essential in the evaluation of haematuria and urothelial malignancies and is valuable for assessing degree and aetiology of obstruction, but contrast must be minimised in at-risk groups (older age, chronic kidney disease with glomerular filtration rate under 30 mL/min per 1.73 m2, volume depletion, diabetes, heart failure), using the lowest necessary volume of low-osmolal or iso-osmolal agent, with normal saline before, during and after. Thinning of the renal parenchyma on CT suggests more chronic obstruction. ^[2]

MRI with and without contrast can be used when avoiding ionising radiation matters, such as in young children or pregnant women, but it is less accurate than ultrasound or CT for stones. MR urography and CT urography provide the anatomy and excretion pattern, and contrast urography has largely been superseded. Antegrade or retrograde pyelography is preferred when contrast cannot be given intravenously because of azotaemia, or when acute obstruction is thought to be caused by calculi, sloughed papillae or a blood clot and the level is not clear. Retrograde studies pass through the cystoscope; antegrade studies need a percutaneous catheter. Duplex Doppler ultrasound, which detects a raised resistive index in unilateral acute obstruction before the system dilates, is less useful in obesity and in bilateral obstruction, which cannot be distinguished from intrinsic kidney disease.

Diuretic renography answers a specific question: is a hydronephrotic system actually obstructed? It is used with suspected hydronephrosis without apparent obstruction, such as when a calculus, urothelial tumour, sloughed papilla or clot is suspected but not seen, or when an incidentally found hydronephrosis needs characterising, and often in children. A loop diuretic (furosemide 0.5 mg/kg intravenously) is given before a radionuclide renal scan. The patient needs enough residual kidney function to respond. ^[1]

• Washout is delayed if obstruction is present: the radionuclide half-time in the collecting system exceeds 20 minutes, against a normal of under 15 minutes. ^[1]
• With a perfusion pressure-flow study, a positive result produces further dilation of the collecting system and a renal pelvic pressure above 22 mm Hg during perfusion at 10 mL/min. ^[1]
• A renogram or perfusion study that reproduces the patient's pain is interpreted as positive; a negative study in a symptomatic patient makes a non-renal cause more likely.
• False-positive results are common, so the index of suspicion and the clinical picture still drive decisions.
• In children, MAG-3 (mercaptoacetyltriglycine), ethylenedicysteine or DTPA are used, and scanning no earlier than 6 to 8 weeks of age is preferred because of the immaturity of renal handling of tracer earlier; in neonates with severe hydronephrosis or cortical thinning it may be done sooner.

The practical rule that follows is quoted by many sources without agreeing: a hydronephrotic system with no obvious obstruction is operated on if the patient has pain and a positive diuretic renogram; no treatment is needed in an asymptomatic patient with a negative renogram, or with a positive renogram but normal kidney function.

These thresholds come from NICE for acute kidney injury, not from an obstructive-urology guideline, but they are widely applied to obstruction, because obstruction and infection must be excluded urgently.

• If pyonephrosis (infected and obstructed kidney) is suspected, ultrasound within 6 hours of assessment. ^[3]
• If there is no identified cause, or a risk of urinary tract obstruction, urgent ultrasound within 24 hours. ^[3]
• Do not request ultrasound routinely when the cause is already clear.
• Nephrostomy or stenting for upper tract obstruction should be carried out as soon as possible and within 12 hours of diagnosis when there is acute kidney injury. ^[3]
• immediate referral to a urologist for upper tract obstruction when any of the following is present: pyonephrosis; an obstructed solitary kidney; bilateral upper urinary tract obstruction; complications of acute kidney injury caused by urological obstruction.

The treatment depends on the cause, and the cause exists on a spectrum from immediate to elective. But one principle runs through all of it: obstruction should be diagnosed and treated as promptly as possible, because delay leads to irreversible kidney damage^[1]. In practice, basic imaging and prompt drainage take priority over elaborate investigation, particularly when infection or a single functioning kidney is involved.

• Drain obstruction now when any of the following is present: pyonephrosis or infected obstructed kidney; obstruction in a solitary or single functioning kidney; bilateral upper tract obstruction; complications of acute kidney injury from obstruction; complete low obstruction with anuria; suspected posterior urethral valves in a neonate. As stated by NICE, upper tract obstruction with these features is referred to a urologist immediately, and nephrostomy or stenting carried out as soon as possible and within 12 hours of diagnosis^[3]. NICE's list also includes bilateral upper urinary tract obstruction and pyonephrosis^[3].
• Catheterisation for the lower tract. Initial management of lower obstructive uropathy is assessment of urethral patency with prompt and complete bladder decompression by catheterisation, or more proximal drainage if that fails. A urethral catheter kept in place can monitor output; where bladder outlet obstruction is not present and the patient is anuric, any catheter is removed promptly to reduce infection risk.
• Percutaneous nephrostomy for the upper tract. Temporary drainage using a nephrostomy technique may be needed in severe obstructive uropathy, with infection, with calculi, or when a ureteral stent cannot be passed into the collecting system. It is the more robust option when infection is present because it decompresses above the point of trouble.
• Ureteric stent (double-J). Indwelling ureteral catheters can be placed for acute or long-term drainage in selected patients, endoscopically, when the anatomy is passable and infection is not uncontrolled.
• Neonates. Lower urinary tract obstruction requires urgent decompression of the bladder; with posterior urethral valves, cystoscopic ablation is usually performed. Other causes such as ureteropelvic junction obstruction often respond to conservative management.
• Intensive treatment of infection and kidney injury. When UTI or acute kidney injury coexists, treat intensively in parallel with drainage, because the combination is the dangerous-state triad.

The choice between catheter, nephrostomy, stent and surgery follows the site and cause, and is often made with a urologist rather than by the admitting clinician, but the principle is uniform: the first hour belongs to decompression.

Drainage buys time; treatment of the cause cures. Depending on the aetiology this consists of surgery, instrumentation (catheters, stents, nephrostomy tubes, endoscopy such as cystoscopy and ureteroscopy), lithotripsy, or medical therapy (for example hormonal therapy for prostate cancer).

• Calculi. Analgesia and fluids, then definitive stone management (ureteroscopy, extracorporeal shock wave lithotripsy, or percutaneous nephrolithotomy depending on size and site) once infection is excluded or controlled.
• Benign prostatic hyperplasia. Alpha-blocker for faster flow, 5-alpha-reductase inhibitor for long-term volume reduction with large glands, catheter if retention, then transurethral resection or another procedure if retention recurs or the gland is large.
• Urethral stricture. Urethral dilation or urethrotomy for short lesions; urethroplasty for longer or recurrent strictures; suprapubic drainage if unpassable.
• Prostate cancer. Hormonal or surgical relief of obstruction alongside oncological treatment.
• Posterior urethral valves (neonates). Catheterisation to decompress then endoscopic ablation of the valves; valve ablation is the standard, with vesicostomy in selected cases.
• Ureteropelvic junction obstruction and non-refluxing megaureter. Conservative management is appropriate for an obstructive renogram pattern when differential renal function exceeds 40 percent, with serial ultrasound and repeat renography if findings worsen; surgery (pyeloplasty or reimplantation) is considered with pain, recurrent febrile infection, falling differential function, or a reduction in differential function of more than 5 to 10 percent on repeat studies. Most experts consider pyeloplasty when obstructed drainage is combined with differential renal function below 40 percent, while others propose surgery below 35 percent or with prolonged half-time above 20 minutes; conservative management is appropriate above 40 percent. These thresholds come from paediatric and antenatal guidelines^[4].
• Retroperitoneal fibrosis. Medical therapy (for example corticosteroids or tamoxifen) with ureterolysis, plus stenting; the encasement pattern means that obstruction should be suspected even without dilation.
• Vesicoureteral reflux. Not an obstruction, but a cause of hydronephrosis-like dilation: antibiotic prophylaxis through the first year for screened infants with reflux, parents counselled about infection risk, with surgery for breakthrough infections or high-grade reflux with scarring.
• Pregnancy. Physiological hydronephrosis is observed. True obstruction (most often a calculus) is managed conservatively first (analgesia, fluids, drainage only if infection or falling function supervenes); if a stone and ureteric stent is needed, the intervention is chosen to minimise radiation and anaesthetic risk. A ureteric stent during pregnancy is a judgement call made by obstetric and urology teams together.

Once a high or low obstruction is relieved, a brisk diuresis often follows. Much of that is physiologically appropriate: extracellular fluid expanded during the obstructed phase is being excreted, and it does not compromise volume status. But the picture can be dangerous when polyuria is accompanied by large losses of sodium, potassium, magnesium and other solutes, producing hypokalaemia, hyponatraemia, hypernatraemia (if free water is not provided), hypomagnesaemia or marked contraction of extracellular fluid with peripheral vascular collapse.

• Close attention to fluid and electrolyte balance is mandatory in this postoliguric phase after relief.
• Do not over-replace salt and water. Overzealous administration after relief can prolong the diuresis by sustaining the driving osmotic load.
• Replacement formula. When postobstructive diuresis occurs, replacement of urine output with 0.45 percent saline at about 75 percent of urine output prevents volume depletion and the tendency for excessive free water loss, while allowing the body to eliminate excess volume if that is the cause of the polyuria^[2].
• Monitor potassium, sodium and magnesium closely, rechecking renal function and correcting dehydration or deficiency. Much of the problem is potassium, and falling potassium can be silent until the ECG changes.
• Expect the unexpected creatinine course. Urine output may return dramatically while serum creatinine keeps falling for several days, because recovery of tubular concentrating ability lags behind clearance.

If the obstruction has produced acute kidney injury, hyperkalaemia may be present and is treated with the standard regimen: an intravenous infusion of 10 mL of 10 percent calcium gluconate, 50 g of dextrose and 5 to 10 units of insulin, which stabilises the myocardium and shifts potassium intracellularly but does not remove it, followed by binders, diuretics or dialysis as needed^[2]. Calcium can be repeated if the electrocardiographic changes persist. Severe, unresponsive or worsening hyperkalaemia is an indication for dialysis.

A practical checklist for the first 24 to 72 hours after decompression:

• Urine output, serially, charted rather than estimated.
• Serum electrolytes including potassium, sodium and magnesium several times a day while the diuresis is brisk.
• Renal function (creatinine, urea) at least daily.
• Weight and fluid balance, to judge the true volume state.
• Blood pressure, because postobstructive diuresis can convert hypertension to hypotension.
• Infection markers (temperature, white cell count, CRP, urine culture) where infection was present.
• Imaging to confirm that the system has actually decompressed; a nephrostogram or check ultrasound if drainage appears inadequate.
• Late renal function at 3 months, because even a transient episode of obstructive acute kidney injury raises the long-term risk of chronic kidney disease.
• For neonates and young children, sequential ultrasound to watch for recurrence.

Neonates and children. This is where hydronephrosis is most often first seen, usually without symptoms, and managed most conservatively. Antenatal diagnosis thresholds used by widely cited guidelines are: hydronephrosis present when the fetal renal pelvic anteroposterior diameter is 4 mm or more in the second trimester and 7 mm or more in the third trimester, graded mild, moderate or severe. Almost 80 percent of cases detected in the second trimester resolve or improve in utero. All newborns with antenatal hydronephrosis should undergo postnatal ultrasound in the first week, within 24 to 48 hours when posterior urethral valves, oligohydramnios or severe bilateral hydronephrosis is suspected. Normal postnatal studies should be repeated at 4 to 6 weeks. Infants with isolated mild hydronephrosis (renal pelvic diameter under 10 mm or Society of Fetal Urology grade 1 to 2) can be followed with ultrasound alone, because clearance is the rule. Moderate or severe hydronephrosis (over 10 mm, grade 3 to 4) or dilated ureters warrant investigation for obstruction or reflux, with diuretic renography for suspected ureteropelvic junction or ureterovesical junction obstruction. Antibiotic prophylaxis is given to infants with moderate or severe hydronephrosis or dilated ureters while awaiting evaluation, and to all with vesicoureteral reflux through the first year. Surgery is reserved for obstructed drainage with reduced or falling differential renal function; ordinary conservative management is appropriate above 40 percent differential function. Most hydronephrosis resolves during the first two years of life, occasionally later. Antenatal hydration thresholds shown to benefit therapeutic intervention (fetal vesicocentesis electrolytes) include falling sodium below 100 mEq/L, calcium below 8 mg/dL and osmolarity below 200 mOsm/L, facts that belong to fetal medicine teams rather than general practice^[4].

Pregnancy. Hydronephrosis is often physiological and right-sided. Suspected true obstruction is investigated bearing in mind that ultrasound and MRI avoid radiation, while CT and renography add radiation and contrast exposure. Management is conservative first, with drainage only when infection or falling renal function appears.

Older adults. Obstruction in men over 60 is usually benign prostatic hyperplasia, calculi, stricture or cancer working together. These patients can be oligo-asymptomatic, creatinine rising quietly, and a small or normal-looking bladder on imaging does not exclude obstruction when retroperitoneal fibrosis or an encased ureter is present.

Immunocompromised patients. They develop infection at lower thresholds and should be drained earlier.

• Infection, from simple cystitis to pyelonephritis and pyonephrosis, becoming life-threatening when obstruction is present; and septicemia.
• Kidney damage, acute and, if prolonged, chronic: tubular atrophy, fibrosis and fixed loss of glomerular filtration rate.
• Hypertension, from long-standing obstructive nephropathy.
• Vesicoureteral reflux and scarring, when lower tract obstruction leads to valve and ureteric dysfunction.
• Bladder complications, such as trabeculation and diverticula formation from chronic bladder outlet obstruction, which raise residual volume and infection risk.
• Acute kidney injury and its companions: hyperkalaemia, metabolic acidosis, fluid overload and pulmonary oedema.
• Post-obstructive diuresis, which can produce dehydration, electrolyte disturbance and hypotension after relief.
• Vesicoureteral reflux alone doubles infection risk and predisposes to renal scarring in children, and it coexists with obstruction in 7 to 18 percent of paediatric cases.

Most obstruction can be corrected. The prognosis depends on the cause, location, degree and duration of the obstruction, and on whether an infection supervenes and is treated. In general:

• Acute kidney injury from a ureteral calculus is reversible with adequate return of kidney function.
• Chronic progressive obstructive uropathy may produce partial or complete, and largely irreversible, loss of function.
• Untreated infection accompanying obstruction worsens prognosis materially: pyonephrosis and urosepsis convert a mechanical problem into an organ failure emergency.
• Recovery after relief is more likely with shorter duration; weeks to months of obstruction may leave permanent tubulointerstitial damage.
• Even after apparent recovery, recheck kidney function around 3 months, because prior obstructive acute kidney injury raises the long-term risk of chronic kidney disease. Nephrology review is warranted if the estimated glomerular filtration rate is 30 mL/min per 1.73 m2 or less. ^[3]

• Antenatal ultrasound has turned obstructive uropathy from a silent disease discovered at end stage into a treatable childhood condition, and its thresholds (4 mm in the second trimester, 7 mm in the third) are now standard.
• In older men, an annual review of voiding symptoms and a prostate examination catch prostatic obstruction before acute retention.
• Avoiding unnecessary bladder catheterisation removes one surprisingly common, and instantly reversible, cause of obstruction.
• Maintaining fluid intake and avoiding nephrotoxins protects patients with known chronic kidney disease or prior obstructive acute kidney injury.
• Serum creatinine is monitored at least daily in hospital inpatients at high risk, because obstruction becomes diagnosable only after someone asks whether there is one.

The principles do not change when the equipment does, but priorities become sharper.

• Think clinically before technologically. Pulse, lying and standing blood pressure, jugular venous pressure, skin turgor, mucous membranes and measured urine output establish volume status and detect obstruction. A post-void residual above 50 mL on a bladder scan, or a palpable bladder on examination, is a prompt to catheterise.
• Urine dipstick and microscopy separate brown granular casts of tubular injury from red cell casts of glomerular disease, and detect infection.
• Plain ultrasound is the most powerful single test available, even on battery power or at a distant centre. It detects hydronephrosis, measures residual urine and guides nephrostomy placement. A post-void residual above 100 mL in the setting of acute kidney injury suggests inadequate emptying. A full bladder with a stone or mass on ultrasound localises the problem to the low tract. ^[2]
• Fluids. Oral and enteral hydration suit many prerenal components. Intravenous normal saline restores perfusion in shock; absent more sophisticated tests, restoring volume first and adding water by mouth follows the same physiology.
• Obstruction is usually relievable with a catheter or temporary nephrostomy, buying time for definitive surgery or stenting at a referral centre. The technique of percutaneous nephrostomy does not require advanced infrastructure once ultrasound and operator skill are present.
• Dialysis is not always available. Manage the dangerous complications medically: restrict potassium and protein, remove potassium with binders if they exist, control acidosis with bicarbonate where it is stocked, and refer early when any indication for dialysis persists. Peritoneal dialysis with locally prepared solution is sometimes the more available option.
• Refer early and clearly. Anuria, no response to fluids, suspected intrinsic or obstetric acute kidney injury, stage 3 acute kidney injury, hyperkalaemia or acidosis that will not correct, and the need for dialysis are all reasons to move a patient to a setting with the next level of care. NICE's criteria (pyonephrosis, an obstructed solitary kidney, bilateral upper tract obstruction, complications of acute kidney injury from obstruction) are a sensible trigger list anywhere.
• Antenatal scanning is unevenly available but transformative where present; even a single third-trimester ultrasound can identify the fetuses with bilateral severe hydronephrosis who will need prompt postnatal management.
• Patient education is cheap and effective: teach at-risk groups (older men, patients with stones) to seek care early for urinary retention, colic or fever, because delay is what converts a correctable obstruction into permanent kidney loss.

• Hydronephrosis is dilation of the collecting system by urine that cannot drain; urinary tract obstruction is the blockage behind it, at any level from renal tubules to urethra. The two are linked, the first is a finding, the second a mechanism.
• Causes are age-dependent: congenital anomalies in children, calculi in young adults, prostatic enlargement, cancer, stricture, calculi and neurogenic bladder in older men.
• The kidney above the obstruction enlarges under pressure, filtration falls, tubules atrophy and fibrosis develops, with irreversible damage possible after weeks to months of unrelieved high obstruction.
• Unilateral obstruction does not reduce urine output unless the other kidney is absent; absolute anuria means complete obstruction at the bladder or urethra and is a surgical emergency.
• Pain localises by level: flank for the renal pelvis and ureter, groin or scrotal or labial radiation for the lower ureter, suprapubic for the bladder.
• Infection plus obstruction, obstruction of a solitary kidney, bilateral upper obstruction, complete low obstruction with anuria and complications of acute kidney injury are the urgent states that demand drainage before completion of investigations.
• Investigations proceed from post-void residual to ultrasound to CT for stones to diuretic renography when the cause is unclear.
• Ultrasound is the first imaging test of choice but can be falsely negative early, in hypovolaemia and in encased ureters such as retroperitoneal fibrosis or tumour.
• Relief uses catheterisation for the low tract, stenting or percutaneous nephrostomy for the high tract, and surgery, endoscopy, lithotripsy or medical therapy for the cause. With pyonephrosis or a single functioning kidney, drain within 12 hours, sooner if possible.
• After relief, post-obstructive diuresis is common: replace fluid carefully using 0.45 percent saline at about 75 percent of urine output, and monitor potassium, sodium and magnesium closely, without over-replacing.
• Conservative management is appropriate for an obstructed diuretic renogram pattern when differential renal function exceeds 40 percent, with surgery below that or with falling function.
• Complications are infection, kidney damage, hypertension from long-standing disease, bladder changes, post-obstructive diuresis and the electrolyte complications of acute kidney injury.
• Antenatal ultrasound detects hydronephrosis in 0.6 to 5.4 percent of pregnancies and has shifted serious lower-tract obstruction from a late discovery to an early, correctable diagnosis; most mild hydronephrosis resolves.
• Even after recovery, recheck kidney function at around three months and refer for specialist review if the estimated glomerular filtration rate is 30 mL/min per 1.73 m2 or less.

Acute kidney injury; nephrolithiasis; benign prostatic hyperplasia; urethral stricture; vesicoureteral reflux; neurogenic bladder; urinary tract infection and pyelonephritis; antenatal hydronephrosis; chronic kidney disease.