Acute Urinary Tract Obstruction

Acute urinary tract obstruction is sudden impairment of urine flow at any level from the renal pelvis to the urethral meatus, producing a rise in pressure proximal to the block, hydronephrosis in most cases and a fall in glomerular filtration.

It is classified by level (upper tract proximal to the vesicoureteric junction, lower tract at or distal to the bladder neck), by laterality (unilateral, bilateral, or in a solitary kidney), and by completeness (partial or complete). Obstruction causes about 5% to 10% of acute kidney injury in hospital and a larger share in community onset injury in older men.

Obstruction raises intratubular and Bowman capsule pressure, which opposes filtration. Renal blood flow follows a triphasic response that explains why a kidney that appears to be functioning on day 1 can fail later if the block persists.

Recovery depends on duration and completeness. After complete obstruction, function is largely recoverable if relief occurs within 1 to 2 weeks; recovery falls progressively beyond 6 weeks and is usually negligible beyond 12 weeks. Partial or intermittent obstruction can erode function silently for years through interstitial fibrosis, tubular atrophy and loss of concentrating ability.

Causes by level:

• Intrarenal and pelvic: calculi, blood clot, papillary necrosis, fungal ball, pelvic ureteric junction obstruction (congenital or acquired), urothelial tumour
• Ureteric (intrinsic): calculi, stricture (post-surgical, radiation, schistosomiasis, tuberculosis), tumour, clot, sloughed papilla, ureterocele
• Ureteric (extrinsic): retroperitoneal fibrosis, retroperitoneal or pelvic malignancy and nodes (cervical, prostate, bladder, colorectal, lymphoma), aortic aneurysm, pregnancy, endometriosis, iatrogenic ligation or injury during pelvic surgery
• Bladder: outlet obstruction from prostatic enlargement, urethral stricture, clots, neurogenic bladder, bladder stone or tumour, anticholinergic or opioid induced retention, constipation
• Urethra: stricture, posterior urethral valves in boys, phimosis, urethral calculus, foreign body

Presentation depends on level, speed of onset and laterality.

• Upper tract: severe colicky loin pain radiating to the groin, nausea and vomiting, haematuria; painless in slow onset obstruction
• Lower tract: suprapubic pain and distension, inability to void (acute retention), hesitancy, poor stream, dribbling, nocturia; overflow incontinence in chronic retention
• Urine output: normal or increased in partial obstruction (loss of concentrating ability), reduced or fluctuating in high grade lesions, anuria in complete bilateral or solitary kidney obstruction or complete outlet obstruction
• Palpable or percussable bladder, tender loin, enlarged smooth prostate or hard irregular prostate on rectal examination, pelvic mass on vaginal examination
• Infection: fever, rigors and loin pain in obstructed pyelonephritis; hypotension and sepsis in pyonephrosis
• Metabolic: hyperkalaemia, hyperchloraemic acidosis from distal tubular dysfunction, hyponatraemia, rarely hypertension from volume expansion or renin release

Red flags: infected obstructed system, anuria, solitary or transplanted kidney, bilateral obstruction, potassium above 6.0 mmol/L, creatinine above 500 micromol/L, hypotension, retention volume above 1 L, haematuria with clots, spinal cord compression signs (cauda equina syndrome with saddle anaesthesia).

• Immediate bladder scan: post-void residual above 100 mL abnormal, above 300 mL acute retention, above 800 mL chronic high pressure retention.
• Bloods: creatinine, urea, electrolytes, bicarbonate, calcium, urate, full blood count, C-reactive protein, lactate; blood cultures when febrile.
• Urinalysis and culture; urine pH (above 7.5 suggests urease producing organisms and struvite).
• Renal tract ultrasound: dilated calyces and pelvis, cortical thickness, perinephric fluid, bladder wall thickness, prostate size. Hydronephrosis is absent in up to 5% of true obstruction (early, volume depleted, staghorn calculi, retroperitoneal fibrosis encasing the collecting system) and present without obstruction in pregnancy, vesicoureteric reflux, high urine flow and after relief of a previous block.
• Doppler resistive index above 0.7 or a difference between kidneys above 0.1 supports obstruction.
• Low dose non-contrast CT of the kidneys, ureters and bladder identifies the level and cause of stones with sensitivity and specificity above 95%; CT urogram or contrast CT characterises masses, strictures and extrinsic lesions.
• Diuretic renography with mercaptoacetyltriglycine (MAG3): washout half time above 20 minutes indicates obstruction, below 10 minutes excludes it; differential function below 40% marks a poorly functioning kidney.
• Antegrade or retrograde pyelography at the time of drainage; magnetic resonance urography where radiation is undesirable (pregnancy, children).
• Pressure flow urodynamics for lower tract obstruction when the cause is unclear.

Differentials: non-obstructive hydronephrosis (reflux, pregnancy, polyuria), acute tubular injury, renal artery occlusion, aortic dissection, pyelonephritis without obstruction, ovarian torsion, appendicitis, testicular torsion (loin or groin pain).

Emergency: infected obstruction

• Obstruction with fever, sepsis or pyonephrosis is a surgical emergency: decompress within hours by percutaneous nephrostomy or retrograde ureteric stent (equivalent efficacy); do not delay for definitive stone treatment.
• Blood cultures, then antibiotics within 1 hour: piperacillin-tazobactam 4.5 g intravenously every 6 to 8 hours, or ceftriaxone 2 g daily; meropenem 1 g every 8 hours if extended spectrum beta lactamase risk. Add a single dose of gentamicin 5 mg/kg for septic shock if eGFR permits. Adjust to cultures from urine drawn at the time of drainage.
• Resuscitate with balanced crystalloid in 250 to 500 mL aliquots; noradrenaline 0.05 to 0.5 microg/kg/min if hypotension persists after volume.

Lower tract obstruction

• Urethral catheter (14 to 16 Fr, 18 Fr if clots); suprapubic catheter if urethral passage fails (stricture, false passage) or is contraindicated (urethral injury).
• Drain the bladder completely; clamping is unnecessary. Record hourly output for 24 to 48 hours.

Upper tract: non-infected

• Urgent decompression if bilateral, solitary kidney, acute kidney injury, refractory pain, potassium above 6.0 mmol/L or high grade obstruction.
• Calculus up to 10 mm with good function and controlled pain: analgesia (diclofenac 50 mg three times daily for 5 days when eGFR above 60 and no volume depletion, or paracetamol 1 g four times daily plus an opioid), antiemetic (ondansetron 4 mg), tamsulosin 400 microg daily for up to 4 weeks for distal ureteric stones 5 to 10 mm, and review at 2 weeks with imaging.
• Definitive treatment: ureteroscopy with laser lithotripsy, shock wave lithotripsy for stones up to 20 mm in the kidney or upper ureter, percutaneous nephrolithotomy for stones above 20 mm.
• Retroperitoneal fibrosis: ureteric stents or nephrostomy to relieve obstruction, then prednisolone 0.5 to 0.75 mg/kg daily (maximum 60 mg) for 4 to 6 weeks, tapering over 6 to 12 months; steroid sparing agents (methotrexate 15 to 25 mg weekly, mycophenolate mofetil 1 g twice daily, azathioprine 2 mg/kg daily, tamoxifen 20 mg twice daily) in relapse.
• Malignant obstruction: stent or nephrostomy chosen by life expectancy, anticipated chemotherapy and patient wishes.

Metabolic and renal

• Hyperkalaemia: calcium gluconate 10% 10 mL, insulin 10 units with glucose 25 g, salbutamol 10 to 20 mg nebulised.
• Stop NSAIDs, ACE inhibitors and angiotensin receptor blockers during the acute phase; avoid contrast unless it alters management.
• Anticipate post-obstructive diuresis after relief of bilateral or high grade obstruction (urine output above 200 mL/h for 2 consecutive hours); monitor hourly output, sodium, potassium, magnesium and creatinine every 6 hours.

Follow up. Creatinine and ultrasound at 4 to 6 weeks; split renography to quantify recovery; metabolic stone screen for recurrent stones; urology follow up for the cause.

Special groups. Pregnancy: physiological right sided hydronephrosis is common; for symptomatic obstruction with infection or kidney injury insert a ureteric stent or nephrostomy and treat with cefalexin 500 mg four times daily or ceftriaxone 1 to 2 g daily. Children: posterior urethral valves in boys need catheter drainage, then valve ablation; pelvic ureteric junction obstruction is assessed with renography and managed by pyeloplasty if function declines.