Bladder Outlet Obstruction

Bladder outlet obstruction is impaired emptying of the bladder due to anatomical or functional obstruction at the bladder neck, prostate or urethra. It increases detrusor pressure, causes hypertrophy and trabeculation, and leads to incomplete emptying, retention and, if prolonged, bilateral hydroureteronephrosis and chronic kidney disease. It is the commonest cause of postrenal acute kidney injury in men over 50 years.

Causes by sex and age:

• Men: benign prostatic enlargement (the commonest; present in over 50% of men aged 60 years), prostate cancer, urethral stricture (post-instrumentation, infection, trauma), bladder neck contracture after surgery, phimosis, urethral calculus, clots
• Women: pelvic organ prolapse (cystocele), uterine fibroids and retroverted gravid uterus, urethral stricture or diverticulum, post-surgical (anti-incontinence procedures), Fowler syndrome (impaired sphincter relaxation in young women), bladder neck dysfunction
• Children: posterior urethral valves in boys, urethral atresia, ureterocele prolapse, phimosis, meatal stenosis
• Functional and neurological: detrusor sphincter dyssynergia (spinal cord injury, multiple sclerosis), diabetic neuropathy, anticholinergics, opioids, alpha agonists (pseudoephedrine), calcium channel blockers, antipsychotics, tricyclic antidepressants, constipation, post-operative retention, delirium

Patterns of retention:

• Acute retention: painful inability to void, retained volume usually 500 mL to 1 L
• Chronic retention: painless, residual volume above 300 mL (high pressure chronic retention: above 800 mL, hydronephrosis, impaired renal function, nocturnal enuresis)
• Retention with overflow incontinence

• Lower urinary tract symptoms: hesitancy, weak stream, intermittency, straining, terminal dribbling, incomplete emptying, frequency, nocturia, urgency
• Acute retention: severe suprapubic pain and distress, palpable tender bladder
• Chronic retention: painless distended bladder, overflow dribbling, nocturnal enuresis, recurrent urinary tract infection
• Haematuria from bladder stones, tumours or prostatic vessels
• Renal impairment, hypertension, hyperkalaemia, anaemia in chronic high pressure retention
• Rectal examination: size, consistency, nodularity or fixation of the prostate; anal tone and perineal sensation to exclude cord compression
• Examination of the external meatus and genitalia for phimosis, meatal stenosis, stricture; abdominal and pelvic examination in women

Red flags: acute retention with fever, anuria, hydronephrosis, creatinine above 200 micromol/L, haematuria with clots, new back pain with leg weakness or saddle anaesthesia (cauda equina compression), hard fixed prostate, weight loss, bone pain, retention in a young man or boy.

• Bladder scan: post-void residual (above 100 mL abnormal, above 300 mL significant).
• Urinalysis, urine culture; creatinine, urea, electrolytes, bicarbonate.
• Prostate specific antigen: interpret after at least 2 weeks following catheterisation or retention (raised falsely by retention and instrumentation); above 4 microg/L, or exceeding the age specific upper limit, needs further evaluation.
• Uroflowmetry: peak flow below 10 mL/s with a voided volume above 150 mL indicates obstruction.
• Symptom score (International Prostate Symptom Score): 0 to 7 mild, 8 to 19 moderate, 20 to 35 severe.
• Renal tract ultrasound for hydronephrosis, bladder wall thickness above 5 mm, bladder diverticula, calculi and prostate volume; transrectal ultrasound for prostate volume above 30 to 40 mL.
• Retrograde urethrography and micturating cystourethrography (stricture, valves); flexible cystoscopy for stricture, bladder neck lesions or tumour.
• Pressure flow urodynamics when the diagnosis is uncertain before surgery or in neurological disease.
• Cauda equina syndrome: urgent magnetic resonance imaging of the spine.

Differentials: detrusor underactivity, neurogenic bladder, drug induced retention, constipation related retention, urinary tract infection, overactive bladder, polyuria.

Acute retention

• Urethral catheter (14 to 16 Fr; 18 Fr if clots) after local anaesthetic gel; suprapubic catheter if urethral catheterisation fails or is contraindicated (suspected urethral injury, stricture, severe prostatitis).
• Drain fully; clamping is not needed. Measure the residual volume. Observe urine output hourly for 24 to 48 hours; expect diuresis if the retained volume is above 1 L or creatinine is raised.
• Alpha blocker before the voiding test: tamsulosin 400 microg once daily (or alfuzosin 10 mg daily, silodosin 8 mg daily) starting 2 to 3 days before removal; remove at 24 to 72 hours; success (voiding after catheter removal) in about 50% when the retained volume is below 1 L and the patient is under 65 years.
• Treat precipitants: constipation (macrogol 1 to 2 sachets daily), anticholinergic and opioid withdrawal, urinary infection, pain control.

Medical therapy for benign prostatic enlargement

• Alpha blocker: tamsulosin 400 microg daily, doxazosin 4 to 8 mg daily (titrate from 1 mg; orthostatic hypotension warning), alfuzosin 10 mg daily; improves flow within days.
• 5-alpha reductase inhibitor for prostate volume above 30 mL or PSA above 1.5 microg/L: finasteride 5 mg daily or dutasteride 0.5 mg daily for at least 6 months; reduces risk of retention and surgery by about 50% over 4 years; halves PSA (double the value to interpret).
• Combination alpha blocker and 5-alpha reductase inhibitor in moderate to severe symptoms with enlarged prostate.
• Antimuscarinic or beta-3 agonist (mirabegron 25 to 50 mg daily) only for storage symptoms after confirming residual volume below 150 mL.
• Phosphodiesterase-5 inhibitor: tadalafil 5 mg daily for coexisting erectile dysfunction (not with nitrates).

Surgery

• Indications: refractory retention, recurrent infection, bladder stones, renal impairment, haematuria, failed medical therapy.
• Transurethral resection or laser enucleation (holmium) of the prostate; bladder neck incision for small prostates; prostatic urethral lift or water vapour therapy in selected men.
• Urethral stricture: dilatation or urethrotomy (recurrence in over 50%), urethroplasty for recurrent or long strictures; intermittent self-catheterisation to maintain patency.
• Suprapubic catheter or intermittent self-catheterisation (12 to 14 Fr, four to six times daily) for neurogenic retention; detrusor sphincter dyssynergia: baclofen, botulinum toxin to the sphincter.
• Posterior urethral valves: catheter and fluid management first, then endoscopic ablation; monitor for post-obstructive diuresis and long term renal function.
• Prostate cancer: androgen deprivation (leuprorelin 22.5 mg every 3 months), radiotherapy, or palliative resection.
• Women: pessary or surgical repair of prolapse; correction of fibroid or retroverted uterus; sacral neuromodulation or intermittent self-catheterisation in Fowler syndrome.

Renal protection

• Check creatinine and electrolytes daily after decompression; monitor for post-obstructive diuresis (output above 200 mL/h for 2 hours): replace 50% to 75% of the previous hour's output with 0.45% saline and review potassium, magnesium and sodium every 6 hours.
• Haematuria ex vacuo after rapid decompression usually settles without treatment; continuous bladder irrigation is needed only for persistent clots.
• Antibiotics only for infection (nitrofurantoin 100 mg modified release twice daily for 3 days if eGFR above 45; trimethoprim 200 mg twice daily for 3 days where susceptible); do not give antibiotics for asymptomatic bacteriuria in catheterised patients.

Referral. Urology for acute or chronic retention, failed voiding test after catheter removal, renal impairment, haematuria, stricture, suspected malignancy. Neurology or neurosurgery for cord compression.