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This is a clinician-written, evidence-based summary aligned to the 2026 MLA Content Map. It is intended for medical students and junior doctors preparing for the UKMLA. Always cross-reference with NICE guidance, local protocols, and clinical judgement.
Key points
- Acute retention: sudden onset, painful, palpable/percussable bladder. Commonest cause in men: BPH. Also drugs (anticholinergics, opioids), constipation, post-operative
- Chronic retention: painless, large residual volumes (>1 L), may present with overflow incontinence or obstructive AKI (high-pressure retention)
- Immediate management: urethral catheterisation (12–14 Ch male, 12 Ch female). If fails → suprapubic catheter
- Acute retention: alpha-blocker (tamsulosin) + TWOC (trial without catheter) at 24–72 hours. ~50% void successfully
- Watch for post-obstructive diuresis after catheterisation for chronic retention — can cause severe dehydration and electrolyte disturbance
Overview
Urinary retention is the inability to voluntarily empty the bladder. Acute retention is characterised by sudden painful inability to void with a distended bladder — a urological emergency requiring catheterisation. Chronic retention is a painless, progressive condition with large residual urine volumes. In men, BPH is by far the commonest cause. Other causes include prostate cancer, urethral stricture, neurological (cauda equina, MS, diabetes), pharmacological (anticholinergics, alpha-agonists, opioids, anaesthetics), faecal impaction, post-operative (especially after spinal/pelvic surgery), and pain. In women, causes include pelvic organ prolapse, gynaecological tumours, and neurological conditions.
Epidemiology
Acute urinary retention affects approximately 1 in 10 men over age 70 during their lifetime. It is the commonest urological emergency. Incidence rises steeply with age and prostate size. Post-operative urinary retention affects 5–70% of surgical patients (depending on the type of surgery and anaesthesia). Chronic retention is often unrecognised until it presents with overflow incontinence or renal impairment.
Clinical Features
Symptoms
Acute: sudden severe suprapubic pain with inability to pass urine
Chronic: often painless with gradual onset of difficulty voiding, poor stream, overflow incontinence (continuous dribbling)
History of LUTS (BPH), constipation, recent surgery, or new medications
Back pain, bilateral leg weakness, saddle anaesthesia, bowel disturbance (suggests cauda equina)
Signs
Palpable distended bladder (suprapubic) — dull to percussion
Acute: patient in distress, unable to sit still
Chronic: painless distended bladder, may reach umbilicus
DRE: enlarged prostate (BPH), hard prostate (cancer), loaded rectum (constipation)
Neurological deficit in lower limbs (cauda equina)
Investigations
First-line
Bladder scan (bedside USS)Confirms retention — residual volume >400 mL in acute, often >1 L in chronic. Quick, non-invasive
Document volume drained on catheterisationImportant — volume >800 mL supports high-pressure chronic retention. Volume drained guides management
U&EsRenal function — chronic retention can cause bilateral obstruction → AKI (raised creatinine, hyperkalaemia)
Second-line
DREProstate assessment (BPH vs cancer), rectal loading (constipation)
Urinalysis and MSUExclude UTI as precipitant
PSACheck after catheterisation has settled (PSA may be acutely raised from retention itself) — wait 1–2 weeks for accurate reading
Specialist
Renal USSAssess for hydronephrosis if raised creatinine or chronic retention suspected
Urgent MRI spineIf any suspicion of cauda equina syndrome (back pain + retention + neurological signs)
1
Acute retention
- Urethral catheterisation (12–14 Ch for males, 12 Ch for females) — immediate relief
- If urethral catheterisation fails (stricture, false passage): suprapubic catheter under USS guidance
- Start tamsulosin 400 µg OD (relaxes prostatic smooth muscle, improves TWOC success)
- Trial without catheter (TWOC): remove catheter at 24–72 hours after starting tamsulosin. ~50% void successfully
- If TWOC fails: long-term catheter or listed for TURP/surgical intervention
2
Chronic retention
- Catheterise — may drain >1 L painlessly
- Monitor for POST-OBSTRUCTIVE DIURESIS: massive urine output (>200 mL/h) after relief of chronic obstruction. Can cause severe dehydration, hyponatraemia, hypokalaemia
- IV fluid replacement guided by urine output and electrolytes (replace ~50–80% of hourly output)
- Correct renal impairment — creatinine often improves significantly after drainage
- Long-term management: catheter (urethral or suprapubic) ± surgical treatment of underlying cause
3
Treat underlying cause
- BPH: medical (tamsulosin ± finasteride) or surgical (TURP)
- Constipation: laxatives and disimpaction
- Medication-related: stop/switch offending drug (anticholinergics, opioids)
- Cauda equina: emergency neurosurgical decompression
- Urethral stricture: urethral dilatation or urethroplasty
Complications
- Post-obstructive diuresis: Massive diuresis after catheterisation of chronic retention — can cause life-threatening dehydration and electrolyte derangement
- Obstructive AKI: Bilateral hydronephrosis from chronic high-pressure retention → reversible with drainage
- UTI and urosepsis: Stagnant urine promotes bacterial growth
- Bladder damage: Chronic overdistension → detrusor muscle decompensation → long-term voiding dysfunction
UKMLA Exam Tips
- 1Acute retention: painful. Chronic retention: painless (bladder gradually stretches). Key clinical distinction
- 2Post-obstructive diuresis: monitor hourly urine output + U&Es after catheterisation of chronic retention. Replace fluids to match ~50–80% of output
- 3Cauda equina syndrome: urinary retention + bilateral leg weakness + saddle anaesthesia → emergency MRI spine. Do NOT miss this
- 4TWOC success is higher with tamsulosin pre-treatment (~50% vs ~25% without)
- 5Do NOT check PSA immediately after catheterisation (falsely elevated). Wait 1–2 weeks
- 6Common precipitants of acute retention: constipation, anticholinergics, opioids, anaesthesia, alcohol, UTI
practicetest your knowledge on Urinary RetentionApply what you've learnt with UKMLA-style questions from the iatroX Q-Bank — Renal and beyond.
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