Acute retention of urine is the sudden inability to voluntarily pass urine, resulting in painful distension of the bladder that requires emergency catheterization.

Acute Retention of Urine (AROU)

Acute urinary retention (AUR / AROU) is defined as the sudden inability to pass urine despite having a full bladder ^[1]. It is a painful condition — the patient is acutely distressed, with a palpable, tense bladder and a desperate urge to void but physically cannot.

Let's break the name down:

Acute = sudden onset (hours, not weeks)
Retention = urine is produced but cannot be expelled — the bladder is full
Of Urine = distinguishing it from bowel/other retention

This is fundamentally different from anuria/oliguria, where the kidneys are simply not producing urine (pre-renal or renal failure). In AROU, the kidneys are working fine — the problem is downstream, at the level of the bladder outlet or detrusor muscle ^[2]^[3].

AROU vs CROU vs Anuria – Don't Confuse Them

	AROU	CROU	Anuria/Oliguria
Onset	Sudden	Gradual	Variable
Pain	Painful (innervation intact)	Usually painless (innervation often abnormal)	No suprapubic pain
Bladder	Distended, tense	Distended, may be massive	Not distended
Mechanism	BOO / acute detrusor failure	Chronic BOO / detrusor underactivity	Failure of urine production
Classic cause	BPH (males)	DM neuropathy	Shock, ATN, GN

Students often confuse AROU with oliguria on the wards. The key distinguishing feature: in AROU, the bladder is full and palpable; in oliguria, the bladder is empty ^[2]^[3].

2. Epidemiology

This is best understood through physiology:

Feature	Male	Female
Physiological bladder outlet resistance	Higher (prostate surrounds urethra)	Lower (no prostate, shorter urethra)
Voiding depends more on	Detrusor contraction	Pelvic floor relaxation
Detrusor contraction strength	Stronger (~60 cmH₂O)	Weaker (~20 cmH₂O)
Common AROU mechanism	BOO more common	Detrusor underactivity more common

Because males have higher outlet resistance (prostate), their detrusor muscle must generate more force to void. This means any additional increase in outlet resistance (e.g., BPH) can tip the balance into retention. Females have low baseline resistance, so obstruction is rarely the issue — instead, their weaker detrusor is more susceptible to underactivity ^[2].

3. Risk Factors

These are the "last straw" triggers that push a patient with borderline compensation into overt retention:

Precipitating Factor	Mechanism
Constipation	Faecal impaction in the rectum mechanically compresses the prostatic urethra/bladder neck from posteriorly (constipation is usually not a standalone aetiology but a precipitating factor with background prostatic enlargement) ^[2]
UTI	Pain and inflammation cause reflex urethral sphincter spasm; mucosal oedema further narrows the urethra
Anaesthesia or analgesia	GA and epidural/spinal anaesthesia block the sacral parasympathetic outflow (S2-4) → detrusor cannot contract; opioid analgesics also suppress the micturition reflex centrally
Immobility	Patients find it difficult to void when supine; prolonged recumbency reduces voiding efficiency
Painful perianal conditions	Thrombosed haemorrhoids, perianal abscess → reflex spasm of pelvic floor and urethral sphincter (pain inhibits micturition reflex)
Excessive fluid intake (especially alcohol)	Alcohol is a diuretic (suppresses ADH) → rapidly fills the bladder; alcohol also has a sedating/CNS depressant effect → patient may not respond to urge to void → overdistension
Drugs	See drug-induced aetiology below

4. Anatomy and Physiology of Micturition

Understanding AROU requires understanding normal voiding. Think of it as a hydraulic system with a pump (detrusor), two valves (sphincters), and a pipe (urethra), all under neural control.

This is critical to understanding both normal voiding and neurogenic causes of AROU.

Afferent pathways ^[1]^[2]:

Aδ fibres for signals of distension (normal filling sensation)
C-fibres for signals of irritation/pain (normally silent; become active in pathological states like infection or neuropathy)
Travel along hypogastric, pelvic (majority), and pudendal nerves
Target: higher centres (pontine micturition centre → cerebral cortex)

Efferent pathways — three key nerve supplies:

Nerve	Origin	Neurotransmitter / Receptor	Target	Function
Hypogastric nerve (sympathetic)	T10-L2	Noradrenaline → α₁ receptors (bladder neck/prostate) and β₃ receptors (detrusor)	Bladder neck smooth muscle, detrusor	Storage: α₁ → contracts bladder neck (↑ outlet resistance); β₃ → relaxes detrusor (↓ tone during filling)
Pelvic nerve (parasympathetic)	S2-S4	Acetylcholine → M₃ muscarinic receptors	Detrusor muscle	Voiding: contracts detrusor
Pudendal nerve (somatic)	S2-S4 (Onuf's nucleus)	Acetylcholine → nicotinic receptors	External urethral sphincter (striated muscle)	Voluntary continence: contracts external sphincter; relaxes during voiding

5. Aetiology

The aetiology of AROU can be divided into three major pathophysiological categories: obstruction, detrusor failure, and neurological impairment. In practice, it is often a mixture of factors ^[2].

Table 2: Aetiology of AUR (Murray et al., 1984) ^[1]:

Cause	Percentage
Benign prostatic hyperplasia	53%
Constipation	7.5%
Carcinoma of the prostate	7%
Urethral stricture	3.5%
Clot retention	3%
Neurological disorders	2%
Post-operative	2%
Calculus	2%
Drugs	2%
Infection	2%
Miscellaneous/unknown	16%

BPH accounts for over half of all AROU cases. This is extremely high-yield.

5.2 Mechanical Bladder Outlet Obstruction (BOO) [1][2][3]

Think of it as: where is the blockage? Extramural (outside the tube), mural (in the wall), or intraluminal (inside the tube)?

Category	Causes	Pathophysiological Basis
Extramural	BPH	Transition zone hyperplasia → compresses prostatic urethra from outside. The prostate wraps around the urethra like a doughnut — as it grows, the "hole" gets smaller
	Cancer of prostate	Malignant infiltration/enlargement of prostate → compresses and invades urethra
	Constipation / Faecal impaction	Loaded rectum directly posterior to prostate → mechanical compression of prostatic urethra against pubic symphysis
	Pelvic tumours	Mass effect compressing bladder neck/urethra
	Prostatitis	Acute inflammation → oedema of prostate → compression of prostatic urethra; also painful voiding reflexly inhibits detrusor
	Pregnancy (females)	Gravid uterus compresses bladder neck
	Pelvic organ prolapse (females)	Cystocele kinks the urethra; rectocele/uterovaginal prolapse distorts anatomy
	Gynaecological tumours (females)	e.g., fibroids compressing bladder neck
Mural	Bladder neck stenosis (usually after previous prostate surgery)	Post-operative scarring/fibrosis at bladder neck → stricture
	Urethral stricture (iatrogenic or infection/inflammation)	Previous catheterisation, instrumentation, or STDs (especially gonococcal urethritis) → fibrosis → narrowing of urethral lumen
	Urethritis	Inflammation → mucosal oedema → narrowing; also painful → reflex sphincter spasm
	Bladder tumour	Tumour at bladder neck can physically obstruct the internal urethral orifice
Intraluminal	Bladder/Urethral stone	Stone lodges at bladder neck or urethra → ball-valve obstruction
	Clot retention (severe gross haematuria)	Blood clots fill the bladder and obstruct the bladder outlet — the bladder becomes full of clot and cannot empty
	Foreign bodies	Rare; inserted objects or migrated devices
	Phimosis	Tight foreskin → cannot retract → obstructs external urethral meatus. "Phimosis" from Greek phimos = muzzle

Drugs are a very common precipitant — always ask about new medications!

Drug Class	Examples	Mechanism
Sympathomimetics – α-agonists	Phenylephrine (in cold/cough medications, nasal decongestants), ephedrine, pseudoephedrine	α₁ agonism → contracts smooth muscle at bladder neck and prostatic urethra → ↑ outlet resistance → cannot void
Sympathomimetics – β-agonists	Terbutaline, salbutamol (bronchodilators), isoproterenol	β₂/β₃ agonism → relaxes detrusor smooth muscle → bladder cannot generate enough contraction force; also some β₃ effect directly relaxes detrusor
Anticholinergics	Atropine, ipratropium (bronchodilators), oxybutynin, tolterodine	Block M₃ muscarinic receptors on detrusor → bladder cannot contract. This is literally the same receptor that parasympathetic nerves use to trigger voiding
Antipsychotics	Chlorpromazine, haloperidol, risperidone	Anticholinergic side effects → same as above
Antidepressants	TCAs (amitriptyline), SSRIs (less common)	TCAs have strong anticholinergic properties; also α-adrenergic effects
Antihistamines	Diphenhydramine, chlorpheniramine	First-generation antihistamines have significant anticholinergic effects
Opioids	Morphine, codeine, tramadol	Central suppression of micturition reflex + smooth muscle spasm + anticholinergic-like effects
Antispasmodics	Various	Anticholinergic effect
Disopyramide	Class Ia antiarrhythmic	Strong anticholinergic side effect
MDMA (Ecstasy)	Recreational drug	Sympathomimetic → α₁ agonism at bladder neck

Exam Pearl: The Cold Medicine Trap

A classic exam scenario: elderly man with known BPH takes an over-the-counter cold remedy containing pseudoephedrine (α-agonist) and chlorpheniramine (antihistamine/anticholinergic) → double hit: ↑ outlet resistance + ↓ detrusor contraction → AROU. Always ask about OTC medications!

Level of Lesion	Causes	Mechanism
Brain	Stroke, Parkinson's disease, multiple sclerosis (MS), normal pressure hydrocephalus (NPH), multiple system atrophy (MSA)	Disruption of cortical/subcortical inhibition of micturition reflex. In stroke, may initially get retention (acute "spinal shock"-like phase) before later developing detrusor overactivity
Spinal cord	Trauma, vertebral metastasis, spinal stenosis, transverse myelitis, spinal cord haematoma/abscess, spinal cord tumour, epidural abscess, epidural metastasis	Interruption between PMC and sacral micturition centre → DSD (detrusor contracts but sphincter won't relax) OR loss of detrusor contraction in acute phase. Must rule out spinal cord compression — this is a surgical emergency!
Peripheral nerves	Diabetic neuropathy, radical pelvic surgery (damage to pelvic plexus), Guillain-Barré syndrome (GBS), genital herpes (sacral nerve involvement)	Damage to pelvic parasympathetic nerves → detrusor cannot contract (areflexic/hypotonic bladder). In DM, insidious autonomic neuropathy → progressive loss of bladder sensation and detrusor tone

Red Flag: Spinal Cord Compression

In any patient presenting with AROU + new neurological deficits (limb weakness, sensory level, saddle anaesthesia, loss of anal tone), you must urgently rule out spinal cord compression. This requires emergency MRI spine. Delay can lead to permanent paralysis and bladder dysfunction. A neurological exam including assessment of sensory level is mandatory in every AROU patient ^[3].

Category	Subcategory	Specific Causes
Mechanical BOO	Extramural	BPH, CA prostate, constipation, pelvic tumours, prostatitis, pregnancy, POP, gynae tumours
	Mural	Urethral stricture, bladder neck stenosis, urethritis, bladder tumour
	Intraluminal	Stones, clot retention, foreign bodies, phimosis
Drug-induced	Sympathomimetics	α-agonists (cold meds), β-agonists (bronchodilators), MDMA
	Anticholinergics	Atropine, antihistamines, antipsychotics, TCAs, antispasmodics, opioids, disopyramide
Neurological	Brain	Stroke, Parkinson's, MS, NPH, MSA
	Spinal cord	Trauma, mets, stenosis, myelitis, abscess, tumour
	Peripheral	DM neuropathy, radical pelvic surgery, GBS, genital herpes
Acute overdistension	↑ production	Excessive fluid/alcohol, IV fluids
	↓ voiding	Post-GA/epidural, opioids, immobility, perianal pain

6. Classification

Feature	Acute (AROU)	Chronic (CROU)
Onset	Sudden	Gradual (weeks–months)
Pain	Painful (intact innervation, rapid distension)	Usually painless (slow distension, often abnormal innervation)
Typical cause	BPH	Hypocontractile bladder (e.g., DM neuropathy)
Bladder	Tense, tender, palpable	Large, non-tender, may be massive
Can lead to	Acute post-renal AKI if untreated	Overflow incontinence, chronic renal impairment

7. Pathophysiology — Bringing It All Together

Pathophysiology of Urinary Retention ^[1] — the lecture specifically dedicates a section to this.

7.1 The Three Mechanisms

The pathogenesis is often a mixture of factors ^[2]:

8. Clinical Features

Symptom	Pathophysiological Basis
Acute suprapubic pain	Rapid bladder distension stretches the bladder wall → activates C-fibre nociceptors and Aδ afferents → severe visceral pain. This is the hallmark symptom distinguishing AROU from CROU (where innervation is often impaired, so pain is absent)
Inability to void	The defining symptom. Urine is being produced but cannot exit — either because of outlet obstruction or detrusor failure
Sensation of fullness / urgency	Stretch receptors in the bladder wall (Aδ fibres) are firing constantly, generating an overwhelming urge to void — but the patient cannot
Restlessness and agitation	Severe pain and sympathetic activation from visceral distension
Overflow dribbling (may occur)	If intravesical pressure exceeds urethral resistance at any point, small amounts of urine may leak out — patients may paradoxically appear to be passing urine despite retention. Don't be fooled!
Previous LUTS (in BPH)	Obstructive LUTS: hesitancy, weak stream, straining, dribbling, incomplete emptying → these suggest pre-existing BOO (especially BPH) that has now decompensated into complete retention ^[2]
Haematuria (in some causes)	Gross haematuria may occur: prostatic venous congestion → ruptured vessels; or bladder tumour; or clot retention causing AROU
Fever + dysuria (if associated UTI)	Stagnant urine is a perfect culture medium for bacteria → UTI is both a complication and a precipitant of AROU
Uraemic symptoms (if prolonged retention)	Bilateral obstruction → post-renal AKI → nausea, confusion, pruritus, anorexia
Strangury (if stone)	Strangury = painful, frequent urination of small volumes, expelled slowly only by straining despite severe urgency and feeling of incomplete emptying ^[2] — characteristic of bladder/urethral stones

Sign	How to Elicit	Pathophysiological Basis
Palpable bladder	Suprapubic palpation — a smooth, rounded, tense mass arising from the pelvis; cannot get below it	Bladder palpable when > 200 mL ^[3]. A full, distended bladder rises above the pubic symphysis and becomes palpable abdominally
Suprapubic dullness to percussion	Percuss from umbilicus downward — dull note over distended bladder	Bladder dull on percussion when > 150 mL ^[3]. Fluid-filled bladder transmits sound differently from surrounding air-filled bowel
Suprapubic tenderness	Palpation elicits pain	Acute distension stretching the peritoneum overlying the bladder dome → somatic pain
Vital signs	Tachycardia, hypertension (pain response); may have fever if associated UTI/prostatitis	Sympathetic activation from pain; sepsis if infected
DRE findings ^[3]	Prostate size/tenderness, masses, anal tone	Enlarged smooth prostate = BPH; hard nodular prostate = CA prostate; tender boggy prostate = prostatitis; ↓ anal tone = neurological cause (cauda equina); rectal mass = constipation/tumour
Neurological examination	Sensory level ^[3]; assess lower limb power, reflexes, perineal sensation (S2-4), bulbocavernosus reflex, anal tone	A sensory level suggests spinal cord compression; absent perineal sensation suggests cauda equina syndrome; absent bulbocavernosus reflex suggests sacral nerve damage
External genitalia	Inspect for phimosis (tight foreskin covering meatus), paraphimosis, meatal stenosis, urethral discharge, blood at meatus	Blood at urethral meatus is a contraindication for urethral catheterisation — suggests urethral trauma. Phimosis can physically obstruct the external meatus
Abdominal examination	Check for masses, scars (previous surgery), signs of renal failure	Flank tenderness may suggest hydronephrosis

Examination Checklist for AROU — Don't Forget!

Vitals — pain response, sepsis?
Abdomen — palpable/percussible bladder, tenderness, scars, masses
DRE — prostate (size, consistency, tenderness, median sulcus), rectal mass, anal tone
External genitalia — phimosis, blood at meatus, discharge
Neurological exam — sensory level, lower limb power/reflexes, perineal sensation, anal tone

These are all explicitly mentioned in the lecture slides ^[1] and senior notes ^[2]^[3] and are classic OSCE stations.

When a patient presents with AROU, your thinking should be:

High Yield Summary

AROU = sudden, painful inability to void despite full bladder — most common urological emergency
Epidemiology: predominantly males > 60; 7/1000 men/year; 10% in 70s, 30% in 80s over 5 years; very rare in females (~3/100k)
BPH is the #1 cause (53%) — always the first differential in an elderly male
Three mechanisms: (a) outflow obstruction (mechanical + dynamic), (b) ↓ detrusor contraction (drugs, neurological, overdistension), (c) DSD
Drug-induced AROU: α-agonists (cold meds) ↑ outlet resistance; anticholinergics ↓ detrusor contraction — always ask about new medications
Constipation is usually a precipitating factor, not standalone cause — mechanical compression of prostatic urethra
Acute overdistension damages detrusor by stretching beyond optimal actin-myosin overlap → vicious cycle
Key exam signs: palpable bladder ( > 200 mL), dull to percussion ( > 150 mL), DRE for prostate, neurological exam for sensory level and anal tone
Must rule out spinal cord compression in any AROU with neurological deficits — emergency MRI
AROU ≠ anuria — in AROU the bladder is full; in anuria the bladder is empty
Distinguish AROU (painful) from CROU (painless) — innervation status determines this
BPH pathophysiology: static component (DHT/5α-reductase → physical enlargement) + dynamic component (α₁ receptors → smooth muscle tone) → rationale for 5-ARI + α-blockers

Active Recall - Acute Retention of Urine

Differential Diagnosis of Acute Retention of Urine

The differential diagnosis of AROU is really about answering two sequential questions:

Does this patient truly have urinary retention? — or is the problem actually that urine is not being produced (anuria/oliguria)?
If it is retention, what is the underlying cause? — obstruction, detrusor failure, neurological, drug-induced, or a combination?

This sounds simple, but in practice a distressed, oliguric elderly patient on a surgical ward can easily be mislabelled as "not passing urine" when they are actually in pre-renal AKI from dehydration, or conversely, a patient with overflow dribbling may be assumed to be voiding normally when in fact they have a massively distended bladder. The first branch in your differential thinking is therefore the most critical.

AROU should be distinguished from anuria or oliguria → lack of urine production, not retention ^[2].

Feature	AROU	Anuria / Oliguria
Bladder	Full, distended, palpable, tender	Empty, not palpable
Pain	Suprapubic pain and urgency	No suprapubic distension
Bladder scan	≥ 300 mL ^[2]	Minimal volume
Urine output	Patient feels urge but cannot void; may have overflow dribbling	Genuinely reduced or absent urine output
Pathophysiology	Outflow problem or detrusor failure (urine is produced but trapped)	Production problem (kidneys not making urine)

Causes of anuria/oliguria to rule out ^[2]^[3]:

Pre-renal ARF due to dehydration and shock — hypovolaemia (haemorrhage, vomiting, diarrhoea, burns, sepsis, cardiogenic shock) → ↓ renal perfusion → ↓ GFR → ↓ urine output ^[5]
Renal ARF due to acute tubular necrosis, interstitial nephritis, glomerulonephritis — intrinsic renal damage → kidneys cannot filter ^[5]
Post-renal obstruction at the ureteric level — bilateral ureteric stones, retroperitoneal fibrosis, bilateral ureteric tumours → urine cannot reach the bladder at all; bladder will be empty on scan

The bedside bladder scan is your instant differentiator: a full bladder ( ≥ 300 mL) = retention; an empty bladder = look upstream (kidneys, ureters) or systemically (shock, dehydration).

Classic Exam Trap

A post-operative patient has not passed urine for 8 hours. The surgical team assumes "AROU from anaesthesia" and calls for catheterisation. But the bladder scan shows only 50 mL. This is NOT retention — this is oliguria, likely pre-renal AKI from intra-operative blood loss or inadequate fluid resuscitation. Catheterising this patient will not help; they need volume resuscitation and assessment of renal function. Always scan the bladder before assuming retention.

The differential for anuria/oliguria masquerading as AROU ^[3]^[5]:

Cause	Mechanism	Key Clue
Hypovolaemic shock (e.g., ruptured AAA, GI bleed, trauma)	↓ circulating volume → ↓ renal perfusion → ↓ GFR → oliguria	Hypotension, tachycardia, empty bladder, history of bleeding/trauma
Dehydration (vomiting, diarrhoea, poor intake)	Same as above — pre-renal mechanism	Dry mucous membranes, ↑ urea:creatinine ratio, concentrated urine
Septic shock	Distributive shock → ↓ effective circulating volume → pre-renal AKI	Fever, hypotension, source of infection
Cardiogenic shock	Pump failure → ↓ cardiac output → ↓ renal perfusion	Raised JVP, pulmonary oedema, hypotension
Acute tubular necrosis (ATN)	Ischaemic or nephrotoxic insult → tubular cell death → intrinsic renal failure	Follows prolonged hypotension or nephrotoxin exposure; muddy brown casts on urinalysis
Bilateral ureteric obstruction	Stones, tumour, retroperitoneal fibrosis → urine cannot reach bladder	Empty bladder; bilateral hydronephrosis on ultrasound

Once you have confirmed urinary retention (full bladder), you must determine whether this is acute or chronic ^[1]:

Feature	AROU	CROU
Onset	Sudden	Gradual (weeks to months)
Pain	Painful	Usually painless, vague lower abdominal distension
Typical cause	Benign prostatic obstruction	Hypocontractile bladder (e.g., DM neuropathy)
Bladder volume	Typically 300–800 mL	Can be ≥ 1 L ^[2] (sometimes > 2 L)
Innervation	Usually intact (that is why it hurts)	Often impaired (autonomic neuropathy → loss of bladder sensation → painless)
Overflow incontinence	Uncommon (too acute)	Common — constant dribbling, especially at night

Why does AROU hurt but CROU doesn't? Because in AROU the sensory nerves (Aδ and C-fibres) are intact and firing intensely as the bladder rapidly distends. In CROU, the underlying cause often involves progressive neuropathy (e.g., diabetes) — the nerves that should signal pain are themselves damaged, so the bladder fills silently to enormous volumes.

Bladder Scan Volume Rule of Thumb

Bladder scan ≥ 300 mL in a patient unable to void → suggests urinary retention ^[2]. ≥ 1 L suggests chronic retention ^[2]. If you catheterise a patient expecting AROU and get 1.5 litres out, think CROU — and look for underlying neurological causes (DM neuropathy, cauda equina).

3. Differential Diagnosis of AROU by Cause — Organised by Mechanism

This is the core differential diagnosis table. Think of it as answering: "Why can't this patient void?"

These are by far the most common causes in males. Organised by anatomical location (extraluminal → intramural → intraluminal):

Location	Cause	Why It Causes AROU	Key Differentiating Feature
Extraluminal	BPH	Transition zone hyperplasia compresses prostatic urethra; 53% of all AROU ^[1]	Age 50–80; DRE: smooth enlarged > 3FB, non-tender, median sulcus present ^[4]; longstanding obstructive LUTS
	Cancer of prostate	Malignant infiltration/enlargement → urethral compression; 7% of AROU ^[1]	DRE: hard, nodular, irregular, median sulcus obliterated; may have bone pain (mets), weight loss, ↑ PSA
	Constipation / faecal impaction	Loaded rectum compresses prostatic urethra from posteriorly; 7.5% ^[1]	Often a precipitating factor with background BPH rather than standalone cause ^[2]; palpable faecal loading on DRE/AXR
	Pelvic tumours	Mass effect compressing bladder neck	History of malignancy; palpable pelvic mass on bimanual/DRE
	Prostatitis	Acute prostatic oedema → urethral compression; also pain inhibits voiding reflex	Fever, tender boggy prostate on DRE, dysuria, pyuria; younger patient (20–50)
	Pelvic organ prolapse (females)	Cystocele kinks urethra; rectocele/uterovaginal prolapse distorts anatomy	Sensation of vaginal bulge, worsens with standing; visible on pelvic exam
	Gynaecological tumours (females)	e.g., large fibroid compresses bladder neck/urethra	Palpable pelvic mass; menorrhagia history
	Pregnancy (females)	Gravid uterus compresses bladder outlet, especially retroverted uterus in 1st trimester	Positive pregnancy test; usually self-resolves as uterus rises out of pelvis
Intramural	Bladder neck stenosis	Usually after previous prostate surgery ^[1]; scarring narrows bladder neck	History of TURP, radical prostatectomy, or radiotherapy
	Urethral stricture	Iatrogenic or infection/inflammation ^[1]; fibrosis → luminal narrowing; 3.5% ^[1]	History of catheterisation, urethral instrumentation, gonococcal urethritis; thin stream/spraying
	Urethritis	Mucosal oedema narrows lumen; pain causes reflex sphincter spasm	Urethral discharge, dysuria; history of STI
	Bladder tumour	Tumour at bladder neck physically occludes internal urethral orifice ^[1]	Painless haematuria; risk factors: smoking, occupational dye exposure
Intraluminal	Bladder/urethral stone	Stone lodges at bladder neck or along urethra → ball-valve obstruction; 2% ^[1]	Strangury (painful straining to void small volumes); haematuria; history of urolithiasis
	Clot retention (severe gross haematuria)	Blood clots fill bladder and plug outlet; 3% ^[1]	Obvious gross haematuria preceding retention; causes include bleeding BPH, bladder tumour, post-TURP
	Foreign body	Object inserted or migrated into urethra/bladder	Psychiatric history; embarrassed to disclose; visible on imaging
	Phimosis	Tight foreskin occludes external urethral meatus ^[1]	Visible on inspection; cannot retract foreskin; history of recurrent balanitis

Drug Class	Examples	Mechanism	Differentiating Clue
Sympathomimetics (α-agonists)	Phenylephrine, ephedrine (cough mixture) ^[4]	α₁ stimulation → contracts bladder neck/prostatic smooth muscle → ↑ outlet resistance	Recent cold/flu; started OTC cold remedy
Sympathomimetics (β-agonists)	Terbutaline, salbutamol	β₂/β₃ agonism → relaxes detrusor → ↓ contractile force	Recent asthma exacerbation with nebuliser use
Anticholinergics	Atropine ^[4], oxybutynin, ipratropium	Block M₃ muscarinic receptors → detrusor cannot contract	New anticholinergic medication; dry mouth, constipation, blurred vision (systemic anticholinergic features)
Antipsychotics	Chlorpromazine, olanzapine	Anticholinergic side effect	Psychiatric medication history
Antidepressants	TCAs (amitriptyline), SSRIs	TCAs: strong anticholinergic + α-adrenergic effects	Recent initiation or dose increase
Antihistamines	Diphenhydramine, chlorpheniramine	1st-generation: significant anticholinergic effect	OTC allergy/sleep medication
Opioids	Morphine, codeine, tramadol	Central suppression of micturition reflex + smooth muscle effects	Post-operative setting ^[1]; recent analgesia
MDMA (Ecstasy)	Recreational	Sympathomimetic → α₁ agonism at bladder neck	Young patient, recreational drug use history

Level	Cause	Why It Causes AROU	Differentiating Features
Brain	Stroke, Parkinson's disease, MS, NPH, MSA	Disruption of cortical/subcortical micturition control; acute stroke can cause initial retention before later developing overactivity	Focal neurological deficits; tremor/rigidity (PD); gait ataxia + dementia + incontinence (NPH triad)
Spinal cord	SCI, vertebral metastasis, spinal stenosis, transverse myelitis, epidural abscess/metastasis	Interruption of descending pathways from PMC → DSD or loss of detrusor reflex	Sensory level on exam; back pain; lower limb weakness; ↓ anal tone; must rule out spinal cord compression ^[4]
Peripheral nerves	DM neuropathy, radical pelvic surgery, GBS, genital herpes (sacral nerves)	Damage to pelvic parasympathetic efferents → areflexic detrusor	DM history (exclude DM in females with hypocontractility) ^[1]; ascending weakness (GBS); vesicular rash in S2-4 dermatome (herpes)

Cause	Why It Causes AROU	Differentiating Features
Post-operative / post-anaesthesia	GA/epidural/spinal → blocks sacral parasympathetic outflow; + IV fluids → rapid filling; + opioid analgesia → central suppression of voiding reflex; 2% ^[1]	Recent surgery/anaesthesia; no prior voiding symptoms
Prolonged immobility	Poor voiding efficiency in supine position; psychological difficulty voiding in bed	Hospitalised, bed-bound patient
Excessive alcohol intake	Diuresis (ADH suppression) + sedation (ignores urge) → overdistension	Binge drinking history; acute presentation
Painful perianal conditions	Thrombosed haemorrhoids, perianal abscess → reflex sphincter spasm from pain	Perianal tenderness; visible pathology on inspection

4. Sex-Specific Differential Diagnosis Framework

Because the differential is so different between males and females, it is worth having a sex-specific mental framework:

6. How to Differentiate at the Bedside — A Practical Approach

When faced with AROU in the emergency department, here is how you systematically narrow the differential:

Assessment	What You're Looking For	Differential Narrowed
Drug history	New cold meds, anticholinergics, opioids, antipsychotics, bronchodilators	Drug-induced
Surgical history	Recent surgery/GA, previous prostate surgery, urethral instrumentation	Post-operative retention, bladder neck stenosis, urethral stricture
LUTS history	Longstanding hesitancy, weak stream, nocturia (IPSS)	BPH (if obstructive predominant)
DRE	Smooth enlarged = BPH; hard nodular = CA prostate; tender boggy = prostatitis; faecal loading = constipation; ↓ anal tone = neurological	Narrows obstructive cause + screens for neurological
External genitalia	Phimosis visible; blood at meatus = urethral injury; discharge = urethritis/STI	Phimosis, urethral trauma, urethritis
Neurological exam	Sensory level, lower limb weakness, absent perineal sensation, ↓ bulbocavernosus reflex	Spinal cord compression, cauda equina, DM neuropathy
Constitutional symptoms	Weight loss, bone pain, anorexia	Malignancy (CA prostate, bladder CA, pelvic tumour)
Haematuria	Preceding gross haematuria with clots	Clot retention; underlying cause = bladder tumour, bleeding BPH
Pelvic exam (females)	Prolapse, pelvic mass	POP, gynaecological tumour

Red Flag	Concern	Action
New neurological deficit (sensory level, weakness, ↓ anal tone)	Spinal cord compression / cauda equina	Emergency MRI spine
Fever + tender prostate	Acute prostatitis / prostatic abscess	Avoid aggressive DRE; antibiotics; avoid catheterisation if possible (suprapubic catheter preferred)
Blood at urethral meatus / high-riding prostate	Urethral trauma (e.g., pelvic fracture)	Contraindication for urethral catheterisation → suprapubic catheter or retrograde urethrogram first
Grossly elevated creatinine	Obstructive nephropathy / post-renal AKI	Urgent decompression + nephrology input
Fever + rigors + retention	Urosepsis	Urgent catheterisation + blood cultures + empirical IV antibiotics

Interestingly, retention of urine appears in the differential diagnosis of delirium ^[6]:

Drugs, Electrolytes/Ears/Eyes, Low oxygen, Infection, Retention of urine or faeces, Ictogenic, Underhydration/Undernutrition, Metabolic

This reminds us that AROU itself can cause delirium in elderly patients — the pain, discomfort, and autonomic disturbance from a grossly distended bladder can precipitate an acute confusional state. Always check for urinary retention in a delirious patient, and always consider delirium in a patient with retention who becomes confused.

High Yield Summary

First differentiate AROU from anuria/oliguria — bladder scan is the key: full bladder = retention; empty bladder = production problem (pre-renal/renal/bilateral ureteric obstruction)
Then differentiate AROU from CROU — AROU is painful (intact innervation, rapid distension); CROU is painless (impaired innervation, gradual distension); ≥ 1 L on catheterisation suggests CROU
In males, think obstruction first — BPH is #1 (53%) → then CA prostate, constipation, stricture, clot retention, stone, phimosis, bladder tumour, bladder neck stenosis
In females, think detrusor failure first — detrusor hypocontractility (exclude DM), neurogenic bladder, idiopathic
Always consider precipitants — drugs (cold meds, anticholinergics, opioids), constipation, post-GA, alcohol, immobility, perianal pain
Neurological red flags mandate emergency MRI — sensory level, lower limb weakness, loss of anal tone → spinal cord compression until proven otherwise
Blood at meatus / high-riding prostate → urethral trauma → contraindication for urethral catheterisation
DRE is the single most important bedside differentiating examination — smooth enlarged = BPH; hard nodular = CA prostate; tender boggy = prostatitis; faecal loading = constipation; ↓ anal tone = neurological
Do NOT check PSA during AROU — it will be falsely elevated; defer 4–6 weeks ^[2]
AROU can cause delirium in elderly — and delirium workup should include checking for retention

Active Recall - Differential Diagnosis of AROU

References

^[1] Lecture slides: GC 180. Benign prostatic hyperplasia, bladder outlet obstruction and urinary retention.pdf (pp. 23, 24, 25, 27, 30, 31, 33) ^[2] Senior notes: Ryan Ho Urogenital.pdf (pp. 164–167); Ryan Ho Fundamentals.pdf (pp. 349–352) ^[3] Senior notes: felixlai.md (AROU section, differential diagnosis of LUTS section) ^[4] Senior notes: maxim.md (AROU section, BPH section) ^[5] Senior notes: Ryan Ho Critical Care.pdf (p. 25 — AKI aetiology) ^[6] Senior notes: Ryan Ho Psychiatry.pdf (p. 74 — DELIRIUM mnemonic)

Diagnostic Criteria, Diagnostic Algorithm, and Investigation Modalities

AROU does not have formal "diagnostic criteria" in the way that, say, rheumatoid arthritis or heart failure do. Instead, it is a clinical diagnosis confirmed by a simple objective measurement. The diagnosis rests on three pillars:

Pillar	How to Confirm	Rationale
1. Inability to void	Patient reports inability to pass urine despite urge	The defining symptom — urine is trapped
2. Full bladder	Bladder scan ≥ 300 mL in a patient unable to void suggests urinary retention ^[2]	Confirms that the bladder is full (urine produced but not expelled) rather than empty (anuria/oliguria). This is THE key differentiator
3. Acute and painful	Sudden onset + painful suprapubic distension	Distinguishes from CROU (painless, gradual, ≥ 1 L suggests chronic retention) ^[2]

So in practical terms:

AROU is diagnosed when: a patient presents with sudden painful inability to void AND the bladder is confirmed to be distended (bladder scan ≥ 300 mL, or palpable/percussible suprapubic mass, or first catheterised volume ≥ 300 mL).

Can You Skip the Bladder Scan?

Can proceed to catheterisation directly if history and physical examination strongly suggest a diagnosis of AROU ^[3]. In practice, if you have a distressed elderly man who cannot void, has a palpable tender suprapubic mass, and has known BPH, you do not need to wait for a bladder scan — go ahead and catheterise. But in ambiguous cases (post-operative patient, uncertain history, no palpable bladder), a quick bedside bladder scan takes 30 seconds and can save you from an unnecessary catheterisation in an anuric patient.

2. The Diagnostic Algorithm — Step by Step

The approach to diagnosing and working up AROU happens in two phases: the acute phase (confirm diagnosis + immediate decompression) and the subsequent workup (identify the underlying cause).

3. Investigation Modalities — Comprehensive Breakdown

Investigations in AROU serve two purposes: (A) confirm the diagnosis and (B) identify the underlying cause and assess for complications. Let's go through each modality systematically.

3.1 Bedside Investigations

When: Should be done in both men and women ^[3]
What you are assessing:

Finding	Interpretation
Prostate: smooth, symmetrically enlarged > 3FB, no nodules, intact median groove, firm consistency ^[7]	BPH
Hard, nodular, irregular, median sulcus obliterated	CA prostate
Tender, boggy, warm	Acute prostatitis
Faecal impaction / loaded rectum	Constipation as precipitant
Anal sphincter tone + perineal sensation	Neurological integrity; ↓ tone suggests cauda equina / sacral nerve pathology ^[3]^[7]
Pelvic mass	Rectal/pelvic tumour

Precautions ^[3]:
- Normal prostate examination does NOT exclude BPH as a cause of obstruction — BPH primarily involves the transition zone which is internal; DRE mainly palpates the posterior peripheral zone. A prostate can feel "normal" on DRE yet have significant transition zone enlargement compressing the urethra
- BPH does NOT necessarily cause outflow obstruction — prostate size alone does not predict symptoms; it is the relationship between prostate, urethra, and detrusor function that matters

Investigations after catheterisation ^[2]:

Test	What You Are Looking For	Why / Interpretation
CBC ^[2]^[4]^[8]	Leukocytosis	Suggests UTI, prostatitis, or urosepsis as a precipitant or complication. Anaemia may indicate chronic disease/malignancy
Clotting profile ^[8]	Coagulopathy	To prepare for surgery ^[8] if intervention is anticipated; also relevant if considering suprapubic catheterisation
RFT (Renal Function Test) ^[2]^[3]^[4]^[8]	Elevated creatinine, elevated urea, hyperkalaemia	Obstructive uropathy / obstructive nephropathy ^[2]^[8] — back-pressure from retention → bilateral hydronephrosis → post-renal AKI. A rising creatinine mandates urgent decompression and renal imaging. High serum creatinine can result from bladder outlet obstruction or underlying renal disease — should prompt USG ^[3]
LFT ^[2]	Baseline	General workup; hepatorenal involvement in chronic disease
PSA	DO NOT CHECK	Do NOT take PSA → AROU can cause false elevation (to be done 4–6 weeks later) ^[2]^[8]. Why? Prostatic distension and inflammation during AROU cause prostate cells to release PSA into the bloodstream artefactually. PSA: only for patients with life expectancy > 10 years and after detailed counselling. DO NOT CHECK PSA during retention or UTI ^[8]. The half-life of PSA is 2–3 days ^[3], so it takes weeks to normalise after the acute event

PSA Trap — Extremely High Yield

Do NOT check PSA during an episode of AROU ^[2]^[8]. This is a classic exam mistake. AROU causes false elevation of PSA due to prostatic distension and ischaemia. Checking it will give a misleadingly high value, causing unnecessary patient anxiety and potentially triggering invasive investigations (biopsy) for a falsely elevated result. Defer PSA measurement to 4–6 weeks after the acute episode ^[2].

Causes of elevated PSA ^[3]:

Prostatitis
BPH
Prostate cancer
Prostate biopsy
AROU (artefactual)
DRE (transiently, minimally)

PSA is prostate-specific but NOT prostate-cancer-specific ^[3].

Catheterised urine: biochemistry, microscopy, C/ST ^[2]^[4]

Test	What You Are Looking For	Why / Interpretation
Urinalysis (dipstick) ^[3]^[4]	Blood, WBCs, nitrites, protein, glucose	Quick screen for haematuria (stone, tumour, infection), pyuria (UTI), glycosuria (undiagnosed DM — relevant as DM neuropathy is a cause of detrusor underactivity)
Urine microscopy ^[3]^[4]	WBC/HPF, RBC/HPF, casts, crystals, organisms	Detect presence of blood, bacteria and WBC ^[3]. Pyuria ( > 5 WBC/HPF) with significant bacteriuria → UTI as precipitant. RBC morphology can hint at glomerular vs non-glomerular source
Urine C/ST (culture and sensitivity) ^[3]^[4]	Organisms and antibiotic sensitivities	Confirms UTI; guides antibiotic choice. Critical because UTI can be both a precipitant and complication of AROU
Urine cytology ^[3]	Malignant cells	Indicated if bladder cancer is suspected in patients presenting with haematuria and predominantly irritative symptoms ^[3]. Sensitivity ~50%; highest for high-grade transitional cell carcinoma and carcinoma in situ; low detection rate for low-grade cancer

3.4 Radiological Investigations

USG bladder (confirm diagnosis) + USG kidney/ureter (hydronephrosis/hydroureter) ^[4]

Component	What It Shows	Why It Matters
USG bladder	Bladder volume, wall thickness, intravesical masses, stones, post-void residual	Confirms retention; thick bladder wall (detrusor hypertrophy) suggests chronic obstruction; intravesical mass suggests tumour or large median lobe of prostate
USG kidney/ureter	Hydronephrosis, hydroureter, renal cortical thinning, renal/ureteric stones	Upper tract imaging indicated if large residual volume, haematuria, or history of stone ^[7]. Hydronephrosis = back-pressure from obstruction → risk of obstructive nephropathy. Cortical thinning suggests chronic damage

3.5 Specialised / Elective Investigations (After Acute Phase)

These are performed once the acute episode has resolved and the catheter has been managed, to determine the underlying cause and guide definitive treatment.

Uroflowmetry: non-invasive investigation of maximal flow, voided volume, and residual volume ^[3]

Parameter	Normal	Abnormal	Interpretation
Voided volume	> 150 mL to be representative ^[7]	< 150 mL	Invalid test if volume too low
Peak flow rate (Qmax)	> 15 mL/s (male); > 30 mL/s (female) ^[7]	< 10 mL/s	< 10 mL/s: 90% obstructed; 10–15 mL/s: 60% obstructed; > 15 mL/s: only 30% obstructed ^[8]
Flow pattern	Bell-shaped curve	↓ peak (BPH) vs plateaued (urethral stricture) ^[7]	Plateaued = fixed narrowing (stricture); ↓ peak = compressive obstruction (BPH) or detrusor underactivity
Post-void residual (PVR)	< 50 mL (normal); up to 100–200 mL acceptable in elderly ^[7]	> 200 mL	Suggests either BOO or detrusor underactivity; large PVR → risk of UTI, stones, upper tract damage

Critical caveats ^[3]^[7]:

Normal maximal flow rate does NOT exclude outflow obstruction — 18% of patients with BOO have Qmax > 15 mL/s ^[7]
Uroflowmetry is NOT sufficient to diagnose outflow obstruction since it cannot distinguish between outflow obstruction and poor detrusor contractility ^[3] — both give ↓ Qmax. To distinguish them, you need urodynamics

Urodynamics: gold-standard for diagnosis of BOO ^[7]

Investigation of ^[3]:

Uroflow rate
Bladder volume
Intravesical and rectal pressure (to calculate detrusor pressure = P_ves − P_abd)
Sphincter function (EMG)

Outflow obstruction is characterised by high pressure, low flow ^[3]

Pattern	Detrusor Pressure	Flow Rate	Diagnosis
High pressure, low flow	↑↑ (detrusor working hard)	↓↓	BOO — detrusor is trying to push through an obstruction
Low pressure, low flow	↓	↓↓	Detrusor underactivity (DUA) — the problem is not obstruction but a weak pump
Normal pressure, normal flow	Normal	Normal	No BOO, no DUA (consider other causes of symptoms)

Why does this matter? Because the treatment is completely different. BOO → relieve obstruction (α-blockers, 5-ARI, TURP). DUA → improve detrusor function (cholinergics, intermittent self-catheterisation). If you do TURP on a patient with DUA, you will not fix anything — the obstruction was never the problem.

Indications for urodynamics ^[8]:

Atypical age for BPH
Suspected neurogenic bladder
History of spinal/pelvic surgery
Failed intervention (e.g., persistent symptoms after TURP)
Uncertain diagnosis where uroflowmetry alone cannot distinguish BOO from DUA

Timing	Investigation	Key Findings / Interpretation
ACUTE	Bladder scan	≥ 300 mL = retention; ≥ 1 L = chronic ^[2]
	Catheterisation (diagnostic + therapeutic)	Volume drained confirms diagnosis; send urine for analysis
	DRE	Prostate (size/consistency/tenderness), faecal loading, anal tone
	Neurological exam	Sensory level, perineal sensation, lower limb power/reflexes
AFTER CATHETERISATION	CBC	Leukocytosis (infection/urosepsis)
	RFT	Elevated creatinine → obstructive nephropathy
	LFT	Baseline
	Catheterised urine: biochemistry, microscopy, C/ST	UTI, haematuria, crystals
	KUB	Stones or faecal loading ^[2]
	DO NOT check PSA	False elevation during AROU — defer 4–6 weeks ^[2]^[8]
ELECTIVE (after acute episode)	IPSS	Quantify LUTS severity; guide treatment
	Uroflowmetry + PVR	Screen for BOO; Qmax < 10 = 90% obstructed ^[8]
	USG kidney/ureter	If large PVR, haematuria, or Hx stone ^[7]
	TRUS	Before 5-ARI or surgery ^[8]
	Cystoscopy	Haematuria, suspected stricture/tumour ^[8]
	Urodynamics	Gold-standard for BOO; atypical cases, suspected neurogenic bladder, failed intervention ^[7]^[8]
	PSA (4–6 weeks later)	Only if life expectancy > 10 years + counselled ^[8]; r/o CA prostate
	MRI spine	Urgently if neurological red flags

Investigation Pitfalls to Remember

PSA during AROU = false elevation → defer 4–6 weeks ^[2]^[8]
Normal uroflowmetry does NOT exclude BOO — 18% still obstructed despite Qmax > 15 mL/s ^[7]
Uroflowmetry cannot distinguish BOO from DUA — both give low Qmax; need urodynamics to differentiate ^[3]^[7]
Normal DRE does NOT exclude BPH — transition zone enlargement is not palpable from the rectum ^[3]
Uroflowmetry requires voided volume > 150 mL to be a valid test ^[7]
BPH does NOT necessarily cause outflow obstruction — size alone doesn't determine symptoms ^[3]

Since PSA is so commonly tested in exams and frequently misunderstood, here is a comprehensive summary for the elective workup phase:

PSA (Prostate-Specific Antigen) ^[3]:

Prostate-specific but NOT prostate-cancer-specific ^[3]
Half-life = 2–3 days ^[3]
PSA > 1.5 ng/mL is a useful marker for prostatic enlargement and predicts increased risk of BPH progression ^[3]

Interpretation ^[3]:

PSA < 4 ng/mL = Normal
PSA ≥ 4 ng/mL = Cutoff for diagnostic prostate biopsy
PSA 4–10 ng/mL = 20% chance of cancer (the "grey zone")
PSA ≥ 10 ng/mL = 50% chance of cancer

When NOT to check PSA ^[3]^[8]:

During AROU or UTI (false elevation)
If patients have < 10 years of life expectancy (unless clinically obvious disease like palpable nodule on DRE) ^[3]

High Yield Summary

AROU is a clinical diagnosis: inability to void + full bladder (scan ≥ 300 mL) + painful + sudden onset
Bladder scan is the first-line differentiator: full = retention; empty = anuria/oliguria
≥ 1 L on catheterisation suggests CROU — look for neurological cause
Acute investigations: CBC, RFT, LFT, catheterised urine (biochemistry, microscopy, C/ST), KUB
DO NOT CHECK PSA during AROU — causes false elevation; defer 4–6 weeks
DRE: must do in all patients; assesses prostate, faecal loading, anal tone; normal DRE does NOT exclude BPH
Uroflowmetry: screening for BOO; valid only if voided > 150 mL; Qmax < 10 = 90% obstructed; but cannot distinguish BOO from DUA
Urodynamics: gold standard for BOO; high pressure + low flow = BOO; low pressure + low flow = DUA
Outflow obstruction = high pressure, low flow on urodynamics ^[3]
TRUS: before 5-ARI or surgical planning; cystoscopy: for haematuria, suspected stricture/tumour
MRI spine: urgently if neurological red flags (sensory level, weakness, ↓ anal tone)
IPSS: quantifies LUTS severity (mild 1–7, moderate 8–19, severe 20–35); guides treatment but is NOT a diagnostic tool

Active Recall - Diagnosis and Investigations of AROU

References

^[1] Lecture slides: GC 180. Benign prostatic hyperplasia, bladder outlet obstruction and urinary retention.pdf (pp. 23, 24, 30, 33) ^[2] Senior notes: Ryan Ho Urogenital.pdf (pp. 164–167, 170–173); Ryan Ho Fundamentals.pdf (pp. 349–352, 355–356) ^[3] Senior notes: felixlai.md (AROU section – diagnosis, physical examination, biochemical tests, specific tests; BPH section – PSA interpretation) ^[4] Senior notes: maxim.md (AROU section – physical examination, investigations) ^[5] Senior notes: Ryan Ho Critical Care.pdf (p. 25 — AKI aetiology) ^[7] Senior notes: Ryan Ho Urogenital.pdf (pp. 170–173); Ryan Ho Fundamentals.pdf (pp. 355–356) — uroflowmetry, IPSS, urodynamics, BPH workup ^[8] Lecture slides: Benign Prostatic Hyperplasia.pdf (pp. 12, 18) ^[9] Senior notes: Ryan Ho Diagnostic Radiology.pdf (p. 17 — IVU)

Management Algorithm and Treatment Modalities

2. Phase 1: Acute Bladder Decompression

This is the immediate priority. Think of it like chest compressions in a cardiac arrest — do it first, everything else follows.

2.1 Urethral Catheterisation (First-Line)

Immediate bladder decompression by urethral catheterisation (first-line) by 14–18Fr Foley's catheter ^[2]

Sex	Standard Size	Rationale
Males	14 or 16 Fr ^[3] (range 14–18Fr ^[2])	Male urethra is longer (~20 cm) and passes through the prostate; 14Fr is comfortable yet large enough for adequate drainage
Females	12 or 14 Fr ^[3]	Female urethra is shorter (~4 cm) and wider; smaller catheter suffices

Catheter material ^[3]:

Latex = short-term use (maximum 2 weeks) — yellow tube
Silicone = long-term use (maximum 4 weeks) — transparent tube

Types of catheter ^[3]:

Standard 2-way: one port for urine drainage, one for balloon inflation — standard for AROU
3-way (triple-lumen): additional irrigation channel for clot retention — used for haematuria with clot formation to allow continuous bladder irrigation ^[3]

Failure to pass into bladder can be due to ^[2]^[4]:

Situation	Likely Cause	Solution	Why It Works
Cannot advance past prostatic urethra	Enlarged prostate (BPH)	Use thicker catheter (20–22 Fr) ^[2]^[4]	A thicker catheter is stiffer — it has more rigidity to navigate through the compressed prostatic urethra. Think of it like a garden hose vs a limp straw — the stiffer one pushes through better
Catheter stuck proximally along penile urethra / Hx of instrumentation or TURP	Urethral stricture	Use thinner catheter (10–12 Fr) ^[2]^[4]	The urethral lumen is narrowed by fibrosis — a thinner catheter can squeeze through the tight segment

If standard troubleshooting fails ^[4]:

Tiemann catheter: curved-tip catheter specifically designed for BPH — the angled tip navigates the elevated prostatic urethra
Cystoscopic-guided Foley insertion: pass a flexible cystoscope first, then railroad the catheter over a guidewire
Urethral dilators: for strictures — progressively dilate the narrowed segment
Suprapubic catheterisation (SPC): the definitive fallback (see below)

Type	Contraindication	Rationale
Absolute	Urethral injury — signs: blood at urethral meatus, high-riding prostate on DRE ^[2]^[3]^[4]	Passing a catheter through an injured urethra can convert a partial tear into a complete transection → devastating complication. If you suspect urethral injury (e.g., pelvic fracture + blood at meatus), do a retrograde urethrogram first
Relative	Urethral stricture ^[3]	Risk of creating false passage; may need specialist guidance or SPC
Relative	Recent urinary tract surgery: radical prostatectomy, urethral reconstruction, presence of artificial sphincter ^[3]	Fresh anastomoses can be disrupted by catheter passage → urethral damage, incontinence
Relative	Acute prostatitis ^[3]	Catheterisation through an acutely inflamed prostate is painful and may worsen infection; SPC is preferred in severe prostatitis

3. Phase 2: Subsequent Management

Once the catheter is in and the patient is stabilised, the management pivots to:

Treat reversible precipitants
Start medical therapy (α-blocker)
Plan trial without catheter (TWOC)
Address definitive management of the underlying cause

Treat reversible causes: stop offending drugs, treat constipation (e.g., Fleet enema) and UTI ^[4]

Precipitant	Management
Offending drugs (cold meds, anticholinergics, opioids)	Stop or switch medications
Constipation/faecal impaction	Fleet enema, lactulose, manual disimpaction if needed
UTI	Appropriate antibiotics based on C/ST results
Immobility	Mobilise patient as early as possible
Post-operative (anaesthesia-related)	Often self-resolving once anaesthetic wears off; TWOC usually successful

TWOC is the pivotal management decision after the acute episode. The catheter is typically left in for ~2 days ^[3], during which time the α-blocker takes effect, precipitants are corrected, and the overdistended detrusor has time to recover some contractile function.

Process ^[4]:

Remove the catheter (usually in the early morning to allow the whole day for voiding assessment)
Observe urine output and perform post-void bladder scan ^[4]
Patient attempts to void naturally
Check post-void residual (PVR) by bladder scan

Outcomes:

Outcome	Next Step
Successful TWOC (patient voids with acceptable PVR < 200–400 mL)	Discharge with α-blocker; follow-up in urology OPD for definitive BPH management
Re-catheterise if bladder scan > 400 mL ^[4]	Re-TWOC / long-term Foley / clean intermittent self-catheterisation (CISC) ^[4]
Failed repeated TWOC	Arrange definitive surgical intervention (e.g., TURP)

TWOC is contraindicated if obstructive uropathy present (RFT improves after Foley insertion) ^[4] — if the creatinine drops significantly after catheterisation, this means the obstruction was causing bilateral back-pressure → post-renal AKI → upper tract damage. Removing the catheter would re-obstruct and cause further renal injury. These patients need to keep the catheter in until definitive surgery.

Why Not Just Remove the Catheter Immediately?

The overdistended detrusor needs time to recover. Remember, overdistension stretches actin-myosin beyond optimal overlap → ineffective contraction. Leaving the catheter in for 48–72 hours (while on an α-blocker) gives the muscle fibres time to recover their length-tension relationship. Additionally, α-blockers need ~48 hours to reach therapeutic effect.

If the patient fails TWOC and is not fit for surgery (or while awaiting surgery):

Option	Description	Preferred When
Long-term indwelling Foley catheter	Indwelling urethral catheter, changed every 4–6 weeks	Short-term bridging; patient preference
Suprapubic catheter (SPC)	Preferred for long-term drainage > 3 weeks ^[2]	Better for long-term use; fewer urethral complications
Clean intermittent self-catheterisation (CISC)	Patient self-catheterises 4–6× daily using a clean single-use catheter	Gold standard for long-term bladder management in neurogenic bladder; requires adequate dexterity and cognition

4. Definitive Management of the Underlying Cause — BPH

Since BPH accounts for 53% of AROU ^[1], BPH management is the most commonly tested definitive treatment.

Medical therapy indications ^[4]^[7]:

Drug Class	Drug Examples	Mechanism	Indications	Key Side Effects	Contraindications/Cautions
α₁-adrenergic blockers (most commonly used)	Non-selective: prazosin (Minipress), terazosin (Hytrin), doxazosin (Cardura), alfuzosin (Xatral) ^[4] Selective (α₁A): tamsulosin (Harnal), silodosin (Rapaflo) ^[4]	Relax smooth muscles in prostate and bladder neck (NOT bladder body) ^[4] → reduces dynamic obstruction → improves flow within days	Moderate-to-severe LUTS (not storage-predominant) ^[7]	Non-selective: more orthostatic hypotension, nasal congestion, dizziness, tiredness ^[4] Selective (α₁A): more retrograde ejaculation ^[4]	To reduce side effects: slow titration, subtype-selective (α₁A), slow-release formulations ^[4]
5α-reductase inhibitors (5-ARI)	Finasteride, dutasteride ^[4]	Reduce DHT → decrease size of prostate + decrease vascularity (less bleeding) + progression prevention ^[4]. DHT is the active androgen that drives prostatic stromal growth; blocking its production causes prostate to shrink by ~20–30% over 6–12 months	Moderate-to-severe LUTS + large prostate ( ≥ 30–40 mL on TRUS) / IPSS ≥ 12 ^[4] 2nd line or in combination with α₁-blockers ^[4]	Erectile dysfunction (10%), gynaecomastia ^[4] 50% decrease in PSA → must multiply PSA by 2 when screening for CA prostate ^[4]	Slow onset: 3–6 months for maximum effect ^[4] — not useful for acute symptom relief; mainly for long-term disease modification
α₁-blocker + 5-ARI combination	Above drugs combined	Dual mechanism: immediate α₁-blockade + long-term prostate shrinkage	Moderate-to-severe LUTS + increased risk of disease progression ^[7]	Combination of above side effects	Large prostate with significant symptoms and risk factors for progression
PDE5 inhibitors	Tadalafil (Cialis) ^[4]	PDE5-mediated reduction in smooth muscle and endothelial cell proliferation; increases smooth muscle relaxation and perfusion to prostate and bladder ^[3]	Especially useful for those with erectile dysfunction ^[7]; moderate-to-severe LUTS	Hypotension, blue/blurred vision, hearing loss, flushing, headache, dyspepsia ^[3]	Avoid if using nitrate ^[4] (severe hypotension risk)
Muscarinic blockers / β₃ agonists	Oxybutynin, solifenacin / Mirabegron (β₃ agonist) ^[3]	Muscarinic blockers: block M₃ on detrusor → reduce involuntary contractions β₃ agonists: activate β₃ receptors → relax detrusor during filling	Storage-predominant moderate-to-severe LUTS; residual storage symptoms after α₁-blocker/PDE5I treatment ^[7]	Anticholinergics: dry mouth, constipation, urinary retention β₃ agonists: hypertension, UTI	Caution if post-void residual > 150 mL ^[7] — anticholinergics can worsen retention! β₃ agonist (Mirabegron) does not have the same concern for urinary retention as anticholinergics ^[3]

Why α-Blockers Work Fast but 5-ARIs Take Months

α-blockers relax smooth muscle (the dynamic component) — this is like turning off a switch. The effect is almost immediate (days). 5-ARIs reduce the physical size of the prostate (the static component) — this requires blocking DHT-driven growth and waiting for prostate tissue to involute. Tissue remodelling takes months. This is why the two drugs are complementary: α-blockers for immediate relief, 5-ARIs for long-term disease modification.

Modality	Mechanism
Thermotherapy: HIFU, TUMT (transurethral microwave therapy)	Heat-induced coagulative necrosis of prostate tissue
UroLift	Mechanical implants that hold prostatic lobes apart, away from the urethra
Steam treatment (Rezum)	Inject water vapour into prostate → induces coagulative necrosis
Prostatic artery embolisation (PAE)	Reduce part of the blood supply to the prostate → ischaemic shrinkage

4.4 Surgical Treatment

Indications for surgery ^[3]^[4]^[7]:

Absolute indications (complications of BPH) ^[7]:

Recurrent acute retention of urine (AROU) — failed TWOC ^[3]
Recurrent urinary tract infection (UTI) ^[3]
Recurrent haematuria ^[3]
Renal insufficiency secondary to BPH (obstructive uropathy) ^[3]
Bladder stones ^[7]

Relative indication ^[7]:

Bothersome LUTS refractory to or cannot tolerate medical treatment ^[3]

TURP decreases urinary symptom score (IPSS), decreases PVR and increases maximal urinary flow rate ^[3]

Surgical procedure ^[3]^[4]:

Monopolar TURP ^[3]:

Resectoscope loaded with monopolar diathermy loop is introduced into bladder
Continuous irrigation using non-conductive solution containing glycine is used to distend the bladder and wash away blood and tissue fragments
Glycine is better than distilled water as non-conductive irrigant since it causes less TUR syndrome comparatively
Saline solution CANNOT be used for monopolar resection because it is a good conductor of electricity, diffuses power and does not allow cutting or cauterisation of tissue ^[3]
Strips of prostate tissue are resected under direct vision until the prostatic fibrous capsule is reached
Prostate chips evacuated and bleeding controlled with electrocautery
Post-operative: 3-way Foley to irrigate bladder with NS to avoid clot retention ^[4]

Bipolar TURP ^[3]^[4]:

Saline is used as the irrigant in bipolar — eliminates the risk of hyponatraemia in TUR syndrome ^[3]
More expensive than monopolar TURP ^[3]
Process is slower due to smaller probe size; poorer haemostasis ^[7]
Usually only use bipolar for larger prostates ^[7]

Surgical modality by prostate size ^[7]:

Prostate Size	Recommended Procedure
Small prostate < 30 mL + no middle lobe	TUIP (Transurethral Incision of Prostate) — longitudinal incision to widen bladder neck without removing tissue; lower morbidity ^[7]
Moderate prostate 30–80 mL	Bipolar or monopolar TURP ^[7]
Large prostate > 80 mL	Transurethral enucleation (e.g., Holmium/Thulium laser enucleation) ^[7] or open prostatectomy

Complication	Mechanism / Details	Incidence
Bleeding / haematuria	Trauma to prostatic venous sinuses; secondary infection (prostatitis)	Bleeding requiring transfusion: ~1% ^[3]
TUR syndrome (Post-prostatectomy syndrome)	Hyponatraemia due to systemic absorption of hypotonic irrigating fluid (glycine) ^[3]^[4] Pathophysiology: dilutional hyponatraemia + fluid overload + glycine toxicity ^[4] Risk factors: long operating time, e.g., massive prostate ^[4] Signs/symptoms: nausea (first symptom), confusion, cerebral oedema, visual disturbance ^[4] Management: manage as hyponatraemia (electrolytes, serum osmolality, volume status), hypertonic saline ^[4] Prevention: use bipolar (NS as irrigating fluid), limit volume < 1 L and irrigation pressure < 60 mmHg ^[4]	Variable
Retrograde ejaculation	70–80% — due to resection of bladder neck ^[4]; semen enters bladder instead of exiting via urethra because the bladder neck can no longer close during ejaculation	Very common
Urethral stricture	Urethral instrumentation ^[4] causes scarring	Variable
Urinary incontinence	~1% ^[3]; urge incontinence (early, due to detrusor irritability) / stress incontinence (late, due to sphincter damage) ^[4]
Erectile dysfunction	Thermal injury to neurovascular bundle adjacent to prostate	Variable

TUR Syndrome — Why Monopolar TURP Is Dangerous in Large Prostates

During monopolar TURP, the irrigant must be non-conductive (glycine). The prostatic venous sinuses are opened during resection, and glycine is absorbed directly into the bloodstream. Longer operating time (larger prostate) = more absorption = greater risk. Glycine causes: (1) dilutional hyponatraemia (water follows glycine osmotically), (2) fluid overload (absorbed volume), (3) direct neurotoxicity from glycine metabolites (ammonia → visual disturbance, confusion). Bipolar TURP uses saline, eliminating this risk entirely, which is why it is preferred for larger prostates ^[3]^[4]^[7].

Technique	Description	Advantage	Indication
Transurethral enucleation (Holmium/Thulium laser)	Laser enucleation of adenoma → morcellation for removal	Saline irrigant (no TUR syndrome); less bleeding; effective for large prostates	Large prostate > 80 mL ^[7]
Ablative techniques (PVP/Green Laser, RFA, Rezum)	Tissue destruction without removal	Less bleeding; alternative to TURP in patients with bleeding tendency or poor operative risk ^[7]; less post-op irritative symptoms	No histological specimen obtained; decreased durability ^[7]
Open / robotic prostatectomy	Open surgical enucleation of adenoma via suprapubic or retropubic approach	Gold standard for very large prostates ( > 80–100 mL); complete removal	Very large prostates; failed TURP
Metallic stent	Temporary urethral stent to keep prostatic urethra open	Avoids surgery	Very unfit patients ^[4]

5. Management of Specific Scenarios

These occur after catheterisation and must be anticipated and managed:

Complication	Definition / Mechanism	Management
Post-obstructive diuresis	Diuresis > 200 mL/h for ≥ 2 hours ^[3]^[4] after decompression. Represents the body's attempt to excrete excess fluid, solutes (urea, Na⁺), and water retained during the period of obstruction. Also due to osmotic diuresis from accumulated urea and loss of medullary concentrating gradient. Primarily a problem of chronic (not acute) retention ^[3]	Monitor hourly urine output closely. Patients usually can manage by increasing oral fluid intake ^[3]. Isotonic saline replacement is indicated if patients are unable to increase fluid intake ^[3]. Monitor electrolytes (risk of hyponatraemia, hypokalaemia). Do NOT remove Foley catheter — may lead to hydronephrosis ^[3]
Haemorrhage ex vacuo	Transient haematuria due to mucosal disruption ^[4] when the overdistended bladder suddenly decompresses. Small mucosal blood vessels that were compressed by the full bladder begin to bleed when pressure is released	Usually self-limiting; maintain catheter drainage; if persistent, consider continuous bladder irrigation with 3-way catheter
Transient hypotension ^[3]	Sudden decompression of a large-volume bladder → abrupt ↓ intra-abdominal pressure → venous pooling in the splanchnic bed → ↓ preload → ↓ BP. Also vagal response from relief of pain	IV fluids; gradual decompression (some advocate clamping catheter after every 500 mL and releasing — though evidence for this is debated)
UTI / CAUTI	Catheter itself is a foreign body that acts as a nidus for biofilm and bacterial colonisation	Aseptic insertion technique; remove catheter as soon as possible; avoid long-term catheterisation if possible
Urethral stricture (late)	Repeated or traumatic catheterisation causes urethral mucosal injury → fibrosis → stricture	Minimise catheter duration; use appropriate size; consider SPC for long-term

Post-Obstructive Diuresis — Don't Panic, But Don't Ignore It

Post-obstructive diuresis is expected after relieving chronic or severe acute retention. It is the kidneys' way of dumping retained solutes and water. The danger is not the diuresis itself — it is under-replacement leading to dehydration and electrolyte disturbance. Monitor Q1H urine output and match IV fluid replacement if the patient cannot drink enough. The diuresis typically resolves within 24–48 hours once the excess is cleared.

Phase	Action
Immediate	Urethral catheterisation (14–18Fr Foley) → record volume → send urine
If catheter fails	Troubleshoot (thicker/thinner catheter, Tiemann) → SPC if all fail
Post-catheterisation	Bloods (CBC, RFT), catheterised urine (C/ST), KUB; NOT PSA
Treat precipitants	Stop offending drugs, treat constipation, treat UTI
Start α-blocker	Alfuzosin/tamsulosin — reduces dynamic obstruction
Monitor	Q1H urine output → watch for post-obstructive diuresis
TWOC	~48 hours later; re-catheterise if PVR > 400 mL
Failed TWOC	Re-TWOC / long-term catheter / CISC
Definitive	Elective TURP 4–6 weeks after AROU ^[4] (lower intra-operative risk than emergency)
Ongoing BPH	IPSS, uroflowmetry, PSA (4–6 weeks), TRUS → guide long-term management

High Yield Summary

Acute management priority = bladder decompression via urethral catheterisation (14–18Fr Foley); SPC if urethral route fails or is contraindicated
Contraindications to urethral catheter: blood at meatus, high-riding prostate (urethral injury); recent urological surgery; severe prostatitis
SPC contraindications: non-distended bladder, bleeding tendency, urothelial cancer
Catheter troubleshooting: stuck at prostate → thicker catheter (20–22Fr); stuck at penile urethra → thinner catheter (10–12Fr)
After catheterisation: treat reversible precipitants (drugs, constipation, UTI), start α-blocker, plan TWOC at ~48 hours
TWOC contraindicated if obstructive uropathy present (RFT improves after catheterisation) — keep catheter, proceed to surgery
α-blockers (alfuzosin, tamsulosin): relax prostatic smooth muscle → immediate relief; 5-ARIs (finasteride, dutasteride): shrink prostate → 3–6 months to work
TURP indications: recurrent AROU (failed TWOC), recurrent UTI, recurrent haematuria, renal insufficiency from BPH, bladder stones, refractory LUTS
TUR syndrome: dilutional hyponatraemia + fluid overload + glycine toxicity; prevented by bipolar TURP (uses saline)
Post-obstructive diuresis: > 200 mL/h for ≥ 2h; monitor Q1H urine output; replace fluids if patient cannot drink
Elective TURP best done 4–6 weeks after AROU (not emergency) — lower intra-operative risk
Long-term options if unfit for surgery: long-term Foley/SPC, CISC, metallic stent

Active Recall - Management of AROU

References

^[1] Lecture slides: GC 180. Benign prostatic hyperplasia, bladder outlet obstruction and urinary retention.pdf (p. 54) ^[2] Senior notes: Ryan Ho Urogenital.pdf (pp. 167, 176); Ryan Ho Fundamentals.pdf (p. 352) ^[3] Senior notes: felixlai.md (AROU treatment section, urinary catheterisation section, TURP section) ^[4] Senior notes: maxim.md (AROU subsequent management, BPH management, TURP complications) ^[5] Senior notes: Ryan Ho Critical Care.pdf (p. 26 — AKI management) ^[7] Senior notes: Ryan Ho Urogenital.pdf (pp. 173, 176); Ryan Ho Fundamentals.pdf (pp. 355–356) — BPH treatment algorithm, surgical indications

Complications of Acute Retention of Urine

Complications of AROU fall into two broad categories that are conceptually distinct:

Complications of the retention itself — what happens to the urinary tract (and the patient) if urine sits trapped in a distended bladder
Complications of decompression (catheterisation) — what happens after you relieve the retention

Both must be anticipated, monitored for, and managed. Let's work through each from first principles.

1. Complications of the Retention Itself

These are the consequences of a distended, obstructed urinary system. Think of it as a plumbing system under back-pressure: the blockage at the outlet causes pressure to build up backwards through the entire system — bladder → ureters → kidneys.

Consequence of BOO ^[1]:

Retention of urine — acute or chronic
Recurrent UTI
Formation of bladder calculi
Hydroureter and hydronephrosis
Renal impairment / ARF (Obstructive uropathy) ^[1]

Feature	Detail
Mechanism	Stagnant urine in a distended bladder is a perfect culture medium for bacteria. Normal voiding is one of the body's most important defence mechanisms against UTI — the "washout effect" flushes bacteria out of the bladder regularly. When voiding stops, bacteria proliferate unopposed. The presence of a catheter (a foreign body) further compounds the risk by providing a surface for biofilm formation and acting as a direct conduit for ascending bacteria
Clinical features	Fever, dysuria, cloudy/malodorous urine, suprapubic pain (which may be masked by the existing retention pain); can escalate to urosepsis (SIRS with urinary source): rigors, tachycardia, hypotension, confusion
Why it matters	Recurrent UTI is listed as a consequence of BOO ^[1] and is an absolute indication for surgical intervention (TURP) in BPH ^[3]. Urosepsis in the context of obstructed, infected urine is a life-threatening emergency requiring urgent decompression + IV antibiotics
Management	Catheterisation (drain the infected reservoir), empirical IV antibiotics (e.g., IV co-amoxiclav or piperacillin-tazobactam depending on local protocol), blood cultures, adjust based on C/ST. Long-term catheterisation carries risk of UTI/urosepsis, trauma, stones, urethral strictures, prostatitis and SCC of bladder ^[2]

Feature	Detail
Mechanism	Formation of bladder calculi ^[1]. Stagnant urine becomes supersaturated with crystalloids (calcium, oxalate, phosphate, uric acid). Normally, regular voiding prevents crystal nucleation. In retention, prolonged contact time allows crystals to nucleate, grow, and aggregate into stones. Infection (particularly with urease-producing organisms like Proteus mirabilis) further promotes stone formation by splitting urea into ammonia → alkalinising the urine → precipitating struvite (magnesium ammonium phosphate) stones
Clinical features	Strangury (painful, frequent urination of small volumes expelled only by straining), terminal haematuria, suprapubic pain that worsens at end of micturition (stone shifts to bladder neck), recurrent UTI with urease-producing organisms
Why it matters	Bladder stones are a consequence of BOO ^[1] and an absolute indication for surgery ^[2]^[7]. They also perpetuate a vicious cycle: stones irritate the bladder wall → detrusor overactivity → worsening LUTS; stones harbour bacteria → recurrent UTI
Management	Cystolitholapaxy (endoscopic stone fragmentation and removal); address underlying BOO (TURP) to prevent recurrence

Feature	Detail
Mechanism	Hydroureter and hydronephrosis ^[1]. When the bladder cannot empty, intravesical pressure rises progressively. Once intravesical pressure exceeds the pressure in the distal ureters, urine can no longer drain from the kidneys into the bladder. Urine "backs up" → ureters dilate (hydroureter) → renal pelvis and calyces dilate (hydronephrosis). In the context of BOO, this is bilateral because the obstruction is below the level of both ureteric orifices. In chronic retention, intravesical pressure can remain elevated for weeks to months, causing sustained bilateral upper tract dilatation
Clinical features	Often insidious and asymptomatic until late. May present as bilateral loin pain, renal impairment discovered incidentally, or bilateral ballotable kidneys on examination (in severe cases) ^[2]. Fever with hydronephrosis = pyonephrosis (infected hydronephrosis) — a surgical emergency requiring urgent drainage (PCN or JJ stent)
Why it matters	Hydronephrosis from retention leads directly to the next complication — obstructive uropathy/renal failure. It is detectable on ultrasound (USG KUB) and its presence indicates significant back-pressure
Management	Bladder decompression (catheterisation) relieves the distal obstruction, and the hydronephrosis typically resolves. If it persists despite bladder drainage, suspect a concurrent ureteric pathology

Feature	Detail
Mechanism	Renal impairment / ARF (Obstructive uropathy) ^[1]. Sustained back-pressure from the distended bladder → bilateral hydronephrosis → ↑ pressure in Bowman's capsule → opposes the filtration pressure driving GFR → GFR drops → creatinine rises → post-renal AKI. If prolonged, the renal parenchyma undergoes tubular atrophy and interstitial fibrosis → irreversible chronic kidney disease. Post-renal disease ( < 10%) due to obstructive uropathy (must be bilateral) ^[5], but in BOO, the obstruction IS bilateral by definition (single point of obstruction below both ureteric orifices)
Clinical features	Elevated creatinine on blood tests, uraemic symptoms (vomiting, lethargy, drowsiness ^[2], confusion, pruritus, anorexia), oliguria (paradoxically — the kidneys cannot filter effectively), fluid overload, hyperkalaemia. Usually rare in AROU, more likely in CROU ^[2] — because chronic retention allows prolonged sustained back-pressure, whereas AROU is typically relieved quickly
Why it matters	Renal impairment from BPH is an absolute indication for surgery ^[3]. TWOC is contraindicated if obstructive uropathy present (RFT improves after Foley insertion) ^[4] — because removing the catheter would re-obstruct and cause further renal damage
Management	Immediate bladder decompression → monitor renal function (creatinine should start improving within 24–48 hours). Management of life-threatening complications of AKI ^[5]: hyperkalaemia (IV calcium, insulin-dextrose, polystyrene sulphonate), fluid overload (diuretics, dialysis if refractory), metabolic acidosis (bicarbonate, dialysis). Anticipate post-obstructive diuresis (see below). Consider haemodialysis if refractory (indications: Acidosis, Electrolyte imbalance, Intoxication, Overload, Uraemia — "AEIOU") ^[5]

Feature	Detail
Mechanism	Acute overdistension stretches detrusor smooth muscle fibres beyond their optimal actin-myosin overlap (the length-tension relationship). The bladder wall becomes thin, atonic, and unable to generate effective contraction. In chronic retention with BOO, the detrusor initially compensates by hypertrophy (thicker muscle to overcome higher resistance) → bladder wall becomes trabeculated (thickened muscle bundles visible as ridges). Eventually, the detrusor decompensates → diverticula (outpouchings of mucosa between hypertrophied muscle bundles) → further residual urine → worsening cycle. Complications of chronic retention: bladder stones, UTI, overflow incontinence, hernia due to chronic straining, obstructive uropathy ^[7]
Clinical features	Failed TWOC (detrusor too weak to void even after obstruction is relieved), persistent elevated post-void residual, overflow incontinence (constant dribbling from an overfull, atonic bladder), bladder diverticula on imaging
Why it matters	If the detrusor is severely decompensated, even a successful TURP may not restore normal voiding — the patient may need long-term CISC. This is why we try not to let AROU persist for too long

Feature	Detail
Mechanism	When the bladder is so full that intravesical pressure intermittently exceeds urethral resistance, small amounts of urine "overflow" and leak out. Overflow incontinence: constant dribbling (especially at night) with associated retention of urine. Mechanism: BOO/DUA → bladder over-distension with continuous dribbling ^[7]
Clinical features	Paradoxical — the patient appears to be passing urine (dribbling) but is actually in retention. The bladder remains palpable. Patients may not recognise that they are retaining urine because they are "passing something"
Why it matters	Can be mistaken for normal voiding → retention goes unrecognised → progressive upper tract damage. Complications: UTI, bladder stones, obstructive uropathy ^[7]

Feature	Detail
Mechanism	Pain, discomfort, and autonomic disturbance from a grossly distended bladder can precipitate acute confusion in elderly patients with reduced cerebral reserve. "DELIRIUM" mnemonic includes R = Retention of urine or faeces as a recognised cause of delirium ^[6]
Clinical features	Acute confusion, agitation, disorientation (time > place > person), fluctuating consciousness, worse at night
Why it matters	Always check for urinary retention in a delirious elderly patient (bladder scan!) — treating the retention may resolve the delirium

Feature	Detail
Mechanism	In patients with spinal cord injury at T6 or above, bladder distension triggers a massive sympathetic response below the level of the lesion (uninhibited by descending control) → severe vasoconstriction below the lesion → dangerous hypertension. Baroreceptors above the lesion detect the hypertension and trigger a compensatory vagal (parasympathetic) response → bradycardia, flushing, and sweating above the lesion — but this cannot counteract the sympathetic storm below
Clinical features	Severe headache, hypertension (can be > 200 mmHg systolic), bradycardia, flushing/sweating above the lesion, pallor/gooseflesh below. Can cause stroke, seizures, or death if untreated
Why it matters	A medical emergency. Bladder distension (including AROU) is the most common trigger. Immediate management: sit the patient upright (orthostatic reduction in BP), catheterise immediately to relieve the distended bladder, loosen tight clothing. If hypertension persists: sublingual nifedipine or GTN patch

2. Complications of Decompression (Post-Catheterisation)

These occur after you have done the right thing and catheterised the patient. They are largely predictable and manageable if anticipated.

Post-obstructive diuresis: > 200 mL/h urine × ≥ 2h or > 3L urine in 24h ^[2]

Feature	Detail
Definition	Diuresis that persists after decompression of the bladder ^[3]. Defined as diuresis > 200 mL/hr for 2 hours ^[3]^[4] or > 3L urine in 24 hours ^[2]
Mechanism	Tubular damage → decreased concentrating ability → rapid fluid and solute loss ^[2]. During the period of obstruction, excess sodium, urea, and water accumulate in the body. Once the obstruction is relieved, the kidneys attempt to excrete this excess — this is the physiological component and is appropriate. However, the sustained back-pressure may have damaged the renal tubules (especially the concentrating mechanism in the loop of Henle and collecting ducts) → the kidneys lose the ability to reabsorb water and solutes appropriately → pathological diuresis that can lead to dangerous fluid and electrolyte depletion
Significance	Represents a physiological response to remove excess fluid in body (especially in CROU), but may result in fluid and electrolyte imbalance (pathological POD) ^[2]. Primarily a problem of chronic but not acute urinary retention ^[3] — because chronic retention allows greater accumulation of excess fluid and more tubular damage
Investigations	May have hypoNa, hypoK, hypovolaemia ^[2]. Check electrolytes Q4–6h during active diuresis
Management	Close monitoring of I/O, fluid/electrolyte status with appropriate replacement and resuscitation (prefer oral hydration, aim to replace 1/2 of urine output in the past hour) ^[2]. Patients usually can manage the increase in urine output by increasing oral fluid intake ^[3]. Isotonic saline replacement is indicated if patients are unable to increase fluid intake ^[3]. Do NOT remove Foley catheter since it may lead to hydronephrosis ^[3]. Remember to chart urine output Q2h ^[2]

POD: The Practical Rule

After catheterising a patient for AROU (especially if high-volume or chronic), chart Q1–2H urine output. If UO > 200 mL/h for ≥ 2 hours: (1) check electrolytes urgently, (2) replace approximately half of the previous hour's urine output as IV normal saline if the patient cannot drink, (3) continue monitoring until diuresis settles (usually 24–48 hours). The physiological diuresis will self-resolve once the excess is cleared; the pathological diuresis from tubular damage may persist longer and needs ongoing replacement.

Haemorrhage ex-vacuo (transient haematuria due to mucosal disruption) ^[2]^[4]

Feature	Detail
Mechanism	Bladder mucosal disruption with sudden emptying of a greatly distended bladder ^[2]. When the bladder is massively distended, the mucosal capillaries are compressed against the detrusor wall. Sudden decompression releases this tamponade effect → previously compressed vessels suddenly re-expand and bleed → haematuria. Think of it like releasing a tourniquet — the re-perfusion causes bleeding
Significance	Usually self-limiting, rarely significant ^[2]. Typically clears within hours. If it persists or is heavy, consider other sources (bladder tumour, BPH bleeding)
Management	Observation; ensure catheter is draining freely (clots can block the catheter → clot retention, which requires 3-way catheter with continuous bladder irrigation). If severe: manual bladder washout; cystoscopy with clot evacuation

Transient hypotension due to vagovagal response or relief of pelvic venous congestion ^[2]^[3]

Feature	Detail
Mechanism (1): Vasovagal	Sudden relief of pain and bladder distension triggers a parasympathetic (vagal) surge → bradycardia + vasodilation → ↓ BP. This is the "fainting with relief" mechanism
Mechanism (2): Pelvic venous decompression	A massively distended bladder compresses the pelvic veins, increasing venous return (and therefore preload) to the heart. When the bladder empties abruptly, this compression is released → pelvic venous pooling → ↓ venous return → ↓ preload → ↓ cardiac output → hypotension. Same principle as inferior vena cava compression by a gravid uterus
Management	Lie the patient flat; IV fluids if symptomatic. Some clinicians advocate gradual decompression (clamping the catheter after every 500 mL and releasing after 10–15 minutes) to prevent this, though evidence for this practice is limited and most guidelines do not recommend it routinely. The key is to monitor vitals during decompression

Feature	Detail
Mechanism	The catheter is a foreign body that disrupts normal urethral defence mechanisms, provides a surface for bacterial biofilm, and creates a direct conduit for ascending infection. Risk increases with duration of catheterisation (~5% per day for indwelling catheters)
Clinical features	Fever, pyuria, cloudy/malodorous urine, new confusion (in elderly). Note: asymptomatic bacteriuria is extremely common with indwelling catheters and should NOT be treated with antibiotics unless symptomatic
Management	Aseptic insertion technique; minimise catheter duration (daily evaluation of need); consider CISC over indwelling catheter; consider SPC for long-term drainage; treat symptomatic CAUTI with appropriate antibiotics
Long-term risk	Long-term catheterisation carries risk of: UTI/urosepsis, trauma, stones, urethral strictures, prostatitis, and SCC (squamous cell carcinoma) of bladder ^[2] — this is why long-term indwelling catheters are avoided whenever possible

SCC of Bladder from Long-Term Catheterisation

Squamous cell carcinoma of the bladder is a rare but real complication of long-term catheterisation ^[2]. Chronic irritation by the catheter causes squamous metaplasia of the transitional epithelium → dysplasia → SCC. This typically requires years of continuous catheterisation and is most relevant in patients with chronic neurogenic bladder (e.g., spinal cord injury patients with permanent catheters). This is one reason CISC is preferred over indwelling catheters for long-term management.

Feature	Detail
Mechanism	Urethral stricture ^[3] from traumatic or repeated catheterisation. The catheter can cause mucosal abrasion → inflammation → fibrosis → scarring → stricture formation. More common with larger catheters, difficult insertions, and long-term use
Clinical features	Subsequent difficulty with catheterisation, thin urinary stream, recurrent UTI, recurrent retention
Prevention	Use appropriate catheter size; use adequate lubrication (lignocaine jelly); gentle technique; minimise catheter duration; SPC for long-term drainage

Since TURP is the definitive treatment for BPH-related AROU, its complications are essential exam knowledge.

Early complications of TURP: significant complications in ~15–20%, mortality in 0.2–2.5% ^[7]

Complication	Mechanism / Details	Incidence
Bleeding	Prostatic venous sinuses are opened during resection → haemorrhage	3–7% require transfusion ^[7]
Infection and urosepsis	Instrumentation of an infected/colonised urinary tract → bacteraemia	Variable
TUR syndrome ( < 1%)	Irrigation fluids used in TURP: ionic fluid cannot be used for monopolar → often used 1.5% glycine ^[7]. Fluid low in sodium enters venous channels → dilutional hyponatraemia + decreased osmolarity (aggravated by post-op stress-related ADH release) ^[7]. Lowest 1–2h after surgery, but slowly increases afterwards due to glycine uptake into cells ^[7]. S/S: N/V, confusion, hypertension, visual disturbance (flashing lights), giddiness, seizures ^[7]. Prevention: avoid non-conductive irrigants (use bipolar technique), monitor fluid absorbed (glycine deficit), limit operation time to < 1h, minimise irrigation pressure < 60 mmHg ^[7]. Management: stop OT immediately + Na replacement + Lasix ^[7]	< 1% ^[7]
Local injury	Incontinence (1%) ^[7]; strictures or bladder neck stenosis ^[7]; fistulation due to perforation of urethra or bladder dome ^[7]	Variable
Retrograde ejaculation	Incompetent bladder neck → retrograde flow of semen ^[7]; penile erection and sexual function rarely affected ^[7]	40–60% ^[7]
Recurrence	Regrowth of prostatic tissue	5% need re-operation in 5 years ^[7]

Late complications of TURP ^[7]:

Bladder neck stenosis and urethral strictures (7–8%)
Urinary incontinence (2%): urge incontinence (early — detrusor irritability from surgery) / stress incontinence (late — sphincter damage)
Regrowth of prostate
Sexual dysfunction: ejaculatory dysfunction (especially retrograde ejaculation 40–60%); erectile dysfunction (5%): uncommon ^[7]

Timing	Complication	Key Features
During retention (untreated)	UTI / urosepsis	Stagnant urine → bacterial proliferation
	Bladder stones	Supersaturation + stasis + infection
	Hydronephrosis / hydroureter	Back-pressure from distended bladder
	Obstructive uropathy / post-renal AKI	Bilateral hydronephrosis → ↓ GFR
	Detrusor decompensation	Overdistension → loss of actin-myosin overlap
	Overflow incontinence	Intravesical pressure exceeds outlet resistance intermittently
	Delirium (elderly)	Pain, discomfort, autonomic disturbance
	Autonomic dysreflexia (SCI T6+)	Uninhibited sympathetic storm from bladder distension
Immediately post-catheterisation	Post-obstructive diuresis	Tubular damage + accumulated solute excretion → > 200 mL/h
	Haemorrhage ex vacuo	Mucosal decompression bleeding
	Transient hypotension	Vasovagal + pelvic venous pooling
Short-term catheterisation	CAUTI	Foreign body → biofilm → ascending infection
	Urethral trauma / false passage	Traumatic insertion
Long-term catheterisation	Urethral stricture	Chronic mucosal irritation → fibrosis
	Bladder stones (catheter-related)	Catheter as nidus for crystal deposition
	SCC of bladder	Chronic irritation → squamous metaplasia → malignancy
Post-TURP	TUR syndrome	Glycine absorption → dilutional hyponatraemia
	Retrograde ejaculation	Bladder neck incompetence
	Bleeding, incontinence, stricture	Surgical trauma

High Yield Summary

Complications of retention (the disease itself): recurrent UTI, bladder calculi, hydroureter/hydronephrosis, renal impairment/ARF (obstructive uropathy) ^[1] — these are also the absolute indications for TURP
Post-obstructive diuresis: > 200 mL/h × ≥ 2h; primarily a problem of chronic retention; due to accumulated solute excretion + tubular damage; manage with close I/O monitoring, replace ~1/2 of UO with oral fluids or IV NS; do NOT remove catheter
Haemorrhage ex vacuo: transient haematuria from mucosal decompression; usually self-limiting
Transient hypotension: vasovagal response + pelvic venous decompression → monitor vitals
Long-term catheterisation risks: UTI/urosepsis, urethral stricture, stones, prostatitis, SCC of bladder — this is why CISC is preferred over indwelling catheters
TUR syndrome ( < 1%): dilutional hyponatraemia + fluid overload + glycine toxicity from monopolar TURP; prevented by bipolar technique (saline irrigant), limiting OT time < 1h, keeping irrigation pressure < 60 mmHg
Retrograde ejaculation after TURP: 40–60%; due to incompetent bladder neck; counsel patients before surgery
Autonomic dysreflexia: life-threatening hypertension in SCI patients (T6+) triggered by bladder distension — immediate catheterisation is the treatment
Detrusor decompensation from prolonged overdistension may be irreversible → patient may need lifelong CISC even after obstruction is relieved
Overflow incontinence is paradoxical — patient appears to void (dribbles) but is actually in retention; always check for a palpable bladder / scan PVR

Active Recall - Complications of AROU

References

^[1] Lecture slides: GC 180. Benign prostatic hyperplasia, bladder outlet obstruction and urinary retention.pdf (pp. 37, 46) ^[2] Senior notes: Ryan Ho Urogenital.pdf (pp. 168, 172, 177); Ryan Ho Fundamentals.pdf (pp. 351, 353) ^[3] Senior notes: felixlai.md (AROU treatment/complications section, BPH complications section) ^[4] Senior notes: maxim.md (AROU complications, TURP complications) ^[5] Senior notes: Ryan Ho Critical Care.pdf (p. 26 — AKI management and complications) ^[6] Senior notes: Ryan Ho Psychiatry.pdf (p. 74 — DELIRIUM mnemonic) ^[7] Senior notes: Ryan Ho Urogenital.pdf (pp. 172, 177); Ryan Ho Fundamentals.pdf (p. 353) — TURP complications, BPH complications

History Taking: Acute Retention of Urine (AROU)

This is your first job. Before you launch into differential diagnosis, you need to establish that the patient actually has a full bladder they cannot empty, rather than simply not producing urine.

Key Questions

Question	Why It Matters	Cantonese Phrasing
"When did you last pass urine?"	Establishes timeline and acuity	你上一次小便係幾時？(nei5 soeng6 jat1 ci3 siu2 bin6 hai6 gei2 si4?)
"Is your lower tummy swollen and painful?"	Painful suprapubic distension = AROU; painless = CROU ^[1]^[2]	你個肚下面有冇脹住痛？(nei5 go3 tou5 haa5 min6 jau5 mou5 zoeng3 zyu6 tung3?)
"Are you still producing urine at all — even small amounts or dribbling?"	Differentiates retention from anuria/oliguria ^[2]^[3]	你仲有冇少少尿出到？(nei5 zung6 jau5 mou5 siu2 siu2 niu6 ceot1 dou2?)
"Is this the first time, or has this happened before?"	First episode vs recurrent — recurrent suggests progressive pathology ^[1]	係第一次定之前都試過？(hai6 dai6 jat1 ci3 ding6 zi1 cin4 dou1 si3 gwo3?)

AROU vs Anuria — Don't confuse them

AROU should be distinguished from anuria or oliguria → lack of urine production, not retention ^[2]^[3]. A patient in shock (e.g. ruptured AAA) or dehydration may also not be passing urine, but the bladder will be empty. Always confirm with a bladder scan (≥300mL unable to void suggests retention; ≥1L suggests chronic retention) ^[2].

2. Characterizing the Current Episode

This is a high-yield area in the OSCE. There is almost always a precipitant on top of a background predisposition.

Precipitant	Question to Ask	Cantonese
Recent surgery / anaesthesia	"Have you had any operations recently?"	你最近有冇做過手術？(nei5 zeoi3 gan6 jau5 mou5 zou6 gwo3 sau2 seot6?)
New medications	"Have you started any new medications — especially cold medicines, painkillers, or psychiatric drugs?"	你有冇食新藥？特別係傷風藥、止痛藥？(nei5 jau5 mou5 sik6 san1 joek6?)
Alcohol	"Have you been drinking large amounts of alcohol recently?"	你最近有冇飲好多酒？(nei5 zeoi3 gan6 jau5 mou5 jam2 hou2 do1 zau2?)
Constipation / faecal impaction	"When did you last open your bowels?"	你上次大便幾時？(nei5 soeng6 ci3 daai6 bin6 gei2 si4?)
Prolonged immobility	"Have you been bedbound or less mobile recently?"	你最近有冇長時間臥床？
Pelvic trauma	"Have you had any injury to your lower tummy, back passage, or groin?"	你有冇撞親下面？

Cold medications — a classic OSCE trap

Sympathomimetics such as α-agonists found in cold medications (e.g. phenylephrine, pseudoephedrine) and β-agonists in bronchodilators can precipitate AROU by increasing outflow resistance ^[2]^[3]^[4]. This is a favourite viva question: "What over-the-counter drugs can cause urinary retention?"

3. Identifying the Underlying Cause

BPH is the most common cause of AROU in males (53%) ^[2]^[3]. Ask about complications:

Complication	Question	Why
Gross haematuria	"Have you noticed blood in your urine?" (有冇血尿 jau5 mou5 hyut3 niu6)	Suggests BPH, CA bladder, or CA prostate ^[1]
UTI (fever/dysuria)	"Any burning when you pass urine? Any fever?"	Stasis predisposes to infection ^[1]
Bladder/urethral stone	"Any sudden sharp pain when urinating that makes you stop mid-stream?" (strangury)	Stones can cause BOO or complicate BPH ^[1]
Renal impairment	"Any nausea, loss of appetite, itching, or fatigue?" (uraemic symptoms)	Obstructive nephropathy from chronic retention ^[1]^[2]

This is the one you cannot miss. Neurogenic bladder can present as AROU, and if spinal cord compression is the cause, delay in diagnosis → irreversible paraplegia ^[3]^[4]^[6].

Question	What You're Looking For	Cantonese
"Any back pain?"	Vertebral metastasis, epidural abscess, disc prolapse	有冇背脊痛？(jau5 mou5 bui3 zek3 tung3?)
"Any weakness or numbness in your legs?"	Motor/sensory deficit from cord compression	對腳有冇冇力或者痺？(deoi3 goek3 jau5 mou5 mou5 lik6 waak6 ze2 bei3?)
"Any numbness around your back passage/saddle area?"	Saddle anaesthesia → cauda equina syndrome	屎窟附近有冇痺？
"Any problems with bowel control?"	Faecal incontinence = red flag for cauda equina	有冇失禁？

Drug-induced AROU is extremely common and very testable ^[2]^[3]^[4]:

Drug Class	Mechanism	Examples
Sympathomimetics (α-agonists)	↑Urethral smooth muscle tone → ↑outflow resistance	Phenylephrine (cold medications), pseudoephedrine
Sympathomimetics (β-agonists)	Detrusor relaxation	Terbutaline, salbutamol (bronchodilators)
Anticholinergics	Impair detrusor contractility	Atropine, ipratropium, tiotropium, antihistamines (promethazine), antispasmodics (opioids)
Antipsychotics	Anticholinergic side effects	Chlorpromazine, olanzapine
Antidepressants	Anticholinergic / serotonergic effects	TCAs (amitriptyline), SSRIs
Disopyramide	Anticholinergic (class Ia antiarrhythmic)	Disopyramide
MDMA (Ecstasy)	Sympathomimetic	Recreational drug use

Ask specifically: "Are you taking any cold medicines, cough syrups, asthma inhalers, sleeping tablets, pain medications, or psychiatric medications?" (你有冇食傷風藥、咳藥水、哮喘噴劑、安眠藥、止痛藥、精神科藥？)

Question	Target
"Any unexplained weight loss or loss of appetite?"	Constitutional symptoms for CA prostate, CA bladder ^[4]
"Any bone pain, especially in the back or hips?"	Bony metastases from prostate CA
"Any blood in your urine — when does it appear?"	Haematuria — beginning = lower tract, throughout = above bladder, terminal = prostate/bladder neck ^[7]
"Have you ever worked with chemicals, dyes, or rubber?"	Occupational risk for bladder CA

System	Questions	Relevance
Urological	Dysuria, haematuria, urethral discharge, scrotal pain	UTI, STD, urethritis causing urethral oedema
GI	Constipation, change in bowel habit, rectal bleeding	Constipation (7.5% of male AROU) ^[2]; rectal mass
Neurological	Back pain, limb weakness, sensory changes, gait difficulty	Cord compression, cauda equina, MS, GBS
Constitutional	Fever, weight loss, night sweats, fatigue	Infection, malignancy
Gynaecological (females)	Pelvic heaviness, prolapse symptoms, vaginal bleeding	Pelvic organ prolapse (cystocele, rectocele), gynaecological tumours ^[2]^[3]

5. Background History

Diagnosis	Key Differentiating Question	Expected Finding
BPH	Long history of progressive obstructive LUTS, nocturia	Most common; older male, gradual worsening
CA Prostate	Bony pain (back, hips), weight loss, haematuria	Rapid progression, hard irregular prostate on DRE
Urethral stricture	History of previous catheterization, urethral instrumentation, STDs	Young/middle-aged male, very slow stream
Drug-induced	New medication started within days/weeks	Temporal relationship; resolves when drug stopped
Constipation/faecal impaction	Not opened bowels for days, abdominal distension	Common precipitant in elderly, often overlooked
Spinal cord compression	Back pain + leg weakness + sensory level + bowel dysfunction	EMERGENCY — needs urgent MRI
Cauda equina syndrome	Saddle anaesthesia, bilateral leg pain, bowel/bladder dysfunction	EMERGENCY — needs urgent MRI and decompression
Post-operative	Recent GA/epidural, onset within hours of surgery	Bladder overdistension from reduced detrusor function post-anaesthesia ^[3]^[4]
Genital herpes	Painful vesicular lesions around genitalia	Sacral nerve involvement ^[4]

What You Ask	Why It Matters for the OSCE
Painful vs painless	Acute = painful (innervation intact, e.g. BPH); Chronic = painless (innervation abnormal, e.g. DM neuropathy) ^[1]^[2] — this is a classic opening viva question
First catheterized volume	>500mL = genuine AROU; >1000mL = likely chronic retention ^[4]
IPSS score	Quantifies LUTS severity; mild 0–7, moderate 8–19, severe 20–35 ^[4]^[5] — shows you understand grading
Drug history	Demonstrates clinical reasoning — if you identify a culprit drug, you can propose stopping it as first-line management
Neurological symptoms	Shows you are thinking about dangerous differentials — examiners love this
Constitutional symptoms	Demonstrates you are screening for malignancy — a must in any urological history
Sexual history / STDs	Links to urethral stricture — shows depth of knowledge
Bowel history	Shows you know about constipation as a precipitant — a subtle but important point

"Mr Chan is a 72-year-old gentleman with a background of hypertension, type 2 diabetes mellitus, and benign prostatic hyperplasia (on tamsulosin), who presented this evening to Queen Mary Hospital with a 6-hour history of inability to pass urine associated with severe suprapubic pain and distension.

In terms of his presenting complaint, he reports progressive lower urinary tract symptoms over the past 2 years, including hesitancy, weak stream, nocturia three times per night, and a sensation of incomplete emptying. His IPSS score is approximately 22 out of 35, indicating severe symptoms. He denies any prior episodes of retention. He denies haematuria, dysuria, fever, or weight loss. There are no neurological symptoms — no back pain, no leg weakness, no saddle anaesthesia, and no change in bowel control. He has been constipated for the past 4 days and has not opened his bowels. He was started on a new cold medication (containing pseudoephedrine) by his GP 3 days ago for an upper respiratory tract infection.

His past medical history includes hypertension, type 2 diabetes mellitus (well controlled, HbA1c 6.8%), BPH diagnosed 3 years ago, and hyperlipidaemia. He has no history of STDs, stroke, malignancy, or neurological disease. He has no previous surgical history.

His regular medications are tamsulosin 0.4mg daily, metformin 500mg BD, amlodipine 5mg daily, and atorvastatin 20mg daily. The new pseudoephedrine-containing cold medication was started 3 days ago. He has no known drug allergies.

His family history is significant for his father being diagnosed with prostate cancer at age 78.

Socially, he is a retired bus driver, lives with his wife, is independently mobile, and manages all activities of daily living. He is a non-smoker and drinks 1–2 beers per week. He denies any illicit drug use.

In summary, this is a case of acute retention of urine in an elderly gentleman with known BPH and severe baseline LUTS, likely precipitated by the combination of constipation and a recently commenced sympathomimetic cold medication. I would like to confirm the diagnosis with a bladder scan, proceed with urethral catheterization for bladder decompression, stop the pseudoephedrine, manage his constipation, check bloods including RFT and CBC, and send catheterized urine for microscopy and culture. PSA should be deferred for 4–6 weeks."

High Yield Summary

AROU = sudden, painful inability to void despite a full bladder. It is the most common urological emergency.

History-taking framework — ask yourself 5 questions in order:

Is it true AROU? — Confirm full bladder (painful suprapubic distension), rule out anuria/oliguria, differentiate from chronic retention (painless)
What characterizes this episode? — First vs recurrent, precipitants (surgery, drugs, alcohol, constipation, immobility)
Is there underlying BOO? — Preceding obstructive LUTS using IPSS; BPH complications (haematuria, UTI, stones, renal impairment)
Is there a neurological cause? — MUST rule out spinal cord compression (back pain, leg weakness, sensory level, saddle anaesthesia, bowel dysfunction)
What drugs might be responsible? — Sympathomimetics (cold meds), anticholinergics, antipsychotics, antidepressants, opioids

Critical pearl: Do NOT check PSA during an acute episode — it will be falsely elevated. Defer 4–6 weeks.

Most common cause in males: BPH (53%). In females: detrusor underactivity / neurogenic bladder.

Acute Retention Of Urine

1. Definition

2. Epidemiology

2.1 Incidence and Prevalence

2.2 Sex Differences – Why the Massive Discrepancy?

3. Risk Factors

3.1 Risk Factors for Male AUR (Olmsted County Study) [1]

3.2 Risk Factors for Female AUR

3.3 Common Precipitating Factors [2]

4. Anatomy and Physiology of Micturition

4.1 Lower Urinary Tract Anatomy

4.2 Two Phases of Bladder Function [1][2]

4.3 Receptive Relaxation — Laplace's Law Applied to the Bladder [1][2]

4.4 Innervation of the Lower Urinary Tract

4.5 Normal Micturition Reflex — Step by Step

4.6 Detrusor Sphincter Dyssynergia (DSD) [1][2]

5. Aetiology

5.1 Tabular Breakdown of Aetiologies from Epidemiological Data [1]

5.2 Mechanical Bladder Outlet Obstruction (BOO) [1][2][3]

Common obstructive causes in males [1]:

Common causes in females [1]:

5.3 Drug-Induced AROU [2][3]

5.4 Neurological Causes (Neurogenic Bladder) [2][3]

5.5 Acute Overdistension of the Bladder [2]

5.6 Aetiology Summary Table — Comprehensive

6. Classification

6.1 Acute vs Chronic Retention [1][2]

6.2 By Mechanism

6.3 Spontaneous vs Precipitated AROU

7. Pathophysiology — Bringing It All Together

7.1 The Three Mechanisms

Mechanism 1: Outflow Obstruction [2]

Mechanism 2: Decreased Detrusor Contraction [2]

Mechanism 3: Detrusor-Sphincter Dyssynergia [2]

7.2 Consequences of AROU (if untreated)

7.3 BPH-Specific Pathophysiology Leading to AROU [3]

8. Clinical Features

8.1 Symptoms

8.2 History Taking — Systematic Approach [2]

8.3 Signs — Physical Examination

9. BPH-AROU Connection: The Most Common Scenario in Hong Kong

10. Approach Summary — Putting It All Together

Active Recall - Acute Retention of Urine

References

1. The First Branch: AROU vs Anuria/Oliguria

2. The Second Branch: AROU vs Chronic Retention of Urine (CROU)

3. Differential Diagnosis of AROU by Cause — Organised by Mechanism

3.1 Obstructive Causes (Mechanical Bladder Outlet Obstruction)

3.2 Drug-Induced Causes

3.3 Neurological Causes

3.4 Acute Overdistension / Functional

4. Sex-Specific Differential Diagnosis Framework

Males — Think Obstruction First

Females — Think Detrusor Failure First

5. Differential Diagnosis Algorithm

6. How to Differentiate at the Bedside — A Practical Approach

Step 1: Confirm Retention

Step 2: Acute or Chronic?

Step 3: Look for the Cause

Step 4: Red Flags Requiring Urgent Action

7. Differential Diagnosis of LUTS — Broader Context

8. The "DELIRIUM" Mnemonic Connection

Active Recall - Differential Diagnosis of AROU

1. Diagnostic Criteria for AROU

2. The Diagnostic Algorithm — Step by Step

Phase 1: Acute Diagnosis and Immediate Management

Phase 2: Subsequent Workup to Identify the Underlying Cause

3. Investigation Modalities — Comprehensive Breakdown

3.1 Bedside Investigations

Bladder Scan (Portable Ultrasound)

First Catheterised Urine Volume

Digital Rectal Examination (DRE)

Neurological Examination

3.2 Blood Investigations

3.3 Urine Investigations

3.4 Radiological Investigations

KUB (Plain Radiograph of Kidneys, Ureters, and Bladder)

USG Bladder + USG Kidney/Ureter

CT KUB (Non-Contrast CT)

Intravenous Urogram (IVU) [9]