• CROU is substantially underdiagnosed because it is painless and patients often do not present until complications (overflow incontinence, renal impairment, recurrent UTI) bring them to medical attention.
• In men, the prevalence increases steeply with age — mirroring the prevalence of BPH and neurological conditions:
  • 50% of patients at age 50 have LUTS related to BPH ^[3], and a proportion of these will progress to CROU if untreated.
  • Up to 10% of men in their 70s and ~30% of men in their 80s will develop some form of urinary retention over a 5-year period ^{[1] [2]}.
• In women, CROU is much rarer since the female urethra is short and bladder outlet obstruction (BOO) is anatomically uncommon; when it occurs, detrusor underactivity (DUA) is the dominant mechanism.

^{[1] [2]}

Risk factors for male urinary retention (Olmsted County Study) ^[1]:

• Increasing age
• Increasing prostate size
• Increasing BPH symptoms (IPSS)
• Decreasing maximal urine flow rate (Qmax)

Additional risk factors relevant to CROU specifically:

• Diabetes mellitus — diabetic autonomic neuropathy → detrusor underactivity (this is a major cause, especially in Hong Kong where DM prevalence is ~10%)
• Neurological disease — multiple sclerosis, Parkinson's disease, spinal cord pathology
• Chronic obstruction left untreated — decompensated bladder that cannot recover contractility
• Medications — chronic use of anticholinergics, opioids, or drugs with bladder-relaxant effects
• Advanced age — independent effect on detrusor muscle contractility (sarcopenia of detrusor)

Drugs rarely cause CROU alone but frequently precipitate or worsen chronic retention:

^{[1] [2] [3]}

This is the classic pathway, best exemplified by BPH:

1. Early compensated phase: BOO → detrusor must work harder → detrusor hypertrophy (just like the heart hypertrophies in response to aortic stenosis)

   • Clinically: patient has obstructive LUTS (hesitancy, weak stream, straining) but can still void
   • The thickened detrusor also develops trabeculation (visible ridges of muscle bundles on cystoscopy)

2. Intermediate phase: continued obstruction → detrusor develops instability (overactivity)

   • Clinically: irritative/storage LUTS develop (frequency, urgency, nocturia) — because the hypertrophied detrusor generates involuntary contractions
   • Diverticula may form where mucosa herniates between hypertrophied muscle bundles

3. Late decompensated phase: collagen replaces smooth muscle → loss of compliance and contractility → the bladder becomes a large, floppy, atonic sac

   • Clinically: rising post-void residual volume → CROU
   • The patient may still void, but incompletely — they dribble urine (overflow incontinence)
   • The detrusor can no longer generate sufficient pressure to overcome the obstruction

4. Consequences of CROU (see Complications section in next installment):

   • Recurrent UTI (stagnant urine = culture medium for bacteria)
   • Bladder stone formation (urinary stasis → crystallization)
   • Hydroureter and hydronephrosis (back-pressure transmitted to upper tracts)
   • Obstructive uropathy / renal impairment / ARF (bilateral obstruction → post-renal AKI)

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

In diabetic cystopathy (the prototype):

1. Sensory neuropathy → impaired perception of bladder fullness → patient does not feel the urge to void → bladder stretches beyond normal capacity
2. Autonomic (parasympathetic) neuropathy → impaired detrusor contraction → inadequate voiding pressure
3. Chronic overdistension → mechanical disruption of detrusor muscle (sarcomere stretch beyond optimal length → reduced actin-myosin overlap = Frank-Starling-like failure of the bladder)
4. Result: large-capacity, low-pressure, poorly contractile bladder with huge PVR

A special pathophysiological pattern in suprasacral spinal cord lesions:

• Normally, the PMC coordinates simultaneous detrusor contraction + sphincter relaxation
• In DSD, the spinal cord lesion interrupts descending PMC control → the detrusor contracts (via local spinal reflex arc) BUT the sphincter also contracts simultaneously
• Result: very high intravesical pressures (detrusor pushing against a closed outlet)
• This is dangerous because high pressures are transmitted to the upper tract → hydronephrosis → renal damage
• CROU in DSD is characterized by: incomplete emptying, high PVR, high voiding pressures, and risk of upper tract deterioration and autonomic dysreflexia (in lesions above T6)

This is a crucial teaching point:

• AROU is dramatic and painful → patients present quickly → catheterized and treated promptly → reversible
• CROU is insidious and painless → patients present late → often with established complications:
  • Renal impairment (may be advanced by the time of diagnosis — "silent" obstructive nephropathy)
  • Bilateral hydronephrosis (chronically elevated back-pressure)
  • Overflow incontinence (often misdiagnosed as stress or urge incontinence)
  • Recurrent UTIs (stagnant urine)
  • Bladder stones
  • Post-obstructive diuresis after catheterization (the kidneys, previously obstructed, produce large volumes of dilute urine — can cause dangerous fluid/electrolyte shifts)

This distinction is critical because:

• High-pressure CROU → greater risk of upper tract damage (hydronephrosis, renal impairment)
• Low-pressure CROU → lower risk of upper tract damage (bladder acts as a low-pressure reservoir) but still causes UTI, stones, overflow incontinence

• Obstructive (BOO)
• Non-obstructive (DUA — neurogenic, myogenic, idiopathic)
• Mixed (most common in elderly men with BPH + age-related DUA)

Based on detrusor pressure at maximum flow (PdetQmax) and maximum flow rate (Qmax):

• Obstructed: high PdetQmax, low Qmax
• Equivocal: intermediate values
• Unobstructed: low PdetQmax, low Qmax (= detrusor underactivity)

AROU (and by extension CROU) should be distinguished from anuria or oliguria → lack of urine production, not retention ^[2]

The definitive test is a bedside bladder ultrasound scan: if the bladder is full (large volume), it is retention; if the bladder is empty, it is anuria/oliguria ^{[2] [3]}.

Pre-renal ARF due to dehydration and shock; Renal ARF due to acute tubular necrosis, interstitial nephritis, glomerulonephritis — these all present with oliguria but an empty bladder ^[2].

Acute retention of urine (AROU): sudden onset, painful → occurs when innervation is normal, e.g. BPH ^[1] Chronic retention of urine (CROU): usually painless with vague lower abdominal distension → occurs when innervation is abnormal, e.g. DM ^[1]

Bladder scan: ≥ 300 mL in a patient unable to void suggests urinary retention; ≥ 1 L suggests chronic retention of urine ^[2]

A patient may also present with acute-on-chronic retention — someone with longstanding CROU who suddenly becomes completely unable to void (e.g. precipitated by a cold medication containing pseudoephedrine). They will have features of both: painful acute presentation + very large bladder volume + pre-existing complications (hydronephrosis, elevated Cr).

A palpable suprapubic mass is not always a bladder:

Overflow incontinence: leakage of urine when bladder is abnormally distended with large residual volume, especially in men with chronic retention ^[5]

Overflow incontinence is essentially the presenting symptom of CROU, but you must distinguish it from other types of incontinence (because treatment differs radically):

^{[3] [5]}

BOO typically presents with predominantly voiding symptoms ^[3]

Common causes of AROU/CROU in females: detrusor hypocontractility — exclude DM; neurogenic bladder; idiopathic ^[1]

Drug-induced causes ^[2]:

• Sympathomimetics: α-agonists (cold medications), β-agonists (bronchodilators), MDMA
• Anticholinergics: anticholinergics (bronchodilators), antipsychotics, antispasmodics (opioids), antihistamines, antidepressants, disopyramide (class Ia antiarrhythmic)

Always review the drug chart. In clinical practice, drugs rarely cause CROU de novo, but they frequently tip a borderline patient into retention (e.g. a man with moderate BPH who takes an OTC cold medicine containing pseudoephedrine and goes into retention).

Posterior urethral valve (PUV) — boys only; congenital cause of BOO; presents with bilateral hydronephrosis, trabeculated bladder on antenatal USS; management is valve ablation ^[6]

• CROU from any cause (BOO or DUA) — the most important to exclude
• Stress incontinence
• Urge incontinence
• Continuous incontinence (fistula, ectopic ureter)
• Functional incontinence

Overflow incontinence: esp in men with chronic retention; may be complicated with UTI, bladder stone formation; may eventually lead to obstructive uropathy and deterioration of renal function ^[5]

• Post-renal AKI from CROU → bilateral hydronephrosis
• Pre-renal AKI (dehydration, sepsis, cardiogenic shock)
• Intrinsic renal AKI (ATN, AIN, GN)
• Other causes of post-renal AKI: bilateral ureteric stones, retroperitoneal fibrosis, pelvic malignancy with bilateral ureteric involvement

Post-renal disease ( < 10%) due to obstructive uropathy (must be bilateral) ^[7] Prolonged post-renal disease will progress to become tubulointerstitial fibrosis, i.e. intrinsic renal disease ^[7]

• CROU with stagnant residual urine
• Structural abnormalities (stones, vesicoureteric reflux)
• Immunocompromised state
• Poor hygiene / catheter-associated UTI

This is the broadest DDx and includes both BOO and non-BOO causes:

Differential diagnosis of LUTS ^[3]:

• Bladder outlet obstruction: bladder stones, bladder cancer, bladder neck contracture, interstitial cystitis, ketamine cystitis; BPH, prostatic cancer; urethral stricture
• Overactive bladder (detrusor overactivity): neurogenic (stroke, SCI, MS, Parkinson's) or non-neurogenic (idiopathic, post-operative, secondary to BOO)

Additional DDx for LUTS includes:

• UTI/prostatitis — irritative symptoms with dysuria, pyuria, significant bacteriuria on culture
• Bladder cancer — painless haematuria, irritative symptoms, smoking history
• Interstitial cystitis / bladder pain syndrome — chronic pelvic pain relieved by voiding, sterile urine, young women
• Ketamine cystitis — important in Hong Kong given ketamine abuse; severely contracted bladder, young patients
• Nocturnal polyuria — isolated nocturia, normal daytime frequency; causes include CCF, OSA, DI, excessive evening fluid intake ^[3]

The diagnosis of CROU requires all three of the following:

1. Elevated post-void residual (PVR) volume — the bladder retains a significant volume of urine after the patient has attempted to void (or cannot void at all)
2. Chronicity — the condition has developed gradually over weeks to months (not acutely)
3. Painless or minimally symptomatic bladder distension — distinguishing it from AROU

Bladder scan: ≥ 300 mL in a patient unable to void suggests urinary retention; ≥ 1 L suggests chronic retention of urine ^[2]

This distinction is made on pressure-flow studies (urodynamics) and is clinically crucial because it determines the risk of upper tract damage:

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

This is important: two patients can both have a PVR of 800 mL, but the one with high-pressure CROU (e.g. from BPH with DSD) is at far greater risk of renal damage than the one with low-pressure CROU (e.g. from diabetic cystopathy).

Step 1: Confirm urinary retention ^[2]

• Bedside bladder ultrasound scan → if bladder volume is large (≥ 300 mL post-void), retention is confirmed
• Alternatively, the first catheterized urine volume confirms retention if a catheter is passed directly
• If bladder is empty → this is NOT retention → consider anuria/oliguria (pre-renal, renal, or bilateral ureteric obstruction)

Step 2: Differentiate AROU from CROU ^{[1] [2]}

• AROU: sudden onset, painful → innervation intact
• CROU: usually painless, vague lower abdominal distension → often abnormal innervation
• Volume ≥ 1 L strongly suggests chronic ^[2]
• Presence of pre-existing complications (bilateral hydronephrosis, elevated Cr, bladder stones) confirms chronicity

Step 3: Assess for complications immediately

• Bloods: RFT (obstructive uropathy?) → High serum creatinine can result from bladder outlet obstruction or underlying renal disease, which should prompt an USG ^[3]
• Urine: urinalysis, microscopy, C/ST (UTI?) ^[3]
• Imaging: USG kidney (hydronephrosis?)

Step 4: Determine the underlying cause

• History: LUTS profile (IPSS), neurological symptoms, DM, drug history, surgical history ^[3]
• Physical examination: DRE (prostate), neurological exam (sensory level, anal tone, perineal sensation), external genitalia ^[3]
• Focused investigations: uroflowmetry, PSA (once retention resolved), urodynamics if diagnosis uncertain, cystoscopy if indicated

Bedwetting (high-pressure chronic retention) ^[4]

Why does bedwetting occur in high-pressure CROU? During sleep, voluntary sphincter tone reduces. In high-pressure CROU, the detrusor generates high pressures but cannot overcome the daytime sphincter tone. At night, with reduced sphincter tone, the high intravesical pressure overcomes the lowered resistance → urine leaks out → nocturnal enuresis/bedwetting. This is a red flag for high-pressure retention and upper tract damage.

Investigations — CBP, L/RFT, CSU × C/ST, KUB, No PSA — Likely falsely elevated ^[1]

This is reiterated here because it is a high-yield exam point: in the acute setting of CROU (or AROU), do NOT check PSA. It will be falsely elevated. Check it 4–6 weeks after decompression and resolution of any infection.

When a catheter is inserted for CROU:

• Record the initial volume drained — this is the first catheterized volume
• Volume > 800 mL–1 L in a painless patient essentially confirms chronic retention
• Monitor hourly urine output after catheterization → watch for post-obstructive diuresis (will be covered in Management section)

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

Procedure ^{[2] [3]}:

1. Aseptic technique: clean genital area with aqueous hibitane (chlorhexidine), drape surrounding areas
2. Intraurethral local anaesthetic: apply xylocaine jelly around meatal opening → milk jelly down urethra → wait 5 minutes
3. Insertion: use forceps to hold 14Fr Foley's catheter → insert all the way down using no-touch technique
4. Fixation: inject 10 mL of water-for-injection into balloon → withdraw catheter until resistance encountered (balloon sitting at bladder neck)
5. Connect to drainage bag and record first catheterized volume

Key points for CROU specifically:

• First catheterized volume > 1000 mL indicates possible chronic retention ^[3]
• Monitor first catheterization volume and hourly urine output (Q1H) ^[3]
• Do NOT clamp the catheter intermittently — gradual decompression has NOT been shown to reduce complications compared to complete drainage; just let it drain freely
• Send catheterized urine for microscopy, culture and sensitivity (C/ST) ^[2]

Catheter sizing ^[3]:

• Males: 14–18 Fr (start with 14Fr)
• Females: 12–16 Fr

Failure to pass catheter into bladder ^{[2] [3]}:

Contraindications to urethral catheterization ^{[2] [3]}:

• Absolute: urethral injury (signs: blood at urethral meatus, high-riding prostate on DRE — typically associated with pelvic fracture)
• Relative: urethral stricture, recent urinary tract surgery (radical prostatectomy, urethral reconstruction), presence of artificial sphincter

Suprapubic catheterization (SPC) ^[2]:

• Indications: failed urethral catheterization, history of urethral trauma (e.g. straddle injury), long-term bladder drainage expected (> 3 weeks)
• Contraindications: non-distended bladder, uncorrected bleeding tendency, known/suspected urothelial CA

Procedure ^[2]:

• LA injected 2 finger-breadths above pubic symphysis
• Small incision made in skin/fascia → insert trocar-type suprapubic tube → catheter advanced over trocar → sutured in place → look for gush of urine

Complications of SPC ^[2]:

• Bowel perforation, rectal injury (overshooting), haematuria

Advantages of SPC over long-term urethral catheter ^[3]:

• Prevents urethral trauma and stricture formation
• Prevents urinary sphincter dysfunction leading to incontinence
• Reduces catheter-associated UTI (CAUTI)
• Allows assessment of patient's ability to void before removing catheter

Post-obstructive diuresis: defined as diuresis > 200 mL/hr for ≥ 2 hours ^[3]

This is primarily a problem of chronic but not acute urinary retention ^[3]. Let me explain why from first principles:

Why does post-obstructive diuresis happen?

1. Accumulated solute load: during the period of obstruction, the kidneys continue to filter (albeit impaired) → urea, sodium, and water accumulate in the body → once obstruction is relieved, the kidneys excrete this accumulated load → osmotic diuresis (urea acts as an osmotic diuretic)
2. Impaired tubular concentrating ability: chronic back-pressure damages the renal tubules → the kidneys lose their ability to concentrate urine → produce large volumes of dilute urine (similar to nephrogenic DI)
3. ANP and natriuretic peptide release: volume overload during obstruction → atrial/ventricular stretch → ANP/BNP release → promotes sodium and water excretion once obstruction is relieved

Clinical significance: the patient can lose litres of fluid and electrolytes rapidly → dehydration, hypotension, hypokalaemia, hyponatraemia, hypernatraemia → potentially life-threatening

Management ^[3]:

• Monitor urine output Q1H — if > 200 mL/hr for ≥ 2 hours, this is significant post-obstructive diuresis
• Do NOT remove Foley catheter — the diuresis needs to resolve; removing the catheter may lead to re-retention and worsening hydronephrosis ^[3]
• Patients normally 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]
• Monitor electrolytes: serial RFT (Cr, K⁺, Na⁺, urea) at least twice daily
• Fluid replacement strategy: replace ~50–75% of the previous hour's urine output with IV normal saline; do NOT match 100% output (this perpetuates the diuresis) — allow gradual physiological correction
• Watch for: hypokalaemia (may need K⁺ supplementation), hyponatraemia, hypernatraemia (free water deficit if tubular concentrating defect)

• Presents with acute painful retention superimposed on a chronically decompensated bladder
• Management follows AROU protocol initially (catheterize urgently) BUT:
  • Expect very large first catheterized volume (> 1 L)
  • High risk of post-obstructive diuresis
  • Pre-existing complications likely (hydronephrosis, elevated Cr)
  • Do NOT attempt TWOC if obstructive uropathy was present

Post-renal disease: consider and reverse — S/S: palpable enlarged bladder with ↑ residual volume, blocked catheter; Mx: directed to underlying cause ^[9]

• Urgent catheterization is the priority
• Life-threatening complications of AKI must be managed concurrently:
  • Hyperkalaemia: IV calcium gluconate, NaHCO₃ infusion, dextrose/insulin drip, oral polystyrene sulphonate, dialysis ^[9]
  • Fluid overload: IV loop diuretics (furosemide), dialysis ^[9]
  • Metabolic acidosis: IV bicarbonate, dialysis ^[9]
• Haemodialysis indications (mnemonic: AEIOU): Acidosis (pH < 7.1 refractory to bicarb), Electrolyte (K⁺ > 6.5 refractory to medical Rx), Intoxication, Overload (refractory to diuretics), Uraemia (pericarditis, neuropathy, altered mental status) ^[9]

PCN indications: hydronephrosis due to benign or malignant obstruction; urine leakage secondary to trauma, infection or neoplasm ^[8]

• In CROU, PCN is rarely needed because bladder catheterization decompresses the entire urinary system (since the obstruction is at the bladder outlet, not the ureters)
• PCN is indicated when:
  • Catheterization fails AND SPC is contraindicated
  • Concurrent ureteric obstruction (e.g. from pelvic malignancy) in addition to CROU
  • Infected hydronephrosis requiring direct drainage of the renal pelvis

Overflow incontinence: constant dribbling (especially at night) with associated retention of urine ^[2] Overflow incontinence: superimposed in CROU with ↑↑ PVR in bladder ^[2]

• Mechanism: as the bladder fills beyond its maximum capacity, intravesical pressure eventually exceeds the outlet resistance → urine passively leaks out. This is like an overfilled cup — the water spills over the rim.
• Why worse at night: during sleep, the voluntary component of external sphincter tone decreases (somatic pudendal nerve relaxes), lowering the outlet resistance threshold → overflow occurs more easily. Additionally, supine position increases venous return → increased renal perfusion → more urine production.
• Clinical features: constant dribbling, wet underwear/bedding, patient may be unaware (especially if sensory neuropathy present)
• Impact: skin maceration, perineal dermatitis, social embarrassment, misdiagnosis as urge or stress incontinence

Recurrent UTI ^[1]

Chronic retention with overflow incontinence may be complicated with urinary tract infection, bladder stone formation ^[5]

• Mechanism: stagnant residual urine provides an excellent culture medium for bacteria. Normal bladder defence mechanisms rely on regular, complete emptying (washout effect). In CROU:
  1. The washout effect is lost → bacteria that enter the bladder are not flushed out
  2. Bladder wall ischaemia from chronic overdistension impairs local mucosal immunity (ischaemic mucosa cannot mount an effective immune response)
  3. Mucosal oedema and trabeculation create crevices where bacteria can harbour and form biofilms
• Common organisms: E. coli (most common), Klebsiella, Proteus mirabilis (important because it is a urease-producer — splits urea into ammonia → alkaline urine → struvite stones)
• Clinical features: dysuria, cloudy/foul-smelling urine, fever, suprapubic discomfort; may be atypical in elderly (confusion, falls, reduced oral intake)
• Risk of urosepsis: if infection ascends to the kidneys (pyelonephritis) in the setting of obstructed drainage → pus under pressure (pyonephrosis) → gram-negative sepsis → life-threatening

Formation of bladder calculi ^[1]

• Mechanism: urinary stasis allows supersaturation of dissolved solutes → crystallization and stone formation. This is the same principle as why a stagnant lake develops mineral deposits while a flowing river does not.
  • Additional factors: UTI with Proteus mirabilis → urease splits urea → ammonia → alkaline pH → precipitation of magnesium ammonium phosphate (struvite) and calcium phosphate stones
  • Foreign body (catheter) also acts as a nidus for stone formation
• Clinical features: worsening irritative LUTS (frequency, urgency, dysuria — from stone irritating the bladder mucosa), intermittent haematuria, strangury (painful desire to void with passage of only small amounts), intermittent urinary stream (stone rolls over the bladder outlet and intermittently blocks it — classically the stream cuts off when standing and resumes when lying down)
• Diagnosis: KUB (radio-opaque stones), USG bladder, cystoscopy
• Treatment: cystolitholapaxy (endoscopic stone fragmentation and removal) + treat the underlying retention

Other BPH complications: gross haematuria, UTI (fever/dysuria), bladder/urethral stone (strangury), renal impairment (uraemic symptoms) ^[1]

• Mechanism: chronic BOO → the detrusor hypertrophies to overcome increased resistance → the muscle bundles become thickened and prominent (trabeculation). Between these thickened bundles, the mucosa herniates outward through weak points → bladder diverticula

Bladder changes: hypertrophy of bladder musculature (trabeculated), bladder diverticulum, secondary detrusor overactivity ^[2]

• Clinical significance of diverticula:

  • Diverticula have no muscular wall (just mucosa) → they cannot contract → urine stagnates within them → acts as a reservoir for infection and stone formation
  • Diverticula can harbour bladder cancer (difficult to detect on standard cystoscopy)
  • Large diverticula can themselves cause BOO by compressing the bladder outlet or ureteric orifices

• Secondary detrusor overactivity: the hypertrophied, irritable detrusor develops involuntary contractions during filling → this is why patients with chronic BOO develop storage/irritative symptoms (frequency, urgency, urge incontinence) in addition to their voiding symptoms

Hydroureter and hydronephrosis ^[1]

• Mechanism: chronically elevated intravesical pressure is transmitted retrograde up the ureters to the renal pelvis. The normal anti-reflux mechanism at the vesicoureteric junction (VUJ) — the submucosal tunnel — can be overwhelmed by sustained high pressure.

  • In high-pressure CROU (e.g. BOO, DSD): back-pressure is significant → higher risk of hydronephrosis
  • In low-pressure CROU (e.g. DUA): back-pressure is lower → hydronephrosis is less common but can still occur if the bladder is grossly overdistended

• Pathological progression:

  1. Initially: dilated ureters (hydroureter) and renal pelvis (hydronephrosis) with preserved renal parenchyma
  2. Prolonged: cortical thinning from chronic compression of the renal parenchyma between the distended collecting system and the renal capsule → reduced nephron mass
  3. Eventually: irreversible renal parenchymal loss → ESRD if bilateral and untreated

• Clinical features: often asymptomatic until advanced (this is why CROU is dangerous — "silent" hydronephrosis). May present with loin pain (if acute-on-chronic), features of renal failure (fatigue, nausea, oedema, pruritus), or incidental finding on USG.

• Diagnosis: USG kidneys (dilated collecting system, cortical thinning), CT urogram if needed

• Management: bladder decompression (catheterization) → most cases of bilateral hydronephrosis from CROU will resolve once the bladder is drained. If unilateral or persistent, consider percutaneous nephrostomy (PCN) or JJ stent ^[8].

Renal impairment / ARF (Obstructive uropathy) ^[1]

Chronic retention caused by benign prostate hyperplasia and other cause of outlet obstruction may eventually lead to obstructive uropathy and deterioration of renal function ^[5]

This is the most serious complication of CROU and the reason it must never be dismissed as a benign condition.

• Mechanism: bilateral hydronephrosis → chronic tubular compression → tubulointerstitial fibrosis → progressive loss of GFR → CKD → potentially ESRD

  • Initially, the tubular damage impairs concentrating ability (loss of medullary gradient) → polyuria, nocturia (paradoxically, the patient with obstructive uropathy may produce large volumes of dilute urine)
  • Later, glomerular filtration decreases → rising creatinine, uraemia
  • The renal damage is partially reversible if decompressed early, but becomes irreversible once fibrosis is established

• Clinical features: elevated creatinine (often an incidental finding), uraemic symptoms (fatigue, anorexia, nausea, pruritus, confusion), fluid overload (peripheral oedema, hypertension, pulmonary oedema), electrolyte derangements (hyperkalaemia — potentially fatal)

• Life-threatening complications of the renal failure ^[9]:

  • Hyperkalaemia: symptoms (arrhythmia, weakness) usually only when K > 6 — management: IV calcium gluconate, NaHCO₃ infusion, dextrose/insulin drip, oral polystyrene sulphonate, dialysis ^[9]
  • Fluid overload: peripheral oedema, hypertension, pulmonary oedema — management: IV loop diuretics (furosemide), dialysis ^[9]
  • Metabolic acidosis: Kussmaul's breathing — management: IV bicarbonate, dialysis ^[9]
  • Uraemia: pericarditis, neuropathy, altered mental status → indication for haemodialysis ^[9]

• Mechanism: chronically elevated intravesical pressure can disrupt the anti-reflux mechanism at the VUJ → urine refluxes from the bladder up to the kidneys
• This is secondary VUR (as opposed to primary VUR which is a congenital anomaly of the submucosal tunnel) ^[10]
• Secondary VUR results from high bladder pressure, e.g. bladder outlet obstruction, neurogenic bladder dysfunction ^[10]
• Significance: reflux of infected urine into the upper tracts → reflux nephropathy → renal scarring → CKD

Hernia due to chronic straining ^[2]

• Mechanism: patients with CROU often strain (Valsalva manoeuvre) to void → chronically elevated intra-abdominal pressure → pushes abdominal contents through weak points in the abdominal wall → inguinal hernia, umbilical hernia
• This is the same mechanism that causes haemorrhoids and rectal prolapse with chronic straining

Haematuria due to ruptured dilated bladder veins at base (significant BPH → ↑ blood flow) ^[2]

• Mechanism: in BPH, the dilated prostatic venous plexus (bladder neck veins) is engorged due to venous congestion from the enlarged prostate. These fragile veins can rupture, causing macroscopic haematuria.
• Also: bladder stones cause mucosal irritation and bleeding; chronic mucosal congestion from overdistension
• Important: haematuria should be worked up for other pathologies, especially CA, even if known BPH ^[2] — never assume haematuria is "just from BPH" without ruling out bladder cancer

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

• Primarily a problem of chronic but not acute urinary retention ^[3]
• Mechanism: tubular damage → ↓ concentrating ability → rapid fluid and solute loss ^[2]
• 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]
• Investigations: may have hypoNa, hypoK, hypovolaemia ^[2]
• Management: close monitoring of I/O, fluid/electrolyte status with appropriate replacement and resuscitation (prefer oral hydration, aim to replace 1/2 of UO in the past hour) ^[2]
• Do NOT remove Foley catheter since it may lead to hydronephrosis ^[3]

Why is POD especially dangerous?

• Rapid fluid loss can cause hypovolaemic shock if not replaced
• Electrolyte losses (K⁺, Na⁺, Mg²⁺, PO₄³⁻) can cause cardiac arrhythmias (hypokalaemia → U waves, ventricular ectopics, torsades de pointes)
• Hypernatraemia can occur if the kidneys preferentially lose free water (tubular concentrating defect produces dilute urine)

Haemorrhage ex-vacuo (transient haematuria): MoA — bladder mucosal disruption with sudden emptying of greatly distended bladder; usually self-limiting, rarely significant ^{[2] [3]}

• Mechanism: when the grossly distended bladder is suddenly decompressed, the previously stretched and compressed submucosal blood vessels suddenly expand → mucosal tearing → transient haematuria
• Think of it like removing a tight bandage — the compressed tissue bleeds briefly once pressure is released
• Management: usually self-limiting. If persistent or heavy, use a 3-way catheter with continuous bladder irrigation (NS) to prevent clot formation and retention

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

• Mechanism (two components):
  1. Vasovagal reflex: rapid bladder decompression stimulates parasympathetic afferents → vagal response → bradycardia + peripheral vasodilation → hypotension
  2. Relief of pelvic venous congestion: the chronically distended bladder compresses pelvic veins → obstructs venous return. When the bladder is drained, the venous compression is relieved → blood pools in the newly decompressed pelvic venous bed → reduced effective circulating volume → transient drop in BP
• Management: lie patient flat, IV fluids if needed; usually resolves spontaneously