Detrusor Underactivity

Detrusor underactivity is a urodynamic diagnosis: a bladder contraction of reduced strength, reduced duration, or both, so that voiding is prolonged or the bladder does not empty fully within a normal time. The associated symptom syndrome is often called underactive bladder.

It is found in roughly 10 to 40% of older men and 12 to 45% of older women who undergo urodynamics for lower urinary tract symptoms, the range reflecting differing definitions. It is commonly mistaken for outlet obstruction, and the two often coexist.

Pathophysiology. Normal contraction needs intact afferent signalling, a sacral reflex arc with parasympathetic efferent drive, and healthy detrusor smooth muscle. Loss at any level, or ischaemic and fibrotic degeneration of the muscle, lowers contraction pressure or shortens its duration.

Chronic retention then causes overdistension injury, which worsens contractility further.

Causes by mechanism:

• Myogenic: ageing, diabetes mellitus, long standing outlet obstruction with decompensation, chronic overdistension, ischaemia.
• Neurogenic: diabetic autonomic neuropathy, conus or cauda equina lesions, lumbosacral disc disease, multiple sclerosis, Parkinson disease, peripheral neuropathies, and pelvic plexus injury after abdominoperineal resection, radical hysterectomy or radical prostatectomy.
• Drug related: opioids, anticholinergics, tricyclic antidepressants, antipsychotics, first generation antihistamines, vincristine.
• Contributor: faecal loading.

Slow or interrupted stream, hesitancy, straining, incomplete emptying, post-micturition dribble, reduced bladder sensation, infrequent voiding, recurrent urinary tract infection and overflow incontinence. Acute or chronic retention may be the first presentation.

Examination: palpable or percussible bladder, prostate size, perineal and perianal sensation, anal tone, bulbocavernosus reflex, lower limb power and reflexes, gait, and signs of peripheral neuropathy.

Saddle anaesthesia, bilateral leg weakness or faecal incontinence (cauda equina or cord compression), hydronephrosis or a rising creatinine, and painless retention with new neurological signs need same day assessment.

• Frequency volume chart for 3 days, urinalysis and culture, serum creatinine and electrolytes, HbA1c, vitamin B12.
• Post-void residual by bladder scan, repeated on separate occasions. Under 50 mL is usually normal, over 100 mL is usually abnormal, and 50 to 100 mL is indeterminate. No single cut off defines the condition.
• Uroflowmetry: low peak flow, with an interrupted, straining or prolonged curve.
• Renal and bladder ultrasound for hydronephrosis, wall thickness, diverticula and stones.
• Pressure flow urodynamics confirm the diagnosis. Bladder contractility index equals detrusor pressure at peak flow plus 5 times peak flow in mL/s. Under 100 indicates weak contractility, 100 to 150 normal, and over 150 strong. Low voiding efficiency (voided volume divided by total bladder volume) and a long voiding time support the diagnosis. Combined with fluoroscopy, urodynamics also excludes concurrent obstruction.
• Flexible cystoscopy when stricture, bladder neck or prostatic obstruction is possible.
• Neurological examination, with spinal MRI when a conus, cauda equina or cord lesion is suspected.

Differential diagnosis.

• Bladder outlet obstruction (prostatic enlargement, urethral stricture, bladder neck obstruction)
• Acontractile detrusor from neurological disease
• Detrusor sphincter dyssynergia
• Dysfunctional voiding
• Fowler syndrome in young women (failure of sphincter relaxation)
• Pelvic organ prolapse or faecal loading causing obstruction
• Drug induced retention
• Psychogenic retention

The aims are safe low pressure emptying, prevention of infection and protection of the upper tracts. No drug has reliable proven benefit.

• Remove contributors: reduce or stop opioids, anticholinergics and tricyclic antidepressants where possible, clear faecal loading and keep stools soft, and improve glycaemic control.
• Behavioural: timed voiding every 3 to 4 hours, double voiding, relaxed seated posture. Suprapubic pressure or straining is used only when the upper tracts are normal and there is no reflux.
• Clean intermittent catheterisation when residual is persistently over 150 to 200 mL with symptoms, or with recurrent infection, hydronephrosis or renal impairment. Use a 12 to 14 Fr hydrophilic catheter 4 to 6 times daily, aiming for volumes under 400 to 500 mL.
• Long term catheter drainage when intermittent catheterisation is not possible: suprapubic placement is preferred to urethral, with changes on the manufacturer's schedule, commonly every 4 to 12 weeks.
• Bethanechol 10 to 50 mg orally three or four times daily, started low and titrated, gives modest and inconsistent benefit. Contraindications: any outlet obstruction, recent gastrointestinal or bladder surgery, peptic ulcer, asthma, bradycardia, hyperthyroidism, epilepsy, Parkinson disease, coronary disease. Never give it parenterally. Cholinergic toxicity (sweating, salivation, cramps, bradycardia) is treated with atropine 0.6 mg subcutaneously or intravenously, repeated as needed.
• Tamsulosin 400 micrograms orally once daily after a meal is used off label to lower outlet resistance, with inconsistent benefit and orthostatic hypotension as the main adverse effect.
• Sacral neuromodulation is an option for non-obstructive retention. Test stimulation runs for 1 to 2 weeks, and the device is implanted if symptoms or catheterised volumes improve by 50% or more.
• Outlet surgery only when obstruction is proven by pressure flow study, as results are poorer when the detrusor is also weak.

Special groups. Women under about 50 with unexplained retention and no neurological disease need assessment for Fowler syndrome, which responds to sacral neuromodulation rather than drugs.

Monitoring. Post-void residual, creatinine and renal ultrasound at 3 to 6 months, then every 6 to 12 months if catheterising or residual stays high. Treat only symptomatic infection. Do not give routine antibiotic prophylaxis.

Referral and escalation. Refer to urology for urodynamics and catheter planning. Urgent referral for retention, hydronephrosis, creatinine rise, or neurological features.