What is an underactive bladder?

An underactive bladder occurs when there is insufficient power in the detrusor muscle to expel the urine from the bladder when voiding.

If there are no contractions, this is known as an atonic bladder.

If the contraction strength is poor, this is known as a hypotonic bladder.

Therefore with an atonic bladder the individual is unable to void any urine and with a hypotonic bladder they pass small amounts only, leaving a residual in the bladder.

This residual amount will gradually increase over time and the detrusor muscle relaxes, resulting in a large flaccid bladder.

What is a high compliant bladder?

The bladder can stretch to hold residual volumes from 500ml to 2 or more litres of urine, but for many cases underactivity is identified at significantly lower levels.

The smooth detrusor bladder muscle can stretch to accommodate the increase in volume of urine without an increase in bladder pressure. This is termed a highly compliant bladder

If the bladder is stretched to over 1 litre, this can lead to permanent damage to the detrusor muscle. Therefore early/prompt identification of incomplete bladder emptying is vital.

This will be identified by a post void bladder scan or if no scanner is available an ‘in/out catheterisation’ to empty the bladder.

What causes an underactive bladder?

Underactive bladder has many potential causes:

• Side effects of medications: including antimuscarinics, antihistamines, antipsychotics, tricyclic antidepressants
• Neurological conditions that affect the nerve supply to the detrusor muscle
• Damage to the lower spinal cord in conditions such as diabetes neuropathy, pelvic injury, cauda equina syndrome and multiple sclerosis
• Changes affecting the detrusor muscle due to ageing, resulting in impaired contractility of the detrusor
• Post epidural anaesthetic
• Chronic constipation, pelvic tumours and prolapses can all affect the bladder’s ability to empty properly due to obstruction of the urethra

If an hypotonic/ atonic bladder is not treated (by draining the urine from the bladder with an intermittent catheter), there is a danger that the urine will ‘back flow’ up to the kidneys and cause renal damage that could be fatal.

Medical referral is required to identify the cause of the underactive bladder, which is commonly missed. If, for example, a medication could be the cause, the first line treatment would be to discontinue the medication and re-scan the bladder.

What are the symptoms of underactive bladder?

The key lower urinary tract symptoms (LUTS) of an underactive bladder are:

• Poor urine flow
• Hesitancy
• Nocturia/nocturnal enuresis
• Daytime frequency
• Small voided volumes
• Large post-void residual urine volume
• Dribbling/overflow incontinence

This occurs when the bladder pressure exceeds the urethral pressure. The bladder pressure is high due to the large residual volume in the bladder.

The overflow leakage often starts with nocturnal enuresis, ie when asleep at night time. The person is less aware of the signals of bladder filling. In the day there is more awareness of bladder signals and the person toilets frequently, passing only a small amount to give relief.

The overflow incontinence happens when additional pressure is put on the bladder, such as from walking, bending, coughing, sitting down on toilet.

Are there any additional symptoms?

In addition to the above LUTS, people with underactive bladder may also have:

• Urgency
• Intermittent stream
• Feeling of incomplete emptying
• History of repeated urinary tract infections – repeated infection is often a presenting symptom