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.
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
Did you know? 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.
Causes of 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. For example a medication could be the cause, which if so, the first line treatment would be to discontinue the medication and re-scan the bladder.
What medications are you aware off that can cause retention of urine? Share your findings with fellow learners.
Symptoms of underactive bladder
The key lower urinary tract symptoms (LUTS) of an underactive bladder are:
- Poor urine flow
- Nocturia/nocturnal enuresis
- Daytime frequency
- Small voided volumes
- Large post-void residual urine volume
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, 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.
In addition to the above LUTS, people with underactive bladder may also have:
- Intermittent stream
- Feeling of incomplete emptying
- History of repeated urinary tract infections – repeated infection is often a presenting symptom
Did you know? Underactive bladder is commonly missed. Why do you think this is the case?
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