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Causes of overactive bladder (OAB)

Overactive bladder (OAB) is due to an abnormality in the nerve signalling to the bladder and messages to the bladder muscle (detrusor muscle).

The bladder muscle (detrusor muscle) normally contracts when nerve signals send a message to the bladder when it is full and a person voluntarily wishes to pass urine. In OAB there is excessive nerve signalling both to and from the bladder muscle that causes it to contract out of a person’s control, even when the bladder is not full.

Excessive messages in the signalling nerves of the bladder from the brain and spinal cord cause the symptoms of frequency, urgency and nocturia.

The symptoms of an overactive bladder are usually associated with involuntary contractions of the bladder (or detrusor muscle) known as “detrusor overactivity”. Detrusor overactivity is definitively diagnosed on urodynamic testing and is often inferred by the typical history, examination and findings on investigations such as urine testing and bladder diary.

Most cases of OAB and detrusor overactivity are “idiopathic” i.e. the person does not have a defined or identifiable neurological abnormality but the bladder nerve pathways are misbehaving.

Some cases of OAB are due to “neurogenic” detrusor overactivity i.e. the person has a definite neurological problem identified e.g. a spinal cord injury, past stroke or neurological condition such as Multiple Sclerosis (MS) or Parkinson’s disease.

Some cases of OAB are caused by blockage i.e. they are the result of the bladder’s reaction to chronic blockage. This can happen in men who have blockage of urine flow due to prostate enlargement. Blockage occurs less commonly in women but can potentially be caused by a large pelvic organ prolapse or surgical treatment for stress urinary incontinence with a sling that is too tight.

Risks factors for OAB

  • Both women and men are at risk of developing OAB.
  • As a person gets older, they are at greater risk of developing symptoms of OAB but not all people develop OAB symptoms with ageing.
    • Remember that treatment options are available at all ages and
    • Even young people can develop problems with OAB
  • Women who have gone through the menopause have higher rates of OAB.
  • Men with prostate problems due to prostate enlargement also have a higher risk of OAB.
  • People with neurological problems that affect the brain and spinal cord are at increased risk of OAB
    • E.g. strokes, spinal cord injuries, Parkinson’s disease, Multiple Sclerosis (MS)

The difference between OAB and Stress Urinary Incontinence (SUI)

OAB is different from another very common problem called stress urinary incontinence (SUI).

Stress urinary incontinence (SUI) is the involuntary leakage of urine caused by physical activity that causes a rise in the pressure in the abdomen (“stress”) such as coughing, straining, sneezing, lifting and exercise.

Unlike OAB, urgency is not a feature of stress urinary incontinence.

Some people can have a combination of both OAB and SUI – which is known as mixed incontinence.