Epidemiology

It is more common in males and accounts for 8.4% of all causes of syncope.^[1] Furthermore, 61% of patients with micturition syncope also experience other kinds of syncope.^[1] Peak age of presentation is 30-49 years.

Aetiology

• The underlying cause of micturition syncope is thought to be related to vasovagal syncope with hypotension and possibly bradycardia.^[2][3] This may involve postural hypotension and increased vagal tone as a result from straining (Valsalva manoeuvre).
• However, the trigger of the vasovagal response is unclear. It has been hypothesised that the bladder becomes hyper-reflexic. This is supported by spinal cord injury patients who develop hypotension and syncope when intermittent urinary catheterisation is performed.^[4]
• Furthermore, the risk of developing hypotension is enhanced by any hypotensive medication, eg alpha-blockers and even antidepressants.
• Excess ethanol intake and excess warmth are also precipitating factors for micturition syncope. Again it is thought that these lead to hypotension.
• Furthermore, research in patients with multiple system atrophy (MSA), who have frequent pre-syncope and syncope on micturition, reported that during bladder filling they experienced a slight rise in blood pressure with no change in heart rate (both increased in controls).^[5] This is thought to result from activation of the sympathetic nervous system. In controls, at the beginning of micturition this sympathetic activity increased with a further rise in blood pressure and heart rate. Following this, there is a decrease in blood pressure and heart rate (back to baseline). Patients with MSA showed a similar pattern but with less of an increase in blood pressure at the beginning of urination, followed by a fall during micturition. However, the fall is more marked and the duration is longer in MSA.^[5] These changes are similar to those seen in neurally mediated syncope.

Presentation

Feeling dizzy or light-headed, or having short-lived loss of consciousness when passing urine or straight afterwards. A collateral history is vital.

Differential diagnosis

Other causes of syncope:

• Cardiac arrhythmias.
• Structural heart disease, eg aortic stenosis, hypertrophic cardiomyopathy.
• Hypovolaemia.
• Postural hypotension.

Investigations

These are mostly directed towards excluding other more sinister causes, eg Holter monitoring, lying and standing blood pressures. Tilt table testing can be used to determine the extent of the autonomic instability and for training to desensitise the patient from the stimulus.

Management

• Safety measures - eg standing up slowly from a lying position, keeping the bathroom door open, moving sharp objects away.
• Stop any precipitating medications, eg antihypertensive medication (if possible - especially alpha-blockers), antidepressants with hypotensive side-effects.
• Botulinum A toxin injections have been injected into the detrusor muscle of the bladder of patients with spinal cord injuries with some success.^[4][6]
• Fludrocortisone has been used and enhances blood pressure on standing - its role in micturition syncope has not been established.
• Drugs such as selective serotonin reuptake inhibitors (SSRIs) have been suggested but there is no evidence of their benefit and they can also aggravate hypotension.