Syncope is a temporary loss of consciousness caused by transient global cerebral hypoperfusion (a temporary reduction of blood flow to the entire brain). There is a rapid onset, short duration and complete recovery.
Vasovagal Syncope
Vasovagal syncope (simple “fainting”) is caused by a temporary imbalance in the autonomic nervous system. In response to a strong stimulus, such as an emotional event (e.g., needle phobia) or pain, the parasympathetic nervous system is activated, and the sympathetic nervous system is reduced.
Parasympathetic signals cause a drop in heart rate, and reduced sympathetic tone causes blood vessels to relax, leading to a drop in blood pressure and reduced perfusion of brain tissue. When cerebral perfusion drops below a certain threshold, the person loses consciousness.
Vasovagal syncope has three Ps:
- Posture – episodes typically occur when standing for long periods
- Provoking factors – episodes have a clear trigger (e.g., seeing blood, prolonged standing or dehydration)
- Prodrome – episodes are preceded by prodromal symptoms
Patients can usually recall experiencing progressive symptoms leading up to a vasovagal episode, such as feeling:
- Hot, clammy or sweaty
- Dizzy or lightheaded
- Heavy
- Nausea
- Vision going blurry or dark
Other Causes of Syncope
There is a long list of causes of a temporary loss of consciousness. It is essential to establish whether a syncopal episode is due to an uncomplicated faint in an otherwise healthy patient or underlying pathology.
Causes of reflex syncope (due to disruption of the autonomic nervous system) include:
- Vasovagal syncope
- Situational (e.g., on micturition, coughing or defecation)
- Carotid sinus hypersensitivity (typically in older patients, triggered by head movement or pressure on the neck)
Causes resulting in orthostatic hypotension (a drop in blood pressure on standing) include:
- Hypovolaemia (e.g., dehydration or infection)
- Post-prandial
- Medications (e.g., antihypertensives and diuretics)
- Adrenal insufficiency
Cardiac causes of syncope include:
- Arrhythmias (e.g., heart block)
- Structural heart disease (e.g., hypertrophic obstructive cardiomyopathy)
- Ischaemic heart disease (e.g., myocardial infarction)
Other causes of a temporary loss of consciousness that are not classed as syncope include:
- Seizure
- Hypoglycaemia
- Intoxication (e.g., alcohol or drugs)
- Psychogenic pseudosyncope (apparent loss of consciousness without abnormal perfusion or brain function)
- Sleep disorders (e.g., narcolepsy)
Syncope Versus Seizure
A collateral history from a witness helps give an impression of the episode. During a vasovagal episode, the person may be described as:
- Looking pale or unwell before the event
- Losing consciousness and falling to the ground
- Unconscious for up to a minute
- Some twitching, shaking or convulsion activity
The patient may experience confusion or grogginess following a faint. However, this is different from the postictal period that follows a seizure. A typical postictal state involves a prolonged period of confusion, drowsiness, irritability and disorientation.
Incontinence may occur with both seizures and syncopal episodes.
| Vasovagal Syncope | Seizure | |
| Before | Sudden trigger (e.g., fear) | Trigger (e.g., poor sleep or missed meds) |
| Prolonged standing | Epilepsy aura (smells, tastes or deja vu) | |
| Lightheaded | ||
| Sweating | ||
| Blurring or clouding of vision | ||
| During | Reduced tone | Head turning or abnormal limb positions |
| Tonic clonic activity | ||
| Tongue biting | ||
| Cyanosis | ||
| Duration | Seconds | Minutes |
| After the event | Progressive recovery | Prolonged postictal period |
Investigations
Investigations are not always required if a detailed history and examination suggest a vasovagal episode with no underlying pathology. Possible investigations include:
- Full blood count (anaemia)
- Urea and electrolytes (electrolyte imbalances or dehydration)
- Blood glucose (hypoglycaemia)
- 12-lead ECG (arrhythmia) and ambulatory ECG (paroxysmal arrhythmia)
- Echocardiogram (structural heart disease)
Management
Ruling out underlying pathology and establishing the diagnosis is essential. Vasovagal syncope is common in children and teenagers. Education, reassurance and advice can be given to:
- Avoid dehydration
- Avoid missing meals
- Avoid standing still for long periods
- When experiencing prodromal symptoms, sit or lie down, drink, eat and rest until feeling better
Suspected seizures or underlying pathology require referral for further investigation and management.
Last updated January 2026
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