Fainting (syncope): Causes, diagnosis, and treatment

Fainting, or syncope, refers to a sudden and temporary loss of consciousness, usually because of a lack of oxygen in the brain.

Oxygen deprivation to the brain has many possible causes, including hypotension, or low blood pressure.

Often, a fainting episode has no medical importance, but sometimes it results from a serious illness, condition, or disorder. Every case of fainting should be treated as a medical emergency until the cause is known and signs and symptoms have been treated.

Anybody who has recurring fainting episodes should see a doctor.

Fast facts on fainting

Here are some key points about fainting. More detail is in the main article.

Fainting, or syncope, is usually caused by a lack of oxygen in the brain.

Fainting should be treated as a medical emergency until the cause is known.

Symptoms can include falling down, blurred vision, and confusion.

Possible causes include dehydration, low blood pressure, alcohol use, and diabetes.

Treatment depends on the cause of the underlying condition.


[Woman fainting]
Fainting happens when not enough blood and oxygen reaches the brain.

Syncope, or fainting, usually results from hypoxia, or a lack of oxygen to the brain.

Causes include problems with the lungs or blood circulation and carbon monoxide poisoning.

Fainting is a mechanism to help a person survive.

If brain blood and oxygen levels drop considerably, the brain immediately shuts down all other non-vital parts of the body so that resources can focus primarily on vital organs.

When the brain detects lower levels of oxygen, the body will start breathing faster, or hyperventilating, to bring levels back up again.

The heart rate will also rise in order to get more oxygen into the brain. This rise in the heart rate results in hypotension, or a drop in blood pressure, in other parts of the body. The brain receives extra blood, at the expense of other parts of the body.

Hyperventilation combined with hypotension may result in short-term loss of consciousness, muscle weakening, and fainting.

Underlying causes

There can be different underlying causes of syncope.

Neurocardiogenic syncope results when something triggers a short-term malfunction of the autonomous nervous system (ANS). It is also known as neurally mediated syncope (NMS).

The ANS affects heart rate, digestion, respiration rate, salivation, perspiration, diameter of the pupils, urination, and sexual arousal. Most of these actions are involuntary, but some, such as breathing, can also be done consciously.

There will be a drop in blood pressure and the heartbeat and pulse rate will slow down. This causes a temporary interruption in the brain’s blood and oxygen supply.

Possible triggers include:

suddenly seeing something that is unpleasant or shocking, such as blood

sudden exposure to an unpleasant sight or experience

sudden emotional upset, for example when receiving bad news

extreme embarrassment

standing still for a long time

being in a hot and stuffy place for a long time

Occupational, or situational, syncope is a type of neurocardiogenic syncope, but the link is physical rather than emotional, mental, or abstract.

Triggers may include:

coughing or sneezing

laughing or swallowing

defecating or urinating

demanding physical activities, such as lifting a heavy weight

Orthostatic hypotension can happen when a person faints after stands up rapidly from a seated or lying down position.

Gravity pulls blood down to the legs, resulting in lower blood pressure elsewhere.

Normally, the body’s nervous system reacts by raising the heart beat and narrowing blood vessels. This stabilizes blood pressure.

However, if something undermines this stabilization process, there may be poor blood and oxygen supply to the brain, leading to fainting.

Triggers include:

Dehydration: If body fluid levels drop, so will blood pressure. This can make it harder for the body to stabilize blood pressure, resulting in less blood and oxygen reaching the brain.

Untreated diabetes: A person with diabetes may urinate more often, leading to dehydration. High blood glucose levels can lead to damage in some nerves, especially those that regulate blood pressure.

Some medications: Diuretics, beta-blockers and anti-hypertensive drugs may cause orthostatic hypotension in some people.

Alcohol: Some people faint if they consume too much alcohol in a short space of time.

Some neurological conditions: Parkinson’s disease and other conditions affect the nervous systems, and this can lead to orthostatic hypotension.

Carotid sinus syndrome: Pressure on the pressure sensors in the carotid artery can cause fainting. These pressure sensors are known as the carotid sinus. The carotid artery is the main artery that supplies blood to the brain.

If the carotid sinus is over-sensitive, blood pressure may drop if it is physically stimulated, resulting in fainting. Examples include turning the head to one side, wearing a tight collar or tie, or pressing over the carotid sinus while shaving.

This is more common among males aged over 50 years.

Cardiac syncope: An underlying heart problem can cause a drop in blood and oxygen supply to the brain.

Possible conditions include:

arrhythmias, or abnormal heart beat

stenosis, a blockage of the heart valves

hypertension, or high blood pressure

a heart attack, when a heart muscle dies because of a lack of blood and oxygen

This cause of fainting will usually need treatment and monitoring.

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