What Causes Fainting?

Reasons for Syncope are Varied

Apr 14, 2009 John Richard Roberts

Fainting is a common problem with numerous causes. Although not generally serious and easily managed, it's important to find out why it's occurring.

The immediate cause of fainting or syncope is lack of blood flow to the cells of the brain. This brings on a partial or complete temporary loss of consciousness. Prior to the faint people usually feel unwell, nauseous or light-headed. Occasionally muscle spasms can occur which may be confused with an epileptic fit.

The word temporary in the definition is important. Unlike stroke which is also due to restricted brain blood flow, fainting doesn't cause permanent damage to the brain – unless of course a head injury results from the fall.

Maintenance of Brain Blood Flow

Anything that interferes with blood circulation has the potential to cause fainting. Three things need to be functioning well to ensure proper blood circulation: the heart must be pumping efficiently, the muscle tone in the walls of the blood vessels has to be properly controlled by the nervous system and the blood pressure needs to be adequate. This last is mainly determined by the first two. The major controlling pathway for heart and blood pressure is the vagus nerve.

Over-activity of Vagus Nerve

This nerve, which runs from below the brain and either side of the spinal column to the chest and abdomen, has an inhibitory effect on the circulatory system: it slows down heart beat and relaxes blood vessel muscle tone and thereby reduces blood pressure. Excessive activity in this nerve is the chief cause of fainting and can occur for a number of reasons.

• Psychogenic syncope: strong emotional stimuli or pain can trigger a reflex which stimulates the vagus nerve leading to slowing of the heart and a drop in blood pressure and therefore reduced blood supply to the brain. Prolonged standing in a hot environment can also be a trigger. More common in young people.
• Cough syncope: coughing raises the pressure in the chest cavity and this temporarily interferes with blood return to the heart, this plus reflex stimulation of the vagus nerve can cause fainting.
• Micturation syncope: similar mechanism to cough syncope. Straining to pass urine pushes up chest cavity pressure and a full bladder stimulates the vagus. More common in men and after alcohol.
• Hypoglycaemia: low blood sugar can increase vagus nerve activity.
• Carotid sinus syndrome: the carotid arteries situated either side of the neck house receptors which help regulate vagus activity. In arterial disease this can become over-sensitive and trigger vagus activity.

Less Common Causes of Fainting

Like the above causes these are due in some way to circulatory problems but not directly to vagus activity.

• Orthostatic hypotension: a rapid reduction in blood pressure on standing. Usually caused by diseases which effect the nerves that control blood vessel tone, diabetes is the most common. Other causes are anaemia and medication for high blood pressure.
• Heart disorders: for example valve disease and heart rhythm defects.
• Hyperventilation syndrome: over-breathing, usually as a result of anxiety. Too much carbon dioxide builds up in the blood and triggers a reflex which alters blood flow in the brain.

This article is for information only. If you have any health concerns you should consult your doctor.

Resource

Neurology: A Concise Clinical Text Michael Swash and Martin Schwartz. Pub. Bailliere Tindall 1989

The copyright of the article What Causes Fainting? in General Medicine is owned by John Richard Roberts. Permission to republish What Causes Fainting? in print or online must be granted by the author in writing.

Vagus Nerve