Causes a Loss of Consciousness

Reading time: 2 min

Syncope is caused by a temporary reduction in blood flow to the brain. This temporary reduction in the blood supply in the brain can be caused by heart diseases and, more commonly, due to other conditions that do not directly involve the heart.

Non-cardiac causes

Person with persistent cough

Vasovagal syncope. It is the most common cause. It is produced by an involuntary reflex of the nervous system. The causes of blackout are: after taking a blood sample; an intense pain, or after certain situations, such as to urinate, defecate, or cough intensely (situational syncope). This situation can also occur with extreme emotional stress, hunger and thirst

Person lying in bed resting

Postural hypotension. It is the lowering of blood pressure due to a change from one body position to a more vertical position, after waking up or sitting down.

Crossed-out water bottle

Dehydration. Causes a decrease in blood volume, and thus, blood flow in the brain.

Green and white pills

Blood pressure drugs. They can lead to an excessive lowering of blood pressure at any time.

Geriatric patient with cane and nerve involvement in legs

Diseases of the nerves of the legs in older people (especially with diabetes or Parkinson’s disease). When there is a lack of tonicity in the nerves of the legs, the blood does not return to the brain adequately.

Brain with secondary haemorrhages due to cerebral vascular malformations

Stroke. Ischaemic stroke, cerebral stroke, transitory ischaemic attack. 

Person holding a hand to his head with a symbol of a lightning bolt above it indicating a headache

A migraine attack.

Glucometer with a down arrow indicating low blood sugar or blood glucose

The lowering of sugar in the blood, or hypoglycaemia.

Beer mug and cannabis

Alcohol consumption and abuse of other drugs.

Cardiac causes

Heart and electrocardiogram, heart palpitations

Abnormal heart rhythms (heart beats too fast or too slow).

Inside view of the heart valves

Abnormalities of the heart valves (aortic stenosis or pulmonary valve stenosis).

Blood vessel obstructed by an atheromatous plaque

Disease of the aorta (aortic dissection due to aneurysms).


Damaged heart muscle as seen under the microscope

Disease of the heart muscle (cardiomyopathy).

Substantiated information by:

Elisenda Gomez Angelats

Published: 20 February 2018
Updated: 20 February 2018


Receive the latest updates related to this content.

Thank you for subscribing!

If this is the first time you subscribe you will receive a confirmation email, check your inbox

An error occurred and we were unable to send your data, please try again later.