Syncope

Syncope

Definition:

Partial or complete loss of consciousness due to a temporary reduction in blood flow and therefore a shortage of oxygen to the brain.
Non-cardiac causes:
Postural hypotension: Drop in blood pressure due to changing body position to a more vertical position after lying or sitting;
Dehydration causing a decrease in blood volume.
Blood pressure medications leading to low blood pressure.
Diseases of the nerves to the legs in older people (especially with diabetes or Parkinson's disease) when poor tone of the nerves of the legs draws blood into the legs from the brain.
High altitude.
Brain stroke or "near-stroke" (transient ischemic attack).
A migraine attack.
Emotions (e.g. fear, anxiety)
A reflex of the involuntary nervous system (the vasovagal reaction) triggered by certain situations (situational syncope) such as blood drawing, urinating, defecating, swallowing and coughing.
Cardiac causes:
Abnormal heart rhythms (heart beating too fast or too slow).
Abnormalities of the heart valves (aortic stenosis or pulmonic valve stenosis).
High blood pressure in the arteries supplying the lungs (pulmonary artery hypertension).
Tears in the aorta (aortic dissection).
Widespread disease of the heart muscle (cardiomyopathy).

Taken from:

Britannica.com
MedicineNet.com
(For detailed information: http://www.medterms.com/script/main/art.asp?articlekey=5612)