Fainting (syncope) is classified by the mechanism that causes the reduction in blood flow to the brain, such a rapid drop in blood pressure. It is important to understand that these descriptions of fainting are not diseases themselves, but ways for physicians to classify the type of fainting a person is experiencing. Once a physician has identified the type of fainting, he or she will be better able to diagnose the underlying condition.

Although there is some variation in the classification of fainting, most physicians generally break it down into cardiac and noncardiac causes. Noncardiac syncope is usually caused by disorders of blood volume and vascular tone. Cardiac syncope is caused by a reduction in cardiac output due to heart disease. In general, only cardiac syncope is associated with increased mortality. Specific types of syncope include:

• Neurocardiogenic syncope. Neurocardiogenic syncope accounts for about half of all episodes of fainting. The term includes vasovagal syncope and vasodepressor syncope. In general, neurocardiogenic syncope is caused by abnormalities in the autonomic nervous system, which controls the heart rate and circulation. Technically speaking, vasovagal syncope is caused by both widening of the arteries and a depressed heart rate (e.g., bradycardia), whereas vasodepressor syncope is caused by widening of the arteries alone. In either case, the syncope may be preceded by a presyncopodal syndrome that might include weakness, nausea, lightheadedness, blurred vision, and a rapid heartbeat followed by cardiac slowing. Neurocardiogenic syncope may be caused by standing for a long time, alcohol, a hot and crowded environment, hunger, pain, and stressful or emotional situations. Episodes of neurocardiogenic syncope usually last only a few minutes, with varying degrees of consciousness ranging from a turgid state of awareness to complete unconsciousness.
  
  
• Orthostatic syncope. This form is syncope is caused by rising rapidly from a reclined position, which results in a rapid decline in blood pressure in the head. It is thought to account for about 30 percent of all episodes of syncope among the elderly. The use of antihypertensive and antidepressant medications may contribute to orthostatic syncope.
  
  
• Carotid sinus syncope. Carotid sinus syncope is caused by a sudden loss of consciousness due to compression of the carotid sinus, a blood vessel in the neck. It can be caused by turning the head to one side or wearing clothing or jewelry too tight on the neck. This is sometimes called carotid sinus syndrome. It occurs mostly in men over 50 years of age.
  
  
• Situational syncope. A number of normal activities, such as urinating, coughing, swallowing and defecating, may also cause syncope. Situational syncope may have elements of both vasodepressor syncope and vasovagal syncope. The straining that accompanies these activities also contributes to the reduction of blood flow. Conditions associated with situational syncope include lung diseases, benign prostatic hyperplasia (BPH), bladder obstruction, esophageal disorders and constipation.
  
  
• Cardiac syncope. This form of syncope is caused by a sudden reduction in cardiac output, usually as a result of an abnormal heart rhythm, or arrhythmia. This condition can be caused be either a slow heart rate (bradycardia) or abnormally rapid heart rate (tachycardia). People who suffer from a number of diseases may be susceptible to cardiac syncope, including cerebrovascular disease, anemia, coronary artery disease or heart valve disease.
  
  
• Syncope of unknown origin, or idiopathic syncope. As the name implies, this describes syncope for which a cause cannot be found. This applies to about a third of cases of syncope. In most of these cases, the syncope is a singular episode from which the patient recovers quickly and suffers no long-term damage. When injury does occur, it is usually due to falling.

Despite its name, local syncope is not a loss of consciousness. It is when an area of the body becomes numb and/or white from lack of blood flow. “Local” refers to the fact that it is occurring only in a section of the body, usually the fingers or toes. It is associated with Raynaud syndrome.