Syncope is the brief loss of consciousness and posture caused by a temporary decrease in blood flow to the brain. Syncope may be associated with a sudden fall in blood pressure, a decrease in heart rate or changes in blood volume or distribution. The person usually regains consciousness and becomes alert right away, but may experience a brief period of confusion.
Syncope is often the result of an underlying medical condition that could be related to your heart, nervous system or blood flow to the brain.
Syncope is a common condition, affecting 3 percent of men and 3.5 percent of women at some point in life. It becomes more prevalent with advancing age, occurring in as many as 6 percent of people over age 75. Syncope affects patients of all ages, both with and without other medical conditions.


The most common symptoms of syncope include:

  • “Blacking out”
  • Light-headedness
  • Falling for no reason
  • Dizziness
  • Drowsiness
  • Grogginess
  • Fainting, especially after a meal or after exercise
  • Feeling unsteady or weak when standing

Syncope is often preceded by other symptoms (called premonitory symptoms), such as light-headedness, nausea and palpitations (irregular heartbeats that feel like “fluttering” in the chest).
Many people with syncope learn, on their own, to avoid a syncopal event or “passing out.” They recognize the premonitory symptoms and sit or lie down quickly and elevate their legs.
Because syncope could be the sign of a more serious condition, it is important to seek treatment right away after a syncope episode occurs.
With accurate diagnosis and appropriate treatment, syncope can be resolved in most patients.


There are many causes of syncope. If blood does not circulate properly, or the autonomic nervous system does not work the way it should, changes in blood pressure and heart rate can cause fainting. Metabolic abnormalities and anemia may also cause syncope.

Autonomic Nervous System (ANS)


The ANS automatically controls many functions of the body such as breathing, blood pressure, heart rate and bladder. In most situations, we are unaware of the workings of the ANS because it functions in an involuntary, reflexive manner.

Types of Syncope

Vasovagal syncope (also called cardio-neurogenic syncope)

Vasovagal syncope is the most common type of syncope that occurs when the blood pressure drops suddenly, reducing blood flow to the brain. When you stand up, gravity causes blood to settle in the lower part of your body, below the level of the diaphragm. In response, the heart and autonomic nervous system (ANS) react to maintain your blood pressure.
Vasovagal syncope may occur in patients who have a condition called orthostatic hypotension. In this condition, the blood vessels do not constrict normally when the patient stands, causing blood to pool in the legs and the blood pressure to drop quickly.

Situational syncope

Situational syncope is a type of vasovagal syncope that occurs only during particular situations that cause unusual patterns of stimulation to certain nerves. The “stimulus” that triggers an exaggerated neurological reflex can be a wide range of different events such as dehydration, intense emotional stress, anxiety, fear, pain, hunger or use of alcohol or drugs. Hyperventilation associated with panic or anxiety also can cause syncope. Other stimuli include coughing forcefully, turning the neck or wearing a tight collar (carotid sinus hypersensitivity), or urinating (micturition syncope).

Postural syncope (also called postural hypotension)

Postural syncope occurs when the blood pressure drops suddenly due to a quick change in position, such as from lying down to standing. Postural syncope can be related to certain medications or dehydration.

Cardiac syncope

Cardiac syncope is the loss of consciousness due to a heart or blood vessel condition that interferes with blood flow to the brain. These conditions may include an abnormal heart rhythm (arrhythmia), obstructed blood flow in the heart or blood vessels, valve disease, aortic stenosis, blood clot, or heart failure.

Neurologic syncope

Neurologic syncope is the loss of consciousness due to a neurological condition such as seizure, stroke, transient ischemic attack (TIA),degerative disease of the brain affecting autonomic centers.
In about one-third of cases, the cause of syncope is unknown.

How is syncope diagnosed?