What Causes Fainting (Syncope)?

Fainting (syncope, passing out) is a temporary loss of consciousness that occurs when there is inadequate blood flow to the brain. Many conditions can cause fainting, including sudden cardiac death, reflex syncope, serious cardiovascular conditions (cardiac syncope), orthostatic hypotension (low blood pressure when standing), and postural Orthostatic Tachycardia Syndrome (POTS).

Fainting, also called syncope or passing out, is a symptom that occurs when there is inadequate blood flow to the brain. It is temporary, with a loss of consciousness that comes on quickly and can lead to falling. In true syncope, recovery is prompt and complete.

It is generally an uncommon occurrence for most people and for the most part, is not a sign of serious illness. But in some people, fainting can be the first and only sign before sudden cardiac death. 

Unconsciousness due to seizures, heart attacks, head injury, stroke, intoxication, a blow to the head, diabetic hypoglycemia, or other emergency condition is not considered syncope.

The most common causes of fainting include:

• Reflex syncope (also called neurally mediated syncope, vasovagal syncope, vasodepressor syncope, the common faint)
  • The most frequent cause of fainting
  • Results from a reflex response to some trigger
  • The heart slows or blood vessels dilate (widen) and blood pressure drops, so less blood flows to the brain and fainting (syncope) or near-fainting (pre-syncope) occurs
  • Triggers for vasovagal syncope can include:
    • Getting an injection or having blood drawn 
    • Seeing blood 
    • Blood donation
    • Standing up quickly (a “head rush” is considered pre-syncope)
    • Standing upright for a long time
    • Sudden and unexpected trauma, stress or pain, such as being hit
  • Other types of reflex syncope include
    • Situational syncope, a sudden reflex response to a trigger other than those listed above
    • Triggers include:
      • Coughing, sneezing, laughing, swallowing
      • Eating a meal
      • Sudden abdominal pain
      • Defecating
      • Urinating (post-micturition syncope: occurs in men while standing to urinate)
      • Pressure on the chest after exertion or exercise
      • Playing a brass instrument 
      • Weight lifting
  • Carotid sinus syncope 
    • Occurs in older people when pressure is applied to the carotid artery in the neck such as by a hard twist of the neck, wearing a tight collar, and pressing on the artery 
• Serious cardiovascular conditions (cardiac syncope), which may be caused by:
  • Irregular heartbeat (arrhythmia) and abnormal heart rhythm
  • Aortic dissection, a tear in the aorta; very rare but life-threatening
  • Aortic valve stenosis, a narrowing of the valve between the heart and the aorta
• Orthostatic hypotension (low blood pressure when standing)
  • A drop in systolic blood pressure (the top number) of 20 mmg Hg or more on standing
• Postural Orthostatic Tachycardia Syndrome (POTS) (increased heart rate when standing)
  • Characterized by an increase in heart rate of at least 30 beats per minute on standing and orthostatic intolerance 
  • Rare, and occurs more often in young women