Fainting (Syncope): Symptoms & Signs

Medically Reviewed on 9/10/2019

Fainting (syncope) is the partial or complete loss of consciousness with interruption of awareness of oneself and ones surroundings. When the loss of consciousness is temporary and there is spontaneous recovery, it is referred to as syncope or, in nonmedical terms, fainting. Syncope accounts for one in every 30 visits to an emergency room. It is pronounced sin-ko-pea.

Syncope is due to a temporary reduction in blood flow and therefore a shortage of oxygen to the brain. This leads to lightheadedness or a "black out" episode, a loss of consciousness. Temporary impairment of the blood supply to the brain can be caused by heart conditions and by conditions that do not directly involve the heart.

Related Symptoms & Signs

Other causes of fainting

Non-heart Causes: Syncope is most commonly caused by conditions that do not directly involve the heart. These conditions include the following:

  • Postural (orthostatic) hypotension: drop in blood pressure due to changing body position to a more vertical position after lying or sitting
  • 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
  • Fainting after certain situations (situational syncope) such as
    • blood drawing,
    • urinating (micturition syncope),
    • defecating (defecation syncope),
    • swallowing (swallowing syncope),
    • coughing (cough syncope) that trigger a reflex of the involuntary nervous system (the vasovagal reaction) that slows the heart and dilates blood vessels in the legs and cause one to feel nausea, sweating, or weakness just before fainting.

Heart Causes: Heart conditions that can cause syncope or fainting due to temporary loss of consciousness include:

Medications can cause fainting by altering blood pressure or by affecting the heart.

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Kasper, D.L., et al., eds. Harrison's Principles of Internal Medicine, 19th Ed. United States: McGraw-Hill Education, 2015.