Fainting
(Syncope)

Medical Author: Benjamin C. Wedro, MD, FAAEM
Medical Editor: Melissa Conrad Stöppler, MD

• Introduction to fainting (syncope)
• What causes fainting (syncope)?
  • Heart rhythm changes
  • Heart structural conditions
  • Heart valve conditions
  • Sudden cardiac death
  • Postural hypotension
  • Vasovagal syncope
  • Anemia
  • Dehydration
  • Orthostatic hypotension
  • Vertebrobasilar system
  • Electrolyte imbalance
  • Other medications and drugs
  • Pregnancy
• What are the signs and symptoms of fainting (syncope)?
• How is fainting (syncope) diagnosed?
• What is the treatment for fainting (syncope)?
• Can fainting (syncope) be prevented?
• Fainting (Syncope) At A Glance

Introduction to fainting (syncope)

Fainting, "blacking out," or syncope is the temporary loss of consciousness followed by the return to full wakefulness. This loss of consciousness is accompanied by loss of muscle tone that can result in falling or slumping over. To better understand why fainting can occur; it is helpful to explain why somebody is awake.

The brain has multiple parts, including two hemispheres, the cerebellum, and the brain stem. The brain requires blood flow to provide oxygen and glucose (sugar) to its cells to sustain life. For the body to be awake, an area known as the reticular activating system located in the brain stem needs to be turned on, and at least one brain hemisphere needs to be functioning. For syncope to occur, either the reticular activating system needs to lose its blood supply, or both hemispheres of the brain need to be deprived of blood, oxygen, or glucose. For the brain to stop functioning, the flow of blood must be briefly disrupted to the whole brain or to the reticular activating system.

Fainting is not caused by head trauma, since loss of consciousness after a head injury is considered a concussion. However, fainting can cause injury if the person falls and hurts themselves, or if the faint occurs while participating in an activity like driving a car.

What causes fainting (syncope)?

Decreased blood flow to the brain can occur because 1) the heart fails to pump the blood; 2) the blood vessels don't have enough tone to maintain blood pressure to deliver the blood to the brain; 3) there is not enough blood or fluid within the blood vessels; or 4) a combination of reasons one, two, or three above.

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