Vasovagal Syncope

What Is It, Causes, Prevention, and More

Author:Anna Hernández, MD

Editors:Alyssa Haag,Emily Miao, PharmD

Illustrator:Jillian Dunbar

Copyeditor:Sadia Zaman, MBBS, BSc

What is vasovagal syncope?

Syncope, from Greek sunkopē (sun-, ‘together’ and koptein, ‘strike, cut off’), refers to a sudden loss of consciousness as a result of a temporary decrease in the blood flow supplying the brain. When syncope is caused by certain triggers, like the sight of blood or an intense emotion, it is called ‘vasovagal syncope’ or ‘reflex syncope’. Most individuals with vasovagal syncope experience a prodrome, which is a period of symptoms lasting at least a few seconds just prior to losing consciousness. The prodrome is usually associated with a precipitating event and may include dizziness, lightheadedness, a sense of being warm or cold, pallor, nausea, sweating, palpitations, and blurry vision, as well as poorer hearing or being able to hear strange sounds.

What causes vasovagal syncope?

Vasovagal syncope is a benign condition caused by the activation of the parasympathetic nervous system in response to certain emotional or environmental triggers. Typically, vasovagal syncope episodes occur after standing for a prolonged period of time; they can be triggered by fasting, dehydration, being in crowded or excessively warm environments, or following stressful events, like seeing blood and needles. Although it is not clear why, these triggers stimulate a vasovagal reaction that consists of bradycardia (slowing of the heart rate) and vasodilation of peripheral blood vessels. In turn, there is a drop in blood pressure that ultimately causes a reduction in the brain’s oxygen supply, resulting in cerebral hypoperfusion and loss of consciousness

Another common cause of syncope is orthostatic hypotension, which refers to a sudden drop in blood pressure that occurs when a person abruptly transitions from lying down or sitting to standing up. This happens as a result of a delay in constriction of the lower body veins, which is needed to maintain adequate blood pressure when changing to a standing position. When the delay occurs, it is due to blood pooling in the veins of the legs resulting in less blood returning to the heart, causing a drop in cardiac output and blood pressure.

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How is vasovagal syncope diagnosed and treated?

Diagnosis of vasovagal syncope begins with a thorough medical history and physical examination. This may include assessing the individual’s vital signs, measuring their blood pressure standing and laying down, and obtaining an ECG to rule out other causes of syncope - like an arrhythmia, structural heart abnormalities, or a myocardial infarction (i.e., heart attack). Next, it is important to rule out any conditions that could potentially mimic a syncope, including seizures, a stroke, sleep disturbances, and accidental falls that resulted in head injury. In individuals where orthostatic hypotension is suspected, a tilt table test can be performed. It involves an individual laying flat on a special table while their ECG and their blood pressure are being monitored. The table creates a change in posture from lying to standing in an attempt to cause syncope. If an individual experiences symptoms associated with a drop in blood pressure during a tilt table test, a diagnosis of orthostatic hypotension can be made.

The immediate treatment of vasovagal syncope involves laying the individual down with their legs elevated, in order to help increase the venous return to the heart and to restore adequate brain perfusion. In most cases, vasovagal syncope only lasts for a few seconds and there is usually a spontaneous recovery requiring no further medical attention. 

Does vasovagal syncope ever go away?

Vasovagal syncope occurs episodically and although it can go away for some time, new syncope episodes may occur if the individual is exposed to precipitating triggers. Vasovagal syncope is highly common, as 1 in 3 individuals will experience a syncope episode at least once throughout their lifetime. Luckily, these fainting spells occur occasionally and are self-limited, so they generally do not require any treatment besides preventive measures. 

How do you prevent vasovagal syncope?

Preventing a vasovagal syncope includes avoiding any potential triggers, such as standing for a long time or the sight of blood; drinking plenty of fluids to maintain adequate blood volume; and standing up slowly from a lying or sitting position to prevent orthostatic hypotension. In addition, recognizing the prodromes of syncope may allow individuals to prepare by laying down on the floor, thereby preventing the evolution of the episode into a full fainting spell, as well as reducing the risk of falling. Finally, individuals with multiple syncope episodes may be treated with medications, including mineralocorticoids (e.g., fludrocortisone), vasoconstrictors (e.g., disopyramide, midodrine), or selective serotonin-reuptake inhibitors (e.g., fluoxetine). However, the efficacy of these medications varies between individuals. 

What are the most important facts to know about vasovagal syncope?

Vasovagal syncope is a benign condition caused by a drop in heart rate and blood pressure that is triggered by certain environmental or emotional stimuli (e.g., the sight of blood). Diagnosing vasovagal syncope involves performing a medical history and physical examination, along with an ECG to rule out cardiogenic causes of syncope. When an individual has an acute syncope episode, the goal is to ensure that blood returns to their brain by positioning the person on the ground, with their legs slightly elevated. Preventing vasovagal syncope is done by avoiding any potential triggers, drinking plenty of fluids, and taking specific medications, if needed. 

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Related links

Introduction to the cardiovascular system
Cardiovascular system anatomy and physiology
Parasympathetic nervous system
Syncope: Clinical practice

Resources for research and reference

Aydin, M. A., Salukhe, T. V., Wilke, I., & Willems, S. (2010). Management and therapy of vasovagal syncope: A review. World Journal of Cardiology, 2(10), 308–315. DOI: 10.4330/wjc.v2.i10.308

Brignole, M., Moya, A., de Lange, F. J., Deharo, J. C., Elliott, P. M., Fanciulli, A., Fedorowski, A., Furlan, R., Kenny, R. A., Martín, A., Probst, V., Reed, M. J., Rice, C. P., Sutton, R., Ungar, A., van Dijk, J. G., & ESC Scientific Document Group (2018). 2018 ESC Guidelines for the diagnosis and management of syncope. European Heart Journal, 39(21), 1883–1948.

Kenny, R. A., & McNicholas, T. (2016). The management of vasovagal syncope. QJM, 109(12), 767–773. DOI:10.1093/qjmed/hcw089 

Runser, L. A., Gauer, R. L., & Houser, A. (2017). Syncope: Evaluation and Differential Diagnosis. American Family Physician, 95(5), 303–312.