Vasovagal Syncope · What Is It, Causes, Prevention, and More

Vasovagal syncope is caused by overstimulation of the parasympathetic nervous system, in particular the vagus nerve, which controls functions like heart rate, blood pressure, and digestion. Excessive vagal activity causes a reduction in heart rate and dilation of blood vessels. Together, these effects lead to a significant reduction in blood flow to the brain, which causes a transient loss of consciousness.  

In most cases, vasovagal syncope occurs in response to certain triggers, which commonly involve emotional stimuli, like stress, intense pain, unpleasant smells, or the sight of blood. Other common triggers include prolonged standing, especially in crowded or excessively warm places; a sudden change in position, like standing up too fast; or straining during bowel movements. 

Factors that can make the body more susceptible to a vasovagal response include being dehydrated or ill, lack of sleep, and physical exertion.  

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 lying 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’s important to rule out any conditions that could potentially mimic syncope, including seizures, stroke, sleep disturbances, and accidental falls that resulted in head injury.   

In individuals where fainting episodes occur due to sudden changes in position, orthostatic hypotension may be suspected, which is when there is a drop in blood pressure after standing from a sitting or lying position. In these cases, a-tilt table test can be performed to evaluate how the body responds to changes in position. The tilt-table test is performed with an individual lying flat on a special table while their ECG and blood pressure are monitored. The table creates a change in posture from lying to standing in an attempt to trigger a vasovagal response. If an individual experiences symptoms associated with a drop in blood pressure during the test, a diagnosis of orthostatic hypotension can be made.  

Immediate treatment of vasovagal syncope involves lying the individual down with their legs elevated, in order to help increase venous return to the heart and restore blood flow to the brain. In most cases, vasovagal syncope only lasts for a few seconds and there is usually a spontaneous recovery requiring no further medical attention. Some individuals may feel weak, tired, or nauseous after regaining consciousness, but that sensation generally resolves on its own after a few hours. 

Preventing a vasovagal syncope includes avoiding any potential triggers, such as standing for a prolonged 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.  

Once the prodrome starts, individuals may take preventive measures to avoid progression into a full fainting spell. For example, counterpressure exercises aim to increase venous return to the heart by decreasing the amount of blood pooling in the legs and pushing it back to the heart. These exercises can be performed standing, sitting, or lying down, and involve squeezing the muscles of the arms or legs tightly until the symptoms resolve. Alternatively, squatting down, squeezing the fists together, or clasping the hands against each other may achieve the same effect.  

Finally, individuals with multiple syncope episodes may be treated with medications, including mineralocorticoids (e.g., fludrocortisone), vasoconstrictors (e.g., midodrine), or selective serotonin-reuptake inhibitors (e.g., fluoxetine). However, the efficacy of these medications varies between individuals.