Vasovagal Syncope and Vomiting

Pathophysiology and Mechanism

The vasovagal response includes activation of the autonomic nervous system, which can trigger gastrointestinal symptoms including nausea and vomiting 1, 2, 3

Nausea is a common prodromal symptom of vasovagal syncope 1, 2
Vomiting is specifically listed as a typical sign of presyncope in the 2019 American Heart Association guidelines 2, 3
Other autonomic activation symptoms include sweating (diaphoresis) 1, pallor/paleness 2, 3, feeling warm/hot or cold 2, and abdominal discomfort 2
The European Society of Cardiology guidelines identify that gastrointestinal stimulation can be both a trigger for situational syncope and a symptom during the episode 1, 4

When evaluating a patient with syncope and vomiting, it's essential to differentiate between vomiting as a prodromal symptom of vasovagal syncope 2, 3 and vomiting as a trigger for situational syncope 1, 4
Other causes of syncope with vomiting, such as cardiac or neurological conditions, should also be considered 4, 5

Recognition of vomiting as a potential sign of presyncope allows for early intervention 2
If presyncope symptoms, including vomiting, occur, the patient should assume a safe position and consider physical counterpressure maneuvers (PCMs) to prevent progression to syncope 2
Lower-body PCMs are preferable to upper-body maneuvers when possible 2

Vomiting alone is not diagnostic of vasovagal syncope and must be considered in the context of other symptoms and triggers 1, 4
Volume depletion from persistent vomiting can exacerbate orthostatic hypotension and increase syncope risk 4

Vasovagal syncope occurs when a triggered reflex causes inappropriate vasodilation and bradycardia, resulting in systemic hypotension and cerebral hypoperfusion 6, 7
The contribution of vasodilation and bradycardia varies between individuals and episodes 8, 9
Cerebral blood flow reduction below critical levels leads to loss of consciousness 7

Fear, pain, instrumentation, and blood phobia are common emotional triggers 10, 6
Emotional distress activates neural pathways that initiate the vasovagal reflex 7
Prolonged standing or orthostatic stress is a major trigger for vasovagal syncope 10, 6
Specific situations that can trigger the vasovagal reflex include coughing or sneezing 10, 8, gastrointestinal stimulation 10, 7, micturition 8, 6, post-exercise 10, 7, and post-prandial 10, 6
Carotid sinus stimulation can trigger the vasovagal reflex 6, 7

Prodromal symptoms often precede loss of consciousness, including lightheadedness, dizziness, blurred vision, nausea, diaphoresis, and pallor 9, 6
Vasovagal syncope must be differentiated from other causes of syncope, such as cardiac arrhythmias 8, 11, structural heart disease 8, 11, and orthostatic hypotension without reflex component 9

The European Society of Cardiology suggests that patients pass out in vasovagal syncope because a triggered brain reflex causes blood vessels to dilate excessively and the heart to slow down, resulting in a critical drop in blood pressure and insufficient blood flow to the brain 12, 13
Blood vessels throughout the body open too widely, causing blood to pool in the lower body and blood pressure to drop precipitously, reducing systemic vascular resistance and central blood volume available for the heart to pump 12, 13
The brain sends inappropriate signals to the heart, instructing it to slow down or even temporarily stop beating, further reducing cardiac output and compounding the hypotension 12, 13
The brain requires substantial blood flow to function and is located at the top of the body, making it the most vulnerable organ when blood pressure drops 12, 13

Vasovagal syncope is brought about by an inappropriate autonomic nervous system reflex in response to specific triggers, including pain, emotion, seeing blood, blood draws, prolonged standing, heat exposure, and various situational factors 12, 13
The reflex represents either a "hypersensitive" autonomic system that over-responds to stimuli or a "hyposensitive" system unable to maintain steady-state adaptation 14

Loss of consciousness typically lasts less than one minute, with rapid recovery once the person is horizontal and blood can return to the brain 12, 13
The horizontal position eliminates the gravitational challenge of pumping blood upward, allowing cerebral perfusion to restore quickly 12, 13
Prodromal symptoms, such as nausea, sweating, pallor, and lightheadedness, occur because the brain is experiencing marginal perfusion before complete loss of consciousness 12, 13