From the Guidelines
Yes, an increase in parasympathetic tone can cause vasovagal syncope. Vasovagal syncope occurs when there is an excessive activation of the vagus nerve, which is a key component of the parasympathetic nervous system. This heightened parasympathetic activity leads to a sudden drop in heart rate (bradycardia) and dilation of blood vessels (vasodilation), resulting in reduced blood flow to the brain and subsequent fainting. The mechanism typically involves a trigger such as emotional stress, pain, prolonged standing, or the sight of blood, which initiates an inappropriate neural reflex. Initially, there may be sympathetic activation with increased heart rate and blood pressure, followed by a paradoxical and excessive parasympathetic response. This vagal overactivity causes the heart to slow down dramatically while blood vessels dilate, leading to decreased cardiac output, reduced cerebral perfusion, and ultimately loss of consciousness.
Key Points
- Vasovagal syncope is the most common form of reflex syncope, mediated by the vasovagal reflex 1.
- The diagnosis is made primarily on the basis of a thorough history, physical examination, and eyewitness observation, if available 1.
- Management includes avoiding triggers, maintaining hydration, performing counter-pressure maneuvers like leg crossing or hand gripping when prodromal symptoms occur, and in severe recurrent cases, medications such as fludrocortisone, midodrine, or beta-blockers might be prescribed by a physician 1.
- The 2017 ACC/AHA/HRS guideline for the evaluation and management of patients with syncope provides a comprehensive approach to the diagnosis and management of vasovagal syncope 1.
- The 2018 ESC guidelines for the diagnosis and management of syncope also provide practical instructions for the diagnosis and management of vasovagal syncope, including the use of tilt-table testing and orthostatic challenge 1.
Recommendations
- Patients with vasovagal syncope should be advised to avoid triggers and maintain hydration 1.
- Counter-pressure maneuvers like leg crossing or hand gripping can be effective in preventing syncope when prodromal symptoms occur 1.
- Medications such as fludrocortisone, midodrine, or beta-blockers may be prescribed in severe recurrent cases, but their use should be individualized and guided by the patient's response to treatment 1.
From the Research
Vasovagal Syncope and Parasympathetic Tone
- Vasovagal syncope is a common cause of recurrent syncope, characterized by transient failure of physiologic mechanisms responsible for maintaining systemic arterial pressure and cerebral blood flow 2.
- The condition is often associated with systemic arterial vasodilation and bradycardia, which can be triggered by various stimuli such as orthostatic stress, pain, or emotions 3, 4.
- The role of the autonomic nervous system, including the parasympathetic tone, in the development of vasovagal syncope is complex and not fully understood.
Parasympathetic Tone and Vasovagal Syncope
- An increase in parasympathetic tone can contribute to the development of vasovagal syncope, as it can lead to vasodilation and bradycardia 2, 3.
- However, the relationship between parasympathetic tone and vasovagal syncope is not straightforward, and other factors such as orthostatic stress, pain, and emotions can also play a role 3, 4.
- Studies have shown that patients with vasovagal syncope often exhibit an imbalance between the sympathetic and parasympathetic branches of the autonomic nervous system 4.
Diagnostic and Therapeutic Implications
- The diagnosis of vasovagal syncope is often made by medical history and head-upright tilt table testing 5, 6.
- Treatment strategies for vasovagal syncope may include the use of medications such as fluoxetine, which has been shown to be effective in preventing recurrent episodes of syncope in some patients 5.
- Understanding the role of parasympathetic tone in vasovagal syncope can help guide the development of more effective diagnostic and therapeutic strategies for this condition 2, 3.