Reflex syncope is a brief loss of consciousness due to a neurologically induced drop in blood pressure or heart rate. Before an affected person passes out, there may be sweating, a decreased ability to see, or ringing in the ears. Occasionally, the person may twitch while unconscious. Complications of reflex syncope include injury from falling. Reflex syncope is divided into three types: vasovagal, situational, and carotid sinus. Vasovagal syncope is typically triggered by seeing blood, pain, emotional stress, or prolonged standing. Situational syncope is often triggered by urination, swallowing, or coughing. Carotid sinus syncope is due to pressure on the carotid sinus in the neck. The underlying mechanism involves the nervous system slowing the heart rate and dilating blood vessels, resulting in low blood pressure and thus insufficient blood flow to the brain. Diagnosis is based on the symptoms after ruling out other possible causes. Recovery from a reflex syncope episode happens without specific treatment. Prevention of episodes involves avoiding a person's triggers. Drinking sufficient fluids, salt, and exercise may also be useful. If this is insufficient for treating vasovagal syncope, medications such as midodrine or fludrocortisone may be tried. Occasionally, a cardiac pacemaker may be used as treatment. Reflex syncope affects at least 1 in 1,000 people per year. It is the most common type of syncope, making up more than 50% of all cases.
Signs and symptoms Episodes of vasovagal syncope are typically recurrent and usually occur when the predisposed person is exposed to a specific trigger. Before losing consciousness, the individual frequently experiences early signs or symptoms such as lightheadedness, nausea, the feeling of being extremely hot or cold (accompanied by sweating), ringing in the ears, an uncomfortable feeling in the heart, fuzzy thoughts, confusion, a slight inability to speak or form words (sometimes combined with mild stuttering), weakness, and visual disturbances such as lights seeming too bright, fuzzy or tunnel vision, black cloud-like spots in vision, and a feeling of nervousness can occur as well. The symptoms may become more intense over several seconds to several minutes before the loss of consciousness (if it is lost). Onset usually occurs when a person is sitting up or standing. Auditory and visual hallucinations are common just before, and during, vasovagal syncope. Often compared to near-death experiences, syncope-induced hallucinations are thought to be triggered by reduced oxygen flow to the brain. Patients may report hearing sounds of rushing wind or water and seeing bright light, peaceful natural landscapes, or vivid images of loved ones. In most cases, they report feeling calm or euphoric during the experience. When people lose consciousness, they fall down (unless prevented from doing so) and, when in this position, effective blood flow to the brain is immediately restored, allowing the person to regain consciousness. If the person does not fall into a fully flat, supine position, and the head remains elevated above the trunk, a state similar to a seizure may result from the blood's inability to return quickly to the brain, and the neurons in the body will fire off and generally cause muscles to twitch very slightly but mostly remain very tense. The autonomic nervous system's physiological state (see below) leading to loss of consciousness may persist for several minutes, so
If patients try to sit or stand when they wake up, they may pass out again The person may be nauseated, pale, and sweaty for several minutes or hours Though rare, transient global amnesia is sometimes reported after vasovagal syncope. After returning to consciousness, the individual may be unaware that they fainted.
Causes Reflex syncope occurs in response to a trigger due to dysfunction of the heart rate and blood pressure regulating mechanism. When heart rate slows or blood pressure drops, the resulting lack of blood to the brain causes fainting.
Vasovagal Typical triggers include:
Prolonged standing Emotional stress Pain The sight of blood Receiving a tattoo Fear of needles Time varying magnetic field (i.e. transcranial magnetic stimulation)
Situational After or during urination (micturition syncope) Straining, such as to have a bowel movement Coughing Swallowing Lifting a heavy weight
Carotid sinus Pressing upon a certain spot in the neck. This may happen when wearing a tight collar, shaving, or turning the head.
Pathophysiology Regardless of the trigger, the mechanism of syncope is similar in the various vasovagal syncope syndromes. The nucleus tractus solitarii of the brainstem is activated directly or indirectly by the triggering stimulus, resulting in simultaneous enhancement of parasympathetic nervous system (vagal) tone and withdrawal of sympathetic nervous system tone. This results in a spectrum of hemodynamic responses:
On one end of the spectrum is the cardioinhibitory response, characterized by a drop in heart rate (negative chronotropic effect) and in contractility (negative inotropic effect) leading to a decrease in cardiac output that is significant enough to result in a loss of consciousness. It is thought that this response results primarily from enhancement in parasympathetic tone. On the other end of the spectrum is the vasodepressor response, caused by a drop in blood pressure (to as low as 80/20) without much change in heart rate. This phenomenon occurs due to dilation of the blood vessels, probably as a result of withdrawal of sympathetic nervous system tone. The majority of people with vasovagal syncope have a mixed response somewhere between these two ends of the spectrum. One account for these physiological responses is the Bezold-Jarisch reflex. Vasovagal syncope may be part of an evolved response, specifically, the fight-or-flight response.
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