A more complete discussion of this common disorder can be found in the section of this website entitled “All About Fainting.” There are many names for this condition. The word syncope simply refers to the fainting, blacking out, or passing out. Neurocardiogenic syncope, sometimes referred to as “reflex” syncope, is one of many causes of fainting grouped together in a classification known as “Neurally-Mediated Syncope.” These also include situational syncope such as cough syncope or micturition syncope (fainting that occurs during or immediately after urination). Loss of consciousness occurs when there is a sudden decrease in blood flow to the brain, generally due to either a drop in pressure and/or heart rate.
Our cardiovascular system can hold many times more blood than we actually carry around with us. Most adults have between 5 and 6 quarts of blood, but the miles and miles of blood vessels in our body can accommodate much more than that due to the blood vessels’ ability to stretch (dilate) and constrict when needed. It is up to our brain, specifically the autonomic nervous system, to make sure all the organs receive their fair share of the blood flow. There are autonomic nerve endings in many parts of the cardiovascular system. These nerve endings are like pressure sensors, and they send signals to the brainstem, so the brain can figure out where the blood is and where it needs to go.
One of the most important jobs of the autonomic nervous system is to deal with the effects of a change in body position (posture) on the distribution of blood in our body. When we are lying down, the blood only has to travel sideways in order to get to all of our organs. However, when we stand up, gravity has a significant effect on the distribution of the blood. In fact, about one third of our blood is pulled down into the bottom half of our body away from the heart. This leaves less blood available to be pumped to the brain, and generally results in a slight drop in blood pressure. The autonomic nervous system must compensate for the change in posture. The brain senses a reduced flow of blood through the carotid arteries in the neck. These pressure receptors send signals to the brain to tell it that the blood pressure has dropped. The brain immediately sends signals out to the blood vessels in the body, getting them to constrict to increase the blood pressure and help to drive the blood upwards back towards the heart. Sometimes the autonomic nervous system malfunctions, and instead of maintaining an appropriate blood pressure there is a sudden withdrawal of sympathetic nerve activity and an increase in parasympathetic signals over the vagus nerve. This causes the blood flow to the brain to drop even further, and results in loss of consciousness due to the brain shutting down.
Vasovagal syncope is often triggered by emotional stresses such as hearing bad news, seeing something upsetting, or physical stresses like pain or sometimes exercise. No one is entirely sure why this abnormal reflex occurs in some patients, but it is a common problem that can recur. There seems to be a problem with the brainstem mis-interpreting signals it is receiving, and there may be a component of abnormally high blood levels of a sympathetic nervous system hormone called epinephrine (also called adrenaline).
The most important objective in treating neurocardiogenic syncope is preventing injury. Falling while unconscious can result in broken bones (including skull fractures), serious internal injuries, and even death. Driving an automobile becomes dangerous if the problem is not controlled, and the impact of recurrent fainting on a person’s quality of life and their ability to work is substantial. Luckily, most people with this problem can sense the warning signs or prodrome of an impending faint, and they can be taught to get into a defensive position to prevent injury. Squatting helps by squeezing the leg muscles, driving some blood back to the heart to increase the blood pressure. Ideally, a person who is about to faint should immediately lie down (to preserve blood flow to the brain) and elevate the legs (to increase blood flow back to the heart). Unfortunately, well-meaning friends and family members often make the mistake of keeping the fainting victim sitting up, which only makes the symptoms last longer (because the blood has to go uphill to get to the brain). Preventing recurrent vasovagal fainting involves a number lifestyle changes such as ensuring an adequate intake of fluids and salt (to maintain a generous blood volume), exercise, and avoiding triggers if possible. If fainting spells continue to occur, medical therapy is sometimes needed, especially if the patient has little or no warning before loss of consciousness. In most cases, a normal lifestyle can be restored, but it is very helpful for people to be evaluated by a physician who is experienced in the evaluation and management of this potentially dangerous problem.