People lose consciousness for a variety of reasons and from a variety of causes. Some are psychological, i.e. the fear response causes them to faint, or physical, such as the fainting that occurs when we over exert ourselves. Vagal nerve fainting (also known as vasovagal syncope) is quite simply any loss of consciousness caused by the vagus nerve.
The vagus nerve runs directly from the brain stem, in front of the ears and down the neck towards the chest and abdomen. It branches off in several places to provide communication with the brain to a number of organs, such as the heart and the stomach. Its main function is to direct blood towards the digestive system when food is eaten, but if it is overactive then the vagus nerve can take too much blood away from the brain, causing a loss of consciousness (vagal nerve fainting).
This type of fainting is not generally limited to people with 'defective' vagal nerves, but it is a reaction that can happen to any human. Most of us have experienced significant dizziness or a 'head rush' after vomiting which means we need to lie down. Some of you reading this will have fainted after vomiting too and this is a classic vagal nerve reaction. Similarly, if we have a particularly large bowel movement the vagus nerve can become over stimulated again, taking blood away from the brain to help with what's happening elsewhere in the body. It's an unpleasant reaction and one that can be embarrassing but medical professionals recognize it readily and know the very simple treatments required for vagal nerve fainting like the backs of their hands.
So what does it feel like to have a vagal nerve fainting episode? You'll most likely feel dizzy, nauseous and as if you have a head rush. Your hearing may feel 'muffled' and your eyesight blurry or even tunnel-like. You may try to articulate that something is wrong, but the words come out garbled. Don't worry, this is just a result of your brain not having enough blood, but once you return to consciousness this will right itself. You will also most likely feel hot at first, but then cool and clammy once you wake up. Don't try to get up right away but relax for a while, have a little water and get some cool, fresh air.
Over stimulation of the vagal nerve can also be triggered if a person is 'squeamish' i.e. they may pass out at the sight of blood, something particularly gory or sexually explicit. This type of response is only a worry when it happens very often, or if the individual has a heart condition, as the vagus nerve causes slowing of the heart when overstimulated. If you are fainting regularly then you should visit your doctor as soon as possible to find out what the possible causes are and what treatments are available to you.