Vasovagal response

Vasovagal response is any cause of fainting related to the function and mediation of the vagus nerve. In a patient with the condition, one or more specific trigger stimuli can quickly lead to light-headedness, nausea, a feeling of being very hot or very cold and, in most cases, fainting.
Several conditions can cause fainting, including several that are transient and do not constitute a pathology. As such, a physician must go through a detailed differential diagnosis in order to rule out any other causes for the fainting, and in most cases must track the triggering event. Tests can include a tilt table test and an echocardiogram.
In all cases, the trigger event causes a cascade in the systems responsible for automatic responses. As the trigger event reaches the brainstem, a signal is sent back through the vagus nerve and another signal causes a withdrawal of the mechanisms in the sympathetic nervous system. This causes a reduction of the heart rate and/or vasodilation, either of which causes a lack of blood flow to the brain until after the patient loses consciousness. Once prone, the patient quickly recovers as the heart rate and blood pressure return to normal.
There can be dozens of triggers to the condition, but most patient have just a single trigger. Some of the more common are standing or sitting upright for an extended period of time, urinating while standing up, standing up quickly, stress, arrhythmia, trauma, pain, and extreme emotional response.
Once the condition is diagnosed, the patient is usually coached to avoid triggers for the condition. In addition, the patient will be taken off medication that may enhance the negative response (such as vasodilators). 
Vasovegal response at Wikipedia
Category:Neurology