Neurally Mediated Hypotension

NMH is the acronym for neurally mediated hypotension, which is also known by the following names: the fainting reflex, neurocardiogenic syncope, vasodepressor syncope, the vaso-vagal reflex, and autonomic dysfunction. Neurally mediated hypotension occurs when there is an abnormal reflex interaction between the heart and brain, both of which are usually structurally normal.

In individuals with neurally mediated hypotension, there is a "miscommunication" between the heart and the brain. Just when the heart needs to beat faster (for example, to pump blood to the brain preventing fainting), the brain sends out the message that the heart rate should be slowed, and that the blood vessels in the arms and legs should dilate. The latter actions take even more blood away from the central part of the circulation, where it is needed. In response, individuals feel lightheaded or may faint because not enough blood is getting to the brain.

We are all susceptible to activation of the vaso-vagal reflex, which results in a lowered blood pressure. However, each person's susceptibility is affected by his or her genetic make-up, dietary factors, psychological make-up and acute triggers such as infection and allergy. The clinical problem of NMH occurs when there is sufficiently early triggering of this reflex to cause symptoms.