By Howard LeWine, M.D.
Q: Over the last couple months, I began getting dizzy. My blood pressure gets very low. If I don’t sit down right away, I pass out. My doctor thinks I have neurally mediated hypotension. What is this, and how is it treated?
A: Neurally mediated hypotension (NMH) is a condition of the autonomic nervous system. The disorder is also called vasodepressor syncope, vasovagal syncope and neurocardiogenic syncope. Syncope is the medical term for passing out or fainting.
The autonomic nervous system includes centers in the brain plus nerves that carry signals between the brain and the body. The autonomic nervous system controls what are called “vital functions” of the body. For example, it controls your body temperature, how fast your heart beats, your blood pressure, and the contractions of your intestines.
When a person who has been lying or sitting down suddenly stands up, gravity pulls blood down into the legs and away from the heart. This causes the blood pressure to drop. However, the autonomic nervous system senses this happening, and reacts to make the drop in blood pressure both small and brief.
In NMH, however, the system fails to make this correction, and the blood pressure falls to an unusual and prolonged degree. That can make a person feel lightheaded, or even faint, because the brain needs a good blood pressure to function normally.
Sometimes, blood pressure falls sharply on standing because a person is dehydrated, or is on a medicine that causes blood pressure to fall on standing. When there is no such clear cause, a person is said to have NMH. The cause of NMH is not known. It comes and goes, for no clear reason.
There are a number of different treatments. Drinking extra fluid and taking extra salt can help. So can wearing special support stockings.
A medicine called fludrocortisone holds extra salt in your body. Other medicines commonly used to treat NMH include ephedrine and midodrine. These drugs raise blood pressure by tightening the walls of blood vessels.
Finding the best treatment plan for you may take some time. You may need to try a few different strategies to help prevent your blood pressure from getting too low.
(Howard LeWine, M.D., is an internist at Brigham and Women’s Hospital and assistant professor at Harvard Medical School. For additional consumer health information, please visit www.health.harvard.edu.)