QUESTION: Last week I was running errands when I suddenly broke out in a sweat. Before I knew it, I had fainted. This has never happened to me before. What made me faint? Could something serious be wrong? And how can I prevent this from happening again?

ANSWER: Few things are more alarming than a sudden and complete loss of consciousness and control. Often, you start out feeling a little nauseated and the room grows dim. You may feel clammy and sweaty. Then you wake up -- on the floor. It's all over in a minute or two, and you quickly get back to normal. But it's been a frightening and embarrassing experience.

Most people would call it fainting, blacking out or passing out. But doctors call it "syncope"(SIN-co-pee). By definition, it's a brief loss of consciousness that resolves without medical treatment within minutes.

In many cases, syncope is a harmless event. But in some people it can be a red flag. Even if healthy functions return quickly and spontaneously, a fainting spell should not be ignored. Anyone who faints should notify a doctor. And if you have previously diagnosed heart disease, you should get prompt medical attention.

People pass out when the brain doesn't get enough blood and becomes deprived of fuel and oxygen. This is usually caused by a sudden drop in blood pressure. The question is what caused your blood pressure to drop suddenly?

You can help your doctor figure out what caused you to faint by providing a full account of the events that occurred before and after you fainted. A witness may be able to fill in the details.

Your doctor will review your medical history and list of current medications. He or she will perform a physical exam, including measuring your blood pressure both lying down and standing, and an electrocardiogram (EKG). If there are any suggestions of heart disease, neurological abnormalities or other serious problems, you'll need other tests.

Your doctor will try to confirm or rule out a few major causes for your fainting.

The first is a temporary malfunction of the nerves and arteries that doctors call neurovascular syncope. You'll know it as a garden-variety fainting spell.

Neurovascular syncope is by far the most common type of fainting. It occurs when the heart slows down instead of speeding up and the blood vessels widen instead of narrowing.

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Harvard Medical School Adviser: Fainting could signal larger medical issue