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Q: I was recently told that I had a rare disease called thrombocytopenic purpura. I had a mild case, and it was going away even before I was seen by a doctor. What is this disease, and what can I do to prevent it in the future?

- F.U., Alto Loma, Calif.

A: Thrombocytopenic purpura is a condition in which the platelets (also called thrombocytes) are low. This causes bleeding just beneath the skin (purpura). It is not as rare as some think, especially if all of the minor cases associated with viral infections are included.

Although platelets are the smallest blood cells, they perform very vital functions and they can be very deadly. As a major mediator of blood clotting, they protect us from bleeding to death from major trauma and from the hundreds of small bleeds that occur in each of us every day.

But if the platelets overreact, blood clots can form and cut off blood flow throughout the body. The most obvious example is coronary heart disease, in which the clotting causes heart attacks.

Medical science has a lot more to learn about the function of platelets. For example, why can some people have one-tenth as many platelets as someone else and yet show no detectable symptoms or problems?

In people with thrombocytopenic purpura, blood leaks from very small arteries just beneath the skin and mucous membranes. This causes small red spots (petechiae) and small bruises (echymoses) viewable through the skin.

Reduction in the number of platelets can be either the result of abnormal function of the bone marrow, where the platelets are formed, or the result of increased destruction of the platelets out in the bloodstream.

The latest research indicates the problem is commonly caused by certain immune cells, called T cells, which produce antibodies against the platelets. This destroys the platelets and their ability to stop bleeding.

Although thrombocytopenic purpura is associated with certain drugs, especially heparin, and with immunologic diseases, such as lupus or lymphocytic leukemia, it is often called idiopathic because the underlying cause is not known.

Idiopathic thrombocytopenic purpura (ITP) also occurs in children and is usually caused by a viral infection. Fortunately, it almost always goes away rapidly. The adult form of ITP usually lasts longer.

Most adults with ITP will require medical treatment. Major categories of ITP treatment are steroids, plasmapheresis, surgery and immunosuppressive drugs.

High doses of the steroid prednisone are effective in controlling but not eliminating ITP. About 80 percent of people with the disease will respond favorably to prednisone and the platelet count will return to normal. This treatment is usually continued until the platelet count is normal, then gradually tapered off.

Replacing the plasma part of the blood in a process called plasmapheresis is a very effective treatment. But some adults will eventually require splenectomy if ITP continues. As many as 80 percent of those having the surgery will have full or partial remission.

The immunosuppressive drugs WinRho SDF and anti-D are new alternatives. They maintain platelet counts in a safe range for up to a year and have minimal side effects.