Idiopathic thrombocytopenic purpura - Definition and Overview

Idiopathic thrombocytopenic purpura is the condition of having a low platelet count (thrombocytopenia) of no known cause (idiopathic). As most causes appear to be related to antibodies against platelets, it is also known as immune thrombocytopenic purpura. Although most cases are asymptomatic, very low platelet counts can lead to a bleeding diathesis and purpura.

Signs and symptoms

Often, low platelet counts are picked up by coincidence when a full blood count has been ordered for routine testing.

Occasionally, ITP patients suffer from bruising, nosebleeds, and bleeding gums; this is the characteristic pattern of bleeding in platelet disorders. Bleeding normally does not occur unless the platelet count is very low (10,000 per mm³).

Diagnosis

When measuring the platelet count, one has to bear in mind that the "normal values" for laboratory measures are all statistical. They are defined by the upper and lower 2.5th percentile. It is therefore possible to be completely healthy but to have a decreased platelet count. There is, however, a higher chance of pathology.

Low platelet count can be a feature of a large number of diseases and, when serious, warrants investigation by a hematologist. In many cases, the cause is not actually idiopathic but autoimmune, although antibodies against platelets are only detected in a minority and have little influence on management.

Secondary causes include lupus erythematosus and some other autoimmune disorders, congenital causes, and antiphospholipid syndrome.

Treatment

Mild ITP does not require treatment. When platelet counts fall under 10,000 per milliliter, or under 50,000 when hemorrhage occurs (e.g. in the digestive tract or in a severe nosebleed) treatment is generally initiated with steroids, and later with so-called steroid-sparing agents (alternatively called DMARDs). When these strategies fail, splenectomy (removal of the spleen) is often undertaken, as platelets targeted for destruction will often meet their fate in the spleen. A relatively new strategy is treatment with anti-D, an agent used in mothers who have been sensitised to rhesus antigen by a Rh+ baby. Extreme cases (very rare, especially rare in children) may require vincristine, a chemotherapy agent, to stop the immune system from destroying platelets.

Further reading

• Cines DB, Blanchette VS. Immune thrombocytopenic purpura. N Engl J Med (http://www.nejm.com) 2002;346:1008.