A Low Platelet Count

Idiopathic Thrombocytopenic Purpura (ITP) is the term used when the platelet count is low enough to cause very easy bruising.  ITP has two basic origins: an autoimmune attack against platelets (primary or autoimmune ITP) and a bone marrow disorder (secondary ITP).

Causes and Development

Primary ITP. The bone marrow produces platelets as fast as usual (at least in the early stages of the disease), but even before they have a chance to mature, they are taken out of circulation.  An IgG antibody attaches to the platelets and marks them to be removed from circulation.  It is likely that individuals who suffer this disease have a genetic propensity to autoimmunity and that a viral disease triggers it.  Many autoimmune disorders have this characteristic.

Primary (or 'autoimmune') thrombocytopenia occurs mostly in children and young adults (typically before age 30), though it can rarely occur later in life.  Many times, it manifests as an acute disease, lasting a few weeks and then clearing up completely.  It might recur again later after another viral infection or with reactivation of a chronic virus, but eventually it ceases to be a problem in the majority of children who experience it.

Secondary ITP. This condition involves a defect in the production of platelets by the bone marrow that can occur as part of a general bone marrow dysfunction, in which both red and white blood cells are also produced insufficiently.  It can occur secondary to leukemia, in which the stem cells that yield white blood cells proliferate and crowd out the stem cells that produce platelets and red blood cells (yielding high white cell count and low RBC and platelet counts).  Secondary ITP can also occur as the result of chemotherapy treatments.

Some chronic diseases that affect the immune system – such as HIV, hepatitis C or systemic lupus – may yield a combination of inhibited platelet production and shortened time that platelets persist in the blood, resulting in ITP.  Because in this case it is some other factor (disease process or medical treatment) that is causing the clinical result, this type of platelet deficiency is termed secondary ITP and may be resolved by stopping this other factor.

British researchers have confirmed that the measles-mumps-rubella (MMR) vaccine can cause this bleeding disorder in children.  However, ITP can also occur in children following the viral infections the vaccinations were intended to prevent.  The risk of post-vaccination ITP is quite low when compared to ITP that occurs after catching measles, rubella or mumps.  For instance, one case of ITP occurs for about every 3000 rubella infections. [Archives of Disease in Childhood March 2001; 84: pp.227-229]

Treatment and Prevention

Conventional treatment is often aimed at inhibiting the immune system with corticosteroids such as prednisone.  Because the spleen filters the platelet-immune complex it may be necessary to remove it in order to reduce the production of anti-platelet antibodies and slow the clearance of platelets from the system.  In some people, however, more platelets are destroyed in the liver than in the spleen and in this case removing the spleen is less likely to result in remission.  Note that after the spleen has been removed and the platelet levels elevated, the liver can take over the removal function and the platelet count can drop again.

The acute manifestation can usually be controlled by a course of therapy using steroids to inhibit the immune response for a period of several weeks.  Chronic autoimmune thrombocytopenia develops in a small percentage of patients.  In that case, steroid therapy eventually fails (due to the side-effects from prolonged administration).  Until recently, the main therapy for chronic autoimmune thrombocytopenia has been splenectomy, which is sometimes curative and at the very least reduces the disease severity.

More recently, intravenous infusion of normal IgG to replace the body's anti-platelet IgG has been tried with some success and may replace splenectomy for some patients.  This has also been proposed as an alternative to the initial therapy with prednisone.  Other therapies are also being developed.