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Immune Thrombocytopenic Purpura (ITP)
Immune thrombocytopenic purpura (ITP; also termed idiopathic thrombocytopenic purpura) is an acquired disorder leading to immune-mediated destruction of platelets and possibly inhibition of platelet release from the megakaryocyte. In children it is usually an acute disease, most commonly following an infection, and with a self-limited course. In adults it usually runs a more chronic course.
The exact nature of the immune dysfunction is generally not known. ITP is termed secondary if it is associated with an underlying disorder; autoimmune disorders, particularly systemic lupus erythematosis (SLE), and infections, such as HIV and hepatitis C, are common causes. The association of ITP with Helicobacter pylori infection is unclear.
ITP is characterized by mucocutaneous bleeding and a low, often very low, platelet count, with otherwise normal peripheral blood cells and smear. Patients usually present either with ecchymoses and petechiae, or with thrombocytopenia incidentally found on a routine CBC. Mucocutaneous bleeding, such as oral mucosa, gastrointestinal, or heavy menstrual bleeding, may be present. Rarely, life-threatening bleeding, including in the central nervous system, can occur. Wet purpura (blood blisters in the mouth) and retinal hemorrhages may herald life-threatening bleeding.
Laboratory Testing in ITP
Laboratory testing for antibodies (serologic testing) is usually not helpful due to the low sensitivity and specificity of the tests. Bone marrow examination can be reserved for older adults (usually >60 years) or those who have other signs or laboratory abnormalities not explained by ITP, or in patients who do not respond to initial therapy. The peripheral blood smear may show large platelets, with otherwise normal morphology. Depending on the bleeding history, iron deficiency anemia may be present.
Laboratory testing is performed to evaluate for secondary causes of ITP and should include testing for HIV infection and hepatitis C (and other infections if indicated); serologic testing for SLE; serum protein electrophoresis and immunoglobulin levels to potentially detect hypogammaglobulinemia, IgA deficiency, or monoclonal gammopathies; and, if anemia is present, direct antiglobulin testing (Coombs test) to rule out combined autoimmune hemolytic anemia with ITP (Evans's syndrome).
Immune Thrombocytopenic Purpura: Treatment
The treatment of ITP utilizes drugs that decrease reticuloendothelial uptake of the antibody-bound platelet and/or decrease antibody production. However, the diagnosis of ITP does not necessarily mean that treatment must be instituted. Patients with platelet counts >30,000/L appear not to have increased mortality related to the thrombocytopenia.
Initial treatment in patients without significant bleeding symptoms, severe thrombocytopenia (<5000/L), or signs of impending bleeding (such as retinal hemorrhage or large oral mucosal hemorrhages) can be instituted as an outpatient using single agents. Traditionally this has been prednisone at 1 mg/kg, although Rh0(D) immune globulin therapy (WinRho SDF) at 50–75 g/kg is also being used in this setting. Rh0(D) immune globulin must be used only in Rh+ patients as the mechanism of action is production of limited hemolysis, with antibody-coated cells "saturating" the Fc receptors, inhibiting Fc receptor function. Hemoglobin levels usually decrease (mean 1.7 g/dL), although severe intravascular hemolysis is a rare complication. Doses are reduced if given to anemic patients. Intravenous gamma globulin (IVIgG), which is pooled, primarily IgG antibodies, also blocks the Fc receptor system, but appears to work primarily through different mechanism(s). IVIgG has more efficacy than anti-Rh0(D) in post-splenectomized patients. IVIgG is dosed at 2 g/kg total, given in divided doses over 2–5 days. Side effects are usually related to the volume of infusion and infrequently include aseptic meningitis and renal failure. All immunoglobulin preparations are derived from human plasma and undergo treatment for viral inactivation.
For patients with severe ITP and/or symptoms of bleeding, hospital admission and combined modality therapy are given using high-dose glucocorticoids with IVIgG or anti-Rh0D therapy, and, as needed, additional immunosuppressive agents. Rituximab, an anti-CD20 (B cell) antibody, has shown efficacy in the treatment of refractory ITP.
Splenectomy has been used for treatment of patients who relapse after glucocorticoids are tapered. Splenectomy remains an important treatment option; however, more patients than previously thought will go into a remission over time. Observation, if the platelet count is high enough, or intermittent treatment with anti-Rh0(D) or IVIgG may be a reasonable approach to see if the ITP will resolve.
Vaccination against encapsulated organisms (especially pneumococcus, but also menningococcus and Haemophilus influenzae, depending on patient age and potential exposure) is recommended before splenectomy. Accessory spleen(s) are a very rare cause of relapse.
New drugs for ITP include TPO receptor agonists. This approach to treatment of ITP stems from the finding that many patients with ITP do not have increased TPO levels, as was previously hypothesized, nor do they all have increased platelet destruction. Two agents, one administered subcutaneously and another orally, have shown response in many patients with refractory ITP. Roles for these agents in ITP treatment are not fully defined.