Immune thrombocytopenia: No longer ‘idiopathic’
ABSTRACTImmune thrombocytopenia (ITP) is a common hematologic disorder. Its pathogenesis involves both accelerated platelet destruction and impaired platelet production. First-line agents are usually effective initially but do not provide long-term responses. Splenectomy remains an effective long-term therapy, as does rituximab (Rituxan) in a subset of patients. Thrombopoietic agents offer a new alternative, although their place in the overall management of ITP remains uncertain.
KEY POINTS
- ITP is defined as an isolated platelet count of less than 100 × 109/L (100,000/μL) and usually presents without symptoms.
- Patients without symptoms who have a platelet count above 30 × 109/L should generally not be treated unless they have an increased risk of bleeding.
- Recent studies suggest that viruses and other pathogens play an important role in secondary ITP.
- Initially, corticosteroids are usually given as prednisone (1–2 mg/kg/day, then tapered), though recent studies suggest that dexamethasone pulses (40 mg/day for 4 days) may provide more durable responses when used in this setting.
- Thrombopoietic agents are important new treatments, although their place in the overall therapy of ITP has not been established.
BLEEDING MAY NOT BE PRESENT AT DIAGNOSIS
ITP is now recognized as a diverse syndrome with a constellation of signs and symptoms.
Petechiae are pinpoint microvascular hemorrhages that do not blanch with pressure. This distinguishes them from small hemangiomas, which look similar but blanch transiently with pressure. Petechiae tend to occur on dependent areas, particularly the hands and feet, when the platelet count drops below approximately 15 × 109/L.
Ecchymoses (dry purpura) appear as large bruises.
Mucosal bleeding (wet purpura) involves the oral mucosa. Particularly in children, wet purpura tends to be associated with systemic bleeding complications, involving the gastrointestinal tract for example. The incidence of intracranial hemorrhage, though very low, may also be increased in patients with wet purpura.
Other bleeding manifestations may include heavy menstrual bleeding, oral bleeding, and epistaxis.
Bleeding is generally but not entirely proportional to the platelet count. In a study of adults with newly diagnosed ITP and a platelet count of less than 50 × 109/L,4 the presenting symptom was hemorrhage in 12% and purpura in 58%.4 Remarkably, 28% of cases were asymptomatic, with some patients remaining free of symptoms for years despite very low platelet counts. More than half of patients with a platelet count of 30 to 50 × 109/L have no symptoms.3,4
A PARADOXICAL RISK OF THROMBOSIS
Although ITP is primarily a bleeding disorder, it is paradoxically also associated with thrombosis. Sarpatwari et al,5 in a study in the United Kingdom, found that the 4-year incidence of thromboembolic events was about 1.3 times higher in patients with ITP than in matched controls.
The reason for the increased risk of thrombosis is not clear. It is possible that in some patients, antiphospholipid antibodies may contribute to the development of thrombosis, although this has not been confirmed in all studies.
A DIAGNOSIS OF EXCLUSION
The evaluation of any patient suspected of having ITP should include the following:
- Personal history, with special attention to drugs and to medical conditions that could cause thrombocytopenia.
- Family history. ITP may occasionally be mistaken for an inherited cause of thrombocytopenia. The presence of the latter can often be confirmed by review of the peripheral blood film of the patient as well as other family members with thrombocytopenia. ITP is generally not considered to be an inherited disorder, although some HLA alleles may be more prevalent in ITP patients.
- Physical examination, with special attention to lymphadenopathy or splenomegaly, which may suggest an underlying malignancy such as a lymphoproliferative disorder. In general, patients with ITP have a normal physical examination, except for signs of bleeding or bruising in some.
- Laboratory tests, including a complete blood cell count, blood smear, reticulocyte count, Rh typing, and direct antiglobulin (Coombs) test.
In ITP, the peripheral blood smear should appear normal except for the presence of thrombocytopenia, although platelets may be mildly enlarged in some individuals. Red cell and leukocyte morphology is normal. It is important to exclude the presence of schistocytes (red cell fragments) and nucleated red blood cells, which often indicate a microangiopathic hemolytic anemia caused by disorders such as thrombotic thrombocytopenic purpura.
International guidelines suggest that testing for reduced immunoglobulin levels (as seen in common variable hypogammaglobulinemia) and HIV, HCV, and H pylori infections should also be considered. Coincident HCV infection is particularly high in some regions. Other cytopenias or abnormalities in the history or physical examination may prompt bone marrow examination. Testing for antiphospholipid antibodies, antinuclear antibodies, parvovirus, and cytomegalovirus may also be indicated in specific individuals. Testing for antiplatelet antibodies is not commonly performed in the current era because of its relatively low sensitivity and specificity.
Ultimately, the diagnosis of ITP is clinical, however, and cannot be established by any specific laboratory assay. Perhaps the best diagnostic study is assessment of the patient’s response to ITP therapy.
ITP INVOLVES ACCELERATED PLATELET DESTRUCTION
In 1951, William Harrington, a fellow at Washington University, infused blood from a patient with ITP into himself and, subsequently, into normal volunteers.6 The majority of recipients demonstrated significant reductions in the platelet count, sometimes severe. This fascinating and bold experiment provided the first demonstration that ITP was caused by a factor that circulates in blood. What is often not emphasized, however, is that some recipients did not develop thrombocytopenia, suggesting an alternative mechanism.
Later, Luiken et al7 and Hirschman and Shulman8 demonstrated that the transmissible agent in the blood was immunoglobulin, primarily immunoglobulin G (IgG). We now understand that much of the pathogenesis of ITP is caused by antibodies against platelet glycoproteins, most commonly platelet glycoprotein IIb/IIIa, the platelet fibrinogen receptor. Most patients, especially those with chronic ITP, also have antibodies against other platelet glycoproteins, including glycoprotein Ib/IX (the receptor for von Willebrand factor), and glycoprotein Ia/IIa, a collagen receptor. It is commonly believed that ITP may begin with antibodies against a single glycoprotein, which leads to accelerated clearance of antibody-coated platelets in the spleen. Degradation of cleared platelets by splenic macrophages leads to the release and subsequent presentation of antigenic peptides from proteolyzed platelet components, including glycoproteins, on the macrophage or dendritic cell. This may lead to recruitment and activation of specific T cells that in turn interact with and stimulate B cells to produce new antibodies against the platelet-derived peptides. This phenomenon, known as epitope spreading, may be responsible for the fact that most patients with long-standing, chronic ITP develop autoantibodies against multiple platelet glycoprotein targets.9
Several agents used in the treatment of ITP may work by impairing clearance of antibody-coated platelets by the reticuloendothelial system. One of many potential mechanisms underlying the therapeutic efficacy of intravenous immunoglobulin (IVIG) may be its ability to interact with a specific type of Fc gamma receptor, Fc gamma RIIb. IVIG therapy stimulates increased expression of this receptor, which in turn may impair the function of other “activating” Fc gamma receptors responsible for platelet clearance.10,11
ITP associated with infection may arise due to molecular mimicry. HCV, HIV, and H pylori contain amino acid sequences that may have structural similarity to regions within platelet glycoproteins. Thus, antibodies directed against the pathogen may cross-react with the glycoprotein, leading to thrombocytopenia.12–15
HCV has been found in up to one-third of cases of ITP in some centers.16–20H pylori-associated ITP is very common in some regions, particularly in Japan, and may often resolve after eradication of the infection. However, in the United States, eradication of H pylori generally does not improve the course of ITP. This may reflect antigen mimicry, in particular the fact that different cagA proteins are expressed by different strains of H pylori in certain regions of the world.
Our understanding of the immunologic basis of ITP has greatly expanded over the last decade. Although it has long been known that B cells produce autoantibodies, T cells have more recently been shown to play a critical role in regulating B-cell-mediated autoantibody production in ITP. In some situations, T cells may directly lyse platelets, or suppress megakaryopoiesis. This may explain why some patients who do not respond to standard B-cell-targeted therapy may respond to cyclosporine or other T-cell-directed agents.
