Table 12.13 . Clinically important thrombocytopenia is confined to disorders with reduced production or increased destruction of platelets. When the cause is the accelerated destruction of platelets, the bone marrow usually shows a compensatory increase in the number of megakaryocytes. Also of note, thrombocytopenia is one of the most common hematologic manifestations of AIDS. It can occur early in the course of HIV infection and has a multifactorial basis, including immune complex–mediated platelet destruction, anti-platelet autoantibodies, and HIV-mediated suppression of megakaryocyte development and survival.
Immune thrombocytopenic purpura (ITP) includes two clinical subtypes. Chronic ITP is a relatively common disorder that tends to affect women between the ages of 20 and 40 years. Acute ITP is a self-limited form seen mostly in children after viral infections.
Antibodies directed against platelet membrane glycoproteins IIb/IIIa or Ib/IX complexes can be detected in roughly 80% of cases of chronic ITP. The spleen is an important site of anti-platelet antibody production and the major site of destruction of the IgG-coated platelets. Although splenomegaly is not a feature of uncomplicated chronic ITP, the importance of the spleen in the premature
Table 12.13 Causes of Thrombocytopenia
Aplasticanemia:congenitalandacquiredMarrowinfiltration:leukemia,disseminatedcancer
Drug-induced:alcohol,thiazides,cytotoxicdrugsInfections:measles,HIVinfection
Autoimmune:ITP,systemiclupuserythematosus
Isoimmune:posttransfusionandneonatal
Drug-associated:quinidine,heparin,sulfacompounds
Infections:infectiousmononucleosis,HIVinfection,cytomegalovirusinfection
Multipletransfusions(e.g.,formassivebloodloss)
DIC, Disseminatedintravascularcoagulation;HIV, humanimmunodeficiencyvirus;ITP, immunethrombocytopenicpurpura;TTP, thromboticthrombocytopenicpurpura.
destruction of platelets is proved by the benefits of splenectomy, which normalizes the platelet count and induces a complete remission in more than two-thirds of patients. The bone marrow usually contains increased numbers of megakaryocytes, a finding common to all forms of thrombocytopenia caused by accelerated platelet destruction.
The onset of chronic ITP is insidious. Common findings include petechiae, easy bruising, epistaxis, gum bleeding, and hemorrhages after minor trauma. Fortunately, more serious intracerebral or subarachnoid hemorrhages are uncommon. The diagnosis rests on the clinical features, the presence of thrombocytopenia, examination of the marrow, and the exclusion of secondary ITP. Reliable clinical tests for anti-platelet antibodies are not available. Treatment usually involves the use of immunosuppressive agents and, in some cases, splenectomy.