Primary
immune thrombocytopenic purpura (
ITP) remains a diagnosis of exclusion both from nonimmune causes of
thrombocytopenia and
immune thrombocytopenia that develops in the context of other disorders (secondary
immune thrombocytopenia). The pathobiology, natural history, and response to
therapy of the diverse causes of secondary
ITP differ from each other and from primary
ITP, so accurate diagnosis is essential.
Immune thrombocytopenia can be secondary to medications or to a concurrent disease, such as an autoimmune condition (eg,
systemic lupus erythematosus [SLE],
antiphospholipid antibody syndrome [APS], immune
thyroid disease, or
Evans syndrome), a lymphoproliferative disease (eg,
chronic lymphocytic leukemia or large granular T-lymphocyte
lymphocytic leukemia), or
chronic infection, eg, with Helicobacter pylori, human immunodeficiency virus (HIV), or hepatitis C virus (HCV). Response to
infection may generate
antibodies that cross-react with platelet
antigens (HIV, H pylori) or
immune complexes that bind to platelet Fcgamma receptors (HCV), and platelet production may be impaired by
infection of megakaryocyte (MK) bone marrow-dependent progenitor cells (HCV and HIV), decreased production of
thrombopoietin (TPO), and splenic sequestration of platelets secondary to
portal hypertension (HCV). Sudden and severe onset of
thrombocytopenia has been observed in children after vaccination for
measles,
mumps, and
rubella or natural
viral infections, including Epstein-Barr virus, cytomegalovirus, and varicella zoster virus. This
thrombocytopenia may be caused by cross-reacting
antibodies and closely mimics acute
ITP of childhood. Proper diagnosis and treatment of the underlying disorder, where necessary, play an important role in patient management.