Idiopathic Thrombocytopenic Purpura

Basics

Idiopathic or immune thrombocytopenic purpura (ITP) is an autoimmune syndrome characterized by the following:
- Isolated thrombocytopenia (platelet count formally <100,000/mm³, typically <20,000/mm³)
- Shortened platelet survival
- Platelet autoantibodies
- Increased number of megakaryocytes in the bone marrow
Primary ITP implies absence of other causes for thrombocytopenia; secondary ITP indicates another diagnosis associated with autoimmune thrombocytopenia.
Phases of ITP
- Newly diagnosed (acute) ITP: within 3 months of initial diagnosis
- Persistent ITP: transient improvement or continued thrombocytopenia for 3–12 months
- Chronic ITP: persistent thrombocytopenia >12 months after initial presentation

Most common acquired platelet disorder in childhood
Often follows viral syndrome by a few weeks; this may be associated with higher likelihood of spontaneous recovery.
Males and females are equally affected in childhood ITP (mild male predominance in younger children; female-to-male ratio is 3:1 in adult and chronic ITP).
Median age at diagnosis is 4 years. Children <1 year or >10 years are more likely to develop chronic ITP.
>70% of childhood ITP resolves within 6–12 months.
Risk of severe bleeding is <5% and of intracranial bleeding is ∼0.5%.

Incidence is 1–10/100,000 children per year (<15 years of age).

Thrombocytopenia due to increased destruction of antibody-coated platelets in the reticuloendothelial system, particularly the spleen
Hypothesized that antibodies generated in response to foreign antigen or drug cross-react with platelet membrane glycoproteins (most commonly IIb/IIIa and Ib/IX)
Other mechanisms of immune dysregulation have been implicated, including possible inhibition of thrombocytopoiesis, limiting ability to compensate for destruction.
Typical bone marrow aspirate shows increased numbers of immature megakaryocytes.

In younger children, primary ITP is the most common presentation of ITP.
Secondary ITP is seen with autoimmune disorders (e.g., systemic lupus erythematosus [SLE], autoimmune lymphoproliferative syndrome [ALPS]).
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