Immune Thrombocytopenic Purpura (ITP)

Epidemiology:

•  Idiopathic
• Most likely an inappropriate immune response to an environmental trigger
• Platelets rapidly destroyed by immune complexes or autoantibodies cleared by reticuloendothelial system
• Autoimmune
• antiplatelet antibodies to membrane glycoproteins
• Acute ITP:
• self-limited disease of children
• resolves by 6 mo.
• M=F
• Post-viral often
• Chronic ITP:
• adult women < 40y
• F:M = 3:1
• Secondary causes of thrombocytopenia:
• Autoimmune disorders (SLE, rheumatoid, thyroid disease)
• Lymphoproliferative disorders (CLL, NHL)
• Solid malignancies
• Viral infection
• HIV, measles, mumps, rubella, EBV, varicella
• Drug induced (ex. heparin)
• Alloimmune (post-transfusion purpura, neonatal alloimmune thrombocytopenia

Common Sites:

Gross features:

• petechial hemorrhages and ecchymoses
• skin (dependent areas)
• spleen normal size (150g)
• brain:
• subarachnoid hemorrhage (rare if treated)
• intracerebral hemorrhage (rare if treated)

Histologic features:

• normal or increased megakaryocyte numbers in marrow
• peripheral blood:
• ?megathrombocytes often

• bone marrow:
• modest increase in megakaryocytes
• should not see a decrease in megakaryocytes
• some immature megakaryocytes
• large, non-lobated, single nuclei

• spleen:

• congestion of sinusoids
• hyperactivity and enlargement of splenic follicles (prominent germinal centres)
• foamy histiocytes in red pulp (gobbling up platelets)
• hemosiderin in red pulp (if concomitant RBC destruction)
• scattered megakaryocytes often

Immunophenotype:

Marker:

Sensitivity:

Specificity:

Molecular features:

Other features:

• Isolated thrombocytopenia (often < 10 x 10⁹/L) but otherwise normal blood count 
• antiplatelet antibodies (80%)
• most IgG
• symptoms:
• easy bruising
• nose bleeds
• gum bleeding
• melena
• hematuria
• excessive menstrual flow
• immunosuppressive doses of glucocorticoids often effective (almost all)
• splenectomy markedly improves condition (75-80%)

References:

• Robbins and Cotran Pathologic Basis of Disease Online (2007)
• Porwit A, McCullough J, Erber WN. Blood and bone marrow pathology (2011)