Thrombocytopenia

Thrombocytopenia means a reduction in the platelet count below the normal lower limit of 100,000 per cubic mL. This can have a variety of causes, including a reduction in platelet production, a reduction in platelet survival, and dilution of platelet numbers resulting from the transfusion of platelet-poor blood.

Disorders Affecting Platelet Production

Congenital

• Megakaryocytic hypoplasia - underdevelopment of the megakaryocytes developed in bone marrow which fragment up produce platelets - usually of autoimmune or infectious origin
• Thrombocytopaenia/absent radii syndrome (TAR) - radial aplasia or hypoplasia and thrombocytopaenia
• Wiskott-Aldrich syndrome - an X-linked recessive disease characterised by thrombocytopenia,
  lymphopenia and depressed cellular immunity, immunosuppression, eczema, malignant lymphoma
• Congenital leukaemia e.g. Down's syndrome
• Fanconi's anaemia

• Viral infections - e.g. herpes simplex, cytomegalovirus, varicella-zoster, Epstein-Barr, rubella, enterovirus, mumps, hepatitis
• Aplastic anaemia
• Marrow infiltration by a primary neoplasm e.g. leukaemia or metastatic malignant disease
• Drugs and chemicals - e.g.:
  • Quinine
  • Quinidine
  • Aspirin
  • Antibiotics such as chloramphenicol and sulphonamides
  • Heparin
  • Gold
  • Digoxin
  • Phenylbutazone
  • Indometacin
  • Sulphasalazine
• Alcohol
• Paroxysmal nocturnal haemoglobinuria
• Megaloblastic anaemia
• Myelofibrosis

Decreased Platelet Survival

• Immune - idiopathic thrombocytopaenia purpura, systemic lupus erythematosus, post-transfusion purpura, drug-induced (same drugs as affect platelet production)
• Thrombotic thrombocytopenic purpura
• Haemolytic uraemic syndrome
• Disseminated intravascular coagulation
• Cardiopulmonary bypass
• Hypersplenism
• Strawberry naevi, in Kasabach-Merritt syndrome (cavernous haemangiomata with severe thrombocytopaenia, and features of disseminated intravascular coagulation)

Dilutional Thrombocytopaenia

This is caused by transfusion of large volumes of blood which may be depleted of functioning platelets resulting from prolonged storage.

See Thrombocytopathy.