During the haemostatic response, the formation of a primary platelet plug limits bleeding and provides a surface for clotting factors to assemble and become activated. The initial platelet plug is stabilized by fibrin monomers, covalently cross-linked by FXIII, forming a platelets-fibrin thrombus. Defects in platelets as well as inherited deficiencies of coagulation factors including fibrinogen, FII, FV, FV + FVIII, FVII, FX, FXI and FXIII deficiencies, generally lead to lifelong bleeding disorders, whose severity of bleeding symptoms is heterogeneous in platelets abnormalities but generally inversely proportional to the degree of the factor deficiency in rare bleeding disorders (RBDs). The prevalence of platelet defects among the general population has not been established, whereas for RBDs it ranges from approximately 1 in 2 million to 1 in 500000, being higher in countries where consanguineous marriages are diffused. As a consequence of the rarity of these deficiencies, the type and severity of bleeding symptoms, the underlying molecular defects, and the actual management of bleeding episodes are not well established. In this review the main features, diagnosis, available treatment options and treatment complications of the platelet disorders, caused by abnormalities in platelet receptors for adhesive proteins, platelet receptors for soluble agonists, platelet granules, signal transduction pathways, or procoagulant phospholipids will be discussed by Dr Cattaneo, whereas fibrinogen deficiency and FXIII deficiency will be described by Dr Inbal and Dr de Moerloose, respectively. Finally, the update of the Rare Bleeding Disorders Database will be presented by Dr Spreafico.
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