Bleeding and thrombosis are the 2 extremes of the physiological process of coagulation disorders. Despite major differences in the levels of individual components of the hemostatic system, neonatal coagulation is equal to or somewhat more rapid than that observed in adults. This suggests the existence of a delicately balanced hemostatic system in neonates, with uncommon bleeding or thrombosis in healthy term infants. However, a number of perinatal or neonatal conditions can disrupt this balance and increase the risk for either hemorrhage or thrombus formation. The presence of bleeding in a healthy term or late preterm infant, especially in an infant with a normal platelet count, is strongly suggestive of a congenital bleeding disorder. Hemophilia A and B and von Willebrand disease account for 95–98% of congenital bleeding disorders.
I. NORMAL PHYSIOLOGY OF HEMOSTASIS
The primary phase. The production of the platelet plug. This involves platelet adhesion (to the injured vessel's subendothelium) and their activation mediated by platelet surface glycoprotein (Ib, IIb/IIa) and von Willebrand factor (vWF).
The secondary phase. Results in the formation of a cross-linked fibrin clot. Coagulation proteins (factor XII to V) circulating as inactive precursor forms (zymogens) are converted to active forms through limited proteolysis. These activated proteins then further activate other zymogen factors in a chain reaction. Ultimately, the activation of factors V and X leads to cleavage of prothrombin (factor II) to thrombin (factor IIa). Cleavage of fibrinogen (factor I) to fibrin (factor Ia) by thrombin results in the formation of the blood clot.
The third phase. Modulates and limits the interactions of activated platelets and the clotting cascade (and Ca+2) that give rise to a clot. This includes the removal of activated factors (through the reticuloendothelial system) and the control of activated procoagulants by natural antithrombotic pathways (antithrombin III, protein C, protein S). Furthermore, restoration of vessel patency is triggered by the fibrinolytic pathway that generates plasmin from plasminogen. This is stimulated by tissue plasminogen activator and limited by α2-antiplasmin and plasminogen activator inhibitor (PAI). Plasmin is a proteolytic enzyme that degrades fibrin into fibrin split products such as d-dimers. Defects in fibrinolytic factors that result in excessive plasmin generation can lead to bleeding.
Neonatal platelets are reported to be hypo-reactive; however, this deficiency is balanced by increased vWF activity, resulting in overall normal platelet function.
Factor VIII, factor V, fibrinogen, and factor XIII levels are normal at birth.
Vitamin K–dependent (factors II, VII, IX, and X) and contact factors (XI and XII) are reduced to about 50% of normal adult values and are further reduced in preterm infants. Similarly, concentrations of the naturally occurring anticoagulants antithrombin, protein C, and protein S are low at birth. As a consequence, both thrombin generation and thrombin inhibition are reduced in the newborn period.
Neonatal fibrinolytic activity ...