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INTRODUCTION

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This chapter reviews common neonatal bleeding disorders. These disorders can present challenges in diagnosis and management. Most neonatal bleeding defects are acquired; however, some inherited hemorrhagic disorders can present in the neonatal period.

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Understanding the normal hemostatic process and the developmental aspects of the neonatal hemostatic system is important when investigating a neonate with a suspected bleeding disorder. The physiology of hemostasis in newborn infants and the approach to evaluation of neonatal bleeding disorders are discussed in the following introductory sections.

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Overview of Normal Hemostasis

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The key elements of the hemostatic system include the vascular endothelium, platelets, and coagulation. The immediate response to vascular injury is transient arteriolar vasoconstriction due to reflex neurogenic mechanisms and local secretion of vasoactive factors. This is followed by activation of platelets and coagulation proteins. Finally, once bleeding is controlled, blood vessel patency is restored by the fibrinolytic system. Hence, the normal hemostatic response can be viewed to occur in the following three phases (Figure 33-1):

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  1. Initiation and formation of the platelet plug (primary hemostasis)

  2. Propagation of the clotting process by the coagulation cascade followed by termination of clotting by antithrombotic control mechanisms (secondary hemostasis)

  3. Removal of the clot by the fibrinolytic system (tertiary hemostasis)

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FIGURE 33-1

Overview of the three phases of hemostasis. A, Primary hemostasis is dependent on interactions between platelets, the vascular endothelium, and coagulation proteins (von Willebrand factor [vWF] and fibrinogen). vWF associated with collagen in the exposed subendothelial matrix interacts with platelet GpIb/V/IX complex to mediate “platelet adhesion,” while vWF and fibrinogen bind platelet GpIIb-IIIa to mediate “platelet aggregation.” B, Secondary hemostasis involves sequential activation of coagulation factors. Formation of factor VIIa-tissue factor (TF) complex is the major initiating event that results in the generation of small amounts of thrombin (initiation phase or intrinsic pathway). These amounts of thrombin activate platelets and additional coagulation factors (amplification phase or extrinsic pathway), which results in a burst of thrombin formation, so that a stable fibrin clot can be formed. This process is subsequently terminated by three types of natural anticoagulants: antithrombin (AT), which inhibits the activity of thrombin and factors IXa, Xa, XIa, and XIIa; protein C (PC), which is activated by thrombomodulin (TM), thrombin, and its cofactor protein S (PS), inhibits factors Va and VIIIa; and TF pathway inhibitor (TFPI), which inhibits factor Xa and TF/factor VIIa. C, Tertiary hemostasis (fibrinolysis) functions to remove formed clots after hemostasis is secured. α1-AT 1 and 2, α1-antitrypsin 1 and 2, respectively; F, factor; FDP, fibrin degradation products; PAI 1 and 2, plasminogen activator inhibitor 1 and 2, respectively; tPA, tissue plasminogen activator; uPA, urokinase plasminogen activator.

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Vitamin K Physiology in Neonates

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Vitamin K is present in a variety of dietary sources and is produced by intestinal ...

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