Disorders of coagulation

Primary hemostatic mechanism (platelet phase)

Primary hemostasis leads to the formation of a reversible aggregate of platelets: a temporary platelet hemostatic plug with endothelial injury exposing vWF and collagen from the subendothelial matrix to flowing blood and shear forces. Plasma vWF binds to the exposed collagen, uncoils its structure and, in synergy with collagen, supports the adhesion of platelets. Initially, the vWF interacts with the GPIb platelet receptor, tethering the platelets. As the platelet collagen receptors GPVI and α2β1 bind to collagen, the platelets adhere and become activated with a resulting release of platelet alpha and dense granule contents. Platelet activation results in a conformational change in the αIIbβ3 receptor, activating it and enhancing its avidity for vWF, for vessel wall ligands and fibrinogen. The enhanced avidity for vWF and fibrinogen mediates platelet-to-platelet interactions that eventually lead to platelet plug formation.

The fibrin thrombus formation component of hemostasis occurs in three overlapping phases ( Fig. 13.1 ):

• 1.

  initiation

• 2.

  amplification

• 3.

  propagation

The initiation phase begins with cell-based expression of tissue factor (TF) at the site of endothelial injury. FVII binds to the exposed TF and is rapidly activated. The FVIIa/TF complex in turn generates factor Xa (FXa) and factor IXa (FIXa). FXa can activate FV that complexes with FXa and generates small amounts of thrombin. During the amplification phase the procoagulant stimulus is transferred to the surface of platelets at the site of injury. The small amounts of thrombin enhance platelet adhesion, fully activate the platelets, and activate FV, FVIII, and FXI. In the propagation phase the “tenase” complex of FIXa FVIIIa is assembled on the platelet surface and efficiently generates FXa. Similarly, the “prothrombinase” complex of FXa FVa is assembled on the platelet surface and efficiently generates thrombin. Unlike FXa generated from TF FVIIa interactions, FXa complexed to FV is protected from inactivation by TF pathway inhibitor (TFPI), assuring adequate thrombin generation. The resulting procoagulant, thrombin, activates FXIII and cleaves fibrinopeptides A and B from fibrinogen. The residual peptide chains aggregate by means of loose hydrogen bonds to form fibrin monomers. Under the influence of FXIIIa, fibrin monomers are converted into fibrin polymers, forming a stable fibrin clot. In the presence of thrombin the mass of loosely aggregated intact platelets is transformed into a densely packed mass that is bound together by strands of fibrin to form a definitive hemostatic barrier against the loss of blood.

Platelet vessel interaction

The vasculature forms a circuit that maintains blood in a fluid state and free of leaks. With vascular injury, platelets and the coagulation system temporarily close the rent and repair the leak. Blood vessel wall characteristics exhibit properties that contribute to hemostasis or stop hemorrhage as well as prevent thrombosis. The media and adventitia of the vessel wall enable vessels to dilate or constrict. The subendothelial basement membranes contain adhesive proteins that provide binding sites for platelet and leukocytes. Remodeling that occurs after injury is enhanced by extracellular matrix metalloproteinases.

Fibrinolysis

The fibrinolytic system provides a mechanism for the removal of physiologically deposited fibrin. Clot lysis is brought about by the action of plasmin on fibrin. Fibrinolytic events are shown in Fig. 13.2 . Plasminogen from circulating plasma is laid down with fibrin during the formation of thrombin. Plasminogen is primarily synthesized in the liver and circulates in two forms, one with an NH ₂ -terminal glutamic acid residue (glu-plasminogen) and a second form with an NH ₂ -terminal lysine, valine, or methionine residue (lys-plasminogen). Glu-plasminogen can be converted to lys-plasminogen by limited proteolytic degradation.

Lys-plasminogen has a higher affinity for fibrin and cellular receptors; it is also more readily activated to plasmin than glu-plasminogen. Both forms of plasminogen bind to fibrin through specific lysine-binding sites. These lysine-binding sites also mediate the interaction of plasminogen with its inhibitor, α2-antiplasmin (α2AP). TAFI-mediated removal of C -terminal lysine and arginine residues will prevent high-affinity plasminogen binding and will attenuate fibrinolysis. Plasminogen is converted to its enzymatically active form, plasmin, by several activators. These activators are widely distributed in body tissues and fluids. t-PA is the principal intravascular activator of plasminogen. t-PA is a serine protease that binds to fibrin through lysine-binding sites. When t-PA is bound to fibrin, its plasmin generation efficiency increases markedly. Urokinase-type plasminogen activator, a second physiological activator of plasminogen, is present in urine and activates plasminogen to plasmin independent of the presence of fibrin. Plasmin splits fibrin and fibrinogen into fibrin-degradation products (FDPs): fragments X, Y, D, and E, some of which are measured clinically as d -dimers.

Properties attributed to the various fibrin split products (FSPs) include heparin-like effects, inhibition of platelet adhesion and aggregation, potentiation of the hypotensive effect of bradykinin and chemotactic properties (monocytes and neutrophils). Increased fibrinolysis is usually a reaction to intravascular coagulation (secondary fibrinolysis) rather than the initial event (primary fibrinolysis). The action of plasmin is negatively regulated by several inhibitors (shown in Fig. 13.2 ) that include α2AP and α2-macroglobulin. PA is in turn regulated by two inhibitors, plasminogen activator inhibitor-1 (PAI-1) and PAI-2. PAI-1 is the physiologically important of these inhibitors.

Natural inhibitors of coagulation

In addition to the physiologic role of fibrinolysis, other inhibitors play critical roles in the control of hemostasis. Table 13.2 lists the plasma fibrinolytic components and hemostatic inhibitors and their principal substrates. All members of this group have overlapping roles in the control of coagulation and fibrinolysis. Major antiproteases of this group of inhibitors include antithrombin (AT), α2AP, α2-macroglobulin, the inhibitor of the activated first component of complement (C1 inhibitor), and α1-antitrypsin.

AT neutralizes the procoagulant thrombin, FIXa, FXa, and FXIa ( Fig. 13.3 ). When bound to circulating heparin or heparin sulfate on endothelial cells, AT undergoes a conformational change with a dramatic increase in this activity. TFPI is responsible for inactivation of the FXa/FVIIa/TF complex.

The vitamin K–dependent zymogen, protein C (PC), and its cofactor protein S (PS), which is also a vitamin K protein, play an important role in the control of hemostasis by inhibiting activated FV and VIII ( Fig. 13.3 ). Binding of thrombin to thrombomodulin on endothelial cells of small blood vessels neutralizes the procoagulant activities of thrombin and activates PC. PC binds to a specific receptor and the binding augments the activation of PC by thrombin. Activated PC (APC) inactivates FVa and FVIIIa in a reaction that is greatly accelerated by the presence of free PS and phospholipids, thereby inhibiting the generation of thrombin. Free PS itself has anticoagulant effects: it inhibits the prothrombinase complex (FXa, FVa, and phospholipid), which converts prothrombin to thrombin and inhibits the complex of FIXa, FVIIIa, and phospholipid, which converts FX to FXa.

Plasma factors

In newborns, FVIII and vWF are normal or higher than adult levels. Plasminogen levels are only 50% of adult values and α2AP levels are 80% of adult values, whereas PAI-1 and t-PA levels are significantly increased over adult values. The increased plasma levels of t-PA and PAI-1 in newborns on day 1 of life are in marked contrast to values from cord blood, in which concentrations of these two proteins are significantly lower than in adults. Newborns also have decreased levels of vitamin K–dependent procoagulants (FII, FVII, FIX, FX) and activity of anticoagulant factors, especially AT, PC, and PS.

Blood vessels

• Capillary fragility is increased.

• Prostacyclin production is increased.

Platelets

• Platelet adhesion is increased due to increased vWF and increased HMW vWF multimers.

• Epinephrine-induced aggregation is decreased due to decreased platelet receptors for epinephrine.

• Ristocetin-induced aggregation is increased due to increased vWF and increased HMW vWF multimers.

• Platelet activation is increased: as evidenced by elevated levels of thromboxane A2, β-thromboglobulin, and PF4.

Initial screening tests

• 1.

  Complete blood count (CBC): quantitative assessment of platelets and review of blood smear to assess platelet morphology.

• 2.

  Assessments of platelet function:

  • a.

    Platelet function analyzer (PFA-100): assesses flow through a membrane with an aperture coated with platelet agonists; membrane closure time is measured in response to ADP and to epinephrine. The closure time is often prolonged with impaired platelet function and von Willebrand disease (vWD).

  • b.

    Bleeding time should not be used because of difficulties in validation.

• 3.

  Coagulation factor screening tests:

  • a.

    From a laboratory perspective the coagulation system is divided into the intrinsic pathway, the extrinsic pathway, and the common pathway. Such an artificial division is not based on coagulation physiology but is useful for conceptualizing in vitro laboratory testing ( Fig. 13.5 ).

    

• 4.

  Prothrombin time (PT) assay (assesses the extrinsic system): this test utilizes tissue thromboplastin and calcium chloride, to initiate the formation of thrombin via the extrinsic pathway. The international normalized ratio (INR) is used to correct for differences between thromboplastin sources across laboratories.

• 5.

  Activated partial thromboplastin time (aPTT) assay (assesses the intrinsic system): this test utilizes a phospholipid reagent, a particulate activator (e.g., ellagic acid, kaolin, silica, soy extract) and calcium chloride to start the enzyme reaction that leads to the formation of thrombin via the intrinsic pathway.

Common confirmatory coagulation assays

• 1.

  Fibrinogen: quantitative measurement of fibrinogen, deficiency suggested when both the PT and the aPTT are prolonged.

• 2.

  Thrombin time (TT): prolonged when fibrinogen is reduced or abnormal, in the presence of inhibitors (FDPs, d -dimers) and in the presence of thrombin-inhibiting drugs and hypoalbuminemia. This is a useful test to diagnose dysfibrinogenemia (qualitatively abnormal fibrinogen). Also useful when both the PT and aPTT are prolonged. The reptilase time is a modification of the TT, using the purified enzyme reptilase instead of thrombin and is unaffected by heparin and heparin-like anticoagulants.

• 3.

  Mixing studies (performed to evaluate a prolonged PT or aPTT): the respective assay is performed following the addition of normal pooled plasma to patient plasma. Normalization indicates a clotting factor deficiency that was corrected by addition of normal pooled plasma. Continued prolongation indicates the presence of a coagulation inhibitor. Such inhibitors may be physiologically relevant or only detected in vitro.

• 4.

  Clotting factor activity assays: performed to identify clotting factor deficiencies if mixing studies normalize. FXII, FXI, FIX, and FVIII assays are useful if the aPTT normalizes in mixing studies. The FVII assay is useful if the PT normalizes in mixing studies. FX, FV, FII, and fibrinogen assays are useful if both PT and aPTT normalize in mixing studies. (Note: FXIII deficiency does not result in prolongation of the PT or aPTT.); confirmation needs assays for FXIII antigen and FXIII activity; the 5-M urea lysis assay should not be used to confirm FXIII deficiency given its poor sensitivity.

• 5.

  von Willebrand panel: von Willebrand antigen (vWF:Ag) is the quantitative assay for vWF, von Willebrand Ristocetin cofactor activity (vWF:RCo) is the functional or qualitative assay for VWF. They are both useful when PFA-100 closure time is prolonged or when vWD is suspected. vWF multimers are useful to diagnose qualitative VWF abnormalities (type 2 vWD) when discrepancies between vWF:Ag (normal to low) and vWF:RCo (extremely low); ratio of vWF:RCo to vWF:Ag<0.6).

• 6.

  Platelet aggregation studies: a qualitative assessment of platelet function, useful when platelet function disorders are suspected or PFA-100 closure time is prolonged.

Global hemostatic tests

Hemostasis is a complex interplay of simultaneously occurring events, with current assays only reflecting snapshots of this dynamic process. Global hemostatic tests can provide detailed information on thrombin generation and processes downstream, including fibrin polymerization and fibrin dissolution.

• 1.

  Thrombin generation assay: the calibrated automated thrombogram system uses a fluorogenic substrate to continuously measure the generated thrombin. The endogenous thrombin potential, which can be measured by calculating the area under the curve from the thrombogram, has shown correlation with the bleeding phenotypes in hemophilia, in hemophilia patients with inhibitors, and factor XI deficiency.

• 2.

  Viscoelastic tests.

Thromboelastography (TEG) is performed on whole blood, assessing the viscoelastic property of clot formation under low shear condition after the addition of specific coagulation activators. Improvement in the methodology is widely expanding the use of TEG from preclinical research settings to point-of-care testing in intensive care units and operating rooms. The device has a metal pin suspended by a torsion wire immersed into a cup that holds the whole blood. Once clotting starts, fibrin strands formed increase the torque between the pin and the cup that is measured electronically. TEG provides various data relating to clot formation and fibrinolysis (the lag time before the clot starts to form, the rate at which clotting occurs, the maximal amplitude of the trace or clot strength, and the extent and rate of amplitude).

Rotational thromboelastography is almost identical to TEG; however, instead of the cup rotating, here the sensor rod rotates and is operator independent. The output data are named differently as also different assays are available for analysis of different aspects of the coagulation system such as EXTEM (extrinsic pathway), INTEM (intrinsic pathway), FIBTEM (fibrinogen contribution), HEPTEM (heparin effect), and APTEM (thrombolysis reversal).

Preoperative evaluation of hemostasis

• 1.

  History: the history is perhaps the most important element of the evaluation. In an effort to standardize bleeding histories, a number of quantitative bleeding assessment tools (BATs) have been developed and validated in individuals with known vWD (Vicenza-based BAT, the condensed Molecular and Clinical Markers for the Diagnosis and Management of Type 1 vWD Questionnaire (MCMDM-1 vWD) International Society on Thrombosis and Haemostasis (ISTH), and the more pediatric-specific Pediatric Bleeding Questionnaire (PBQ)), although whether these tools can directly predict future bleeding episodes needs further study.

  • a.

    If negative: no coagulation tests are indicated.

  • b.

    If positive or unreliable: the following tests should be performed: CBC, PFAs, PT, aPTT, and fibrinogen, TT, von Willebrand panel.

• 2.

  Abnormal tests require further investigation (see Fig. 13.6 ).

  

• 3.

  In a patient with a significant bleeding history, if all screening tests and von Willebrand panel are normal, consider FXIII, PAI-1 activity, α2AP, and platelet aggregation studies, vitamin C levels and rule out hyperextensibility on clinical examination (see Figure 13.4 ).

Genetics

Given X-linked recessive inheritance females are carriers for hemophilia or may have mild hemophilia depending on factor levels and accompanying bleeding symptoms. Most usually have variable factor levels but typically will have enough levels to be in the hemostatic range. Excessive lionization, however, may lead to symptomatic females with undetectable factor levels and bleeding symptoms requiring management as their male counterparts. The FVIII common intron 22 inversion, resulting from an intrachromosomal precombination, is identifiable in 45% of severe hemophilia A patients. For the remaining 55% of patients with severe hemophilia A, as well as all those with mild and moderate hemophilia A, the molecular defects can usually be detected by efficient screening of all 26 FVIII exons and splice junctions. Targeted mutation analysis is the most accurate test for carrier detection and prenatal diagnosis for severe hemophilia A. For rare patients in whom a precise mutation cannot be identified or gene sequencing is not an option, intragenetic and extragenetic linkage analysis of DNA polymorphisms can be useful with up to 99.9% precision (when an affected male patient and his related family members are available). Preimplantation genetic diagnosis is a reproductive option available to carrier females. When definitive diagnosis of the carrier state cannot be made, determination of the FVIII/vWF:Ag ratio (<1.0) can be used to detect 80% of hemophilia A carriers with 95% accuracy when done in laboratories with careful standardization procedures.

Hemophilia B carriers have a wide range of FIX levels but, in a subset of cases, can be detected by the measurement of reduced plasma FIX activity (60–70% of cases). The FIX gene is located centromeric to the FVIII gene in the terminus of the long arm of the X chromosome. The 34-kb FIX coding sequence comprises eight exons and encodes a 461-amino-acid precursor protein that is approximately one-third the size of the FVIII complementary Deoxyribonucleic Acid (cDNA). Because of the smaller gene size, FIX mutations can be identified in nearly all patients. Direct FIX mutation testing is available through DNA diagnostic laboratories, with linkage analysis used in those cases where the responsible mutation cannot be identified.

Prenatal diagnosis of hemophilia can be performed by either chorionic villus sampling at 10–12 weeks gestation or by amniocentesis after 15 weeks gestation. If DNA analysis is not available or if a woman’s carrier status cannot be determined, fetal blood sampling can be performed at 18–20 weeks gestation for direct fetal FVIII plasma activity level. The normal fetus at 18–20 weeks gestation has a very low FIX level, which an expert laboratory can distinguish from the virtual absence of FIX in a fetus with severe hemophilia B.

Clinical course of hemophilia

Hemophilia should be suspected when unusual bleeding is encountered in a male patient. Clinical presentations of hemophilia A and hemophilia B are indistinguishable. The frequency and severity of bleeding in hemophilia are usually related to the plasma levels of FVIII or FIX ( Table 13.10 ), although some genetic modifiers of hemophilia severity have been identified. The median age for first joint bleed is 10 months, corresponding to the age at which the infant becomes mobile. Table 13.11 shows the common sites of hemorrhage in hemophilia. The incidence of severity and clinical manifestations of hemophilia are listed in Table 13.12 . For hemophilia B the Leyden phenotype (severe hemophilia as a child that becomes mild after puberty) has been described in families with defects in the androgen-sensitive promoter region of the gene.

Managing newborns with known or suspected hemophilia

Known female carriers or females with positive family history should plan deliveries at an obstetric center affiliated with hemophilia centers whenever possible, although 30% of newborns with hemophilia due to de novo mutations will not have a family history. There is no contraindication to vaginal delivery and the option of elective C-section should be considered based on obstetrical factors and individualized. Forceps application, vacuum delivery, fetal scalp electrodes, and fetal blood sampling should be avoided. Uncontaminated cord blood should be collected and sent to a laboratory that can expediently run FVIII/FIX levels. Mild FVIII deficiency may be missed at birth and any low normal levels should be repeated at approximately 6 months of life. FIX levels could be physiologically low at birth and should be repeated beyond age 3 months to confirm a diagnosis of mild/ moderate hemophilia B. Intramuscular injections should be avoided in patients with severe hemophilia to avoid muscle bleeds. If heelsticks are conducted, adequate pressure should be applied to achieve hemostasis. Acute bleeding should be managed by recombinant factor support. 1-Deamino-8- d -arginine vasopressin (DDAVP) is strictly contraindicated in newborns due to the risk for hyponatremic seizures. Screening cranial ultrasound should be performed to rule out intracranial hemorrhage (ICH) in all newborns diagnosed with hemophilia A/B. In a symptomatic newborn with suspected ICH but a normal ultrasound, magnetic resonance imaging (MRI)/computed tomography (CT) scan should be pursued. The decision for circumcision should involve consultation with a hematologist. For newborns without forewarning who have bleeding symptoms and an elevated aPTT, FFP treatment (15–25 mL/kg) can be initiated prior to receiving confirmatory factor levels.

Treatment (factor replacement therapy)

Factor replacement therapy is the mainstay of hemophilia treatment. The degree of factor correction required to achieve hemostasis is largely determined by the site and nature of the particular bleeding episode. Commercially available products for replacement therapy are listed in Table 13.13 . Source plasma for all plasma-derived factor concentrates undergoes donor screening and nucleic acid testing for a variety of viral pathogens. In addition, all plasma-derived and many recombinant factor concentrates undergo a viral inactivation treatment, typically with solvent detergent, wet or dry heat treatment, pasteurization, or nanofiltration. Recombinant factor concentrates are widely accepted as the treatment of choice for previously untreated patients, minimally treated patients, and patients who have not had transfusion-associated infections. These products, however, need frequent administration via intravenous (IV) infusions due to their short half-life.

Extended half-life (EHL) recombinant factor products with the FVIII or FIX protein fused to the Fc portion of immunoglobulin 1 are available, which prolong the half-life of the factor in circulation. This enables prophylaxis dosing regimens occurring every 3–5 days for FVIII-deficient patients and 7–10 days dosing for FIX-deficient patients. PEGylated products are available that have longer half-lives. By increasing the molecular mass, glomerular filtration, proteolytic degradation, and clearance are reduced leading to an increased half-life of the PEGylated protein. Several other strategies being investigated include single-chain rFVIII that more effectively binds to vWF, bioengineered antibodies, and short interfering RNAs that slow down the endogenous AT synthesis.

Emicizumab is a bispecific humanized antibody acting as an FVIIIa “mimic” binding to FIXa and FXa and thereby accelerating progression of the coagulation cascade toward thrombin generation. The HAVEN trials established that emicizumab prophylaxis can achieve remarkable reduction in bleeding rates regardless of age or FVIIIa inhibitor status. Its singular advantage is that it can be given subcutaneously at weekly/bimonthly schedules. Breakthrough bleedings during the trials did require additional hemostatic support. The main safety concerns are from increased thrombotic risks and thrombotic microangiopathy, paucity of assays to assess hemostatic drug levels, and potential for the development of antidrug antibodies. Patients with high activity levels or involved in high-impact sports may not be suitable candidates for this therapy.

Gene therapy offers the promise of curing hemophilia by inserting the deficient gene into the patient’s tissue that can then restore circulating factor levels. Notably, the gene for FIX is small and easy to insert into many vectors. In 2011 Nathwani and colleagues successfully transduced 10 hemophilia B patients using adeno-associated viral (AAV) vectors, with persistent FIX levels in all patients up to 9 years, with 90% reduction in bleeding episodes. Gene therapy in hemophilia A has been slower due to the larger size of the FVIII gene. In 2017 a truncated FVIII cDNA was used to incorporate into AAV vector, which leads to successful FVIII expression up to 2 years. This is currently being reviewed by the US Food and Drug Administration. A number of phase 1–3 gene therapy trials are currently ongoing and the results could potentially change the treatment landscape of hemophilia.

Strategies for hemophilia care include on-demand treatment of acute bleeding episodes or, for severe hemophilia patients, prophylactic administration of clotting factor concentrate to maintain trough factor levels >1% augmented with on-demand treatment of breakthrough bleeding episodes. The latter strategy pharmacologically converts the severe hemophilia phenotype to a moderate phenotype with an attendant reduction in frequency of bleeding episodes. A randomized multicenter US national study suggested a markedly reduced incidence of hemophilic arthropathy when prophylaxis was instituted prior to onset of recurrent joint bleeds. This benefit must be balanced with the need for frequent prophylactic infusions (3–4 times/wk for FVIII, 2 times/wk for FIX), venous access considerations, potential requirements for central venous access devices, increased cost of treatment, and the occasional patients who have a mild clinical course. EHL products are useful to mitigate some of these limitations. However, recovery studies maybe needed to determine adequate dosing frequency and treatment individualized for the patient’s activities. Most providers will still use short-acting products for breakthrough bleeds. Table 13.14 provides generally accepted guidelines for treatment of most types of hemophilic bleeding. When a bleeding episode is suspected, hemostatic treatment should be rendered first and diagnostic evaluation(s) should be performed later to prevent adverse consequence of the bleed.

Ancillary therapy

DDAVP: in hemophilia A patients, DDAVP increases plasma FVIII levels twofold to fivefold. It is commonly used to treat selected hemorrhagic episodes in mild hemophilia A patients.

When used intravenously the dose is 0.3 μg/kg (maximum dose 25 μg) administered in 25–50 mL normal saline over 15–20 minutes. Its peak effect is observed in 30–60 minutes. Subcutaneous DDAVP is as effective as IV DDAVP, facilitating treatment of very young patients with limited venous access. Concentrated intranasal DDAVP (Stimate, 0.1%), available as a 1.5 mg/mL preparation, can be used age 6 years or older. Care should be exercised to avoid inadvertent dispensing of the dilute intranasal DDAVP (0.01%) commonly used for treatment of diabetes insipidus. The peak effect of intranasal Stimate is observed 60–90 minutes after administration.

Recommended dosage for use of intranasal Stimate:

• Body weight <50 kg: 150 μg (one-metered dose).

• Body weight >50 kg: 300 μg (two-metered doses).

Precautions with DDAVP use:

• 1.

  Mild fluid restriction to two-thirds maintenance fluids and drinking to thirst, only electrolyte-containing fluids; avoidance of free water.

• 2.

  Monitoring urinary output and daily weights may be useful to track fluid retention.

A test dose of DDAVP should be administered at the time of diagnosis or in advance of an invasive procedure to assess the magnitude of the patient’s response. DDAVP administration may be repeated at 24-h intervals according to the severity and nature of the bleeding. Administration of DDAVP at shorter intervals results in a progressive tachyphylaxis over a period of 4–5 days. Depending on the response after a trial, DDAVP could be recommended for most minor procedures. For major procedures requiring maintenance of factor activity >80%, factor concentrates may be needed even in mild hemophilia A patients.

Side effects of DDAVP:

Asymptomatic facial flushing.

• Thrombosis (a rarely reported complication).

• Hyponatremia, more common in very young patients, in patients receiving repeated doses of DDAVP or large volumes of oral or IV fluids; hyponatremic seizures have been reported in children under 2 years of age. DDAVP is, therefore, contraindicated in children less than 2 years of age.

Antifibrinolytic therapy

Antifibrinolytic drugs inhibit fibrinolysis by preventing activation of the proenzyme plasminogen to plasmin, mostly by activating TAFI. This intervention is useful for preventing clot degradation in areas rich in fibrinolytic activity, including the oral cavity, the nasal cavity, and the female reproductive tract. Approved antifibrinolytic drugs include:

Epsilon aminocaproic acid (EACA; Amicar): available as a pill and liquid, prescribed in a dose of 50–100 mg/kg every 6 hours (maximum, 20 g total dose/d). GI symptoms may occur at higher doses; therefore the preferred starting dose is 50 mg/kg. The drug is available as 500 mg, 1000 mg tabs, or as a flavored syrup (250 mg/mL).

Tranexamic acid (Cyklokapron, Lysteda): 20–25 mg/kg (maximum, 1.5 g) orally or 10 mg/kg (maximum, 1.0 g) intravenously every 8 hours. This is approved for use in women with bleeding disorders with menorrhagia with expanded use in bleeding disorder patients.

Antifibrinolytic therapy should not be utilized in patients with urinary tract bleeding because of the potential of intrarenal clot formation. To treat spontaneous oral hemorrhage or to prevent bleeding from dental procedures in pediatric patients with hemophilia, either drug is begun in conjunction with DDAVP or factor replacement therapy and continued for up to 7 days or until mucosal healing is complete. Antifibrinolytic drugs also have efficacy as an adjunct treatment for epistaxis and for menorrhagia. Antifibrinolytic drugs are safe to use in hemophilia B patients receiving coagulation FIX concentrates but should not be used concurrently with activated prothrombin complex concentrates (aPCCs) because of the thrombotic potential of the combination. Initiation of oral antifibrinolytic drug therapy 4 hours after the last dose of aPCCs appears to be well tolerated.

Management of inhibitors in hemophilia

Approximately 30% of patients with severe hemophilia A can develop neutralizing alloantibodies (inhibitors) directed against FVIII. Inhibitors are a major cause of morbidity and mortality in hemophilia. Risk factors for inhibitors include early age at exposure, presence of the common inversion mutation, large deletions of the FVIII gene, African-American ethnicity, and a sibling with hemophilia and an inhibitor. The SIPPETT study prospectively randomized previously untreated patients to receive a plasma-derived product or a recombinant product. There was a twofold higher incidence of inhibitors in the recombinant product group. The INSIGHT group demonstrated an incidence of 6.7% by 50 exposure days and 13.3% by 100 exposure days.

Inhibitors are quantified using the Bethesda assay. Low-responder inhibitors have titers <5 Bethesda units (BU) and do not exhibit anamnesis upon repeated exposure to FVIII. A 1 BU neutralizes 50% of FVIII/FIX activity. Approximately half of these patients with inhibitors will be low responders and, of these, approximately half will have transient inhibitors. Hemophilia A patients with low-responder inhibitors can generally be treated with FVIII concentrate, albeit at an increased dosing intensity because of reduced in vivo recovery and a shortened half-life of the FVIII. High-responder inhibitors have titers ≥5 BU, and although the titer may decay in the absence of FVIII exposure, these patients will display an anamnestic rise in titer upon rechallenge with FVIII. The clinical approach is different for high and low responders ( Table 13.15 ).

Low responders

For serious limb- or life-threatening bleeding, a bolus infusion of 100 units/kg FVIII is administered, repeat doses of 100 units/kg are administered at 12-h intervals or, alternatively, the level is maintained with a continuous infusion rate based on the inhibitor titer, recovery and estimated half-life of the factor. An FVIII assay should be obtained 15 minutes after the bolus infusion and trough or steady-state FVIII levels should be followed at least daily thereafter. During prolonged treatment in vivo recovery and half-life may transiently improve as inhibitor antibody is adsorbed by the FVIII.

High responders

Treatment of an acute bleed is achieved by the use of bypassing agents [aPCCs and recombinant VIIa (rVIIa)] and inhibitor eradication is achieved by initiating immune tolerance. In high-responder inhibitor patients with limb- or life-threatening bleeding, if the inhibitor titer is <20 BU, high-dose continuous infusion of FVIII may saturate the antibody permitting a therapeutic FVIII level. The dose can be based on recovery and half-life studies. If an FVIII level is not attainable or the antibody level is greater than 20 BU, then bypassing agents to initiate hemostasis independent of FVIII should be employed.

Agents used to manage bleeding:

• 1.

  aPCC [FVIII inhibitor bypassing activity (FEIBA)]

  These products have increased amounts of activated FVII (FVIIa), FX (FXa), and thrombin and are effective in patients even with high-titer inhibitors (>50 BU). The initial dose of 75–100 units/kg can be repeated in 8–12 hours not to exceed 200 units/kg/d. Generally, for a joint bleed, four to five doses at 12-h intervals are employed after which the risk of thrombogenicity is increased. Approximately 75% of patients with inhibitors respond to aPCC infusions. For some patients, trace amounts of FVIII in aPCC products may cause anamnesis of the inhibitor titer. If multiple doses are administered, the patient should be monitored for the development of DIC and thromboembolic complications. The simultaneous use of antifibrinolytic therapy (e.g., Amicar) should be avoided. An Oral antifibrinolytic drug therapy 4 hours after the last dose of aPCC, however, appears to be safer.

• 2.

  Recombinant FVIIa (NovoSeven)

  Recombinant activated FVII (rFVIIa) concentrate can be administered to achieve hemostasis in patients with high-titer inhibitors. The recommended dose is 90-μg/kg rFVIIa repeated every 2 hours for two to three infusions. Higher doses up to 200 μg/kg for two to three doses have been used in pediatric patients in life-threatening situations to achieve better hemostasis. Single high dose of 270 μg/kg has been shown to be effective in the outpatient setting for the treatment of an acute bleed. The subsequent frequency of infusion and duration of therapy must then be individualized, based on the clinical response and severity of bleeding. Early initiation of hemostatic treatment (within 8 hours of a bleed) with rFVIIa produces response rates on the order of 90%. Treatment failures with conventional doses of rFVIIa may respond to higher doses. The incidence of thrombotic complications with this product has been low and anamnesis of the inhibitor does not occur.

• 3.

  Emicizumab:

  Emicizumab should be considered at the onset of inhibitor development prior to the start of immune tolerance induction (ITI) (see next). Breakthrough bleeding should be treated with rVIIa due to the increased thrombotic risk with aPCC use. Concurrent use of ITI with emicizumab is under investigation to assess its ability to achieve and maintain tolerance.

Modalities used to eradicate inhibitors:

• 1.

  Plasmapheresis with immunoadsorption

  When bleeding persists despite active treatment, extracorporeal plasmapheresis over staphylococcal A columns may rapidly reduce the inhibitor titer (up to 40%) by adsorbing offending inhibitory IgG antibodies. This approach is cumbersome and may produce significant fever and hypotension due to the release of staphylococcal A protein. However, it can be lifesaving in desperate situations and its efficacy can be enhanced by concomitant replacement therapy with FVIII containing concentrates.

• 2.

  ITI

  This intervention is instituted for inhibitor eradication in high-responder inhibitors and involves frequent administration of FVIII concentrate to induce immune tolerance to exogenous FVIII. The goals of ITI are an undetectable inhibitor titer, restoring the ability to treat bleeds with FVIII concentrates and restoration of normal in vivo FVIII recovery and half-life. A variety of regimens have been used for ITI ( Table 13.16 ), including daily high-dose FVIII (100 units/kg twice daily or 200 units/kg daily regimen) with or without immunomodulatory therapy, daily intermediate-dose FVIII (50–100 units/kg/d), and alternate-day low-dose FVIII (25 IU/kg).

  Table 13.16

  Selected immune tolerance induction regimens for hemophilia associated inhibitors.

  Protocol	FVIII dose (IU/kg)	Other agents	% Success rate (no. of patients in trial)	Inhibitor elimination time (range in months)	Predictors of success ( P value)
  High dose
  Bonn ( ) (Brackman et al.)	100–150 twice daily	aPCC as required	100 (60)	1–2
  Malm ( ) (Berntorp/Nilsson et al.)	Maintain FVIII>0.40 units/mL	Cyclophosphamide; 12- to 15-mg/kg IV daily for 2 days followed by 2–3 mg/kg orally given daily for 8–10 days IV Ig; dose is 0.4 g/kg daily for 5 days immunoadsorption	63 (16)	1–2
  Intermediate-dose
  Kasper/Ewing ( )	50–100 daily	Oral prednisone PRN	79 (12)	1–10
  Low-dose
  Dutch ( ) (Mauser-Bunschoten)	25 alternate days		87 (24)	2–28
  Registry
  International, IITR ( )	>200 daily: 32%	Steroids	51%	10.5 (time to success)	Age at ITI start (.008)
  	100–199 daily: 20%				Pre-ITI inhibitor titer (.04)
  	50–100 daily: 23%				Historical peak titer (.04)
  	<50 daily: 25%				VII dose (higher, .03)
  North American NAITR ( )	>200 daily: 14%	Immunomodulators	63%	16.3 (time to success)	Pre-ITI inhibitor titer (.005)
  	100–199 daily: 33%				Historical peak titer (.04)
  	50–100 daily: 28%				Peak titer on ITI (.0001)
  	<50 daily: 25%				FVII dose (lower, .01)
  German GITR ( )	200–300 daily		76%	15.5 (time to success)	Historical peak titer (.0012)

  The international ITI study demonstrated that a higher dose (200 IU/kg/d) prevented more early bleeding events and achieved faster tolerance than the lower dose, but immune tolerance was eventually achieved in 70% of patients. Until tolerance is successfully attained episodic bleeds require treatment with bypassing agents. A low historical peak inhibitor titer, a low inhibitor titer at initiation of ITI (<10 BU), and a low maximum inhibitor titer during ITI all favor success. Success rates may also be higher in young patients and in patients treated on higher dose regimens. Data from the Italian ITI study indicate that the relationship between FVIII mutations and rate of inhibitor development is likely to correlate with ITI outcome. Large FVIII gene deletions known to be associated with the highest risk of inhibitor development also showed the highest ITI failure rates. Cost and venous access are added obstacles to successfully completing immune tolerance. In the subset of children who fail ITI, anti-CD20-antibody (rituximab) has been used with and without ITI with varying success rates. Clinically significant responses were observed with concurrent ITI in 47% of patients with only 14% patients achieving durable responses.