Hematology

Hemophilia A and B

Hemophilia A and B

What every physician needs to know:

Hemophilia A and B are X-linked inherited bleeding disorders caused by mutations or deletions in genes coding for clotting proteins. Although they share a common bleeding phenotype, the clotting proteins themselves serve different, but essential, functions in the intrinsic pathway of coagulation and thrombin generation. Factor VIII is a large heterotrimeric glycoprotein consisting of a light chain of approximately 80,000 daltons and a heavy chain that is 70,000 to 200,000 daltons. It circulates as a trace protein in plasma, normally circulating at concentrations of approximately 20 nanograms/milliliter with its carrier protein, von Willebrand factor (vWF), which stabilizes it against in vivo inactivation caused by spontaneous dissociation of the A2 domain of the factor VIII heavy chain. Factor VIII serves as a cofactor that enhances the activation of factor X by factor IX (a serine protease of 55,000 daltons). Factor IX circulates at nearly 50 times the concentration of factor VIII, a level similar to other serine proteases in the clotting cascade (factors II, VII and X).

The combined presence of factors VIII and IX in physiological concentrations result in an approximately 50,000 fold increase in the rate of thrombin generation. Because factors VIII and IX form a “tenase” complex to activate factor X, the absence of either results in a nearly infinite activated partial thromboplastin time (normal less than 40 seconds) and a normal prothrombin time (which measures only the extrinsic and common pathways of coagulation). The bleeding phenotypes of these two very different chemical entities (enzyme [IX] versus cofactor [VIII]) are similar when the function of either is missing or reduced.

Hemophilia A, factor VIII deficiency, is more common, accounting for approximately 80-85% of all cases. In both conditions, the bleeding phenotype is typically influenced by the type of mutation. Nonsense mutations, gene deletions, gene insertions or other gene defects resulting in no functional clotting protein being produced (null mutations) manifest as a severe bleeding phenotype. Other mutations, such as missense mutations often cause mild or moderate hemophilia A or B since partially functional factor VIII or IX may be present. Although de novo (new) mutations account for new cases of hemophilia A and B in pedigrees without a previous history, this occurs frequently, up to one third of affected patients, in hemophilia A.

As both hemophilia A and B are X-linked recessive mutations, most affected individuals are male. Females who are heterozygous, typically have approximately 50% of normal physiologic levels of factor VIII or IX, respectively. Generally, factor levels are sufficient to confer a non-bleeding phenotype; however, some “carriers” have factor VIII or IX levels in the mild hemophilia range and manifest bleeding symptoms such as heavy menstrual bleeding or post-partum hemorrhage. As with other X-linked conditions, there is random inactivation of one of the X chromosomes in every female somatic cell. Rarely, a female may have significantly fewer than half of her factor VIII- or IX- producing cells express the normal allele. This, according to the Lyon hypothesis, may result in unexpectedly low levels of the relevant factor and a more severe bleeding phenotype. Other rare genetic events such as Turner syndrome or uniparental disomy can also give rise to a female with a hemophilia A or B phenotype.

Until the early 1990s, the most worrisome complication associated with treatment of hemophilia A or B was infection with viruses such as HIV or hepatitis B and C. Prior to the mid-1980s the clotting factor concentrates were made from large pools of human plasma which did not undergo rigorous pathogen screening or viral attenuation/purification processes due to limited knowledge and technology. Emergence of manufacturing techniques to improve the safety profile of plasma-derived factor concentrates and the development of recombinant factor VIII and IX concentrates eliminated the remaining risks of pathogen contamination and increased the availability of replacement therapy.

Currently, the most challenging complication of therapy is the development of neutralizing alloantibodies (inhibitors) to the infused clotting factor proteins. Inhibitors occur in approximately 25-40% of patients with severe hemophilia A, compared to only 3-4% of patients with severe hemophilia B. Development of a neutralizing antibody negatively impacts the pharmacokinetics of infused clotting factor. Approximately half the time, development of these antibodies are anamnestic, that is, repeat exposure to the factor protein increases the inhibitor titer to levels that quickly and completely neutralize the function of the infused clotting factor. Therapeutic efficacy is lost and alternative treatment strategies must be utilized. Hemophilia B patients with inhibitors are at risk of severe allergic reaction/anaphylaxis or nephrotic syndrome due to creation and precipitation of inhibitor-factor IX protein complexes.

Quantification of the inhibitor titer is achieved using the Bethesda assay. For this assay a factor VIII (or IX) assay is performed following a 2-hour incubation (anti-factor VIII antibodies are time and temperature sensitive). Results are quantified as Bethesda units (BUs), with one Bethesda unit defined as the amount of antibody in one milliliter of patient plasma sufficient to reduce the expected amount of factor VIIII (or IX) in a one to one mix with normal plasma by 50% (e.g., 25% of normal, rather than the expected 50%). A titer greater than 5 BUs is classified as “high titer” and is likely to be anamnestic. Titers less than 5 BUs are designated “low-titer” and may be transient, low responding/non-anamanestic, or represent a long interval since the last factor exposure in a patient with a known high responding inhibitor.

Are you sure your patient has hemophilia A or B? What should you expect to find?

Reasons for suspecting hemophilia A or B

The most obvious reason for suspecting hemophilia is a positive family history, particularly when a female whose father or brother was diagnosed with hemophilia gives birth to a male child.

A second reason is unexpected or unexplained bleeding in a newborn male infant, particularly bleeding following circumcision or bruising, cephalohematoma, intracranial hemorrhage with minimal birth trauma, or large ecchymoses in neonates with a normal platelet count.

A third is a spontaneous or traumatic joint hemorrhage (hemarthrosis) following minimal injury in a male of any age. Patients with hemophilia A or B can bleed into virtually any organ or tissue; however, with the exception of joints and superficial mucous membranes, most bleeding episodes occur following trauma. Distinguishing clinical features of hemophilia are continued bleeding, hematoma expansion, and progression of secondary inflammation beyond what is expected for the injury in a normal individual.

Bleeding

If the bleeding is not immediately life or limb threatening, appropriate laboratory coagulation studies should be ordered. The initial panel should include a prothrombin time (PT), activated partial thromboplastin time (aPTT), and platelet count. If the aPTT is prolonged and the PT and platelet count normal, a factor VIII assay should be run immediately (if family history does not indicate known factor IX or XI deficiency).

If the factor VIII assay (which requires approximately 2 hours to perform) is normal, then a factor IX assay should be done. Levels of factor VIII or IX of less than 1% indicate severe hemophilia A or B, respectively. Levels greater than 1% and less than 5%, indicate moderate disease and level greater than 5% but less than the lower limit of normal are diagnostic of mild disease. Age-specific normal ranges for factor levels should be consulted for patients less than one year of age prior to assigning a diagnosis. Upon laboratory confirmation, if the bleeding continues or is clinically significant, an appropriate clotting replacement product should be administered acutely to the patient (see below).

Common symptoms

Common symptoms that emerge in pediatric hemophilia following the neonatal period include soft tissue bleeding (bruising and oral mucous membrane bleeding) or traumatic or spontaneous bleeding in weight-bearing joints and elbows. In patients with severe hemophilia A, the median age of first hemarthrosis is approximately 17 months. Mild mucous membrane bleeding may respond to oral antifibrinolytic therapy (epsilon-aminocaproic acid or tranexamic acid) alone without need for infusion of clotting factor replacement.

Rarely, severe bleeding such as intracranial hemorrhage related to falls or other trauma, presents a true emergency requiring immediate infusion of clotting factor concentrate to achieve and sustain normal physiologic levels of factor VIII or IX, as appropriate (see dosing considerations, below). Confirmatory imaging of the central nervous system is essential after replacement of the deficient clotting factor.

Hemarthroses

Hemarthroses are the hallmark of severe hemophilia A and B, but may be infrequent in the first 2 to 3 years of life. When clotting factor replacement is only administered to treat recurrent hemarthroses (on-demand therapy), rather than infused pre-emptively to prevent initial or recurrent bleeding (prophylactic therapy), progression of joint damage is inevitable. Blood, with the associated iron, is toxic to the synovium of joints resulting in synovitis. Recurrent or persistent inflammation within a serially bleeding "target joint" predisposes to increased bleeding frequency. This results in synovial proliferation, hypertrophy and neovascularization leading to cartilage destruction and the initiation of bone changes (such as cyst formation) due to osteoblastic and osteoclastic bone remodeling. Early in this pathogenetic process the incipient cartilage changes are only visible on magnetic resonance imaging (MRI), later stages are evident on X-ray. Over decades, this can result in complete loss of motion (arthrodesis) and significant loss of function and chronic pain.

Any expectation of preventing the natural history of this joint destruction requires prophylactic administration of clotting factor concentrate on a continuing and long-term basis. In practical terms, prophylaxis (discussed below) should begin before a second bleed occurs within an individual joint, in order for joint structure and function to be maintained into adulthood. When initiated prior to disease in any joint (primarily knees, ankles, and elbows), this preventative therapy is "primary prophylaxis".

Joint disease

For older children and adults with preexisting joint disease, there is no intervention that will reverse the hemarthrotic changes; however, introduction of a prophylactic factor replacement regimen at this stage (secondary prophylaxis), can slow the rate of joint disease progression. While the data supporting secondary prophylaxis are not as robust as those demonstrating the benefit of primary prophylaxis, this strategy has become standard of care. Even with mild joint changes, the use of secondary prophylaxis will not only minimize progression in a target joint, but it will also protect the other joints with minimal (or no) hemarthrotic changes.

What laboratory studies should you order to help make the diagnosis and how should you interpret the results?

The profile in coagulation screening tests that is consistent with both hemophilia A and B is the following:

  • Normal prothrombin time

  • Abnormal partial thromboplastin time

  • Normal platelet count

  • Normal fibrinogen level or thrombin clotting time

When this profile characterizes a person with unexplained bleeding, a factor VIII assay should be sent. If the results show low levels, hemophilia A is the likely diagnosis in a male patient, although von Willebrand disease must be excluded in the absence of a supportive family history. If the factor VIII assay is normal, a factor IX level should be obtained. An abnormally low level (using age-specific normal ranges) indicates hemophilia B. In emergency circumstances, factor VIII and factor IX assays should be performed concurrently.

What imaging studies (if any) will be helpful in making or excluding the diagnosis of hemophilia A or B?

Imaging studies are not required for making the diagnosis of either hemophilia A or B. They are useful for following the natural history of hemarthropathy in any individual with hemophilia, who has experienced repeated joint bleeding (hemarthroses).

Beware of other conditions that can mimic hemophilia A or B:

There are several inherited conditions and at least one acquired condition that should be considered in the differential diagnosis for hemophilia A and B. In each of these, laboratory screening includes a prolonged aPTT with normal PT, platelet count, and functional fibrinogen (which may be screened as a normal thrombin clotting time). These hemostatic defects include von Willebrand disease, types 1, 2 or 3, rare conditions such as factor XI deficiency, and acquired hemophilia A.

Inherited diseases

Von Willebrand disease is an autosomally inherited bleeding disorder that results from one of many possible mutations in the cell adhesive molecule, von Willebrand factor (vWF). Unlike factor VIII, which is produced primarily in hepatocytes, vWF is produced in vascular endothelial cells (with large molecular weight forms assembled in organelles called Weibel-Palade bodies) and in platelets. The phenotypes from vWF mutations, unlike hemophilia A, typically result in abnormal levels of factor VIII, plus abnormal vWF-mediated platelet agglutination in response to the reagent ristocetin. The exception is type 2N vWD (von Willebrand disease), which results in a low factor VIII level (and, accordingly a prolonged aPTT), but normal qualitative vWF function. The low factor VIII level is the result of increased proteolysis of free factor VIII (non-vWF-bound).

Mildly depressed levels of factor VIII require differentiation between mild hemophilia A and type 1 or 2 vWD. A laboratory test showing normal vWF factor by functional vWF assay indicates hemophilia A (the exception is type 2N vWD, which is detected by VWF:FVIII-binding assay or genetic analysis). Severely depressed factor VIII levels are seen in both moderate/severe hemophilia A and type 3 vWD; however, in the latter, vWF levels are also extremely low (instead of normal as in hemophilia A).

Factor XI deficiency requires differentiation from hemophilia A and B. Factor XI assay should be obtained when both the factor VIII and IX assays are normal in an individual with bleeding or when there is a family history of factor XI deficiency.

Acquired hemophilia A

Acquired hemophilia A manifests as acute unexplained bleeding in an individual with no prior history of abnormal bleeding. The age of onset and pattern of bleeding ensure that it is almost never confused with inherited hemophilia A. The exception may be in women with post-partum hemorrhage, who must have both vWD and mild hemophilia A excluded. Acquired hemophilia A occurs most commonly after the sixth decade of life and affects both genders equally. Bleeding symptoms typically include muco-cutaneous bleeding, deep soft tissues hemorrhage and large ecchymosis; hemarthrosis is uncommon. On laboratory evaluation, the patient's prolonged aPTT does not correct in a one to one ratio mixing study with normal plasma as it does in both vWD and inherited hemophilia A.

Which individuals are most at risk for developing hemophilia A or B?

X-linked inherited hemophilia A and B are both pan-racial and pan-ethnic in their distribution. Approximately 1 in 5,000 live male births world-wide result in a boy with hemophilia A. Hemophilia B is approximately one sixth as common. The estimated number of persons with mild, moderate or severe hemophilia A and B in the world exceeds 400,000.

If you diagnose hemophilia A or B, what therapies should you initiate immediately?

Acute

Severe bleeding, particularly when the bleeding is determined to be life or limb threatening (for example, an acute limb compartment syndrome), requires immediate infusion of factor VIII concentrate for hemophilia A or factor IX concentrate for hemophilia B.

Treatment or “on-demand” dosing of factor VIII or IX concentrates is based on achieving or maintaining adequate hemostatic potential in the bleeding patient. In contrast, preventative or prophylactic factor replacement provides either pre-surgical prophylaxis or prevention of joint bleeding in a patient without joint disease. Effective treatment of either hemophilia A or B at home requires that the caregiver or patient be instructed in home infusion therapy. Instructing patients and/or family members in factor replacement therapy is often best achieved by a specialized hemophilia treatment center nurse and team. This is important because delay in therapy invariably means a worse clinical outcome, regardless of the type or severity of the hemophilic bleed.

Recently licenced factor VIII and IX clotting factor concentrates are costly but reduce the burden of infusion frequency needed for ongoing prophylaxis. The pharmacokinetics and pharmacodynamics of these “extended half-life” factor VIII and IX clotting concentrates have been improved by modifying the native clotting proteins using Fc-receptor or albumin fusion or pegylation. The extension of circulation time has been more robust (and clinical impactful) for factor IX compared to factor VIII.

For major or life-threatening bleeding, such as a central nervous system hemorrhage, initial replacement therapy with clotting factor concentrate should be dosed to achieve a plasma level of at least 100% of the normal physiologic level. This physiologic level is defined as 1 unit of Factor VIII or IX per milliliter of plasma. The plasma volume represents approximately half of the total vascular volume (the other half being red blood cells and other cells). A dose of factor VIII remains exclusively in the vascular space because of the large size of the factor VIII molecule, thus infusing 50 units of factor VIII concentrate per kilogram of body weight should raise the circulating factor VIII level to approximately 100%, assuming the starting factor VIII level is less than 1%. If the patient has a higher baseline clotting factor level, as occurs in mild hemophilia, the resulting dose necessary to achieve a targeted level of 100% is less. Stated another way, 1 unit per kg of infused factor VIII concentrate will raise the plasma factor VIII level by approximately 2%. By contrast, factor IX is a comparatively small molecule that does not remain exclusively in the plasma. As a result, 50 units of factor IX concentrate per kg of body weight typically raises the plasma level to "only" 50% in a patient with severe hemophilia B (less than 1%). Therefore, the resulting dosing scheme for hemophilia B states that 1 unit/kg of infused factor IX concentrate, will raise the factor IX level by approximately 1%.

Not all types of bleeding require replacement of the deficient clotting factor to 100% of normal. Doses of 20 to 35 units per kg body weight, of factor VIII or IX concentrate are sufficient to treat an acute hemarthrosis. Treatment of epistaxis or oral bleeding may require 35 to 40 units of the deficient clotting factor per kg of body weight. In the latter instance, concurrent administration of oral antifibrinolytic therapy (tranexamic acid, 25mg/kg, or epsilon-aminocaproic acid, 100mg/kg every 6 to 8 hours may obviate the need for follow-up factor infusion.

In addition to the therapeutic peak level of an infused clotting factor, the trough or nadir level is important, particularly in controlling severe bleeding or surgical prophylaxis. Although a single infusion may be sufficient to treat some hemarthroses or mild muco-cutaneous bleeding episodes, maintenance of sustained physiologic levels is required when treating intracranial, gastrointestinal, retroperitoneal, spinal, and ophthalmic hemorrhages, as well as bleeding in or near the airway. In these circumstances, repeated dosing of the clotting factor concentrate to maintain near-physiologic trough levels based on individual pharmacokinetics is essential. This may require repeated factor VIII or IX levels in order to determine if desired circulating levels are being achieved. The references provide detailed methods for managing these challenging bleeding problems.

If a factor VIII (or, uncommonly, a factor IX) inhibitor is present, alternative therapy such as a by-passing agent, for example, recombinant factor VIIa (rFVIIa) or activated prothrombin complex concentrate (aPCC, containing some activated factors II, VII, IX, or X) may be required. In the setting of an inhibitor, consultation with experts at a comprehensive hemophilia treatment center is essential; appropriate dosing requires clinical expertise in treating inhibitors.

More definitive therapies?

For mild dental bleeding or epistaxis, either of the oral antifibrinolytic agents (epsilon-aminocaproic acid or tranexamic acid) alone may suffice in achieving and maintaining hemostasis. These agents are especially likely to be effective in patients with mild hemophilia A or B.

For patients with mild hemophilia A, desmopressin or DDAVP (1-desamino-8-D-arginine vasopressin) administered intravenously, subcutaneously or intranasally offers a less expensive and more easily administrated option when the bleeding is mild. DDAVP induces release of (factor VIII-binding) vWF from Weibel-Palade bodies and endothelial cells. A single dose of DDAVP (0.3 micrograms/kg if IV or subcutaneous, or 150-300 micrograms of intranasal) results in a three-fold or greater increase in the baseline factor VIII level. A mild hemophilia A patient with a baseline factor VIII level of 15% should have a plasma factor VIII level increase to approximately 50%. This is usually sufficient for the effective treatment of mild hemorrhage, such as hematoma, epistaxis, or dental prophylaxis (especially when combined with antifibrinolytic therapy).

There is not yet a cure for hemophilia; however, advances in gene therapy technology have hinted at this possibility. The first publication of successful, sustained factor IX protein expression was published by Nathwani et al. in 2011 with follow-up demonstrating stability of factor IX levels in the mild-moderate hemophilia range, 3-7%. Several factor IX gene therapy programs are currently recruiting patients, and the first factor VIII gene therapy program has also begun recruitment. Several challenges remain including: pre-existing antibodies to the adeno-associated viral vector being used in hemophilia gene therapy, development of a T-cell mediated immune response resulting in decreasing factor protein expression (if present), and reducing the cDNA size, particularly for factor VIII, to meet the capacity of the AAV vector.

What other therapies are helpful for reducing complications?

Because all presently licensed products have been free of known pathogens such as hepatitis or HIV for over two decades, there is no reason to fear these therapeutic complications in the 21st century. Inhibitor risks run in families, but concordance among siblings is variable. While a number of risk factors including gene mutation, factor product type used, intensity of early factor product exposure, and immunomodulatory phenotype variations have been identified, the relative risk increase attributable to these elements is unknown and no risk stratification schema is currently employed routinely.

Managing the hemostatic therapy of patients with inhibitors, particularly those with anamnestic or high responding antibodies, is difficult. For the (approximately) 50% of patients with hemophilia A and low responding inhibitors who do not anamnesce, a higher dose of factor VIII may saturate the inhibitor sufficiently to achieve a measurable, hemostatically effective factor VIII level. This must be verified acutely with a factor VIII assay performed following the infusion of the "estimated" dose. Methodology for targeting this estimate is imprecise.

The treatment of patients with high responding inhibitors is a special challenge. In contrast to low titer inhibitors, high responding or anamnestic inhibitors to factor VIII (or rarely factor IX) cannot easily be treated with concentrate of the clotting factor that is the antibody target. It is recommended that these patients be in a comprehensive hemophilia treatment center, if possible. In acute situations, consultation with the hematologists in these centers can be important because of the narrow therapeutic window that exists in the use of these agents. Recent data suggest that patients with inhibitors do not bleed more frequently than severe hemophilia patients without an inhibitor (who are not on prophylaxis); however, bleeding is less well controlled with bypassing agents than are comparable bleeding episodes in non-inhibitor hemophilia patients treated with factor replacement concentrates. Patients with inhibitors are at increased risk of chronic joint morbidity, pain, reduced quality of life and early mortality.

At present there are two bypassing agents licensed to treat individuals with high responding factor VIII or IX inhibitors: recombinant factor VIIa (rFVIIa) and activated prothrombin complex concentrate (aPCC). These agents boost thrombin generation through the extrinsic or common coagulation pathways. Recombinant factor VIIa provides supraphysiologic quantities of activated factor VII that converts significant amounts of factor X to factor Xa, and initiates the common pathway of coagulation (that is, factor Xa-Va complex formation, prothrombin [factor II] activation to thrombin, and fibrinogen conversion to fibrin). The aPCC provides additional factors II, VII, IX, and X to achieve a similar end.

Using dosing recommended for either rFVIIa or aPCC, the resultant thrombin generation rarely equals that observed in a non-inhibitor patient who is successfully treated with sufficient factor replacement, to raise the factor level to normal. Nonetheless, with the limited options presented by a bleeding patient with a high-responding inhibitor, either of these by-passing agents has proven useful in achieving and maintaining hemostasis.

A recombinant porcine factor VIII product (Obizur, Shire) was recently licensed for use in acquired hemophilia A and has undergone only limited (but encouraging) testing in congenital hemophilia A patients with inhibitors. A previously marketed plasma-derived porcine factor VIII product (Hyate-C, Ipsen) was used for bleed management in this population; however, side effects including thrombocytopenia and hypersensitivity reactions and ultimately concerns for viral contamination led to its discontinuation.

Eradication of inhibitors should be attempted using immune tolerance induction (ITI). This entails long and costly administration of factor VIII (or factor IX) infused frequently (typically daily), until the host immune system ceases making the allo-antibody. The process can take months to years and in some cases requires the addition of immunosuppressive agents. The greatest likelihood for success has been observed in patients whose inhibitor titer decreases to less than 10 BU prior to the start of ITI and whose peak inhibitor titer has not exceeded 200 BU. The success rate for patients whose parameters conform to these ranges is approximately 65 to 70% for complete loss of neutralizing antibody. In a small subset of these individuals, however, recrudescence of the inhibitor occurs. Therapeutic strategies for inducing immune tolerance can be found in the Hay, DiMichele reference listed below. The use of prophylactic bypassing agents is becoming increasingly routine in clinical practice, particularly for patients early in their ITI course.

Novel therapies are emerging that may provide alternate options for hemostasis in all hemophilia A or hemophilia B patients, but are particularly exciting for those with neutralizing alloantibodies. Emicizumab (Roche/Chugai) is a humanized bispecific monoclonal antibody (ACE910) engineered to mimic the cofactor function of factor VIII, binding factors IXa and X on a phospholipid membrane. This therapeutic is unaffected by factor VIII inhibitors. Other novel approaches to improve hemostasis (thrombin generation) that are presently in clinical trials target downregulation of anti-coagulant proteins such as antibodies against Tissue Factor Pathway Inhibitor (TFPI) or RNA interference knockdown of antithrombin.

What should you tell the patient and the family about prognosis?

If there is a family history of hemophilia, the child's mutation will, almost without exception, be the same as the affected individual in the same pedigree. This typically means that the bleeding phenotype will be similar, as well. Therapeutic options (for example, primary prophylaxis) can be chosen to attenuate the phenotype.

For young children with severe hemophilia A or B who have pristine joints, significant joint morbidity can be pre-empted by institution of a primary prophylaxis regimen.

Optimal quality of life for individuals is maximized when they receive coordinated care in a comprehensive hemophilia treatment center.

Children born today with hemophilia can expect a normal lifespan, and can participate in all but the most physically violent of activities and sports.

Participation in hemophilia patient organizations can provide important support for families with hemophilia, and can also create opportunities for personal growth and public service to the community.

What if scenarios.

In the event of severe trauma, an individual or a family member skilled in home therapy/self-infusion should try to administer a prescribed "high dose" of factor concentrate (that is, 50 units/kg of factor VIII concentrate for hemophilia, and 70 to 100 units/kg of factor IX concentrate for hemophilia B) concurrent with other emergency efforts, such as calling for emergency responders via 911. The only emergency procedure that should take precedence over infusion of factor concentrate is cardiopulmonary resuscitation (CPR).

Any head injury can become life-threatening in an individual with hemophilia, especially those with severe phenotypes. If such an injury occurs (even trauma that might be considered inconsequential in people without a bleeding disorder), prompt infusion of a "high dose" of the patient's prescribed factor and evaluation quickly in an emergency center with computed tomography (CT) imaging capability. Any evidence of central nervous system hemorrhage on CT requires hospitalization and implementation of scheduled infusions of the appropriate clotting factor concentrate.

Consultation with a hematologist with expertise in hemophilia management is required before a patient with hemophilia undergoes surgery, invasive diagnostic testing, or invasive dental procedures.

Bleeding into the retropharyngeal space can occlude the airway. Extraction of impacted mandibular third molars requires careful planning in an individual with severe hemophilia.

Joint replacement in hemophilia carries a far greater risk for complications than in other orthopedic patients, even with adequate peri-procedure administration of clotting. An established team within a hemophilia treatment center can usually provide ongoing hemostasis recommendations for a safe, successful arthroplasty.

Pathophysiology

Factor VIII and the homologous glycoprotein factor V, in their active forms, are amplifying co-factors of the intrinsic pathway of coagulation. Both factors VIII and V enhance the rate of serine protease activation (factor X activation by factor IXa for factor VIIIa; prothrombin activation to thrombin by factor Xa for factor V). Factor VIII and V activation is promoted by trace amounts of thrombin generated by the extrinsic pathway, activation of the factor VIIa/tissue factor/factor Xa complex. Factor VIII is also activated through the intrinsic pathway by factor XI.

Also indicative of the functional homology between factors V and VIII is the fact that both are inactivated by the vitamin K-dependent protein, activated protein C (APC). The entire coagulation cascade of serial serine protease activation occurs most efficiently on the surface of activated platelets, helping to ensure that thrombin generation is localized at the site of vascular injury. The presence of activated factor V or factor VIII on activated platelet phosphatidylserine surfaces increases downstream serine protease activation. Prior to activation factor VIII circulates in trace (nanomolar) concentrations. Upon activation, it attains a 50,000-fold increase in capacity to participate in thrombin generation. When combined with factor V activation, enough thrombin is generated to cleave two of the three chains of circulating fibrinogen resulting in a fibrin clot at sites where endothelial injury occurs.

The three-dimensional structure of factor VIII is organized into six functional domains: A1, A2, A3, B, C1, and C2. The A1, A2, and B domains constitute the heavy chain, and the remaining sequence (A3, C1, and C2) comprise the light chain. The glycoprotein is unstable in the circulation unless it is associated with its multimeric carrier protein, vWF. The heavily glycosylated B domain is not required for activation of factor VIII and is cleaved away during activation of factor VIII by either thrombin or factor XI. The B domain is important for factor VIII transport from the endoplasmic reticulum (ER) to the Golgi in hepatocytes and other cells where factor VIII is produced. Because of its large size and complex structure, folding of the molecule in the ER requires protein chaperones.

The molecular function of factor VIII also is dependent on strategic tyrosine sulfation and serine phosphorylation, in addition to the essential glycosylation. Upon activation, factor VIII separates from its multimeric carrier protein, vWF, jettisons the B domain, and forms a heavy chain of loosely attached A1 and A2 domains, and a light chain consisting of A3, C1, and C2 domains. Dissociation of the A2 from the complex, partially accounts for the short half-life of activated factor VIII (minutes). In contrast, unactivated factor VIII bound to vWF, has a half-life in circulation from 8 to 12 hours.

Factor VIII is encoded for by a large gene (186 kilobases) containing 26 exons. The gene is unusual in that there are two homologous coding sequences for part of the factor VIII protein sequence in the intron separating exons 22 and 23. During the meiosis occurring in spermatogenesis, these homologues (sometimes referred to as pseudogenes) can undergo homologous recombination with the expressed exon sequence. One of these homologues is transcribed in the opposite direction to the factor VIII gene. When there is a recombination with this homologue, the reading of the factor VIII gene is terminated at the origination of the inverted sequence, resulting in transcription of a truncated messenger RNA (mRNA) and no translation of functional factor VIII protein.

This gene inversion accounts for approximately 50% of severe hemophilia A cases. The other 50% of cases include large deletions, insertions, nonsense mutations, splicing errors, and missense mutations. Not surprisingly, the missense mutations usually produce a moderate or mild phenotype, because a full length protein is translated with varying degrees of dysfunction. The greater the loss of protein expression due to a mutation (e.g., large deletion of factor VIII gene) the more susceptible a hemophilic individual with that mutation is, to the development of a high responding factor VIII inhibitor.

Hemophilia B results from mutations in the factor IX gene. Thirty-three kilobases in length, the gene codes for a protein of 55,000 daltons composed of a propeptide, activation or signal peptide, catalytic domain, two epithelial growth factor domains and a gamma-carboxyglutamic (GLA) domain. The latter domain is post-translationally modified (as are all vitamin K-dependent proteins), to include a sequence of approximately 12 gamma carboxyglutamic residues, essential for the binding of the protein to the phosphatidylserine surfaces of activated platelets. Unlike hemophilia A, most of the mutations giving rise to hemophilia B, are single nucleotide substitutions (64%). Accordingly, nonsense mutations account for the largest percentage of severe phenotypes, by inducing chain termination or aberrant splicing. Similar to hemophilia A, most factor IX mutations occur during spermatogenesis.

Factor IX, like all vitamin K-dependent clotting factors (factors II, VII, and X) and anticoagulants (protein C and S), is synthesized in hepatocytes. Cysteine residues in factor IX that crosslink as disulfide loops are critical for proper three-dimensional structure of the protein and functional activity. The functional domains are essential for factor IX phospholipid binding (GLA), cleavage of factor X (catalytic), factor VIIIa binding, and factor XIa binding. As with factor VIII, mutations such as large deletions that result in no protein production, predispose to inhibitor formation. Inhibitors occur less frequently in hemophilia B, perhaps because these null mutations are present in a smaller subset of the hemophilia B population.

The extraordinary structural differences between the enzyme factor IX and the co-factor VIII likely also account for the differential immunogenicity of the respective mutant proteins, as well. One notable phenotype that can be associated with inhibitor development in hemophilia B patients (especially those with large factor IX deletions), is an anaphylactic syndrome occurring concomitantly with the initial onset of factor IX antibody neutralization. For this reason, infants known to have this type of deletion, require careful monitoring during their first few infusions with factor IX concentrates.

What other clinical manifestations may help me to diagnose hemophilia A or B?

Has the individual or any family member had a hemarthrosis? If so, was it in response to mild trauma?

If the individual is circumcised, did he bleed following the procedure?

Has the individual had unexpected or excessive bleeding following a dental procedure?

Does the individual have oral, nasal bleeding over his lifetime, sometimes exacerbated by aspirin or nonsteroidal anti-inflammatory drugs (NSAIDs)? This may unmask a mild hemophilia phenotype.

Soft tissue bruising (purpura) over trunk or extremities in an otherwise healthy person (particularly male), without accompanying systemic signs such as fever should prompt consideration of a diagnosis of hemophilia. In contrast, a well individual presenting exclusively with petechiae is much more likely to have a platelet defect.

Presence of a hemarthrosis on arthrocentesis. If blood is suspected before the procedure, prudence dictates a screening PT and aPTT be performed before the procedure. If only the aPTT is prolonged, a hemophilia work-up should proceed and, if confirmatory, may obviate the need for the procedure.

Active oral bleeding or epistaxis, in the absence of obvious trauma or minimal trauma, particularly when it has persisted for more than 10 minutes.

The presence of a splenic or retroperitoneal hematoma on ultrasound following modest trauma.

Caution: If the individual presents with compartment syndrome or unexplained intracranial hematoma or hemorrhage, it is essential to rule out hemophilia A and B before surgical intervention.

What other additional laboratory studies may be ordered?

N/A

What’s the evidence?

Berntorp, E, Astermark, J, Baghaei, F. "Treatment of haemophilia A and B and von Willebrand's disease: summary and conclusions of a systematic review as part of a Swedish health-technology assessment". Haemophilia. vol. 18. 2012. pp. 158-165.

(This article provides recommendations that have evolved over the last two decades, with regards to treatment of hemophilia A and B, with and without inhibitors and correlates these with the level of evidence based on scientific research studies. As with many rare diseases, the hemophilias suffer from the lack of large scale, randomized clinical trial data to support the selection of drug specific choices or treatment strategies. However, as this article discusses, prospective data from national and international registries may be the most straight-forward methodology for assessing therapeutic and diagnostic efficacy when randomized clinical trials (RCTs) are not feasible.)

Mahdi, AJ, Obaji, SG, Collins, PW.. "Role of enhanced half-life factor VIII and IX in the treatment of haemophilia". British journal of haematology. vol. 169. 2015. pp. 768-776.

(In this review Mahdi and colleagues succinctly summarize the novel technologies applied to recombinant factor VIII and IX to extend the circulating half-life. Additionally, a brief overview of recently approved or anticipated products is provided including their potential benefits in clinical practice and the challenges of incorporating these products into routine use.)

Sidonio, RF, Mili, FD, Li, T. "Females with FVIII and FIX deficiency have reduced joint range of motion". American journal of hematology. vol. 89. 2014. pp. 831-836.

(This cross-sectional study examined joint range of motion in girls and women with FVIII or FIX deficiency, evaluated as part of Hemophilia Treatment Center Universal Data Collection (UDC), and compared it to historical controls. It demonstrated that even females with mild deficiency had a decreased range of motion compared to healthy controls prompting speculation that subclinical bleeding even with a mild phenotype could lead to a negative impact on joint health and that there is a potential for a greater impact of “hemophilia carrier” status on health outcomes than just a risk of affected progeny.)

Kemball-Cook, G, Gomez, K., Lee, C, Berntorp, E, Hoots, K. "Molecular basis of hemophilia A". Textbook of Haemophilia. Wiley- Blackwell Publishing. 2014. pp. 23-32.

Gomez, K., Chowdary, P., Lee, C, Berntorp, E, Hoots, K. "Hemophilia B-molecular basis". Textbook of Haemophilia. Wiley-Blackwell Publishing. 2014. pp. 97-102.

Callaghan, MU, Kaufmann, RJ., Lee, C, Berntorp, E, Hoots, K. "Cellular processing of Factor VIII and Factor IX". Textbook of Haemophilia. Wiley-Blackwell Publishing. 2014. pp. 9-20.

(These chapters provides a succinct, yet quite comprehensive discussion of how factors VIII and IX are expressed genetically, how the protein is assembled within cells and secreted into the extracellular/vascular system, and the manner in which they interact on a molecular basis to enhance thrombin generation, and thereby clotting. In addition, it provides molecular insights to the protein structures in a manner that elucidates genotypic-phenotypic relationships.)

Manco-Johnson, MJ, Abshire, TC, Shapiro, AD. "Prophylaxis versus episodic treatment to prevent joint disease in boys with severe hemophilia". N Engl J Med.. vol. 357. 2007. pp. 535-44.

(This is the seminal randomized clinical trial that definitively demonstrates the efficacy of primary prophylaxis in alleviating and, in some cases, eliminating hemarthropathy as compared to on-demand replacement therapy. Results are corroborated by joint MRI data from elbows, knees, and ankles performed at study entry (mean age 27 months) and repeated at age 6 years.)

Peyvandi, F, Oldenburg, J, Friedman, KD.. "A critical appraisal of one-stage and chromogenic assays of factor VIII activity". Journal of thrombosis and haemostasis: JTH. vol. 14. 2016. pp. 248-261.

(This article highlights the assay methodologies including the advantages and limitations of one-stage and chromogenic assays for determining factor VIII activity levels. The causative mutation (for mild and moderate hemophilia) and factor product characteristics such as product purity and protein modifications for circulating half-life can impact technical aspects of assay performance. While one-stage assays have been used as the standard clinical assay for decades, these authors discuss why more widespread integration of chromogenic assays may be necessary as patients switch to the newest factor products and to support accurate diagnosis of patients with mild factor deficiencies.)

Shima, M, Hanabusa, H, Taki, M. "Factor VIII-Mimetic Function of Humanized Bispecific Antibody in Hemophilia A". The New England journal of medicine. vol. 374. 2016. pp. 2044-2053.

(Alternatives to factor VIII replacement for the management of hemophilia, such as Emicizumab (ACE910) presented in this article have gained enthusiasm in part due to the ability of subcutaneous and less frequent administration. In this presentation of Phase I Emicizumab data, authors highlight the safety profile of ACE910 in their dose escalation study as well as the marked reduction in annualized bleeding rates for both hemophilia A inhibitor and non-inhibitor patients.)

Sehgal, A, Barros, S, Ivanciu, L. "An RNAi therapeutic targeting antithrombin to rebalance the coagulation system and promote hemostasis in hemophilia". Nature medicine. vol. 21. 2015. pp. 492-497.

(This article presents murine data supporting the novel approach of antithrombin knockdown by RNA interference technology to restore hemostasis in hemophilia. Dose dependent reduction of antithrombin is demonstrated with concurrent increase of thrombin generation and decrease of bleeding symptoms.)

Astermark, J.. "FVIII inhibitors: pathogenesis and avoidance". Blood. vol. 125. 2015. pp. 2045-2051.

(In this article Astermark reviews the current understanding of how the human immune system processes factor VIII and how polymorphisms in immune response genes may provide a key to understanding mechanisms of inhibitor development. Risk factors for inhibitor development are also summarized.)

Peyvandi, F, Mannucci, PM, Garagiola, I. "A Randomized Trial of Factor VIII and Neutralizing Antibodies in Hemophilia A". The New England journal of medicine. vol. 374. 2016. pp. 2054-2064.

(The randomized, controlled trial investigating the immunogenic potential of plasma-derived versus recombinant factor products in previously untreated hemophilia patients is presented. Early generation recombinant factor VIII products demonstrated an 87% higher incidence of all inhibitors and 69% higher incidence of high-titer inhibitors compared to plasma-derived products. How this risk of recombinant products extends to newer generation factor VIII products and extended half-life products has not yet been explored.)

Hay, Cr, DiMichele, DM. "The principal results of the International Immune Tolerance Study: a randomized dose comparison". Blood. vol. 119. 2012. pp. 1335-1344.

(This article presents data from the only powered, multicenter, international, randomized, clinical trial to examine strategies for immune tolerance induction in individuals with high responding factor VIII inhibitors. Inhibitor patients randomized to either a high dose daily regimen (200 units per kg per day of factor VIII) or a thrice weekly dose of 50 units per kg. Efficacy was comparable between arms, but bleeding toxicity was much greater in the low dose arm and resulted in premature closure of the trial by the Data Safety Monitoring Board.)

Di Michele, DM.. "Immune tolerance induction in haemophilia: evidence and the way forward". J Thromb Haemost. vol. 9. 2011. pp. 216-225.

(This article reviews immune tolerance therapy for hemophilic inhibitors. It weighs the use of this therapeutic strategy within the context of the challenges of managing inhibitor-associated bleeding in patients for whom this option is not feasible or not chosen. It also provides a review of the consensus of how and in whom immune tolerance induction provides a viable intervention based on outcome predictors.)

Ingerslev, J, Sorensen, B.. "Parallel use of by-passing agents in haemophilia with inhibitors: a critical review". Br J Haematol.. vol. 155. 2011. pp. 252-62.

(This article examines the simultaneous use of more than one by-passing hemostatic agent, to treat bleeding in patients with congenital and acquired hemophilia. The results support the position that such use confers a high risk of thromboembolic morbidity and even mortality. It provides a cautionary tale about the challenges of treating hemophilic inhibitor patients with persistent bleeding, and supports the argument that such individuals be managed by experienced hemophilia providers.)

Leissinger, CA, Singleton, T, Kruse-Jarres, R.. "How I use bypassing therapy for prophylaxis in patients with hemophilia A and inhibitors". Blood. vol. 126. 2015. pp. 153-159.

(In this article Leissinger and colleagues present the challenge of hemostatic management of patients with inhibitors. Using case-based scenarios they provide guidance of treatment strategies and discuss the potential benefit of bypassing agent prophylaxis in this population.)

Kruse-Jarres, R, St-Louis, J, Greist, A. "Efficacy and safety of OBI-1, an antihaemophilic factor VIII (recombinant), porcine sequence, in subjects with acquired haemophilia A". Haemophilia. vol. 21. 2015. pp. 162-170.

(This article presents the pivotal trial data for recombinant porcine factor VIII in adult patients with acquired hemophilia A, highlighting the impact of cross-reacting human factor VIII antibodies on porcine factor VIII pharmacokinetics, dosing strategies, and efficacy of this novel therapeutic. Although the natural history and response to therapy differs between acquired and congenital hemophilia A, consideration for how this prospective study could impact management of congenital hemophilia A should inform additional investigations.)

High, KA, Anguela, XM.. "Adeno-associated viral vectors for the treatment of hemophilia". Human molecular genetics. vol. 25. 2016. pp. R36-41.

(The clinical development of AAV-gene therapy for hemophilia is reviewed in this article. Early success with (low-level) factor IX protein expression and challenges to expansion of this therapeutic platform including a human-specific AAV immune response and special considerations for factor VIII are discussed.)

Arruda, VR, Samelson-Jones, BJ.. "Obstacles and future of gene therapy for hemophilia". Expert opinion on orphan drugs. vol. 3. 2015. pp. 997-1010.

(This broad review highlights not only a review of AAV gene therapy technology for hemophilia but examines in more depth barriers to success (pre-existing neutralizing antibodies, cellular immune response), implementation of gene therapy in the pediatric population, and safety considerations. Consideration of other approaches to gene therapy using integrating vectors (e.g., lentivirus) and the potential application to special sub-populations such as inhibitor patients are presented.)

Rodriguez-Merchan, EC, Jimenez-Yuste, V, Aznar, JA. "Joint protection in haemophilia". Haemophilia. vol. 17. 2011. pp. 1-23.

(This article summarizes the current thinking about the diagnosis, treatment and prevention of hemarthroses in patients with hemophilia. It describes the pathogenesis of synovitis and the use of diagnostic imaging to ascertain presence and extent of joint injury from recurrent intra-articular bleeding. It also examines both primary and secondary prophylactic replacement therapy for patients with and without inhibitors.)

Haxaire, C, Blobel, CP.. "With blood in the joint-what happens next? Could activation of a pro-inflammatory signalling axis leading to iRhom2/TNFα-convertase-dependent release of TNFα contribute to haemophilic arthropathy?". Haemophilia. vol. 20. 2014. pp. 11-4.

(This article summarizes important new murine haemophilia model analyses of the impact of blood in the joint. It verifies inflammatory pathway activation that is identical to that seen in both animal and human models of rheumatoid arthritis. The authors then extrapolate these biochemical observations about intra-articular activation of TNFα into a hypothesis of hemarthropathy in patients with hemophilia.)
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