Hematology

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. De novo (new) mutations account for new cases of hemophilia A and B in pedigrees without a previous history. This occurs frequently, in 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 prevent a bleeding phenotype; however, some carriers may also have mild deficiency of factor VIII or IX and manifest bleeding symptoms such as heavy menstrual bleeding or post-partum or operative 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 cells express the normal allele for factor VIII or IX. This, according to the Lyon hypothesis, may result in unexpectedly low levels of the relevant factor protein 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 moderate or severe hemophilia A or B.

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 hemophilia 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 neutralizing antibodies 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 VIII (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. Factor VIII or IX levels of less than 1% indicate severe hemophilia A or B, respectively. Levels greater than 1% and less than 5%, indicate moderate disease and levels 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 called "primary prophylaxis".

Joint disease

For older children and adults with preexisting joint disease, there is no intervention that will reverse the hemarthropathy; 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 liver sinusoidal cells, vWF is produced in vascular endothelial cells (with large molecular weight forms assembled in organelles called Weibel-Palade bodies) and in platelets. Pathologic variants in the VWF gene typically result in abnormal levels of vWF, plus abnormal vWF function often measured as platelet agglutination in response to the reagent ristocetin. Mild deficiency of factor VIII requires differentiation between mild hemophilia A and type 1 or 2 vWD (von Willebrand disease). The exception is type 2N vWD, which results in a low factor VIII level (and, accordingly a prolonged aPTT), but normal qualitative vWF function by platelet agglutination. This vWD subtype is diagnosed by VWF:FVIII-binding assay or genetic analysis. The low factor VIII level is the result of increased proteolysis of free factor VIII (non-vWF-bound). Moderate or severe deficiency of factor VIII is found 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. A 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, in whom both vWD and mild hemophilia A must be 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 ecchymoses. 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.

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, a smaller dose is necessary to achieve a targeted level of 100%. 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%.

Recently licenced “extended half-life” 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 concentrates have been extended by modifying the native clotting proteins using Fc-receptor or albumin fusion or pegylation. The extension of circulation time has been more robust (and clinically impactful) for factor IX compared to factor VIII.

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, 25 mg/kg, or epsilon-aminocaproic acid, 50 mg/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 to consider, 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 to treat or prevent bleeding. 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 administered 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%. Subsequent early phase trials for both FIX and FVIII gene therapy programs have demonstrated improving success. Results presented at international hemostasis conferences have fueled enthusiasm that this technology may provide long-term factor IX levels of 20-40% and factor VIII levels of 20-240%. 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 consistent, predictable increase in factor FVIII or FIX levels.

What other therapies are helpful for reducing complications?

Because all presently licensed factor concentrate 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. 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.

It is recommended that patients with inhibitors be managed by a comprehensive hemophilia treatment center, if possible. In acute situations, consultation with the hematologists in these centers can be important because of the challenge of safely and effectively using bypassing 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.

A recombinant porcine factor VIII product (Obizur, Shire) was recently licensed for use in acquired hemophilia A and has undergone 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 patient’s 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 inhibitors. 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. Recent publication of pivotal trial results demonstrated marked reduction of annualized bleed rates for hemophilia A patients with inhibitors. Interference of this biologic with standard clinical laboratories renders them completely unreliable for assessing aPTT, factor VIII levels, and inhibitor titers. Potential increased thrombotic risk when used concomitantly with aPCC also emerged during the phase 3 clinical trial. Other subcutaneous, non-factor replacement options to improve hemostasis (thrombin generation) are presently in clinical trials. Fitusiran employs RNA interference to decrease antithrombin levels. Concizumab and other antibodies against Tissue Factor Pathway Inhibitor (TFPI) are also being investigated.

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. 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 "major 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 " major dose" of the patient's prescribed factor and rapid evaluation in an emergency center with computed tomography (CT) imaging capability following the factor infusion is necessary. Any evidence of central nervous system hemorrhage on CT requires hospitalization and implementation of scheduled infusions of the appropriate clotting factor concentrate to maintain trough factor levels in the normal range.

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 was circumcised, did he bleed following the procedure?

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

Does the individual have oral or 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.

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