Hemophilia A and B

• Genetic mutation and protein deficiency: Hemophilia A results from mutations in the F8 gene (chromosome Xq28) encoding coagulation factor VIII, while hemophilia B results from mutations in the F9 gene (chromosome Xq27) encoding coagulation factor IX (Christmas factor). These mutations can include large deletions, inversions, point mutations, or insertions that may completely ablate protein expression (null mutations causing severe disease) or produce dysfunctional or partially functional proteins (missense mutations causing mild-to-moderate disease). The X-linked recessive inheritance pattern means hemizygous males express the disease phenotype, while heterozygous females are typically asymptomatic carriers; however, females with skewed X-inactivation (lionization) favoring the mutant allele can manifest clinically.

• Disruption of intrinsic tenase complex formation: Factor VIII and Factor IX function as critical components of the intrinsic tenase complex (also called the Factor VIIIa-IXa complex), which forms on the phospholipid membrane surface of activated platelets. In the normal coagulation cascade, tissue factor pathway inhibitor-regulated Factor VIIa (from the extrinsic pathway) initially activates Factor X to Factor Xa; however, the intrinsic pathway provides the bulk of Factor Xa generation through the tenase complex. Factor VIII serves as a non-enzymatic cofactor (serving as a scaffolding protein to enhance catalytic efficiency), while Factor IX is a serine protease that catalyzes Factor X activation. The absence or dysfunction of either component prevents efficient Factor Xa generation, severely diminishing thrombin burst kinetics—the rapid phase of thrombin generation critical for clot stabilization and fibrin cross-linking.

• Impaired thrombin generation and fibrin formation: The coagulation cascade converges toward prothrombin (Factor II) activation to thrombin (Factor IIa), which is catalyzed by the prothrombinase complex (Factor Xa-Factor Va on the membrane). Defective intrinsic tenase function results in reduced Factor Xa generation, limiting prothrombin activation and ultimately reducing thrombin concentration. This leads to insufficient fibrin polymerization, defective fibrin cross-linking by Factor XIII, and inadequate clot stabilization. Consequently, clots that do form are structurally weak and rapidly degraded by fibrinolysis, resulting in early re-bleeding after initial clot formation—the characteristic pattern seen in hemophilia.

• Secondary fibrinolysis and clot remodeling impairment: Beyond the intrinsic pathway deficit, hemophilia patients demonstrate impaired activation of thrombin-activatable fibrinolysis inhibitor (TAFI), an antifibrinolytic enzyme that requires thrombin for activation. The low thrombin levels in hemophilia fail to adequately activate TAFI, leaving clots more susceptible to plasmin-mediated fibrinolysis. Additionally, Factor XIII activation (which cross-links fibrin for mechanical stability) is thrombin-dependent and is therefore insufficient in hemophilia. These combined mechanisms explain the delayed bleeding pattern characteristic of hemophilia—clots initially form but are structurally defective and subject to early dissolution.

• Severity correlation with residual factor activity: Hemophilia severity directly correlates with residual coagulation factor activity: severe hemophilia (<1% factor activity) causes spontaneous hemorrhages and life-threatening bleeding from minor trauma; moderate hemophilia (1-5% activity) presents with bleeding with minor trauma and occasionally spontaneous episodes; mild hemophilia (5-40% activity) typically manifests only with significant trauma or surgery. This graded relationship reflects the non-linear relationship between factor concentration and thrombin generation—below a critical threshold (~5-10% activity), the time to reach a hemostatic level of thrombin becomes prohibitively prolonged, causing spontaneous bleeding.

• Hemophilia A genetic mutations: The F8 gene contains 26 exons spanning 186 kilobases and encodes a 2,351 amino acid protein; mutations account for virtually 100% of hemophilia A cases. Common mutations include intron 22 and intron 1 inversions (accounting for ~45% of severe cases), which disrupt the F8 gene structure and result in severe deficiency. Specific point mutations, deletions, and insertions comprise the remaining 55%, with missense mutations typically producing mild-to-moderate disease while nonsense mutations usually cause severe disease. The large F8 gene size creates a high spontaneous mutation rate, explaining why approximately 30% of severe hemophilia A cases arise from de novo mutations in patients without family history.

• Hemophilia B genetic mutations: The F9 gene spans 33.5 kilobases and contains 8 exons; mutations are generally more heterogeneous than F8 mutations. Point mutations comprise the majority of F9 defects, with distinct mutation hotspots identified. Protein C promoter mutations represent a specific category that affects both Factor IX and Protein C production. Deletions and insertions are less common than in hemophilia A. De novo mutations account for approximately 20% of severe hemophilia B cases, reflecting the gene's relatively smaller size compared to F8.

• X-linked inheritance patterns and female manifestations: Hemophilia A and B follow strict X-linked recessive inheritance, with affected males (XᵃY) transmitting the mutation to all daughters (obligate carriers) but no sons. Carrier females (XᵃX) are typically asymptomatic due to X-inactivation of the mutant allele in most cells; however, unfavorable X-inactivation (skewed lionization) can result in symptomatic heterozygous females with factor levels in the range of 30-70%. Homozygous females (XᵃXᵃ) are rare and result from affected father and carrier/affected mother; such females present with severe hemophilia. Additionally, females with Turner syndrome (45,X) who carry an F8 or F9 mutation express the hemophilia phenotype due to hemizygosity.

• Inhibitor development as acquired complication: While not an etiologic factor, Factor VIII or IX inhibitor formation develops in 25-30% of hemophilia A patients and 1-3% of hemophilia B patients receiving factor replacement therapy. Inhibitors are IgG alloantibodies directed against the exogenous therapeutic factor, arising through T cell-dependent B cell activation. High-responder inhibitors reach titers >5 Bethesda units (BU), while low-responder inhibitors remain <5 BU. This acquired defect transforms treatment from straightforward factor replacement to complex bypass therapy strategies.

• Spontaneous and post-traumatic hemarthroses (joint hemorrhages): The most characteristic clinical manifestation of hemophilia, particularly in severe disease, is recurrent hemarthroses—acute bleeding into joint synovial spaces that typically occurs without trauma or following minor trauma. Hemarthroses present with acute joint pain, swelling, erythema, and warmth, often accompanied by inability to bear weight (if lower extremity) or use the extremity. The knee, elbow, and ankle are most frequently affected due to their high mobility and shear stress. Pathophysiologically, the synovial membrane contains abundant tissue factor and other procoagulants that are overwhelmed by the hemophilic defect; activated platelets and inflammatory mediators within the hematoma trigger a cascade of synovitis, progressive cartilage damage, and fibrin deposition. Recurrent hemarthroses lead to hemophilic arthropathy—irreversible joint destruction with chronic pain, contractures, and functional impairment—particularly when prophylactic factor replacement is not provided. In severe hemophilia, spontaneous hemarthroses can occur monthly or more frequently, severely impacting quality of life.

• Spontaneous intramuscular and soft tissue hemorrhages: Hemophilia frequently manifests with intramuscular hematomas following minor trauma or spontaneously in severe cases. These present with pain, swelling, and functional limitation of the affected muscle; common sites include the iliopsoas, calf (gastrocnemius/soleus), forearm, and upper arm. Large iliopsoas hematomas present as a clinical emergency with acute groin pain radiating down the thigh and mimicking appendicitis or other abdominal pathology; these can compress the femoral nerve (causing femoral neuropathy with hip flexion weakness) or extend into the retroperitoneal space causing hemodynamic compromise. Compartment syndrome represents a major complication of large intramuscular bleeds, particularly in the forearm or calf, requiring emergent fasciotomy to prevent permanent nerve and muscle damage.

• Mucosal and spontaneous gastrointestinal bleeding: Hemophilia manifests with spontaneous gingival bleeding, epistaxis, and oral mucosal hemorrhages that are often the presenting signs in childhood. Gastrointestinal hemorrhage, whether spontaneous or provoked by peptic ulcer disease, can cause significant blood loss and anemia requiring transfusion. Hematuria occurs spontaneously in ~10% of hemophilia patients and warrants investigation to exclude urologic pathology while assuming hemophilia as the primary cause. These mucosal bleeds reflect the inability of the coagulation cascade to seal small vascular breaches, even in areas of high shear stress.

• Central nervous system hemorrhage: Intracranial hemorrhage (ICH) remains a leading cause of death in severe hemophilia, occurring either spontaneously or after head trauma. ICH presents with headache, altered mental status, focal neurologic deficits, or signs of increased intracranial pressure; the mortality rate approaches 30% even with prompt recognition and treatment. The high mortality reflects both the catastrophic nature of ICH and delays in diagnosis if hemophilia is not suspected. Other CNS bleeds include subdural hematomas (which may be chronic and insidious), subarachnoid hemorrhage, and spinal epidural hematomas causing cord compression and paraplegia. CNS hemorrhage represents a medical emergency mandating immediate factor replacement to hemostatic levels (>80% activity) and neuroimaging.

• Post-operative and post-traumatic bleeding: While hemophilia patients may tolerate simple cuts and abrasions due to primary hemostasis remaining intact, significant trauma, surgical procedures, and dental extractions cause severe, life-threatening hemorrhage without factor replacement. Post-operative bleeding typically presents as delayed hemorrhage (hours to days after surgery) rather than immediate intraoperative bleeding, reflecting the gradual dissolution of structurally defective clots. Hemophilia patients undergoing surgery require careful perioperative factor management with trough levels maintained at hemostatic levels throughout recovery.

• Physical exam findings: Ecchymoses (bruising) from minor trauma appear out of proportion to the injury in moderate-to-severe hemophilia and may be the first sign in infants; bruising at non-traumatic sites or with unusual patterns (linear, hand-shaped) should raise suspicion for hemophilia in children. Swollen, warm joints with limited range of motion indicate acute hemarthrosis. Muscle swelling and induration suggest intramuscular hematoma. Long-term complications produce fixed joint contractures, muscle atrophy, and deformity from recurrent hemorrhages. Hepatomegaly develops secondary to chronic hepatitis (particularly hepatitis C from contaminated blood products before screening era).

• Important clinical variants and severity correlation: Mild hemophilia (5-40% factor activity) may remain undiagnosed until adulthood when elective surgery is planned, or may present with delayed wound healing after dental extraction. Moderate hemophilia (1-5% activity) frequently presents in infancy with unexplained bruising or bleeding with routine circumcision; hemarthroses and bleeding with minor trauma become apparent in childhood. Severe hemophilia (<1% activity) typically presents in infancy with spontaneous bleeding (hemarthroses, intramuscular hematomas) or excessive bleeding with minor trauma; these patients are immediately identified and treated. Acquired hemophilia (discussed in complications) presents acutely in elderly patients without prior bleeding history, producing severe bleeding and requiring different diagnostic and therapeutic approaches.

• Coagulation screening and factor assays: Initial laboratory evaluation reveals a prolonged activated partial thromboplastin time (aPTT) with normal prothrombin time (PT), normal bleeding time, and normal platelet count, creating a characteristic pattern that directs diagnosis toward coagulation factor deficiency. The aPTT corrects when normal plasma is mixed with patient plasma (mixing study demonstrating factor deficiency rather than inhibitor), though this correction must be verified by repeat testing since some inhibitors demonstrate time-dependent correction. Definitive diagnosis requires Factor VIII or Factor IX activity assay (one-stage aPTT-based assay or two-stage chromogenic assay), with results expressed as percentage of normal activity or Bethesda units; Factor VIII deficiency confirms hemophilia A, while Factor IX deficiency confirms hemophilia B. Chromogenic assays are increasingly preferred as they directly measure enzymatic activity and are less affected by inhibitors or lupus anticoagulant. The specific factor assay result correlates with severity: <1% activity = severe, 1-5% = moderate, 5-40% = mild, >40% = normal or carriers.

• Factor VIII and IX level interpretation: Normal Factor VIII levels range from 50-150%; levels <1% indicate severe hemophilia A, 1-5% indicate moderate disease, and 5-40% indicate mild disease. Normal Factor IX levels similarly range from 50-150%; proportional cutoffs define severity for hemophilia B. These assays demonstrate variable results within individuals due to stress, exercise, and estrogen effects (in females), so interpretation in context of clinical presentation is essential. Carriers of hemophilia A and B typically have factor levels of 40-60% due to skewed X-inactivation, though some carriers have levels as low as 10-20% and can manifest mild bleeding symptoms; measurement of factor levels in mothers and sisters of affected hemophilia patients defines carrier status and guides reproductive counseling.

• Mixing studies for inhibitor detection: When aPTT remains prolonged after mixing patient plasma with normal plasma (1:1 ratio), an inhibitor (most commonly a Factor VIII or IX alloantibody) is suspected. The Bethesda assay quantifies inhibitor titer by measuring the degree of Factor VIII activity inhibition after incubation; results are reported in Bethesda units (BU), with 1 BU defined as the inhibitor concentration that reduces factor activity by 50% after 2-hour incubation. High-titer inhibitors (>5 BU/mL) are more concerning for immune response and require complex management. Testing for inhibitors is indicated in patients with hemophilia developing reduced responsiveness to factor replacement, new-onset bleeding despite adequate factor levels, or unexplained aPTT prolongation on mixing study.

• Genetic testing and mutation identification: DNA sequencing of the F8 or F9 gene identifies the specific mutation causing hemophilia and is increasingly used for carrier identification in female relatives, prognostic counseling regarding inhibitor risk, and reproductive planning. Certain mutations (e.g., large F8 inversions) are associated with higher inhibitor risk and warrant closer monitoring. Copy number variation analysis detects deletions and insertions not identified by sequencing. Genetic testing is not required for acute diagnosis but provides valuable prognostic information and guides family management.

• Other supportive laboratory findings: Patients with recurrent hemarthroses or chronic hepatitis develop normocytic normochromic anemia from chronic blood loss; iron studies may reveal iron deficiency if gastrointestinal bleeding is present. Liver function tests and **hepat

Immediate stabilisation (life- or limb-threatening bleed)

• Factor replacement before imaging: In suspected intracranial hemorrhage, airway/neck hematoma, iliopsoas bleed, or major trauma, the World Federation of Hemophilia (WFH) 3rd edition guideline directs that factor concentrate be infused immediately — do not delay for CT, consultation, or laboratory confirmation. Target correction to near-normal activity (roughly 80–100%) and maintain trough levels for days, since defective clots re-bleed as thrombin generation wanes.
• Dosing logic: Recombinant factor VIII distributes to plasma volume, so ~1 U/kg raises activity about 2%; factor IX distributes extravascularly, so ~1 U/kg raises activity about 1%. Half-life is shorter for FVIII (~8–12 h) than FIX (~18–24 h), dictating more frequent redosing in hemophilia A.

First-line therapy

• Clotting factor concentrates: Recombinant factor VIII for hemophilia A and recombinant factor IX for hemophilia B, including extended half-life (Fc- or albumin-fused, PEGylated) products. WFH and the ASH/ISTH/NBDF/WFH 2021 guidelines recommend regular prophylaxis over episodic (on-demand) treatment in severe disease to prevent arthropathy.
• Desmopressin (DDAVP): Releases stored vWF–FVIII from endothelial Weibel–Palade bodies. Useful only in mild hemophilia A with a documented response; ineffective in severe hemophilia A and in all hemophilia B.
• Antifibrinolytics: Tranexamic acid or aminocaproic acid as adjuncts for mucosal, dental, and epistaxis bleeding, countering the impaired TAFI/plasmin balance.

Escalation and second-line options

• Emicizumab: Bispecific antibody bridging factor IXa and factor X, substituting for FVIIIa cofactor function; subcutaneous prophylaxis for hemophilia A with or without inhibitors.
• Bypassing agents for inhibitors: Recombinant activated factor VIIa or activated prothrombin complex concentrate (aPCC/FEIBA) for acute bleeds; immune tolerance induction with sustained factor exposure to eradicate the inhibitor.

Definitive and surgical management

• AAV-vector gene therapy: FDA-approved products deliver a functional F8 or F9 transgene to hepatocytes for durable endogenous factor expression.
• Orthopedic surgery: Radiosynovectomy or arthroplasty for end-stage hemophilic arthropathy, always under a perioperative factor-coverage plan.

Contraindicated/avoid

• Aspirin and NSAIDs, intramuscular injections, and arterial/lumbar punctures without coverage.
• aPCC combined with emicizumab (thrombotic microangiopathy risk) and antifibrinolytics with gross hematuria (obstructive urinary clots).
• Fresh frozen plasma/cryoprecipitate as routine therapy when concentrates are available; cryoprecipitate contains no factor IX.

Complications of the disease

• Hemophilic arthropathy: Repeated intra-articular blood leaves hemosiderin in synovium, driving iron-catalyzed free-radical injury, villous synovial hypertrophy, and cartilage loss. Signalled by a target joint (recurrent bleeds in the same joint), chronic effusion, contracture, and radiographic joint-space narrowing with subchondral cysts.
• Intracranial hemorrhage — EMERGENCY: Leading bleeding-related cause of death. Headache, vomiting, altered mental status, or focal deficit after even trivial head trauma; factor first, CT second.
• Compartment syndrome — EMERGENCY: Forearm or calf hematoma raises compartment pressure above capillary perfusion pressure. Pain out of proportion, pain on passive stretch, paresthesias; distal pulses persist until late. Needs factor plus fasciotomy.
• Iliopsoas hematoma — EMERGENCY: Groin/flank pain with hip held in flexion and femoral neuropathy (quadriceps weakness, loss of patellar reflex); retroperitoneal blood loss can be occult and massive.
• Airway/retropharyngeal hematoma — EMERGENCY: Neck trauma or dental block causing dysphagia and stridor.
• Hemophilic pseudotumor: Encapsulated, expanding chronic hematoma in muscle or bone causing progressive mass effect and lytic bone erosion on imaging.
• Chronic anemia: Iron deficiency from gastrointestinal or recurrent mucosal loss.

Complications of treatment

• Inhibitor formation: IgG alloantibody against infused factor. Signalled by bleeding that fails to respond to appropriate factor dosing, a shorter-than-expected factor recovery, and a mixing study that does not correct; quantify with the Bethesda assay.
• Anaphylaxis and nephrotic syndrome: Distinctive to factor IX inhibitors, especially with large gene deletions and during immune tolerance induction in hemophilia B.
• Thrombotic microangiopathy/thrombosis: Occurs when aPCC is given to a patient on emicizumab — excessive thrombin generation from combined FIXa/FX bridging.
• Assay interference: Emicizumab shortens/normalizes the aPTT and invalidates one-stage FVIII activity and Bethesda assays; bovine-reagent chromogenic assays are required. A normal aPTT here does not mean the hemophilia is cured.
• Transfusion-transmitted infection (historical): HIV and hepatitis C from pre-1985/1992 plasma-derived products, with resulting cirrhosis and hepatocellular carcinoma — hepatomegaly and abnormal aminotransferases in an older patient.
• Gene therapy hepatotoxicity: Vector-directed immune response with transaminase elevation and falling factor expression, managed with corticosteroids.

• The lab signature: Isolated prolonged aPTT with normal PT, normal platelet count, and normal platelet function/bleeding time. Intrinsic pathway only — PT stays normal because the extrinsic/tissue factor arm is intact.
• Single best next step in head trauma: Give factor concentrate immediately, then obtain the CT. Any stem where a hemophiliac hits his head and imaging is offered before factor is testing this reflex. The same rule applies to any suspected major bleed.
• Mixing study is the pivot: Corrects → factor deficiency; fails to correct → inhibitor, then order a Bethesda titer. A previously well-controlled patient who suddenly stops responding to factor has developed an inhibitor until proven otherwise.
• DDAVP is only for mild hemophilia A — it releases stored vWF-bound FVIII from Weibel–Palade bodies. Useless in severe hemophilia A (nothing to release) and in all hemophilia B. Watch for hyponatremia and tachyphylaxis.
• The classic distractor is von Willebrand disease: vWD also lowers FVIII and prolongs the aPTT, but gives mucocutaneous bleeding (epistaxis, menorrhagia), abnormal platelet adhesion/PFA-100, and low ristocetin cofactor activity — hemophilia gives deep bleeding into joints and muscles with normal platelet function. Also do not confuse with hemophilia C (factor XI, autosomal recessive, Ashkenazi Jewish).
• The association examiners love: Hemophilia B Leyden — a promoter mutation whose factor IX level rises toward normal after puberty under androgen stimulation, so a severely affected boy becomes a mildly affected adult.
• Inheritance trap: X-linked recessive, but ~30% of severe hemophilia A cases are de novo, so a negative family history never excludes the diagnosis; and a symptomatic female is explained by skewed X-inactivation, homozygosity, or Turner syndrome.
• Things never to do: aspirin/NSAIDs, intramuscular injections, and antifibrinolytics in the setting of gross hematuria (obstructive urinary clots).