Hemophilia is a genetic bleeding disorder resulting from the insufficient levels of clotting factors in the body. The clotting factors irregularity causes a lack of clumping of blood required to form a clot to plug a site of a wound. The genes involved in the pathogenesis of hemophilia include the F8 gene in hemophilia A, F9 gene in hemophilia B, and F11 gene in C. Hemophilia predominantly affects the male population but the sub-type hemophilia C, with an autosomal inheritance pattern, can affect the males as well as females.

The normal physiology of hemostasis can be summarized as follows:

• Hemostasis is a tightly regulated process whereby the body maintains a homeostatic balance to permit normal blood flow, without thrombosis or bleeding.^[1]
• The process of hemostasis involves a fine balance between the procoagulant and anticoagulant factors. It attempts to maintain blood flow within the vascular compartment and promotes the formation of blood clots following vascular injury.^[2]
• It also enables repair after vascular injury, promotes vessel healing, and maintains vessel integrity.
• Hemostasis can be divided into three phases
  • Each phase is explained as follows:

1. Primary hemostasis

• Endothelial damage marks the beginning of this phase.^[3]
• It involves platelet adhesion, activation and aggregation, to form a platelet plug at the injury site.
• Circulating platelets and endothelial cells, both provide the source of von Willebrand factor (VWF).
• Von Willebrand factor (VWF) has two main functions. First, it acts as a mediator in binding of platelets to the sub-endothelium. Then it protects the circulating factor VIII from proteolytic degradation.^[4][5]
• The endothelial cells, which normally promote anticoagulation, switch from anticoagulant to procoagulant upon vascular injury. They promote platelet aggregation by releasing the contents of their Weibel-Palade bodies and hence leading to enhanced local concentrations of von Willebrand factor (VWF) and tissue factor (TF).^[6]
• The released von Willebrand factor (VWF) binds to the collagen on the exposed sub-endothelial surface, and is then utilized for platelet binding via the glycoprotein Ib (GPIb) complex.
• The platelets release their granules after undergoing a shape change. This event marks the formation of a platelet plug.

2. Secondary hemostasis

• The main goal of secondary hemostasis is to stabilize the platelet plug.^[7]
• It involves the activation of coagulation system and coagulation factors to eventually produce cross-linked fibrinogen (“fibrin”).
• The process of platelet plug stabilization has been always referred to as the “Coagulation cascade” which can be separated into the intrinsic, extrinsic, and common pathways.

3. Fibrinolysis

• Fibrinolysis involves the process of physiological lysis of the clot, generated by the actions of primary and secondary hemostasis, to permit tissue repair under the supervision and help of multiple proteins.^[8]

• The cell-based model of hemostasis basically says that blood has to be exposed to cells containing the tissue factor (TF) for the initiation of the clotting process.^[9]
• It better reflects true in vivo hemostasis.
• The model proposes three overlapping phases of hemostasis which are explained as follows:

a. Initiation

• It occurs on the surface of the tissue factor-bearing cell.
• Tissue factor-bearing cells such as the fibroblasts bind to the surface of platelets in an evolving thrombus.^[9]
• Factor VII comes into direct contact with the tissue factor-bearing extravascular cells during vascular injury, and rapidly undergoes activation via the extrinsic pathway.
• Effective initiation means bringing FVIIa/TF activity into close proximity to the activated platelet surfaces.

b. Amplification

• It occurs on the surface of the platelets as they get activated.^[10]
• Platelets adhere at the site of endothelial injury and get activated by thrombin.
• Platelet activation is marked by the execution of the following processes:

• The release of alpha granules which contain Factor V and von Willebrand factor (VWF)
• Binding to plasma proteins including von Willebrand factor (VWF)
• Promoting the assemblage of procoagulant complexes
• Ensuring prompt thrombin generation.

c. Propagation

• Propagation occurs on the surface of activated platelets.^[11]
• It involves the assembly of “tenase” (FVIIa and FIXa/FVIIIa) and “prothrombinase” (FXa/FVa) complexes on the platelet surface, thus allowing thrombin generation to take place on a large scale which is necessary to form a hemostatic fibrin clot.^[12]

• Hemophilia is an X-linked bleeding disorder caused by a deficiency or complete absence of coagulation factor VIII (hemophilia A) or factor IX (hemophilia B).^[13]
• Bleeding in hemophilia occurs due to the failure of secondary hemostasis.^[14]
• Primary hemostasis and the formation of platelet plug occurs normally but stabilization of the plug by fibrin is defective because of the generation of inadequate amounts of thrombin.^[14]
• Clinical expression of hemophilia usually correlates with the activity of the coagulation factor and the disease can be classified as:

• Mild (factor level > 0.05–0.40 IU/mL)
• Moderate (factor level = 0.01–0.05 IU/mL)
• Severe (factor level < 0.01 IU/mL)

• Excessive bleeding in mild hemophilia patients occurs only after major injuries, surgery, or other invasive procedures.^[13]
• In patients with moderate hemophilia, hemarthroses and muscle hematomas may occur after relatively minor injuries.^[13]
• Bleeding occurs frequently and spontaneously in patients with severe hemophilia and this group can rarely also experience life-threatening episodes such as retroperitoneal and intracranial bleeds.^[13][15]
• Peculiar pathology of the types of hemophilia is discussed below:

• Hemophilia A is caused by an absence or deficiency of factor VIII (FVIII) protein activity.^[16]
• It is a lifetime disease that is transmitted from usually asymptomatic carrier females to their male offspring.^[16]
• Hemophilia A is characterized by recurrent bleeding, in particular into joints.^[14]
• The recurrent bleeding in joints leads almost inevitably to severe arthropathy.^[17]
• The molecular causes of FVIII deficiency can be divided into 3 main categories:

1. Classic mutations in the F8 gene that cause structural changes in the FVIII molecule or even produce a truncated protein lacking essential functional domains.^[18][19]
2. Mutations in proteins that interact intracellularly in the correct folding and trafficking of the FVIII protein or mutations in extracellular plasma proteins such as von Willebrand factor (VWF).^{[20][21][22][23]}
3. The third category encompasses patients who have the clinical disease but have no mutations in the F8 gene or in any of the known interacting partners.^[16]

• Less than 1/3 of the patients (mostly elderly with comorbidities) of hemophilia develop autoantibodies (inhibitors) against factor VIII (FVIII) that can lead to spontaneous and severe bleeding.^[24][25]

• Hemophilia B is an X-linked bleeding disorder caused by the deficiency of factor IX (FIX).^[26]
• Factor IX is a vitamin K–dependent plasma protease that plays a role in the intrinsic pathway of hemostasis and whose function is to cleave and activate Factor X.^[27]
• Bleeding tendency in hemophilia B is in good accordance with the severity of factor deficiency.^[28]
• Patients with the severe form (Factor IX <1%), a significant proportion of 30 to 45% of all affected by hemophilia B, usually suffer from recurrent joint, soft-tissue, and muscle bleeds.^[28]

• Hemophilia C is an autosomal genetic disorder involving a lack of functional clotting factor XI.^[29]
• This condition is not completely recessive, as heterozygous individuals also show increased bleeding.^[29]
• The bleeding diathesis in hemophilia C is considerably milder than in hemophilia A or B.^[30][31]
• The spontaneous soft tissue bleeds and hemarthroses characteristic of hemophilia A and B are not features of hemophilia C.^[32]
• Menorrhagia and epistaxis are the most common bleeding episodes encountered in hemophilia C.^[33]
• Factor XI deficiency can especially be a problem when trauma involves the oral and nasal cavities or the urinary tract.^[30][34]
• Symptoms in hemophilia C patients correlate poorly with factor XI activity measured by aPTT-based assays.^{[31][35][36][37]}

• Hemophilia A can be characterized by the detection of inversions of intron 22 (reported in 40–45% of severe patients) and intron 1 (reported in 1–6% of severe patients) of F8 gene (which encodes factor VIII).^[38][39]
• The F8 gene is located on the X chromosome.^[40]
• Point mutations (missense, nonsense, and splice site mutations) account for 67% of molecular defects described.
• Small insertions and deletions represent 25% of such defects.^[41]
• Roughly 6% of all mutations are large deletions.^[41]

• Missense, nonsense, and splice site mutations in the F9 gene (which is located on the X chromosome and encodes factor XI) are the most common, accounting for around 70% of mutations.^[42][43][44]
• Frameshift mutations in the F9 gene account for approximately 17%.^[42]
• Large deletions and promoter region mutations are relatively rare, accounting for 3% and 2% respectively.^[42]

• Hemophilia C, characterized by a deficiency of factor XI, results from mutations (splice site, nonsense, or missense mutation) in the F11 gene.^[45]
• Homozygous or compound heterozygous deficiency of factor XI results in a variable bleeding phenotype but the clinical presentation in heterozygotes is less predictable.^[46]

• Hemophilia can be associated with the following conditions:

• End-stage liver disease^[47][48]
• Autoimmune thyroiditis^[49]
• Autoimmune pancreatitis^[50]
• Hematologic malignancies^[51]
• Hepatocellular carcinoma (HCC)^[52]
• Pregnancy^[53]
• Polymyositis^[25]
• Autoimmune hemolytic anemia^[25]
• Hepatitis A, B, and C^{[54][55][56][57]}
• Osteoporosis^[58][59]
• Rheumatoid arthritis^[25]
• Myasthenia Gravis^[60]
• Systemic lupus erythematosus (SLE)^[25]
• Moyamoya syndrome^[61]

On gross pathology, hemophilia is characterized by the following findings:

• Hemarthroses^[13]
• Muscle hematomas^[13]
• Subcutaneous bleeding^[62]
• Gross hematuria^[62]
• Extremity swelling^[63]
• Epistaxis^[33]
• Menorrhagia^[33]
• Excessive uncontrollable bleeding after major/minor injuries^[13][15]

On microscopic histopathological analysis, hemophilia can be characterized by the following findings:

• Reduced red blood cell (RBC) count^[64]