Laboratory Evaluation

Normal factor VIII:C and factor IX activities range from 50% to 150%. A significantly prolonged activated partial thromboplastin time is the most common abnormal coagulation laboratory assay in hemophilia A and hemophilia B because factor VIII:C and factor IX lie within the intrinsic pathway of coagulation (Fig. 73-1). In North America, the one-stage clotting assay is performed on patient plasma to determine the exact levels of clotting factor VIII:C and factor IX activity. This assay is based on the activated partial thromboplastin time and is reproducible, technically simple and automated, and inexpensive.

Figure 73-1 The coagulation cascade.

Hemophilia A and vWD are associated with deficiencies of factor VIII:C. Factor VIII:C synthesis is deficient in hemophilia A, but not in vWD, where factor VIII:C deficiency is due to defective stabilization of factor VIII:C by von Willebrand factor protein. von Willebrand factor protein complexes with factor VIII:C and chaperones it through the circulation and protects factor VIII:C from naturally occurring proteolytic degradation. Factor VIII:C decreases in vWD represent shortened circulating survival, but not decreased synthesis. Hemophilia A usually can be differentiated from vWD in the laboratory by the presence of normal or increased von Willebrand factor antigen and ristocetin cofactor activity.

Prolonged bleeding times are present in individuals with vWD, but individuals with hemophilia A or hemophilia B have normal bleeding times unless patients have ingested aspirin or nonsteroidal anti-inflammatory drugs (NSAIDs). Platelet aggregation assays typically distinguish vWD from hemophilia A when ristocetin is used as the agonist or platelet activator. Ristocetin mediates platelet–platelet interactions by interacting with von Willebrand factor protein.

An inhibitor is an immunoglobulin, usually IgG, that interacts with and neutralizes the coagulation potential of factor VIII:C or factor IX. If it occurs in an individual with severe or moderately severe hemophilia, it is termed an allogeneic antibody and develops after the patient has been exposed to sources of the missing clotting factor in replacement therapies used to treat or prevent bleeding events. The potency of inhibitors against factor VIII:C and factor IX activities is quantitated in Bethesda units (BU).

From the clinical perspective, an inhibitor targeting factor VIII:C or factor IX is clinically suspected when a bleeding episode fails to respond to an adequate dose of clotting factor concentrate replacement therapy. Surgical bleeding commonly is observed in individuals with alloantibody inhibitors even at extremely low titers (e.g., >0.6 BU). Laboratory confirmation of the potency of the inhibitor is crucial to the diagnosis and subsequent treatment strategy.

Clinical Manifestations

Severe hemophilia A and hemophilia B (<1% normal clotting factor activity) are characterized by the occurrence of spontaneous bleeds into joints (hemarthrosis). These joints are very painful (bleeding into a closed space) and swollen, and repetitive bleeds ultimately result in chronic inflammation of the synovium and deterioration of cartilaginous and bony surfaces. Bleeding originates from the subsynovial venous plexus underlying the joint capsule.

Infants with severe hemophilia usually experience their first bleeding event between 12 and 18 months of age as they enter the toddler stage. Easy bruising also may be apparent. Patients with moderately severe or mild hemophilia have less bleeding and bruising, unless surgical or physical trauma is involved. Although patients with severe hemophilia can bleed from any anatomic site after negligible or unnoticed trauma, intra-articular and intramuscular bleeds are the most prevalent sites. Soft tissue hemorrhages may be life-threatening if they occur in open spaces (e.g., retroperitoneum). Intracranial bleeds are the most common hemorrhagic cause of death. Hematuria and bleeding in the gastrointestinal tract, although usually not life-threatening, are the most common visceral problems. Isolated hemorrhage into closed spaces, such as into the extremities, may result in compartment syndromes with compromised nerve function and blood flow. If these conditions are not treated immediately by clotting factor replacement or surgical decompression, permanent neuropathy or limb-threatening tissue necrosis may develop. Compartment syndromes of the forearm can occur after faulty venipuncture, and retropharyngeal bleeds may compromise the airway after central venous access devices are placed.

Spontaneous intra-articular bleeds occur most commonly in the knees, followed by the elbows and ankles and, less commonly, the shoulders, hips, and wrists. Acute bleeds produce a tingling or burning sensation in the joint area that progresses to intense pain and swelling and reduced joint mobility. The acutely affected joint typically is fixed in a flexed position until the acute swelling subsides and the intra-articular blood resorbs in response to replacement therapy with the deficient clotting factor. With repeat bleeds, arthritic changes and muscle atrophy occur, and pain may be constant.

Factor Replacement Therapy

Cessation of intra-articular bleeding in the hemophilias depends on adequate replacement of the deficient clotting factor protein to achieve hemostasis. The dose and choice of replacement product are influenced by the severity of the coagulopathy, the site and severity of the bleeding event, and the presence or absence of an alloantibody. The premise of replacement dosing relies on the fact that administration of 1 U/kg body weight of factor VIII:C–containing products ideally produces an incremental increase of 2% factor VIII:C activity in the one-stage coagulation assay. In comparison, 1 U/kg body weight replacement of factor IX ideally produces a 1% incremental rise in factor IX plasma activity in coagulation assays.

Commercially available clotting factor concentrates licensed in the United States are listed in Table 73-2. No difference in efficacy has been noted among these concentrates, but there may be perceived and real potential safety advantages of recombinant products over plasma-derived products, so more than 90% of replacement therapy for hemophilia A and hemophilia B in the United States is accomplished with recombinant products. With the advent of virtually safe viral attenuated clotting factor replacement therapies, fresh-frozen plasma and cryoprecipitate are used rarely in the treatment of hemophilia A and hemophilia B. Rather, fresh-frozen plasma is the preferred replacement therapy for the rare clotting factor deficiencies for which no specific replacement product exists (e.g., factor XI deficiency). Cryoprecipitate is hardly used in the United States as a replacement product except in patients with afibrinogenemia.

Table 73-2 Factor VIII and Factor IX Concentrates Available in the United States

* Distributed by ZLB-Behring.

^† Prepared from volunteer donor plasma provided by the American Red Cross.

Potential Risks of Clotting Factor Concentrates: Blood-Borne Pathogens

Recombinant factor VIII:C and factor IX products introduced in the late 1980s and late 1990s are blood-borne viral safe. No transmission of human immunodeficiency virus (HIV); hepatitis A, B, or C; parvovirus; or new variant Creutzfeldt-Jakob disease has ever been reported with recombinant replacement concentrates. Virtually all patients with hemophilia A (and hemophilia B) who were exposed to plasma-derived concentrates manufactured before 1985 were likely to have been infected by hepatitis C, and greater than 80% of severe hemophilia A patients became HIV seropositive. Hepatitis B infections were almost eliminated by aggressive vaccination programs for hemophiliac individuals. Similar more recent vaccine initiatives have reduced the risk of hepatitis A transmission to hemophiliac persons. Currently available plasma-derived factor concentrates for hemophilia A or hemophilia B are virtually viral safe because the plasma donations are nucleic acid tested for the most common and lethal blood-borne viruses. New variant Creutzfeldt-Jakob prions may escape in vitro detection in plasma donations and are not amenable to currently used viral elimination techniques. Health authorities in the United Kingdom have informed individuals with hemophilia and other bleeding disorders that they are considered “at risk” for variant Creutzfeldt-Jakob disease if they used U.K. plasma products manufactured between 1980 and 1998. These products were made from plasma collected from donors in the United Kingdom who were later identified to have variant Creutzfeldt-Jakob disease or possibly from donors who remain asymptomatic for variant Creutzfeldt-Jakob disease. To date, no cases of variant Creutzfeldt-Jakob disease are known to have been transmitted by any plasma replacement product, although transfusion of packed red blood cells has been associated with two cases of variant Creutzfeldt-Jakob disease.

Treatment of Inhibitors

Treatment of alloantibody inhibitors is determined by whether they are high-titer (>10 BU) or low-titer (<5 BU) antibodies. Low-titer inhibitors can be overwhelmed by administration of large enough doses of human factor VIII or IX concentrates to saturate the inhibitor and provide adequate clotting factor activity levels. High-titer factor VIII inhibitors cannot be overwhelmed by specific clotting factor replacement therapy. Rather, coagulation must be achieved by using agents that “bypass” the effects of the inhibitor. Multiple treatment options are available and seem to be successful in about 80% of alloantibody-related bleeding episodes; however, no one agent is universally successful in treating bleeding complications in all patients. Factor IX complex concentrates (formerly prothrombin complex concentrates) of standard (Konyne 80) or activated varieties (FEIBA) and recombinant activated factor VIIa concentrate (NovoSeven) have become first-line therapies for bleeding events.

Pathophysiology of Hemophilic Arthropathy

Hemophilia results in spontaneous bleeding into the knee. Repeated bleeds first cause a synovitis, which leads to articular cartilage destruction. The mechanism of joint damage is thought to be multifactorial. Chemical and inflammatory factors arise from the occurrence of bleeds in the joint. These two factors are followed by physical and biomechanical degradation of the articular cartilage.²⁸

The reaction of the synovium to intra-articular bleeds results in vascular hyperplasia and friability of the synovial lining, which increases the potential for more bleeds. The synovial lining of the joint becomes fibrotic, leading to joint fibrosis and contracture. As this occurs, physiochemical alterations of the synovium cause articular cartilage damage.

When bleeding has occurred in the knee, hemosiderin-stained deposits appear in the synovium (Fig. 73-2). Evidence indicates that iron deposits in the synovium stimulate the production of cytokines. These proteins damage the articular cartilage. It seems likely that phagocytosis by synovial cells leads to stimulation of cytokine production. Roosendal and coworkers⁵⁶ showed significantly higher production of interleukin-1, interleukin-6, and tumor necrosis factor in cultures of hemophilic synovial tissue compared with normal tissue. In addition, supernatant fluid from these cultures showed greater catabolic activity. Proteoglycan synthesis has been shown to be inhibited by these supernatants. Arnold and Hilgartner¹ have shown high levels of acid phosphatase and cathepsin D in hemophilic synovial tissue samples. Stein and Duthie⁶² studied the histochemistry of synovium retrieved from hemophiliac individuals undergoing reconstructive surgery. They showed hemosiderin deposits in the synovium and articular cartilage. They postulated that the hypertrophic synovial membrane led to the release of lysosomal enzymes, such as cathepsin D, resulting in articular cartilage breakdown.

Figure 73-2 Hyperplastic, hemosiderin-laden brown synovium from hemophiliac knee. A, Gross appearance, on arthrotomy. B, Surgical specimen. C, Microscopic specimen.

The direct effect of iron deposits on articular cartilage is not completely understood. Iron deposits within the articular cartilage are shown on histologic examination. Electron microscopy has shown deposits within the cytoplasm of chondrocytes. These deposits have been termed siderosomes. These cells all show signs of degeneration. The exact mechanism is not completely understood. One explanation is that the deposits inhibit the hydrolysis of calcium pyrophosphate, causing the precipitation of insoluble calcium pyrophosphatase crystals in the matrix, which leads to deterioration of the matrix and chondrocyte damage. Experimental evidence indicates that hemarthrosis has an inhibitory effect on proteoglycan synthesis. It is also thought that the deposits themselves may change the biomechanical properties of the matrix, leading to degeneration.

The cause of degeneration of the joint is probably multifactorial. Repeated bleeds into the joint result in synovial hypertrophy. This abnormal synovial tissue releases catabolic enzymes, which degrade the articular cartilage. In addition, evidence suggests that the presence of blood itself within the joint leads to articular cartilage degeneration.

Radiology of Hemophilic Arthropathy of the Knee

Radiographic evaluation is the primary diagnostic tool used to evaluate the knee in patients with hemophilia. Standing anteroposterior and lateral views should be taken. In addition, skyline or Merchant views add valuable information about the presence of patellofemoral disease. Magnetic resonance imaging (MRI) has been shown to be valuable in the evaluation of knees with minimal x-ray changes.^4,75 Often MRI shows evidence of synovial hypertrophy and other pathology before the standard x-ray. MRI may have particular value in the treatment of children with early disease who are candidates for synovectomy.⁴³ Wilson and colleagues⁷⁴ proposed the use of diagnostic ultrasound as an adjunct in the decision-making process for soft tissue swelling around the hemophilic joint. Ultrasound may aid in the diagnosis of pseudotumors.

Two radiographic classification systems are in use today. Arnold and Hilgartner¹ described a five-stage system in 1977. This logical, simple system easily divides the presentation of joint changes into stages that have surgical significance. In 1979, Pettersson and associates⁴⁶ described an eight-stage system for grading arthropathy. Although this system is more detailed, it is not as commonly used today as the five-stage system of Arnold and Hilgartner (Table 73-3):

Table 73-3 Arnold and Hilgartner’s Radiographic Classification of Hemophilic Arthropathy

The classification system of Pettersson and associates⁴⁶ consists of eight different radiographic signs: osteoporosis, epiphyseal enlargement, joint margin erosion, irregular subchondral surface, joint incongruity, subchondral cysts, joint space narrowing, and angular deformity. Each sign is graded 0 points (no change), 1 point (slight change), or 2 points (severe changes). The total score yields a particular stage. Greene and colleagues²⁴ compared both of these systems and found that the Pettersson system was more accurate. A new system of four signs and seven points was proposed. This system was shown to be more accurate; however, the Arnold and Hilgartner system¹ remains the most commonly used system in the United States today.