Difference in Fibrin Clot Structure in Haemophilia A and Haemophilia B

Introduction: Hemophilia A and Hemophilia B are X-linked bleeding disorders characterized by deficiency of coagulation factor VIII and IX respectively. The disorders have traditionally been considered to display identical symptoms. However, recent clinical data suggest that hemophilia B has a less severe phenotype than hemophilia A in patients with similar levels of their deficient factor. There is a distinct lack of laboratory studies comparing the hemostatic potential in patients with the same severity grade of Hemophilia A and Hemophilia B. We assessed fibrin clot quality in Hemophilia A and Hemophilia B patient plasma as a possible explanation to the milder bleeding phenotype in patients with Hemophilia B. To achieve this a liquid permeation technique was used, which has been used to investigate fibrin networks in previous in vitro studies (He et. al. Blood Coagul Fibrinolysis 2005).

Methods: 20 patients with hemophilia A and 28 with hemophilia B were considered for inclusion in the study. Patients were recruited from centers in Stockholm and Belgrade and were thereafter matched according to their deficient factor level. Hemophilia diagnosis and informed consent formed the basis for study inclusion and exclusion criteria were lack of consent and inability to provide a matched patient from the other group. 17 hemophilia A and 17 hemophilia B patients were included in the study. The patient matching according to factor level was tested with Spearman's rank-order correlation, yielding a correlation coefficient of 0.999 (p<0.001).

In vitro fibrin clots were created from centrifugated patient plasma, CaCl₂ and bovine thrombin. They were then permeated with a percolating buffer with a pre-determined viscosity (Tris-hydroxymethyl aminomethane, imidazole, NaCl and trasylol) under three different pre-determined hydrostatic pressures. Fibrin clot permeability was calculated in Darcy constants (Ks) using the equation: Ks = (Q x L x h) / (t x A x ΔP). where Q is the elution volume (cm³), t is the buffer percolation time (seconds), h is the viscosity (poise, dyne x s cm^-2), L is the fibrin clot length (cm), A is the area of the fibrin clot (cm²) and ΔP is the difference of pressure (dyne cm^-2). A high Ks represents high permeability, which is interpreted as porous fibrin clots. In contrast low Ks values represent low permeability, which is interpreted as low porosity.

Results: Ks was higher in hemophilia A than in hemophilia B. Mean Ks for hemophilia A was 12.28 ± 5.92 and for hemophilia B 6.16 ± 3.63 (p<0.0005). Subanalysis of moderate and severe hemophilia patients (8 matched pairs, 16 patients) showed higher mean Ks in hemophilia A. Mean Ks of the hemophilia A group was 14.90 ± 6.88 and for the hemophilia B group 5.78 ± 3.86, p=0.039.

Conclusion: Fibrin clot structure exhibits less permeability in hemophilia B than hemophilia A, which may be one of the explanations to the milder bleeding symptoms observed in hemophilia B at the same level of deficient factor. This finding is in accordance with clinical studies showing that HB has a less severe bleeding phenotype than HA.