Factor VIII inhibitors in mild and moderate‐severity haemophilia A

Summary. Inhibitors are an uncommon complication of mild haemophilia, occurring in 3–13% of patients and usually arising during adulthood. The risk of inhibitor development in this group appears to be associated with relatively few high‐risk factor VIII genotypes clustered in the A2 and C2 domains, especially the Arg 593 ‐Cys and the Trp 2229 ‐Cys mutations. Kindreds with these mutations have an inhibitor incidence of up to 40%. These mutations may induce a stable conformational change in the factor VIII molecule rendering it antigenically distinct from wild‐type factor VIII. Inhibitors in mild haemophilia usually cross‐react with endogenous factor VIII reducing the basal VIIIC to <0.01 IU/ml, and causing spontaneous bleeding. This bleeding is sometimes severe and life‐threatening, two‐thirds of patients developing a pattern of soft tissue, gastrointestinal (GI) and urinogenital bleeding reminiscent of acquired haemophilia. Bleeding has been treated with human and porcine factor VIII, bypass therapy and DDAVP. Recombinant factor VIIa and DDAVP have the advantage that they do not induce an anamnestic rise in inhibitor titre. About 60% of these inhibitors disappear in the remainder over a median of 9 months. Few of these inhibitors recur, suggesting that most such patients have become tolerant. The inhibitors persist long‐term and remain troublesome in about 40% of patients. The limited data available on immune tolerance induction in this group indicate a generally poor response to this approach. Two of nine achieved tolerance, with a partial response in a further four. Inhibitors are an uncommon but life‐threatening complication of haemophilia. This complication should be considered when selecting the treatment modality for patients with a family history of inhibitors, and DDAVP used whenever possible.