Haemophilia and Bleeding Disorders: Replacing the Missing Factor
One missing protein in a chain of a dozen, and a child's blood forgets how to seal a wound. For a century that meant a life measured in hospital infusions and ruined joints. Now a once-weekly injection under the skin stands in for the missing factor entirely — and a single gene infusion may end the deficiency for good. This is the story of what breaks in haemophilia, and how we are learning to fix it.
A seven-year-old boy is carried into the clinic, one knee hot, tight and swollen to twice its size. There was no fall worth mentioning — a knock against a table edge that morning. He holds the leg rigid, terrified to bend it, because the joint is filling with blood from the inside: a haemarthrosis. His grandfather, who had the same disease, walks with a cane and joints fused by decades of such bleeds. But the boy's future is being rewritten. Instead of rushing to hospital for an intravenous infusion of clotting factor every few days, he receives one small injection under the skin each week — an antibody that does the missing factor's job. Two proteins that never meet in his blood are finally introduced by a molecular matchmaker.
The two classic haemophilias: A and B
Clotting is a relay race of proteins, and haemophilia is a dropped baton. Haemophilia A is a deficiency of coagulation factor VIII (Factor VIII); haemophilia B is a deficiency of factor IX (Factor IX), historically called Christmas disease after the first patient described. Both factors work at the same crucial junction of the clotting cascade, so losing either one produces almost identical bleeding. A is about four to five times commoner than B. Both are X-linked recessive: the gene sits on the X chromosome, so the disease overwhelmingly affects males, while females are usually carriers who pass it on. A son of a carrier mother has a one-in-two chance of being affected.
Severity tracks the residual factor level in the blood, not the type. Severe disease means less than 1% of normal factor activity — spontaneous bleeds into joints and muscles with no obvious trigger. Moderate (1–5%) bleeds after minor injury; mild (5–40%) may only reveal itself after surgery, dental extraction or major trauma, sometimes not until adulthood. The hallmark of the severe forms is deep bleeding — into the large joints (haemarthrosis of knees, ankles, elbows) and into muscles — rather than the superficial skin-and-mucosa bleeding of platelet disorders. Repeated joint bleeds are what cripple: blood is toxic to cartilage, and each bleed leaves the joint more damaged and more likely to bleed again.
Two quick discriminators at the bench: haemophilia prolongs the aPTT (the intrinsic-pathway test) while the PT and platelet count stay normal — a prolonged aPTT that corrects on mixing with normal plasma points to a factor deficiency. And clinically, deep bleeding (joints, muscles, delayed after trauma) says coagulation-factor problem; immediate mucosal bleeding (nosebleeds, gums, easy bruising) says platelet or von Willebrand problem.
Von Willebrand disease: the commonest one
The classic haemophilias are famous, but the commonest INHERITED bleeding disorder is von Willebrand disease, affecting up to about 1% of people. Von Willebrand factor (vWF) has two jobs, so a deficient or defective vWF breaks two things at once. First, vWF is the glue that lets platelets stick to a damaged vessel wall — lose it and platelet adhesion fails, giving mucocutaneous bleeding: nosebleeds, gum bleeding, easy bruising, and heavy menstrual periods (menorrhagia), which brings many women to attention. Second, vWF is the carrier and bodyguard of factor VIII in the circulation; without it, factor VIII is degraded and its level drops too — so severe vWF disease can look partly haemophilia-like.
- Haemophilia A = factor VIII deficiency; haemophilia B = factor IX deficiency (Christmas disease).
- Both are X-linked recessive: affect males, carried by females; A is far commoner than B.
- Severity follows factor level: severe <1%, moderate 1–5%, mild 5–40%.
- Hallmark = deep bleeding into JOINTS (haemarthrosis) and muscles, not skin.
- Von Willebrand disease is the commonest inherited bleeding disorder: mucocutaneous bleeding + low factor VIII.
- Labs: prolonged aPTT, normal PT and platelet count in the classic haemophilias.
The old cornerstone: replacing the factor
The logic is disarmingly simple: if a factor is missing, put it back. The mainstay of treatment has long been factor concentrates — purified factor VIII for haemophilia A, factor IX for haemophilia B, given by intravenous infusion. These come as recombinant products (made in cell culture, carrying no risk of blood-borne infection) or plasma-derived products (purified from donated plasma, now heat- and solvent-treated for safety). A crucial modern advance is extended-half-life products, engineered so the factor lasts longer in the blood — fewer infusions to hold the same protective level. These build on the lessons of the earlier Blood products chapter, where whole factor concentrates and prothrombin complex first appeared.
The deeper strategic choice is when to give it. On-demand treatment infuses factor only to STOP a bleed once it has started — it rescues, but each bleed still damages the joint. Prophylaxis gives regular infusions to keep the factor level above the danger threshold at all times, PREVENTING bleeds before they happen. Prophylaxis, especially started in early childhood, is what preserves joints and transforms the natural history of severe haemophilia from progressive crippling to a near-normal life. The catch is the burden: repeated venous access, often several times a week, in small children.
Recombinant factor VIII (e.g., octocog alfa) and recombinant factor IX (e.g., nonacog alfa) are standard replacements. Extended-half-life versions fuse the factor to an Fc fragment or to albumin, or attach polyethylene glycol (PEGylation) — for example efmoroctocog alfa (Fc-fused factor VIII) and eftrenonacog alfa (Fc-fused factor IX) — cutting infusion frequency substantially, especially for factor IX, which tolerates the change well.
The great complication: inhibitors
There is a serious catch to factor replacement. In some patients — most often those with severe haemophilia A — the immune system sees the infused factor as foreign and develops alloantibodies against it: inhibitors. Once an inhibitor appears, the ordinary factor infusions stop working, because the antibody neutralizes the factor as fast as you give it. Inhibitors are the single most feared complication of modern haemophilia care, turning easily-managed bleeds into refractory ones. They are detected and quantified with a blood test (the Bethesda assay).
When an inhibitor blocks the normal factor, we reach for bypassing agents — treatments that trigger clotting further downstream, going AROUND the blocked step so the missing/neutralized factor is no longer needed. Two are standard: activated prothrombin complex concentrate (aPCC, marketed as FEIBA), which supplies activated downstream factors, and recombinant activated factor VIIa (eptacog alfa), which drives thrombin generation directly at the site of injury. Longer term, immune tolerance induction — giving regular factor to retrain the immune system to accept it — can eradicate the inhibitor in many patients.
- Factor concentrates (recombinant or plasma-derived) replace the missing VIII or IX intravenously.
- Extended-half-life products cut infusion frequency.
- Prophylaxis (prevent bleeds, save joints) beats on-demand (treat bleeds after they start).
- Inhibitors = alloantibodies against the infused factor; replacement stops working.
- Manage inhibitors with bypassing agents (aPCC/FEIBA, recombinant factor VIIa) ± immune tolerance induction.
The revolution: non-factor and gene therapies
The biggest change in a century did not replace factor VIII — it impersonated it. Emicizumab is a bispecific antibody: one arm grabs activated factor IX (IXa), the other grabs factor X, and by holding the two together it MIMICS the bridging job that factor VIII normally does. Because it is not factor VIII, existing inhibitors cannot neutralize it — it works even in patients with inhibitors, which is transformative. And because it is an antibody with a long half-life, it is given subcutaneously as prophylaxis as infrequently as once weekly to once monthly, no vein required. For a small child on thrice-weekly IV infusions, a weekly injection under the skin is a different life. This is the once-a-week injection our boy in the clinic now receives.
Beyond mimicry lies cure. Gene therapy delivers a working copy of the factor VIII or factor IX gene, usually packaged in an adeno-associated virus (AAV) vector, as a single one-off intravenous infusion. The liver takes up the gene and begins producing the patient's own factor, in some cases raising levels enough to stop prophylaxis for years. The first such products have now reached the clinic for both haemophilia A and B. Questions remain about how long expression lasts and long-term safety, but the direction is unmistakable: from lifelong replacement, to impersonation, toward a durable fix.
Two familiar drugs from the previous chapter (Antifibrinolytics & desmopressin) fill important niches. Desmopressin (DDAVP) triggers the release of stored factor VIII and vWF from the vessel wall — so it works in MILD haemophilia A and type-1 von Willebrand disease, where stores exist to release, but is useless in severe disease or haemophilia B (no factor IX to release). Tranexamic acid, an antifibrinolytic, stabilizes clots and is excellent for mucosal and dental bleeding, and for menorrhagia in von Willebrand disease — often as an add-on rather than a standalone.
- Giving desmopressin for haemophilia B or SEVERE haemophilia A. It only releases existing stores — useful in mild A and type-1 vWD, useless where there is little or no factor to release.
- Forgetting inhibitors when factor replacement suddenly stops working. A previously well-controlled patient who stops responding needs a Bethesda assay, not just a higher dose.
- Using aspirin or NSAIDs in a patient with a bleeding disorder. They inhibit platelets and irritate the gut mucosa, compounding the bleeding risk — avoid them and use safer analgesia.
A boy with severe haemophilia A on factor VIII prophylaxis suddenly stops responding to his usual infusions, with recurrent joint bleeds. What is the most likely explanation?
- Haemophilia A (factor VIII) and B (factor IX) are X-linked; hallmark is deep joint/muscle bleeding, severity set by factor level.
- Von Willebrand disease is the commonest inherited bleeding disorder: poor platelet adhesion plus low factor VIII, mucocutaneous bleeding.
- Replace the factor (recombinant/plasma-derived, extended-half-life); prophylaxis protects joints; inhibitors are managed with bypassing agents.
- Emicizumab (subcutaneous bispecific antibody) mimics factor VIII and works even with inhibitors; gene therapy offers a one-off cure; desmopressin and tranexamic acid are adjuncts.
- Hoffbrand AV, Moss PAH. Hoffbrand's Essential Haematology — Inherited coagulation disorders: haemophilia A & B and von Willebrand disease.
- World Federation of Hemophilia (WFH). Guidelines for the Management of Hemophilia, 3rd edition — prophylaxis, inhibitors and emicizumab.
- Rang HP, Dale MM, et al. Rang & Dale's Pharmacology — Haemostasis and coagulation-factor replacement.
- Brunton LL, et al. Goodman & Gilman's The Pharmacological Basis of Therapeutics — Coagulation disorders and their treatment.
- Katzung BG. Basic & Clinical Pharmacology — Drugs used in bleeding disorders.
