Inhibitors

You get an inhibitor when your immune system starts rejecting the clotting factor that is being used to treat bleeds. Treatment with that clotting factor no longer works, and other ways of stopping bleeding have to be used.

Inhibitors can happen in any clotting factor deficiency, including von Willebrands. However, they are most commonly experienced by people with severe haemophilia A – around 30% have, or have previously had, an inhibitor to Factor VIII. It is ten times less common in severe haemophilia B.

You can have varying amounts of inhibitor, and it may appear and rapidly disappear. A ‘high titre’ inhibitor is also the most difficult to get rid of. It is not usually possible to be on prophylaxis if you have an inhibitor (except to Factor VIII), so you should expect to have more bleeds.

Any of the following make you are more likely to get an inhibitor

• Severe haemophilia
• A family history of inhibitors
• Aged under 5, or increasingly older than 60
• Recently started treatment
• Using a lot of treatment

How to prevent or manage bleeds with an inhibitor to Factor VIII or IX

If you have an inhibitor to Factor VIII, you may be able to use a new treatment, Emicuzimab (Hemlibra®). Giving yourself weekly injections, just under the skin in a fleshy part of your body, can help prevent bleeds.

There are currently two clotting factor concentrates that can be used as alternatives to Factor VIII or IX. These are known as ‘by-passing agents’, because they by-pass the need for factor VIII or IX:

Recombinant Factor VIIa (or NovoSeven®) is given by a single injection or up to three injections, two hours apart.

FEIBA, which stands for Factor Eight Inhibitor Bypassing Activity, is a type of blood product known as an activated Prothrombin Complex Concentrate (aPCC). It is the only aPCC currently available for use in Scotland. It is given as a single injection followed by more after eight to twelve hours if necessary.

It may be possible to simply use a higher dose of Factor VIII or IX than you used before you had an inhibitor, or if you have mild haemophilia A, to switch to desmopressin. Tranexamic acid can be used in addition, especially for surface bleeds, but is not with FEIBA.

How to get rid of an inhibitor to Factor VIII

The first line of attack if you have severe haemophilia A and an inhibitor is a tactic known as immune tolerance therapy. By infusing large amounts of Factor VIII daily for up to 18 months you try to ‘drown out’ your inhibitor by overloading it with the clotting factor that it is neutralising. If this is not successful, then you can try a higher dose, switch to plasma-derived clotting factor, or use rituximab, which is a drug that reduces the activity of your immune system.