What is haemophilia?

Haemophilia is a general term for a family of inherited blood-clotting disorders. In haemophilia, there is a life-long defect in the clotting mechanism of the blood caused by a deficiency in one of the thirteen proteins in the blood called clotting factors. These work in a chain reaction whenever there is tissue damage to form a blood clot. If one of the factors is missing or defective, the process of forming a clot is slowed down or affected in other ways.

The best known bleeding disorder is so-called classic haemophilia, or haemophilia A, which is caused by a deficiency in factor VIII (NB most clotting factors are referred to using Roman numerals). A smaller number of people are missing factor IX. This is called haemophilia B or Christmas Disease, after Stephen Christmas the first person diagnosed with the condition (CHS 2001). In the UK approximately 6,000 people have a diagnosis of haemophilia A with around 1,100 with haemophilia B.

A more common bleeding disorder is von Willebrand's Disease (vWD) named after the Finnish doctor Erik von Willebrand (1870–1949). This is caused by a deficiency of or a defect in von Willebrand's Factor, which works closely with Factor VIII and the proper functioning of platelets in blood clotting. There are around 3,500 people diagnosed with vWD in the UK, although it is thought to affect as many as 1-3% of the population. It often goes undiagnosed, as it is usually less severe and less well known than haemophilia.

There are differing degrees of severity in both forms of haemophilia and vWD. This severity is measured by the amount of the clotting factor people have and is described in a rather complicated way because a figure was set as the normal amount in a person and described as a level of 100%. In reality most people have a clotting factor level of between twice this level and half of it described as between 200% - 50% of this specified normal amount.

In people with haemophilia this level can vary from 25% (mild haemophilia) to less than 1% (severe haemophilia). With moderate to mild haemophilia of both types, bleeding usually only happens after an injury, and treatment need only be administered if there is a bleed. With severe haemophilia, bleeding happens spontaneously, and must be treated very promptly. vWD is rarely as severe as haemophilia, usually causing only bruising or prolonged bleeding with cuts, tooth extractions, or nose bleeds etc. However, there is a rare and severe form of the condition that can be just as serious as severe haemophilia. There is also not a perfect correlation between clotting factor levels and the number of bleeds people experience, some people with mild to moderate levels of a clotting factor can experience as many bleeds as a person with severe haemophilia and a few fortunate people with very low levels of a clotting factor have very few bleeds (CHS 2001).

There is also an extremely rare condition called 'acquired haemophilia' in which a person develops antibodies to one of the clotting factors in their own blood.