What is haemophilia?
Types of haemophilia
What is the incidence/prevalence?
Haemophilia is a genetic bleeding disorder in which the blood fails to clot normally. Blood clotting is the process by which protein factors and platelets activate one another to form a plug at a site of bleeding such as a torn blood vessel to prevent further loss of blood from the site. Over time the initial plug forms into a scab and healing of the torn vessel occurs. Clots are formed by the activation in the blood of special proteins called clotting factors, these factors activate one another in a sequential manner or ‘cascade’ that results in a blood clot over a few minutes and this stops bleeding. When a clotting factor is missing, only present in low amounts or in an ineffective form the cascade cannot complete. An ineffective clot is formed so bleeding continues.
This bleeding most frequently occurs internally.
Types of haemophilia
There are two types -:
Haemophilia A – this is the commonest type of haemophilia. In this condition clotting factor VIII is abnormally low or absent.
Haemophilia B – this is the less common type of haemophilia. In this condition clotting factor IX is abnormally low or absent. Also called Christmas disease.
These conditions are both sex linked recessive conditions. This means that the abnormal gene is carried on the X chromosome and the condition affects boys. The X chromosome is a long chromosome that pairs with another X chromosome in women (XX) and with a very short Y chromosome in men (XY). The Y chromosome does not carry the genes for clotting factors. Women have two copies of the X chromosome carrying genes for clotting factors VIII and IX. In women with the haemophilia gene on one X chromosome the unaffected X chromosome ensures enough clotting factor is produced. In boys with the affected X chromosome there is no second X chromosome to ensure clotting factor production and they have haemophilia. Boys with the affected chromosome will be affected by haemophilia, girls with the affected X chromosome will not have haemophilia but they will be carriers and their male children have a 50% chance of being affected by haemophilia.
What is the incidence/prevalence?
The prevalence of haemophilia in the UK is 194 cases of haemophilia per million males. Around 6000 boys and men have haemophilia in the UK.
Bleeding disorders caused by blood clotting factor deficiencies
