A group of hereditary genetic disorders that impair the body’s ability to control blood clotting or coagulation, which is used to stop bleeding when a blood vessel is broken.
- Haemophilia A (clotting factor VIII deficiency) is the most common form of the disorder, present in about 1 in 5,000–10,000 male births.
- Haemophilia B (factor IX deficiency) occurs in around 1 in about 20,000–34,000 male births.
Like most recessive sex-linked, X chromosome disorders, haemophilia is more likely to occur in males than females. This is because females have two X chromosomes while males have only one, so the defective gene is guaranteed to manifest in any male who carries it. Because females have two X chromosomes and haemophilia is rare, the chance of a female having two defective copies of the gene is very low, so females are almost exclusively asymptomatic carriers of the disorder. Female carriers can inherit the defective gene from either their mother or father, or it may be a new mutation. Although it is not impossible for a female to have haemophilia, it is unusual: a female with Haemophilia A or B would have to be the daughter of both a male haemophiliac and a female carrier, while the non-sex-linked Haemophilia C, which can affect either sex, is extremely rare.
How it works?
Haemophilia lowers blood plasma clotting factor levels of the coagulation factors needed for a normal clotting process. Thus when a blood vessel is injured, a temporary scab does form, but the missing coagulation factors prevent fibrin formation, which is necessary to maintain the blood clot. A haemophiliac does not bleed more intensely than a person without it, but can bleed for a much longer time. In severe haemophiliacs even a minor injury can result in blood loss lasting days or weeks, or even never healing completely. In areas such as the brain or inside joints, this can be fatal or permanently debilitating. Deficiency in coagulation factor VIII is the most common cause of haemophilia.
Characteristic symptoms vary with severity. In general symptoms are internal or external bleeding episodes, which are called “bleeds”. Patients with more severe haemophilia suffer more severe and more frequent bleeds, while patients with mild haemophilia typically suffer more minor symptoms except after surgery or serious trauma. Moderate haemophiliacs have variable symptoms which manifest along a spectrum between severe and mild forms.
Severe complications are much more common in severe and moderate haemophiliacs. Complications may be both directly from the disease or from its treatment:
- Deep internal bleeding, e.g. deep-muscle bleeding, leading to swelling, numbness or pain of a limb.
- Joint damage from haemarthrosis, potentially with severe pain, disfigurement, and even destruction of the joint and development of debilitating arthritis.
- Transfusion transmitted infection from blood transfusions that are given as treatment.
- Adverse reactions to clotting factor treatment, including the development of an immune inhibitor which renders factor replacement less effective.
- Intracranial haemorrhage is a serious medical emergency caused by the buildup of pressure inside the skull. It can cause disorientation, nausea, loss of consciousness, brain damage, and death.
Types and Causes
- Haemophilia A is a recessive X-linked genetic disorder involving a lack of functional clotting Factor VIII and represents 80% of haemophilia cases.
- Haemophilia B is a recessive X-linked genetic disorder involving a lack of functional clotting Factor IX. It comprises approximately 20% of haemophilia cases.
Haemophilia C is an autosomal genetic disorder (i.e. not X-linked) involving a lack of functional clotting Factor XI. Haemophilia C is not completely recessive: heterozygous individuals also show increased bleeding.