La hemofilia es una enfermedad genética recesiva que impide la buena coagulación de la sangre. Está relacionada con el cromosoma X y existen tres tipos: la hemofilia A, cuando hay un déficit del factor VIII de coagulación, la hemofilia B, cuando hay un déficit del factor IX de coagulación, y la hemofilia C, cuando hay un déficit en el factor XI de coagulación.

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Haemophilia also spelled hemophilia, is a mostly inherited, genetic disorder that impairs the body's ability to make blood clots, a process needed to stop bleeding. This results in people bleeding longer after an injury, easy bruising, and an increased risk of bleeding inside joints or the brain. Those with mild disease may only have symptoms after an accident or during surgery. Bleeding into a joint can result in permanent damage while bleeding in the brain can result in long term headaches, seizures, or a decreased level of consciousness. There are two main types of haemophilia: haemophilia A, which occurs due to not enough clotting factor VIII, and haemophilia B, which occurs due to not enough clotting factor IX. They are typically inheriting from one's parents an X chromosome with a nonfunctional gene. Rarely a new mutation may occur during early development or haemophilia may develop later in life due to antibodies forming against a clotting factor. Other types include haemophilia C, which occurs due to not enough factor XI, and parahaemophilia, which occurs due to not enough factor V. Acquired haemophilia is associated with cancers, autoimmune disorders, and pregnancy. Diagnosis is by testing the blood for its ability to clot and its levels of clotting factors. Prevention may occur by removing a woman's egg, fertilizing it, and testing the embryo before putting it in the uterus. Treatment is by replacing the missing blood clotting factors. This may be done on a regular basis or during bleeding episodes. Replacement may take place at home or in hospital. The clotting factors are made either from human blood or by recombinant methods. by Up to 20% of people develop antibodies to the clotting factors which makes treatment more difficult. The medication desmopressin may be used in those with mild haemophilia A. Studies of gene therapy are in early human trials. Haemophilia A affects about 1 in 5,000–10,000, while haemophilia B affects about 1 in 40,000, males at birth. As haemophilia A and B are X-linked recessive disorders females are very rarely severely affected. Some females with a nonfunctional gene on one of the X chromosomes may be mildly symptomatic. Haemophilia C occurs equally in both sexes and is mostly found in Ashkenazi Jews. In the 1800s haemophilia was common within the royal families of Europe. The difference between haemophilia A and B was determined in 1952. The word is from the Greek haima αἷμα meaning blood and philia φιλία meaning love.

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