Hemophilia type B is a genetic disorder caused by missing or defective factor IX, a blood clotting protein. The disorder can cause prolonged bleeding or oozing after surgery or an injury. In severe cases, spontaneous bleeding may occur in the absence of injury.
Hemophilia type B is about four times rarer than type A and can affect all ethnic groups equally. A subtype of hemophilia type B, known as hemophilia B Leyden, causes episodes of excessive bleeding in childhood, which gradually disappear after puberty.
Symptoms of hemophilia type B include reduced clotting factor IX activity, prolonged or delayed blood clotting, persistent bleeding after trauma, osteoarthritis, bleeding at the joints, gastrointestinal bleeding, and generalized abnormal bleeding.
Hemophilia type B is inherited in an X-linked manner, meaning that the disease-causing mutation is located on the X chromosome, one of two gender-determining chromosomes.
Females inherit two X chromosomes, one from their mother and one from their father. Males inherit an X chromosome from their mother and a Y chromosome from their father. This means that if a boy inherits an X chromosome from his mother that carries a disease-causing mutation, he will develop hemophilia.
Women who have a single copy of a disease-causing mutation may still develop the disease. During development, one of the two X chromosomes is randomly inactivated, to ensure that cells do not make twice as much of some proteins. If the X chromosome carrying the disease-causing mutation is inactivated, women are unlikely to develop hemophilia, although there’s still a risk they will transmit the disease to their children. However, if the X chromosome with the healthy copy of the gene is the one that’s inactivated, girls can also develop the disease.
About one-third of hemophilia type B cases are not inherited but caused by a spontaneous genetic mutation.
Although family medical history can be useful in diagnosing hemophilia type B, about one-third of patients have no family history of the disease. After a symptomatic event (an accident or injury in which slow clotting is noted), patients can be diagnosed using clotting tests that assess the levels of clotting factors in the blood. Genetic testing can also be used to determine whether a patient has a disease-causing mutation in their clotting factor genes.
Hemophilia type B is categorized by severity, ranging from mild to moderate to severe. Mild hemophilia patients have between 6% and 49% of normal levels of clotting factor protein in their blood. Moderate hemophilia patients have between 1% and 5% of normal levels. Severe hemophilia patients have less than 1% of normal levels of clotting factor protein in their blood.
Hemophilia type B is a treatable disease with replacement therapy where an infusion of the missing clotting factor is given to patients, usually after an accident or injury.
Patients with a severe form of the disease may need prophylactic therapy that provides periodic infusions of clotting factor IX. Many patients can be trained to self-administer these infusions.
Some patients may be treated with antifibrinolytics, which are medications that slow the process of breaking down clotting factors in the blood. These treatments can slightly increase the levels of clotting factors but are not enough to treat severe forms of hemophilia.
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