Hemophilia B is a hereditary disorder and is an X-linked recessive trait, which means that the defective gene is on the X chromosome. Females have two copies of the X chromosome, so if the FIX gene on one chromosome does not work, the gene on the other chromosome can sometimes do the job of making enough FIX. One case where a female might have a genetic FIX deficiency is called lyonization or x-inactivation, where some or all of one copy of the X chromosome is inactivated. This is called lyonization because in 1961, a scientist named Mary Lyon proposed the random inactivation of one X chromosome in female mice. One X chromosome can “shut down” or not express itself. If the X chromosome with the defective hemophilia B gene lyonizes the other X chromosome, a female may have lower factor levels. Research is beginning to indicate that 50% or more of carriers have low factor levels. Because males have only one X chromosome, any male born with the FIX gene defect on that chromosome will have hemophilia B. For this reason, most people diagnosed with hemophilia B are male. For more information on the genetics of hemophilia B, please click here.
Hemophilia B may also be acquired, caused by illness or medication (both resolvable with treatment) or a liver transplant from someone who had undiagnosed hemophilia. Acquiring hemophilia B from a liver transplant is extremely rare.
Symptoms and Diagnosis of Hemophilia B
If you have hemophilia B, you likely bleed longer than someone without hemophilia; you can bleed internally or externally. People with hemophilia B do not bleed more than people without hemophilia, they just bleed longer. The most common sites of bleeding are into the joints and muscles. Baby boys with no family history are often diagnosed when they are circumcised. Other bleeding problems usually show up when babies start crawling and walking. However, if you have mild hemophilia B, you might not be diagnosed until much later in life, possibly after a physical trauma or a surgery.
A female carrier of hemophilia who experiences bleeding symptoms is called a symptomatic carrier if her level is over 50%, and she has bleeding symptoms. Carrier females with less than 50% (and greater than 5%) have mild hemophilia B, while those with 1% to 5% have moderate hemophilia B, and those with less than 1% have severe hemophilia B. Women with hemophilia and symptomatic carriers have the same symptoms as anyone with hemophilia. Some carriers of hemophilia B have completely normal levels of FIX and do not exhibit any signs of bleeding.
The most common hemophilia B symptoms are as follows:
- Bleeding into muscles and joints, which can cause pain and swelling
- Prolonged bleeding from cuts, dental work, tooth extraction, or surgery
- Easy bruising (unexplained bruising)
- Blood in the urine or stool
- Gastrointestinal tract and urinary tract bleeding
- Bleeding that starts without cause
- Large bruises/hematomas
- Menorrhagia (heavy or prolonged menstrual bleeding: longer than 5 days, more than 90 ml of bleeding, and/or clots larger than a grape)
Hemophilia B complications may include the following:
- Long-term joint problems, possibly requiring joint replacements
- Bleeding in the brain (intracranial hemorrhage)
- Blood clots due to treatment (this is a rare complication)
- Bloodborne diseases, such as human immunodeficiency virus (HIV) and the hepatitis C virus (HCV) occurred in the past and are currently an extremely low risk for patients who were not previously affected.
If you exhibit any of these symptoms and are not diagnosed, you should pursue a diagnosis. Ask your health care provider to refer you to a hematologist, who will order a series of tests called a coagulation study. If you are diagnosed with hemophilia B, you should meet with the HTC or a genetics counselor to determine who else in your family could be affected. You can also request genetic testing so that you know your specific genetic mutation. For more information on who might be affected, please click here.
A coagulation study to diagnose hemophilia B might include the following:
- Partial thromboplastin time (PTT)
- Prothrombin time
- Fibrinogen level
- Platelet count
- Serum FIX activity
Carrier Testing for Hemophilia B
A mother who has a son with hemophilia B and who has no family history of the condition may or may not be a carrier of hemophilia, depending on where the genetic mutation occurred. A mother who has a son with hemophilia B should consider having her DNA tested for her son’s genetic mutation to see if she is a carrier of hemophilia. She may have low FIX levels herself, but just having a low FIX level is not diagnostic of being a carrier. However, that information could be critical in her well-being in cases of surgery or severe trauma. The genetic information may be important when planning subsequent pregnancies and may be necessary to share with other female family members who are thinking of having children.
Severity of Hemophilia B
Severity of hemophilia B refers to the amount of clotting factor that is present a person’s blood. People with severe hemophilia tend to have more frequent bleeding, while people with mild hemophilia may only bleed with significant trauma, dental procedures, or surgery. There are exceptions to this, and not everyone bleeds “by the numbers.”
|Hemophilia B Severity Ranges|
|Normal factor levels||50% – 150%|
|Mild hemophilia B||5% – 49%|
|Moderate hemophilia B||1%-5%|
|Severe hemophilia B||Less than 1%|