Haemophilia B is a hereditary bleeding disorder caused by insufficient production of clotting factor IX, a protein essential for blood to form clots and stop bleeding. Although this condition predominantly affects males due to its X-linked inheritance pattern, carrier females may also experience bleeding symptoms, and understanding how to manage this lifelong condition can help patients lead full and healthy lives.

Understanding the Numbers: How Common Is Haemophilia B?

Haemophilia B represents the second most common form of haemophilia, though it occurs far less frequently than haemophilia A. This condition earned the name “Christmas disease” after Stephen Christmas, the first person diagnosed with it in 1952. The disorder is sometimes called “the royal disease” due to its notable presence in the royal families of Spain, Germany, England, and Russia throughout history.^[2]

Across the globe, haemophilia B affects approximately one in every 19,000 male births. The condition is about three to four times less common than haemophilia A. In the United States, based on data collected from federally funded haemophilia treatment centres (comprehensive clinics that treat people who have bleeding disorders) between 2012 and 2018, roughly 33,000 males are living with haemophilia of all types, with haemophilia B representing a smaller portion of these cases.^[4]

The condition affects people from all racial and ethnic groups equally, showing no preference for any particular population. About 50 percent of people with haemophilia B have factor IX levels greater than 1 percent, which typically means they experience less severe symptoms than those with lower factor levels.^[8]

What Causes Haemophilia B?

Haemophilia B results from a defect in the F9 gene, which provides instructions for making factor IX, one of the special proteins needed for blood to clot properly. When you bleed, a series of reactions called the coagulation cascade takes place in your body to help blood clots form. This process involves as many as 20 different special proteins called coagulation or clotting factors. When the F9 gene doesn’t work correctly, the body cannot make enough functional factor IX, and blood cannot clot properly to control bleeding.^[2]

The genetic defect can occur in two ways. Most commonly, it happens through X-linked inheritance, where the faulty gene is passed down from parents to their children. Less commonly, it can appear as a spontaneous de novo mutation, meaning the genetic change occurs for the first time in an affected person without being inherited from either parent.^[2]

The inheritance pattern explains why haemophilia B predominantly affects males. Females have two copies of the X chromosome, and if the factor IX gene on one chromosome doesn’t work, the gene on the other chromosome can usually do the job of making enough factor IX. Males, however, have only one X chromosome. If the factor IX gene is missing or not working on a boy’s X chromosome, he will have haemophilia B because he has no backup copy of the gene.^[3]

If a woman carries a variant factor IX gene, she is considered a carrier. Boys born to such women have a 50 percent chance of having haemophilia B, while their daughters have a 50 percent chance of being carriers themselves. All female children of men with haemophilia carry the variant gene, whereas male children do not inherit the condition from their fathers.^[3]

Who Is at Risk for Haemophilia B?

The primary risk factor for haemophilia B is having a family history of bleeding disorders. Because the condition follows an X-linked inheritance pattern, being male significantly increases the risk of developing the disorder if the genetic mutation is present in the family. Women who carry the variant gene can pass it to their children, making family history crucial in identifying risk.^[3]

Heterozygous women who carry one copy of the F9 genetic mutation may have varying levels of factor IX in their blood. Those with factor IX levels at or above 50 percent of normal are usually asymptomatic and may never experience bleeding problems. However, approximately 30 percent of heterozygous females have factor IX clotting activity lower than 40 percent and are at risk for bleeding, even if the affected family member has only mild haemophilia B. For these women, bleeding severity generally correlates with their factor IX levels, and they may experience prolonged or excessive bleeding after major trauma or invasive procedures, regardless of the severity of haemophilia in their male relatives.^[5]

Males born into families with a known history of haemophilia B should be carefully monitored from birth. The severity of symptoms in other family members can provide clues about what to expect, though each individual’s experience may vary. Even in families with no known history of bleeding disorders, spontaneous genetic mutations can occur, making any male infant potentially at risk, though this is rare.^[2]

Recognizing the Symptoms of Haemophilia B

The severity of symptoms in haemophilia B varies depending on the level of factor IX clotting activity in the blood. In any individual with haemophilia B, bleeding episodes may be more frequent in childhood and adolescence than in adulthood. The main symptom is prolonged bleeding, which is often first noticed when an infant boy is circumcised.^[5]

People with severe haemophilia B, defined as having less than 1 percent normal factor, are usually diagnosed during the first two years of life. Without preventive treatment, they may average up to two to five spontaneous bleeding episodes each month. These can include spontaneous joint or muscle bleeds that occur without any obvious injury, as well as prolonged bleeding or excessive pain and swelling from minor injuries, surgery, and tooth extractions.^[5]

Those with moderate haemophilia B, having between 1 and 5 percent normal factor, experience less frequent spontaneous bleeding, although this varies between individuals. They typically have prolonged or delayed oozing after relatively minor trauma and are usually diagnosed before age five to six years. The frequency of bleeding episodes in moderate cases can vary from once a month to once a year.^[5]

Individuals with mild haemophilia B, having more than 5 percent but no more than 40 percent normal factor, generally do not have spontaneous bleeding episodes. However, without pre- and post-operative treatment, abnormal bleeding occurs with surgery or tooth extractions. The frequency of bleeding in mild cases may vary from once a year to once every ten years. People with mild haemophilia B are often not diagnosed until later in life when they undergo surgery or experience significant trauma.^[5]

Common symptoms of bleeding problems include bleeding into joints, which causes swelling, pain, or tightness and often affects the knees, elbows, and ankles. Bleeding into the skin appears as bruising, while bleeding into muscle and soft tissue causes a build-up of blood in the area called a hematoma (a collection of blood outside blood vessels). Other symptoms include bleeding of the mouth and gums, bleeding that is hard to stop after losing a tooth, bleeding after having shots such as vaccinations, blood in the urine or stool, and frequent, hard-to-stop nosebleeds.^[4]

Other bleeding problems usually show up when an infant starts crawling and walking. Babies with this condition who bump their mouths on a toy may bleed from their mouths for extended periods. They may develop large bruises after minor injuries or have goose eggs, which are large, round bumps that develop if they bump their heads. Babies may have bruises or swollen areas on their arms and legs, and they may be fussy or refuse to crawl if they have painful joint bleeds.^[6]

Preventing Bleeding Complications

While haemophilia B cannot be prevented because it is an inherited genetic condition, many strategies can help prevent bleeding episodes and their complications. The most effective approach is prophylactic treatment, which involves regular infusions of factor IX concentrate to maintain factor IX clotting activity higher than 1 percent. This preventive therapy significantly improves outcomes and prevents chronic joint disease.^[5]

For those with severe haemophilia B and those with moderate or mild forms who experience frequent bleeding, prophylaxis has been shown in many studies to prevent or at least reduce the progression of damage to target sites, such as joints. Preventive therapy started early in childhood, compared with on-demand treatment given only when bleeding occurs, can reduce total bleeds and bleeding into joints, decreasing overall joint deterioration and improving quality of life.^[9]

Physical safety measures play a crucial role in preventing injuries that could lead to bleeding. People with haemophilia B should choose low-impact activities such as swimming, walking, or yoga. It’s best to avoid contact sports such as football and hockey, though others like baseball or tennis can be good options when proper precautions are taken, such as wearing a mouth guard and helmet. Talking to a physical therapist at a haemophilia treatment centre before starting an exercise programme ensures the activity is safe and appropriate.^[15]

Regular dental care is essential because complications from teeth and gum problems can cause bleeding that may be more serious for people with haemophilia B. Regular dental cleanings and good oral hygiene can prevent dental problems that might lead to excessive bleeding. Before any dental procedure, patients should inform their dentist about their condition and may need preventive factor IX infusions.^[15]

Vaccination against hepatitis B is important for people with haemophilia B. They are more likely to get hepatitis B because they may receive blood products during their treatment, and preventing this infection protects the liver from additional damage.^[3]

Understanding bleeding risks and recognizing emergencies are fundamental steps in prevention. Patients and their families should learn to recognize early signs of bleeding and know when to seek immediate medical attention. Having a plan for home management of bleeding, including keeping adequate supplies of factor IX concentrate on hand, enables quick treatment that can prevent minor bleeds from becoming serious complications.^[16]

How Haemophilia B Affects the Body

To understand how haemophilia B affects the body, it helps to know how normal blood clotting works. When you bleed, your body initiates a complex series of reactions called the coagulation cascade. This cascade involves approximately 20 different special proteins called coagulation factors, each playing a specific role in forming a blood clot. These factors work together in a precise sequence, with each factor activating the next one in line.^[3]

Factor IX plays a crucial role in this cascade. In the normal clotting process, factor IX is activated and then works with other factors to help convert a protein called prothrombin into thrombin. Thrombin then converts another protein, fibrinogen, into fibrin, which forms a mesh that traps blood cells and creates a stable clot. This clot plugs the wound and stops bleeding.^[2]

In haemophilia B, the body either doesn’t make enough factor IX or makes a version that doesn’t work properly. This deficiency disrupts the coagulation cascade. Without sufficient functional factor IX, the chain reaction that normally leads to clot formation cannot proceed effectively. The blood can still clot to some degree, which is why people with haemophilia don’t bleed faster than others, but they bleed for a longer time because stable clots form too slowly or not at all.^[3]

The mechanical and physical consequences of this clotting deficiency vary depending on where bleeding occurs. Bleeding into joints, called hemarthrosis, is one of the most common and problematic manifestations. When blood accumulates in a joint space, it causes immediate pain, swelling, and restricted movement. Over time, repeated bleeding into the same joint damages the cartilage and bone, leading to chronic joint disease, arthritis, and potentially permanent disability. This is why joints, especially the knees, elbows, and ankles, are often called “target joints” in haemophilia patients.^[4]

Bleeding into muscles creates hematomas that can compress nerves and blood vessels, potentially causing pain, numbness, or even permanent nerve damage if not treated promptly. Large muscle bleeds can also lead to compartment syndrome, a serious condition where pressure builds up in an enclosed muscle space.^[3]

Internal bleeding can occur in any organ system. Gastrointestinal bleeding may result in blood in the stool, while urinary tract bleeding causes blood in the urine. Bleeding in the brain, though rare, is the most dangerous complication because it can cause permanent neurological damage or death. Even minor head injuries that wouldn’t cause problems in people without haemophilia can lead to serious complications in those with the condition.^[3]

The biochemical aspect of haemophilia B involves the specific genetic mutations in the F9 gene. More than 1,000 different mutations have been identified in this gene, and the type and location of the mutation can influence the severity of the disorder. Some mutations result in no factor IX production at all, while others allow some factor IX to be made but in a form that doesn’t function properly. Understanding the specific mutation can help predict disease severity and the likelihood of developing inhibitors, which are antibodies that can form against infused factor IX and make treatment less effective.^[5]

The body’s response to repeated bleeding episodes also creates pathophysiological changes. Chronic inflammation in joints from repeated bleeds leads to synovial hypertrophy, where the joint lining becomes thick and prone to bleeding even more easily. This creates a vicious cycle where damaged joints bleed more frequently, leading to more damage. The iron from broken-down blood cells deposited in joint tissues contributes to the inflammatory process and joint destruction.^[9]