Haemophilia B is characterised by spontaneous and provoked bleeding into joints and muscles. Severity is determined by residual factor IX clotting activity: severe disease (<1% activity) typically presents in the first two years of life with spontaneous joint and muscle bleeding; moderate disease (1-5%) is usually recognised by age 6 years; mild disease (>5% to <40%) often comes to attention later in life with haemostatic challenges such as surgery, dental extractions, or trauma.
Bleeding episodes affect joints and muscles most prominently, with additional mucocutaneous, gastrointestinal, genitourinary, and central nervous system involvement. In severe disease, spontaneous joint bleeding is the most frequent sign; intracranial haemorrhage is the leading cause of bleeding-related mortality, and repeated bleeding into joints is the principal cause of bleeding-related disability. Spontaneous bleeding generally only affects severely and moderately affected patients. Female carriers with factor IX clotting activity below 40% are at risk for bleeding comparable to affected males with similar factor levels. Most common symptoms in females are menorrhagia, bruising, and epistaxis.
