|Classification and external resources|
Deficiency in coagulation factor VIII is the cause of haemophilia A.
Hemophilia A is inherited as an X-linked recessive trait, and thus occurs in males and in homozygous females. However, mild hemophilia A (and B) is known to occur heterozygous females due to X-inactivation, so it is recommended that levels of factor VIII and IX be measured in all known or potential carriers prior to surgery and in the event of clinically significant bleeding.:1701
Hemophilia A (incidence) occurs in approximately 1 in 5,000 males.:1700The incidence of hemophilia B is 1 in 30,000 in male population. :1700 Of these, 85% have hemophilia A and 15% have hemophilia B.:1700
5-10% of patients with hemophilia A are affected because they make a dysfunctional version of the factor VIII protein (qualitative deficiency), while the remainder are affected because they produced factor VIII in insufficient amounts (quantitative deficiency).:1700 Of those who have severe deficiency (defined as <1% activity of factor VIII), 45-50% have the same mutation, an inversion within the factor VIII gene that results in total elimination of protein production.:1700 However, since both forms of hemophilia can be caused by a variety of different mutations, initial diagnosis and classification is done by measurement of protein activity rather than by genetic tests, though genetic tests are recommended for testing of family members once a known case of hemophilia B is identified.:1700-1701
Approximately 30% of patients have no family history; their disease is presumably caused by new mutations.
There are numerous different mutations which cause haemophilia A. Due to differences in changes to the gene involved (and the subsequent resulting protein), patients with haemophilia often have some level of active clotting factor. Individuals with less than 1% active factor are classified as having severe haemophilia, those with 1–5% active factor have moderate haemophilia, and those with mild haemophilia have between 5–40% of normal levels of active clotting factor.
Signs and symptoms 
Characteristic symptoms vary with severity. In general symptoms are internal or external bleeding episodes, which are called "bleeds". Patients with more severe hemophilia suffer more severe and more frequent bleeds, while patients with mild hemophilia typically suffer more minor symptoms except after surgery or serious trauma. Moderate hemophiliacs have variable symptoms which manifest along a spectrum between severe and mild forms.
If forceps or vacuum extraction are used in vaginal births, the first signs of hemophilia may be severe head bruising or hematomas or even intracranial hemorrhage. Prolonged bleeding from a circumcision wound or a venepuncture or heelprick is another common early sign of hemophilia. These signs may lead to blood tests which indicates hemophilia. In other patients, especially those with moderate or mild hemophilia a later trauma will lead to the first serious bleed.
Hemophilia leads to a severely increased risk of prolonged bleeding from common injuries, or in severe cases bleeds may be spontaneous and without obvious cause. Bleeding may occur anywhere in the body. Superficial bleeding such as those cause by abrasions, or shallow lacerations may be prolonged and the scab may easily be broken up due to the lack of fibrin, which may cause re-bleeding. While superficial bleeding may be troublesome, by far the most serious sites of bleeding are:
The muscle and joint hemorrhages are indicative of hemophilia, while digestive tract and cerebral hemorrhages are also germane to other coagulation disorders.
Though typically not life threatening, joint bleeds are one of the most serious symptoms of hemophilia. Repeated bleeds into a joint capsule can cause permanent joint damage and disfigurement resulting in chronic arthritis and disability. Joint damage is not a result of blood in the capsule but rather the healing process. When blood in the joint is broken down by enzymes in the body, the bone in that area is also degraded. Therefore, this exerts massive amounts of pain upon the person afflicted with the disease.
The diagnosis may be suspected as coagulation testing reveals an increased PTT in the context of a normal PT and bleeding time. PTT tests are the first blood test done when haemophilia is indicated. However, the diagnosis is made in the presence of very low (<10 IU) levels of Factor VIII.
Diagnosis of hemophilia A also includes a severity level which can range from mild to severe based on the amount of active and functioning factor VIII detected in the blood. Factor VIII levels do not typically change throughout an individual's life. Severe haemophilia A is the most common form occurring in 60% of patients. Severe haemophilia A is indicated by active Factor VIII levels less than 1 or 2% Severe haemophiliacs have frequent serious bleeds (20+ times per year), these bleeds often occur spontaneously without trauma or injury. Moderate haemophilia is indicated by active Factor VIII levels between 1–5% and a lower, but variable frequency of bleeding episodes, particularly of spontaneous bleeds. Mild haemophilia is indicated by active Factor VIII levels between 5–25%. Patients with mild haemophilia often experience few or no bleeding episodes except in the case of serious trauma (i.e. compound fracture of a bone), tooth extraction, or surgery.
Differential diagnosis 
The two most common differential diagnoses are haemophilia B (also known as Christmas disease) which is a deficiency in Factor IX and von Willebrand Disease which is a deficiency in von Willebrand factor needed for the proper functioning of Factor VIII. Haemophilia C is also a possible, but rare, differential diagnosis.
Most severe hemophilia patients require regular supplementation with intravenous recombinant or plasma concentrate Factor VIII. The prophylactic treatment regime is highly variable and individually determined. Apart from "routine" supplementation, extra factor concentrate is given around surgical procedures and after trauma. In children, an easily accessible intravenous port (e.g. Port-a-Cath) may have to be inserted to minimise frequent traumatic intravenous cannulation. These devices have made prophylaxis in hemophilia much easier for families because the problems of "finding a vein" for infusion two to three times a week are eliminated. However, there are risks involved with their use, the most worrisome being that of infection. Studies differ but some show an infection rate as high as 50%. These infections can usually be treated with intravenous antibiotics but sometimes the device must be removed. Also, there are other studies that show a risk of clots forming at the tip of the catheter. Still, many families choose to use the device because of the benefits.
Some patients with severe hemophilia and most with moderate and mild hemophilia treat only as needed without a regular prophylactic schedule. Due to risk of permanent disability, prophylactic treatment is always indicated if a "target joint" (a joint that has repeated bleeding episodes) is identified.
Mild hemophiliacs often manage their condition with desmopressin, which releases stored factor VIII from blood vessel walls.
A particular therapeutic conundrum is the development of "inhibitor" antibodies against factor VIII due to frequent infusions. These develop as the body recognises the "normal form" factor VIII as foreign, as the body does not have its own "copy". The problem is that in these patients, factor VIII infusions are ineffective. Recently[update] activated factor VII (NovoSeven) has become available as a treatment for haemorrhage in patients with haemophilia and factor inhibitors.
See also 
- Merck Manual of Diagnosis and Therapy Home Edition Hemophilia
- Kliegman, Robert (2011). Nelson textbook of pediatrics. (19th ed. ed.). Philadelphia: Saunders. ISBN 978-1-4377-0755-7. Retrieved 8 January 2012.
- Bowen DJ: Hemophilia A and hemophilia B: molecular insights. Mol Pathol 2002; 55:1
- Hemophilia overview Emedicine.medscape.com, Dimitrios P Agaliotis, Robert A Zaiden, and Saduman Ozturk. Jan. 2, 2008.
- Types of Bleeds National Hemophilia Federation.
- Key facts: what is hemophilia? The Hemophilia Society.
- Kelley, L.A. 2007 "Raising a child with hemophilia: a practical guide for parents, 4th edition." LA Kelley Communications, Inc. USA. Sponsored by CSL Behring.