Haemophilia is caused by an inherited genetic mutation, which mainly affects males.

A genetic mutation is a permanent alteration in the DNA sequence that makes up a gene. This means that some of the body's processes won't work in the normal way.

The type of mutation determines whether a person will experience mild, moderate or severe symptoms.

How the mutation is inherited

The gene mutation is found on the X chromosome. It can be carried by either the mother or father, or both.

You can read more about chromosomes and how we inherit mutations by reading about genetics.

The chances of a child inheriting the haemophilia mutation depend on which of their parents has the mutated gene.

Only the mother has the mutated gene

If a woman with the mutated chromosome and an unaffected man have a baby, there's a:

• one in four chance of having an unaffected baby boy
• one in four chance of having a baby boy with haemophilia
• one in four chance of having an unaffected baby girl
• one in four chance of having a baby girl with an affected X chromosome

In the last situation, the girl becomes a carrier of the mutated gene. This means she can pass it on to her children, but won't usually have any severe symptoms of haemophilia herself.

However, some female carriers sometimes have bleeding problems, such as heavy periods.

Only the father has the mutated gene

If a man with haemophilia has a son with an unaffected woman, there's no chance the boy will get haemophilia.

This is because he always inherits his X chromosome from his mother, who in this case doesn't have the mutated gene.

However, any daughters the man has will become carriers of the mutated haemophilia gene and may pass it on to their children. 

Both parents have the mutated gene

If a woman with the mutated chromosome and a man with haemophilia have a baby, there's a:

• one in four chance of having an unaffected baby boy
• one in four chance of having a baby boy with haemophilia
• one in four chance of having a baby girl who's a carrier of haemophilia
• one in four chance of having a baby girl with haemophilia

This means it's possible for a female to have haemophilia, although it's very rare.

No family history

In some cases, a boy is born with haemophilia even though there's no family history of the condition.

In such cases, it's thought the mutation developed spontaneously in the boy's mother, grandmother or great-grandmother, but until then a male member of the family had never inherited the mutated gene.

Some studies have shown there's no known family history of haemophilia in up to a third of new cases.

How haemophilia affects the blood

Blood cells called platelets are very important for blood clotting. These cells have a sticky surface that allows them to clump together to stop the flow of blood.

Platelets also need clotting factors. These are proteins that form a "web" around the platelets, helping them to stay in place.

The mutated haemophilia gene means a child with the condition doesn't have enough clotting factors in their blood.

Several different clotting factors are present in the blood. They are numbered using roman numerals.

For example, in haemophilia A there's not enough clotting factor VIII (8) in the blood. In haemophilia B, there's not enough clotting factor IX (9) in the blood.

The symptoms of haemophilia depend on how severe the condition is, but the main sign is prolonged bleeding.

The bleeding may occur spontaneously. For example, this could be:

• sudden nosebleeds
• bleeding gums
• joint bleeds (bleeding inside your joints, such as elbows)
• muscle bleeds

The bleeding may also occur after a medical procedure, such as having a tooth removed.

The severity of haemophilia is determined by the level of clotting factors in a person's blood:

• mild haemophilia – between 5 and 50% of the normal amount of clotting factors
• moderate haemophilia – between 1 and 5% 
• severe haemophilia – less than 1% 

Find out when to seek emergency medical help.

Mild haemophilia

Children born with mild haemophilia may not have any symptoms for many years.

The condition usually only becomes apparent after a significant wound or surgery, or a dental procedure such as having a tooth removed. These events could cause unusually prolonged bleeding.

Moderate haemophilia

Children with moderate haemophilia are affected in the same way as those with mild haemophilia, but also bruise easily.

They may also have symptoms of internal bleeding around their joints, particularly if they have a knock or a fall that affects their joints. This is known as a joint bleed.

The symptoms usually begin with a tingling feeling of irritation and mild pain in the affected joint – most commonly the ankles, knees, and elbows. Less commonly, the shoulder, wrist, and hip joints can also be affected.

If a joint bleed isn't treated, it can lead to:

• more severe joint pain
• stiffness
• the site of the bleed becoming hot, swollen, and tender

Severe haemophilia

The symptoms of severe haemophilia are similar to those of moderate haemophilia. However, joint bleeding is more frequent and severe.

Children with severe haemophilia have spontaneous bleeding. This means they start bleeding for no apparent reason.

For example, this could be:

• nosebleeds
• bleeding gums
• joint bleeds
• muscle bleeds

Without treatment, people with severe haemophilia can develop:

• joint deformity, which may require replacement surgery
• soft tissue bleeding
• serious internal bleeding

When to seek emergency medical help

There's a small risk of bleeding inside the skull, known as an intracranial haemorrhage. It's estimated 3% of people with moderate or severe haemophilia will have an intracranial haemorrhage. 

However, spontaneous bleeding inside the skull is uncommon and usually only caused by a head injury.

Bleeding in the skull should be treated as a medical emergency.

The symptoms of an intracranial haemorrhage include:

• a severe headache
• a stiff neck
• vomiting
• a change in mental state, such as confusion
• speaking difficulties, such as slurred speech
• changes in vision, such as double vision
• loss of co-ordination and balance
• paralysis of some or all of the facial muscles

Call 999 for an ambulance if you think someone is bleeding inside the skull.

The recommended treatment plan for haemophilia depends on how severe the condition is.

There are two main approaches to treatment:

• preventative treatment – where medicine is used to prevent episodes of bleeding and subsequent joint and muscle damage
• on-demand treatment – where medicine is used to treat an episode of prolonged bleeding

Haemophilia is usually treated by a team at a specialist haemophilia hospital department.

Preventative treatment

Most cases of haemophilia are severe and need preventative treatment (prophylaxis). This involves regular injections of clotting factor medicine.

If your child has haemophilia, you'll be trained to give them the injections when they're young.

They'll be taught how to inject themselves when they're older to help avoid regular hospital appointments.

In some cases, injections may be given into a device called an implantable port, which can be surgically placed under the skin.

This port is connected to a blood vessel near the heart, so you don't need to try to find a vein for every injection.

People having preventative treatment need regular follow-up appointments with their care team so their progress can be monitored.

Preventative treatment is usually continued for life. It may be possible for someone to change to on-demand treatment, but they may be advised to switch back to preventative treatment if they have any episodes of significant bleeding.

There are different types of haemophilia. This page covers the most common types: haemophilia A and haemophilia B. They have similar symptoms, but need different treatments because different clotting factors are affected. 

Haemophilia A

Preventative treatment for haemophilia A involves regular injections of a medicine called octocog alfa.

This is an engineered version of clotting factor VIII (8), the clotting factor people with haemophilia A don't have enough of. Injections every 48 hours are often recommended.

Side effects of octocog alfa are uncommon, but can include an itchy skin rash, and redness and soreness at the site of the injection.

Haemophilia B

Preventative treatment for people with haemophilia B involves regular injections of a medicine called nonacog alfa.

This is an engineered version of clotting factor IX (9), which people with haemophilia B don't have enough of. Injections twice a week are often recommended.

Side effects of nonacog alfa are uncommon, but include headaches, altered taste, nausea, and discomfort and swelling at the injection site.

On-demand treatment

In mild or moderate cases, treatment for haemophilia may only be necessary as an immediate response to bleeding.

Haemophilia A

People with haemophilia A can be treated on-demand with injections of octocog alfa or a medication called desmopressin.

Desmopressin is a synthetic hormone. Hormones are powerful chemicals that can have a wide range of effects on the body.

Desmopressin works by stimulating the production of clotting factor VIII (8) and is usually given by injection.

Possible side effects of desmopressin include headache, stomach pain, and nausea.

Haemophilia B

On-demand treatment for haemophilia B usually involves injections of nonacog alfa.

Complications of haemophilia

Complications can include developing inhibitors in your immune system, and joint problems.

Inhibitors

Some people who take blood clotting factor medicine develop antibodies in their immune system, called inhibitors, that make the medicine less effective.

People having treatment for haemophilia should be regularly tested for inhibitors.

Inhibitors can be treated with immune tolerance induction (ITI). This involves daily injections of clotting factors so the immune system should begin to recognise them and stop producing inhibitors. 

ITI is usually offered to people with severe haemophilia A. People with haemophilia B may be offered ITI, but it's less effective and carries a risk of anaphylaxis for them.  

People with mild or moderate haemophilia A who develop inhibitors may be offered either bypass therapy or immunosuppressants. 

Bypass therapy uses a medicine called bypass agent to stop bleeding when it happens.

Immunosuppressants are medicines that suppress your immune system.

Joint damage

Joint bleeds can damage the soft spongy tissue in your joints called cartilage, and the thin layer of tissue lining the inside of the joint (synovium). The more damaged a joint is, the more vulnerable it is to bleeding.

Joint damage is more common in older adults with severe haemophilia. It's hoped modern treatments mean children growing up with haemophilia today won't have joint damage.

Surgery can be used to remove damaged synovium so new synovium can grow. Serious damage to a joint may mean it needs to be replaced – for example, a hip replacement or knee replacement.