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About thrombosis
Thrombophilia: congenital thrombophilia
Antithrombin deficiency
Antithrombin (previously known as antithrombin III) is one of the natural anticoagulants found in the blood. People who have insufficient antithrombin are more likely to have a venous thrombosis. It is uncertain whether there may also be a very slight risk of arterial thrombosis.
Inheritance of antithrombin deficiency
Antithrombin deficiency is inherited in an autosomal
dominant way. This means that if one parent has it, there is a 50:50 chance
(1 in 2) that a child will inherit it.
A single dose is known as heterozygous antithrombin deficiency. There is an extremely rare chance of inheriting a double dose of antithrombin deficiency, known as homozygous antithrombin deficiency. In this situation the foetus does not survive.
Other causes of antithrombin deficiency
Occasionally other medical conditions, such as kidney disease, can
cause low levels of antithrombin.
Antithrombin deficiency and venous thrombosis
Of all the congenital thrombophilias, heterozygous antithrombin deficiency
is associated with the greatest risk of venous thrombosis. As many as 1 in
2 individuals with antithrombin deficiency will have a venous thrombosis before
the age of 50, if they do not follow advice to reduce the risks.
During pregnancy, about half of all women with antithrombin deficiency will have a venous thrombosis if they do not receive thromboprophylaxis to reduce the risk. This involves giving small amounts of anticoagulant drugs to prevent blood clots.
The level of risk varies between individuals with
antithrombin deficiency. This is because each family with the condition has
a different mutation within the antithrombin gene.
One of the best guides to risk is to look at other members of the family with antithrombin deficiency. If none if them has had a venous thrombosis, then the risk is probably low unless other risk factors are present. The risk of venous thrombosis increases when one or more additional risk factors are present.
If a number of family members with antithrombin deficiency have had a venous thrombosis, the risk is greater. This risk can be reduced through prevention.
Preventing venous thrombosis
People who have antithrombin deficiency can
reduce the risk of venous thrombosis by making lifestyle changes, such as:
- Leading a physically active life
- Eating a healthy, balanced diet
- Avoiding becoming overweight or obese
- Avoiding long periods of immobility during illness
or when travelling - Stopping cigarette smoking
Individuals with antithrombin deficiency should also seek medical advice before major surgery, or when they are likely to be immobile for a long period, as this increases the risk of venous thrombosis.
Women with antithrombin deficiency should seek medical advice before taking oral contraception or hormone replacement therapy, and when pregnant or planning to become pregnant.
Antithrombin deficiency and pregnancy
It is important that women with antithrombin deficiency who are pregnant discuss
this with their obstetrician. As well as an increased risk of venous thrombosis,
women with antithrombin deficiency may have a slightly higher risk of placental
problems during pregnancy. These include having a smaller baby or pre-eclampsia.
Testing for antithrombin deficiency
There are a number of tests to detect antithrombin
deficiency and these may vary between different laboratories. Sometimes, a
hospital may send the test to a specialist laboratory.
Usually a test is carried out to measure the amount of antithrombin
that is working in the blood. This is known as a functional antithrombin
assay. Other tests may measure the total amount of antithrombin
present in the blood.
Testing other family members
Testing is recommended for close blood relatives of people with a antithrombin
deficiency, including brothers and sisters and sons and daughters, as there
is a 50:50 chance that they will inherit the condition.
Because individuals with congenital thrombophilia do not
tend to have problems until they are adults, children are not usually tested
until they are old enough to understand why the blood sample is being taken.
This is usually after the age of 13.
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