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Essential Thrombocythaemia (ET)

What is it?

Essential thrombocythaemia (ET) is a disorder in which too many platelets are produced in the bone marrow. Platelets are normally needed in the body to control bleeding. However, excess numbers of platelets can lead to abnormal blood clotting which can block the flow of blood in the blood vessels.

There are a number of conditions that can cause a rise in the number of platelets in the circulating blood (thrombocytosis). These include bleeding, infection and some types of cancer. In essential thrombocythaemia however, the blood platelet count is persistently elevated as a result of increased bone marrow production of platelets, in the absence of any identifiable cause.

Essential thrombocythaemia is a rare chronic disease diagnosed in an estimated 3 per 100,000 population. Although it can occur at any age, even (rarely) in children, essential thrombocythaemia usually affects older people, with most patients diagnosed between the ages of 50 and 70 years. It occurs equally in both males and females.

Symptoms and complications of essential thrombocythaemia

Many people have no symptoms when they are first diagnosed with essential thrombocythaemia and their disease is diagnosed incidentally during a routine blood test. However if symptoms do occur they generally include tingling or burning in the hands and feet, headache, visual problems, weakness and dizziness. These symptoms and others result from excessive numbers of platelets causing blockages in small or large blood vessels in different parts of the body.

An enlarged spleen is common and occurs in around 30% of cases. Symptoms include feelings of discomfort, pain or fullness in the upper left-side of the abdomen. An enlarged spleen may also cause pressure on the stomach causing a feeling of fullness, indigestion and a loss of appetite. In some cases the liver may also be enlarged (hepatomegaly). Other symptoms include weight loss and generalised itching.

Blood clots (thrombosis) and bleeding

Thrombosis is a major complication of essential thrombocythaemia. Older patients and those with a high platelet count, or a prior history of thrombosis, may be at increased risk. A major aim of treatment in essential thrombocythaemia is to reduce your platelet count, and therefore your risk of thrombosis.

Blood clots can occur in large or small arteries interfering with the blood and therefore oxygen supply to various organs or tissues. Blockages in the smaller blood vessels in the toes and fingers can cause redness of the skin and burning and throbbing pains. These pains are often made worse by heat or exercise and relieved by cooling and elevating the affected area. These symptoms are often dramatically improved using small daily doses of aspirin, and/or reducing the person's platelet count.

Blockages in the arteries supplying the heart (causing a myocardial infarction or heart attack), kidneys or brain (causing a stroke) can be serious and can lead to significant tissue damage or ischaemia (tissue death). Blood clots can also develop in the veins of the legs (causing deep vein thrombosis), and less commonly, the spleen and liver occluding the blood flow and causing pain in these areas. A blood clot that breaks off the wall of the vein and travels in the blood stream is known as an embolism. When a blood clot travels to the lungs it is known as a pulmonary embolism and can cause breathing problems.

Less commonly, people experience symptoms of abnormal bleeding including bruising for no apparent reason, or exaggerated or prolonged bleeding following minor cuts or injury. Some people notice frequent or severe nose bleeds or bleeding gums and some women may have unusually heavy menstrual periods.

In pregnancy, uncontrolled essential thrombocythaemia can reduce the blood supply to the placenta or foetus. This can cause problems with foetal growth and may in some cases lead to miscarriage.

How is essential thrombocythaemia diagnosed?

The diagnosis of essential thrombocythaemia is only made when other causes of a raised platelet count have been excluded.

Full blood count

A persistently raised platelet count is the most common sign of essential thrombocythaemia. The platelet count can range from slightly higher than normal to many times higher than normal. Under the microscope the platelets may be abnormally large and pale blue-stained. Fragments of megakaryocytes, the cells from which platelets are released, may also be seen in the blood film. Around a third of people with essential thrombocythaemia will also have a mildly raised red cell and / or white cell count.

If the results of your blood test suggest that you may have essential thrombocythaemia, further investigation and tests including a bone marrow examination may be required to help confirm the diagnosis and rule out other secondary or 'reactive' causes of a raised platelet count.

Bone marrow examination

In essential thrombocythaemia the bone marrow is usually found to be overactive, similar to polycythaemia vera. An excess number of abnormal megakaryocytes is a common finding. Cytogenetic and molecular analysis of blood and bone marrow cells may be carried out to help confirm the diagnosis. A mutation in the JAK2 gene is found in a significant
proportion (50-60 %) of people with ET. Mutations in the c-MPL gene (which produces a protein that responds to a growth factor that stimulates platelet production) account for approximately 10% of cases.

Other blood tests may be done to check your general health and how well your kidneys, liver and other vital organs are functioning.

How is essential thrombocythaemia treated?

The goal of treatment for people with essential thrombocythaemia is to prevent complications like abnormal bleeding and bruising and in some cases reducing the number of platelets in the blood. You may not have any symptoms of essential thrombocythaemia when you are first diagnosed and therefore may not require any treatment for some time. Instead your doctor may recommend a 'watch and wait' strategy which involves regular check-ups and blood counts to carefully monitor your health. In addition he or she will advise you on the
steps you can take to stay healthy and reduce any 'lifestyle-related' risk factors you may have that increase your chances of developing a blood clot. You may be advised for example on ways to help you stop smoking, and / or maintain a healthy weight range and blood pressure.

For the majority of people, essential thrombocythaemia will require some form of treatment to reduce their platelet count and therefore their risk of thrombosis. The treatment chosen for you will depend on a number of factors that influence your particular risk of complications due to thrombosis or bleeding. These include your age, platelet count and whether or not you have had any previous episodes of blood clots or bleeding in the past. A history of smoking or high blood pressure can affect your risk of thrombosis. These factors and others are taken into account when planning the most appropriate treatment for your disease.

For people at high-risk of thrombosis, a chemotherapy drug called hydroxyurea (see polycythaemia vera for a discussion of this and other treatments which are also frequently used in essential thrombocythaemia) with or without low-dose aspirin is often used as first-line treatment. Hydroxyurea works by suppressing the function of your bone marrow and thereby controlling platelet production, while aspirin prevents your platelets from aggregating and forming harmful clots in your body.

Anagrelide hydrochloride (Agrylin®) and interferon (see polycythaemia vera) may also be used. Anagrelide slows down platelet production in your bone marrow, thereby helping
to reduce symptoms and your risk of thrombosis. Interferon works by suppressing the abnormal megakaryocyte clone in your bone marrow thereby reducing the overproduction of platelets.

Those at low-risk may be simply treated using low-dose aspirin or an equivalent drug alone. They usually have a very good outlook with no difference to the general population.

Your doctor will be able to discuss with you all of the treatment options suitable for you.

Plateletpheresis

If your platelet count is very high and you have symptoms of clotting or bleeding, your platelet count will need to be reduced quickly to prevent further complications. In these emergency situations, excess platelets can be removed from your bloodstream using a procedure known as plateletpheresis. During this procedure all your blood is passed through a special machine called a cell separator. The blood is drawn from a cannula (plastic needle) placed in a vein in one arm. The machine spins the blood very quickly and removes the excess platelets. This is a continuous process. While platelets are being removed the rest of your blood is being returned to you via another cannula, placed in your other arm. If your veins are not suitable for this procedure, a special catheter device can be inserted into a large vein and might be used instead. This line allows blood to be drawn from one of the
bigger veins in your body when your smaller veins are hard to access.

Plateletpheresis is usually carried out in hospital. It usually takes about two hours to complete.

Prognosis

Essential thrombocythaemia is regarded as an incurable disease but in many people with treatment, the disease remains stable for long periods of time, often 10-20 years or more. In the longer term, a small number of people with essential thrombocythaemia may develop myelofibrosis. The risk of transforming to acute myeloid leukaemia is relatively low (<1 per cent).

Your doctor is the best person to give you an accurate prognosis regarding your disease as he or she has all the necessary information to make this assessment.