Essential Thrombocythemia (ET)
Characteristics
Essential thrombocythemia (ET) is a myeloproliferative neoplasm (MPN) in which mutations in blood stem cells lead to an excessive production of blood platelets. In normal circumstances, platelets slow or stop bleeding by creating blood clots. In ET, however, the bone marrow produces more platelets than the body needs. These excess numbers of platelets can lead to abnormal blood clotting which can block the flow of blood in blood vessels,
Somewhat contra-intuitively, patients with ET and a very high platelet count may also suffer from unusual, or excessive bleeding due to a reduction in certain clotting factors (i.e., acquired Von Willebrand disease).
Researchers have found that 50-60% of ET patients carry a mutation in the JAK2 gene. This JAK2 gene encodes a protein that controls the production of blood cells by stem cells. As a result of this mutation, too much of this protein is made, which stimulates stem cells to make more platelets. In addition to this, a proportion of ET patients presents with mutations in the CALR (20-35%) or MPL (1-5%) gene.
ET typically affects people between the age of 60 and 80, with only about a fifth of cases in patients below 40 years of age. When monitored and treated properly, most ET patients have a normal life expectancy. However, in a small proportion of patients, the disease may transform to myelofibrosis, acute myeloid leukemia (AML), or (less frequently) myelodysplastic syndrome (MDS).
Symptoms
ET is often detected during a routine blood test, before symptoms develop. When symptoms occur, they are mainly linked to the high platelet levels. When a blood clot develops in a vein deep inside the body, this may lead to pain, swelling and redness in the arms or legs. If a clot forms in the lungs, patients may experience shortness of breath, chest pain and cough. In contrast, if a blood clot develops in the arteries that supply blood to the brain, symptoms may include headaches, dizziness, numbness on one side of the body, a blurred or double vision or a slurred speech.
As indicated earlier, an extremely high platelet count may also cause bleeding. If this is the case patients tend to bruise easily, have frequent nosebleeds, or have blood in the urine or stool.
Other signs and symptoms of ET include fatigue, unexplained weight loss, night sweats, redness and swelling in the hands or feet caused by a diminished blood flow and an enlarged spleen.
Diagnostic tests
Following a complete physical examination, the diagnostic process for ET usually starts with a complete blood count (CBC) to measure the level of the different blood cells.
Although a bone marrow examination is not strictly necessary to make a diagnosis, physicians often use it to confirm an ET diagnosis. In patients with ET, the bone marrow will show a significant increase in platelet-forming cells (megakaryocytes). In addition, these cells also have an abnormal shape and size.
Finally, in suspected cases of ET, genetic tests are being performed on blood or bone marrow cells to check for mutations in JAK2, MPL or CALR.
Treatments
In ET patients without symptoms, physicians may decide not to start therapy straight away, but opt for a watchful waiting strategy. For patients with a low to intermediate risk for clots, a treatment with low-dose aspirin can be initiated to reduce the thrombosis risk.
In patients with a higher risk for blood clots, treatment will be initiated to reduce the blood cell levels (i.e., cytoreduction). There are a few drugs that can be used to accomplish this cytoreduction and doctors will base their choice on the age of the patient, the thrombotic risk, and the number of platelets in the blood. The potential options include hydroxycarbamide, immunotherapy with interferon alfa, or anagrelide, a drug that specifically reduces the number of platelets.
Finally, participation in clinical trials may provide access to alternative treatment options to patients.
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