All these disorders begin with a change in the DNA of a blood-forming cell in the marrow, called a stem cell. There are three types of chronic myeloproliferative disorders:
- polycythemia vera
- idiopathic myelofibrosis
- essential thrombocythemia
This is accompanied with overproduction of platelet-forming cells, called megakaryocytes, in the marrow. The function of the platelet-cell is to start the process of blood clotting in response to blood vessel injury. A platelet is also called a thrombocyte. The term thrombocythemia means an excess of platelets in the blood.
Thrombocytosis is classified as either:
- Primary thrombocytosis (or essential thrombocythemia), a single disease entity with unique clinical characteristics, or
- Secondary thrombocytosis, which can be traced to another cause, such as inflammation, severe bleeding, iron deficiency, or cancer.
What is essential thrombocythemia?
All blood cells begin as stem cells in the marrow, and each stem cell has the potential to become a red cell, white cell, or platelet. Normally, some of the stem cells in the marrow will develop into giant cells called megakaryocytes which all break up into small pieces to produce several thousands platelets. Essential thrombocythemia involves an overproduction of megakaryocytes in the marrow, which in turn break up and release a greater than normal number of platelets into the blood. These platelets may not function normally and can cause a blockage in blood vessels, known as a thrombus.
Incidence of essential thrombocythemia
Essential thrombocythemia is an uncommon disease. There is about 0.1 and 2.4 new cases per 100,000 people each year. The condition mostly occurs in older children, but it is also diagnosed in adult men and women.
Possible causes of essential thrombocythemia
This condition is one of several clonal diseases of the bone marrow. The term “clonal” means that the disease began with an acquired, genetic change in a single stem cell. This single stem cell is the parent of all the abnormal cells that develop and accumulate as part of the disease. Currently, the cause of the genetic change is unknown.
Secondary thrombocytosis may develop as a result of:
- acute hemorrhage or infection
- arthritis and other chronic inflammations
- iron deficiency
- removal of the spleen
- polycythemia vera
Causes of thrombocytosis:
Secondary (reactive) thrombocytosis
Infection (acute and chronic)
Essential thrombocytosis (non-familial)
Essential thrombocytosis (familial)
Chronic iron deficiency
Chronic myeloid leukemia
Acute or chronic blood loss
Tissue damage from trauma or surgery
Splenectomy and hypersplenism
Rebound from chemotherapy
Symptoms and Complications
Many patients with essential thrombocythemia do not have any signs or symptoms. Other patients may have:
- Redness of the skin
- Burning or throbbing pain in feet and hands
- Nonspecific symptoms, such as headache, dizziness, weight loss and itching
- A mildly enlarged spleen in about half the patients.
- Unexpected or exaggerated bleeding
- Thrombosis(abnormal clotting of the blood)
- Bloody stools
- Enlarged lymph nodes
Uncontrolled essential thrombocythemia can cause pregnancy complications, including:
- Spontaneous abortion
- Fetal growth retardation
- Premature delivery
- Placental abruption
Thrombosis and bleeding
Thrombosis and bleeding are the most common complication of essential thrombocythemia. A blood clot can occur in either an artery or, less frequently, a vein. The clot can block blood flow to an organ, such as the brain or heart, causing a stroke or heart attack. Clotting complications may occur in patients with only a slightly elevated platelet count.
Major thrombotic events
Deep venous thrombosis
Minor thrombotic events
Transient ischemic attack
Gastrointestinal tract bleeding
How is essential thrombocythemia diagnosed?
Essential thrombocythemia is often identified completely accidentally in symptom-free patients when a routine blood test shows a higher than normal platelet count. A mildly enlarged spleen could also be easily found.
The platelet count is a standard medical blood test, and the normal values range from about 175,000 to 350,000 platelets per microliter of blood. It is important to know that essential thrombocythemia is a consideration if:
- The platelet count is above 600,000/ml of blood
- There is no other evident cause for an elevated platelet count.
Unfortunately, there are no specific tests that can be used to establish the diagnosis of essential thrombocythemia. Some routine tests that will probably be used are a chest X-ray, an ultrasound or other scan of the abdomen, and a bone marrow test.
The bone marrow test involves taking both a liquid and a small pencil-lead like sample under a local anesthetic from the bones at the back of the pelvis.
There are several conditions that can result in an increase in platelets, including:
- Inflammatory disorders, such as arthritis.
- Iron deficiency anemia.
- Certain types of cancer.
In most cases, the diagnosis of essential thrombocythemia is made based on:
- A high platelet count that persists over time
- A slightly lower than normal blood hemoglobin concentration and a slightly higher white cell count
- An examination of the marrow compatible with the diagnosis
- The absence of evidence for other clonal blood diseases that can be accompanied by increased platelets, such as chronic myelogenous leukemia and idiopathic myelofibrosis
Treatment decisions should be guided by the risk of clotting or bleeding complications in a patient. Physicians may use chemotherapy to reduce the high platelet count in patients with previous bleeding or clotting episodes, or those who are at high risk for such complications.
Risks for clotting complications include:
- A previous history of previous clot.
- Cardiovascular risk factors.
- Advanced age
Risk factors for bleeding include:
- A platelet count of 2,000,000 per microliter and above.
- Aspirin or other nonsteroidal inflammatory (NSAID) drug use.
The most commonly used drugs to treat essential thrombocythemia are:
- hydroxyurea (Hydrea)
- interferon alpha
Hydroxyurea is a strong myelosuppressive agent that can be used as initial therapy for essential thrombocythemia. Hydroxyurea is often successful in decreasing platelet count within several weeks, with few short-term side effects. However, there is some evidence that hydroxyurea is associated with an increased risk of developing acute leukemia after long-term therapy.
In rare cases where patients have life-threatening complications, the platelet count can be reduced rapidly using platelet aphaeresis – a procedure that removes platelets from the blood directly.
Aspirin is widely used in essential thrombocytosis, particularly for symptoms likely to be due to clotting or overactive platelets. There are, of course, side effects of aspirin, as it can cause bleeding and ulcers in the stomach and small intestine.
Patients with essential thrombocytosis have near-normal life expectancy because of the low rate of leukemic conversion. The major morbidity factor is the increased risk of hemorrhagic complications. Essential thrombocytosis is a slowly progressive disorder with long asymptomatic periods punctuated by thrombotic or hemorrhagic events. The average survival of patients with this disorder is 12 to 15 years, and may be much longer in younger patients.