You may not have any noticeable symptoms of essential thrombocythemia. The first indication you have the disorder may be the development of a blood clot (thrombus). Clots can develop anywhere in your body, but with essential thrombocythemia they occur most often in your brain, hands and feet.

Signs and symptoms depend on where the clot forms. They include:

• Headache

• Dizziness or lightheadedness

• Chest pain

• Fainting

• Temporary vision changes

• Numbness or tingling of the hands and feet

• Redness, throbbing and burning pain in the hands and feet (erythromelalgia)

Less commonly, essential thrombocythemia may cause bleeding, especially if your platelet count is extremely high (more than 1 million platelets per microliter of blood). Bleeding may take the form of:

• Nosebleeds

• Bruising

• Bleeding from your mouth or gums

• Bloody stool

If a blood clot occurs in the arteries that supply the brain, it may cause a transient ischemic attack (TIA) or stroke. A TIA, or ministroke, is a temporary interruption of blood flow to part of the brain. Signs and symptoms of a stroke or TIA develop suddenly and include:

• Weakness or numbness of your face, arm or leg, usually on one side of your body

• Difficulty speaking or understanding speech (aphasia)>

• Blurred, double or decreased vision

Essential thrombocythemia is a type of chronic myeloproliferative disorder. That means the bone marrow, the spongy tissue inside your bones, makes too many of a certain type of cell. In the case of essential thrombocythemia, the bone marrow makes too many cells that create platelets.

It's not clear what causes this to happen. About 90 percent of people with the disorder have an acquired gene mutation contributing to the disease.

Platelets stick together to help form blood clots. Normally, blood clots stop bleeding when you damage a blood vessel, such as when you get a cut.

A normal platelet count ranges from 150,000 to 450,000 platelets per microliter of blood. Someone with essential thrombocythemia has more than 450,000 platelets per microliter of blood.

Plus, the excess platelets may not function normally, leading to abnormal clotting or bleeding.

If an underlying condition such as an infection or iron deficiency causes a high platelet count, it's called reactive, or secondary, thrombocythemia. Secondary thrombocythemia causes less risk of blood clots and bleeding than does essential thrombocythemia.

Older people with essential thrombocythemia are at risk of complications. People who've had prior blood clots or bleeding problems related to the disease are also at risk of complications.

The abnormal blood clotting of essential thrombocythemia can lead to a variety of potentially serious complications, including:

• Stroke. A clot that blocks blood flow to your brain can cause a stroke. If you develop signs and symptoms of a stroke, get immediate medical attention.

• Heart attack. Blood clots that obstruct blood flow to your heart cause heart attacks. Get immediate attention if you develop signs and symptoms of a heart attack.

• Excessive bleeding. This may appear as nosebleeds, bleeding gums or bruising.

Rarely, essential thrombocythemia may progress to these potentially life-threatening diseases:

• Acute myelogenous leukemia. This is a type of white blood cell and bone marrow cancer that progresses rapidly.

• Myelofibrosis. This progressive disorder results in bone marrow scarring, leading to severe anemia and enlargement of your liver and spleen.

Pregnancy complications

Pregnant women with essential thrombocythemia have a higher risk of complications than women without the condition. But most women who have thrombocythemia have normal, healthy pregnancies.

However, uncontrolled thrombocythemia can lead to miscarriage and other complications. Your risk of complications may be reduced with regular checkups and medication, so be sure to have your doctor regularly monitor your condition.