Many people with polycythemia vera don't have signs or symptoms. Others might have:

• Itchiness, especially following a warm bath or shower
• Headache
• Dizziness
• Bleeding or bruising, usually minor
• Weakness
• Fatigue
• Blurred vision
• Excessive sweating
• Painful swelling of one joint, often the big toe
• Shortness of breath
• Numbness, tingling, burning or weakness in your hands, feet, arms or legs
• A feeling of fullness or bloating in your left upper abdomen due to an enlarged spleen
• Fevers
• Unexplained weight loss

Polycythemia vera is one of a group of blood cancers known as myeloproliferative neoplasms. It occurs when a mutation in a gene causes a problem with blood cell production. Normally, your body regulates the number of each of the three types of blood cells you have — red blood cells, white blood cells and platelets. But in polycythemia vera, your bone marrow makes too many of some blood cells.

The mutation that causes polycythemia vera is thought to affect a protein switch that tells the cells to grow. Specifically, it's a mutation in the protein Janus kinase 2 (JAK2). Most people with polycythemia vera have this mutation. The cause of the mutation isn't known, but it's generally not inherited.

Polycythemia vera can occur at any age, but it's more common in adults older than 60.

Possible complications of polycythemia vera include:

• Blood clots. Increased blood thickness and decreased blood flow, as well as abnormalities in your platelets, increase your risk of blood clots. Blood clots can cause a stroke, a heart attack or a blockage of an artery in your lungs (pulmonary embolism) or in a vein deep within a muscle (deep vein thrombosis).
• Enlarged spleen (splenomegaly). Your spleen helps your body fight infection and filter unwanted material, such as old or damaged blood cells. The increased number of blood cells caused by polycythemia vera makes your spleen work harder than normal, which causes it to enlarge.
• Problems due to high levels of red blood cells. Too many red blood cells can lead to a number of other complications, including open sores on the inside lining of your stomach, upper small intestine or esophagus (peptic ulcers) and inflammation in your joints (gout).
• Other blood disorders. In rare cases, polycythemia vera can lead to other blood diseases, including a progressive disorder in which bone marrow is replaced with scar tissue (myelofibrosis), a condition in which stem cells don't mature or function properly (myelodysplastic syndrome), or cancer of the blood and bone marrow (acute leukemia).