What is polycythemia? Polycythemia is a blood condition in which the bone marrow makes excess blood cells, primarily red blood cells, but also platelets and white blood cells. The extra cells cause a thickening of the blood, which increases the risk of blood clotting, in turn potentially causing strokes, heart attacks, and other complications. While the exact cause of polycythemia is not known, genetic changes are thought to be related to its development. Genetic polycythemia, referred to as polycythemia vera, develops slowly and is generally seen in older adults. It is rare in young adults and children. Although it is the result of genetic mutations or changes in particular genes, these genetic changes are acquired during an individual’s lifetime and are generally not passed from parents to their children. Polycythemia is more common in adults over 60 years of age, and about one in every 200,000 people is diagnosed each year with the condition (Source: NHLBI). In the early stages of polycythemia, symptoms may be mild and include flushed face, dizziness, and impaired senses. In more severe cases, thrombosis (blood clotting) may occur, leading to severe symptoms. In secondary polycythemia, long-term oxygen deprivation, such as from chronic smoking or long periods spent at high altitudes, causes increased production of red blood cells and resultant blood thickening. This form of polycythemia often resolves once the cause of oxygen deprivation is addressed. In all cases of polycythemia, treatment by periodic blood draws or medications to reduce the number of blood cells is generally effective, although there is no cure for the condition. While polycythemia is rare, generally treatable, and usually mild, serious complications such as heart attack or stroke can occur if left untreated. Seek immediate medical care (call 911) for any sudden symptoms of heart attack or stroke, such as sudden numbness, weakness, confusion, vision problems, or chest pain. Polycythemia may also lead to less severe symptoms, such as difficulty breathing when lying down or excessive bleeding. Seek prompt medical care if these symptoms persist, as early diagnosis and intervention are critical to preventing more severe symptoms.