A condition in which increased production of red blood cells leads to an unusually large number of them in the blood. This condition is usually caused by another disorder or by hypoxia, and is called secondary polycythaemia. If it occurs for no apparent reason, it is called polycythaemia vera or primary polycythaemia. Secondary polycythaemia occurs naturally at high altitudes due to the reduced oxygen level. It can also result from a disorder that impairs the oxygen supply to the blood, or can be secondary to liver cancer or some kidney disorders. Descending to sea level, or effective treatment of an underlying disorder, returns the blood to normal. Polycythaemia vera is a rare disorder that mainly affects people over 40. The large number of red blood cells causes increased volume and thickening of the blood, which may lead to headaches, blurred vision, and hypertension. There may also be flushed skin, dizziness, night sweats, and widespread itching. The spleen is often enlarged. Possible complications include a tendency to bleed easily or to form blood clots; stroke; and myelofibrosis or acute leukaemia (see leukaemia, acute). Diagnosis is made from a physical examination and blood tests and by ruling out other causes. Treatment is by venesection, sometimes in combination with anticancer drugs or with radioactive phosphorus. This enables most patients to survive for 10–15 years.


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