Polycythemia (erythrocytosis) is a condition characterized by an increase above normal in the number of red cells in the circulating blood, usually accompanied by an increase in the quantity of hemoglobin and in the volume of packed red cells. The increase may be either an actual rise in the total quantity of red cells in the circulation, or it may be the result of a loss of blood plasma and thus a relative increase in the concentration of red cells in the circulating blood (relative polycythemia). The latter may be the consequence of abnormally lowered fluid intake or of marked loss of body fluid, such as occurs in persistent vomiting, severe diarrhea, or copious sweating or when water is caused to shift from the circulation into the tissue.

Polycythemia occurs in response to some known stimulus for the production of red cells. This is in contrast to a disease called polycythemia vera, in which an increased amount of red cells are produced without a known cause. In polycythemia vera there is usually an increase in other blood elements as well.

Polycythemia is a response by the body to an increased demand for oxygen. It occurs when hemoglobin is not able to pick up large amounts of oxygen from the lungs (i.e., when it is not “saturated”). This may result from decreased atmospheric pressure, as at high altitudes, or from impaired pulmonary ventilation. The sustained increase in red cells in persons who reside permanently at high altitudes is a direct result of the diminished oxygen pressure in the environment. Chronic pulmonary disease (e.g., emphysema—abnormal distension of the lungs with air) may produce chronic hypoxemia (reduced oxygen tension in the blood) and lead to polycythemia. Extreme obesity also may severely impair pulmonary ventilation and thereby cause polycythemia (pickwickian syndrome).

Congenital heart disorders that permit shunting of blood from its normal path through the pulmonary circuit, thereby preventing adequate aeration of the blood, can also cause polycythemia, as can a defect in the circulating hemoglobin. The latter defect may be congenital because of an enzymatic or a hemoglobin abnormality, as mentioned above; or it may be acquired as the result of the excessive use of coal tar derivatives, such as phenacetin, which convert hemoglobin to pigments incapable of carrying oxygen (methemoglobin, sulfhemoglobin). Lastly, polycythemia can develop in the presence of certain types of tumours and as the result of the action of adrenocortical secretions. Treatment of polycythemia due to any of these causes involves the correction or alleviation of the primary abnormality.

In polycythemia vera the number of red cells and often also the numbers of white cells and platelets are increased, and the spleen usually is enlarged. In this disease the stem cell precursor of the bone marrow cells produces excessive progeny. Persons with polycythemia vera have an exceptionally ruddy complexion and may have headaches, dizziness, a feeling of fullness, and other symptoms. Because of the excessive quantities of red cells, the blood is usually thick, and its flow is retarded; it sometimes clots in the blood vessels (thrombosis) of the heart, the brain, or the extremities with serious consequences. One of the simplest methods of treatment is to remove the blood, one pint at a time, from a vein until the cellular level approaches normal and the symptoms disappear. Occasionally it may be necessary to use drugs or radiation therapy, in the form of radioactive phosphorus, to restrain the overactivity of the marrow cells. These treatments must be avoided when possible, however, because of their potential complications.