There are two distinct forms of the disease. There is hyperplasia of the cell-forming tissues of the bone marrow, with resultant elevation of the erythrocyte count and hemoglobin level, and an increase in the number of leukocytes and platelets. Secondary polycythemia is a physiologic condition resulting from a decreased oxygen supply to the tissues. The body attempts to compensate for the oxygen deficiency by manufacturing more hemoglobin and red blood cells. Living at high altitudes can produce polycythemia, as can severe chronic lung and heart disorders, especially congenital heart defects.
Absolute polycythemia refers to an increase in red cell mass from any cause.
Relative polycythemia refers to a loss of plasma volume causing an elevated hematocrit. The symptoms of both primary and secondary polycythemia are much the same. The increased erythrocyte production results in thickening of the blood and an increased tendency toward clotting. The viscosity of the blood limits its ability to flow properly, diminishing the supply of blood to the brain and to other vital tissues. This may cause mental sluggishness, irritability, headache, dizziness, fainting, disturbances of sensation in the hands and feet, and a feeling of fullness in the head.