The causes of polycythemia are various and numerous, initially divided into causes of relative polycythemia and of
absolute polycythemia. The first term identifies all those conditions in which the increased concentration of red blood cells in the blood results from a decrease in plasma volume: for example, in cases of severe dehydration (induced by vomiting, diarrhoea, profuse sweating, diabetes insipidus, inadequate compensated by the introduction of water) the red blood cells – although numerically unchanged – appear more concentrated, due to the reduction of the liquid part of the blood. The causes of absolute polycythemia are more numerous and common, in which there is an actual increase in the total number of red blood cells. In turn, absolute polycythemia is distinguished into primary polycythemia or primitive, when associated with an increased synthesis of erythrocytes at the level of the hematopoietic marrow, and in secondary polycythemia when it depends on a disease unrelated to the bone marrow or on particular environmental conditions (staying at high altitude), therefore susceptible to regression if the cause responsible for it can be eliminated.
Secondary polycythemias are mostly related to chronic hypoxemia, or a lack of oxygen in the blood. This condition induces an adaptive physiological response which – mediated by the increased synthesis of erythropoietin at the renal level – leads to an increase in the synthesis of red blood cells. In this way the organism is able to capture more oxygen from the atmospheric air and make up for deficiencies within certain limits. It is no coincidence that polycythemia is a typical trait of numerous high-altitude ethnic groups, appreciable even in those who stay for several weeks in high altitude; as anticipated, this is an adaptive response to the reduced partial pressure of oxygen that characterizes these environments. There Physiological altitude polycythemia explains why various athletes, especially those practicing cross-country sports (running, cycling, etc.), train for certain periods at high altitude: the increase in red blood cells will ensure an improvement in sports performance.
Causes of secondary polycythemia:
From increased synthesis of erythropoietin in response to arterial hypoxemia
- extended stay in the high mountains
- respiratory diseases with alveolar hypoventilation (e.g. COPD)
- congenital heart disease with right-to-left shunt
- sleep apnea in the course of excessive obesity
From inappropriate secretion of erythropoietin
From increased intake of erythropoietin or other drugs with similar action (epoetin), both for therapeutic and doping purposes
As mentioned, secondary polycythemia is a reversible phenomenon: when the individual descends to a low altitude or the cause of the hypoxia disappears, the number of red blood cells gradually re-stabilises.
For further information: Polycythemia symptoms
Also called primary polycythemia or erythremia/Vaquez-Oslere disease, polycythemia vera is an autonomous myeloproliferative disease, characterized by an abnormal proliferation of hemocytoblasts on a genetic basis (mutation of the JAK2 tyrosine kinase in the stem cells of 90% of patients suffering from polycythemia vera).
It takes the form of an elevated synthesis of red blood cells, generally also accompanied by an enhanced synthesis of white blood cells and platelets. This results in an increase in hematocrit and total blood volume (plasma hyperviscosity and hypervolemia); the increased blood viscosity and blood pressure on the vascular walls can cause important alterations in blood flow and lead to rather dangerous consequences for the health of the patient suffering from polycythemia vera: the capillaries become clogged due to the excessive viscosity of the blood, thrombotic phenomena increase (increases the risk of stroke, angina pectoris, myocardial infarction, superficial and deep thrombophlebitis and, more rarely, pulmonary embolism). Generally, dizziness, headache, mild hypertension, hepatomegaly, splenomegaly and hemorrhagic phenomena (nosebleeds, blood loss from the gums and bruising) are present; the skin takes on reddish (due to the increased presence of oxygenated hemoglobin) and bluish – cyanotic shades (due to the increased presence of deoxygenated hemoglobin), and is often subject to itching after bathing.
The diagnosis of polycythemia vera is based on the study of the blood count:
- hemoglobin and hematocrit values, higher than normal, can respectively reach 22-24 g/dl and 55-60%, while the finding of neutrophilic leukocytosis and platelet disease is common
and other biohumoral parameters:
At the bone marrow biopsy and subsequent morphological examination of the bone marrow, erythroid hyperplasia is recorded; furthermore, it is possible to demonstrate the presence of the aforementioned JAK2 V617F mutation. Ultrasound and objective evaluation may demonstrate an increase in the size of the liver and spleen.
The therapy, originally based on bloodletting or phlebotomy – that is, on the removal of 300-500 ml of blood every 2-3 days until the hematocrit falls below the threshold of 50%, possibly compensated by the reinfusion of plasma or the administration of its substitutes – may use cytotoxic/chemotherapeutic drugs (busulfan, hydroxyurea, cyclophosphamide, chlorambucil, cytosine arabinoside, melphalan) or radiotherapy. These latter interventions have the aim of depressing the abnormal proliferative activity of the bone marrow, in which polycythemia vera recognizes its pathogenetic center. New generation drugs capable of inhibiting the activity of the abnormal protein tyrosine kinase (JAK2) responsible for the disease are being developed and tested.