Polycythemia is due to an excess of red blood cells. This is known as true polycythemia and occurs when some pathology triggers the excessive production of red blood cells. True polycythemia leads to an elevated red blood cell count.
Relative polycythemia or apparent polycythemia is where the total red blood cell count is normal but due to the lower plasma volume the hematocrit is higher – this is the ratio of red blood cells to the volume of whole blood. This type of polycythemia is often caused by dehydration, either due to the excessive use of diuretics or alcohol.
Causes of Polycythemia (True)
Red blood cells are produced in the bone marrow and are manufactured from hematopoietic stem cells. The main stimulus for red blood cell production is hypoxia – a low oxygen state. Erythropoietin (Epo), a hormone primarily secreted by the kidney, has to also be present for red blood cell production.
There are two types of polycythemia based on the cause – it may either be primary or secondary. In primary polycythemia, the disorder lies in the bone marrow and involves the stem cells. There is no other disease process causing the increased red blood cell production. With secondary polycythemia the increase in red blood cell production is due to diseases leading to hypoxia or an abnormally high levels of erythropoietin.
A myeloproliferative disorder is when the body produces an excess of blood cells. Polycythemia vera is a type of myeloproliferative disorder resulting in too many red blood cells mainly but the white blood cell and platelet count is also elevated.
The exact cause of every case of polycythemia vera is unknown but in most cases, there is a mutation of Janus kinase-2 gene (JAK2) that causes cells to grow uncontrollably.
In this case, polycythemia may be due to hypoxia or a high level of erythropoietin.
Due to a low oxygen availability, either within the environment or bloodstream, the body produces an excess of red blood cells. There is an elevation of erythropoietin (Epo) but only as a consequence of the prevailing hypoxia.
- High altitude (physiological polycythemia)
- Heavy cigarette smoking (smoker’s polycythemia)
- Industrial – working in environmental conditions with decreased oxygen concentration
- Lung/airway diseases like chronic obstructive pulmonary disease (COPD) : chronic bronchitis, emphysema
- Chronic heart disease like congestive heart failure
- Cyanotic heart disease (congenital)
- Sleep apnea
- Obesity (Pickwickian syndrome)
Abnormally High Erythropoietin
There may be various causes of inappropriately high erythropoietin (Epo) levels in the bloodstream. It may be related to the disruption of the kidney’s erythropoietin secretion, Epo-secreting tumors or it is artificially induced or administrated (exogenous).
- Kidney disease
- Renal carcinoma (kidney)
- Adrenal adenocarcinoma, pheochromocytoma (adrenal gland)
- Hepatoma (liver)
- Bronchogenic carcinoma (lung)
- Benign uterine growths like uterine fibroids (uterus)
- Cerebellar hemangioblastoma (brain)
Exogenous administration of Epo or blood cells is seen in some athletes who attempt to enhance their physical performance. Polycythemia may occur as a result of blood doping or the use of anabolic steroids.