Polycythemia is a condition where the body has an increased concentration of red blood cells, reflected in higher levels of hematocrit or hemoglobin. Secondary polycythemia specifically indicates this excess is a response to an identifiable underlying health condition or an external factor.
Differentiating Secondary Polycythemia
Secondary polycythemia is a reactive process where the body produces excess red blood cells in response to a stimulus outside the bone marrow. This increased production is driven by elevated erythropoietin (EPO), a hormone primarily produced by the kidneys. EPO acts as a chemical messenger, signaling the bone marrow to generate more red blood cells to meet the body’s oxygen demands.
This mechanism distinguishes secondary polycythemia from primary polycythemia, also known as Polycythemia Vera (PV). PV is a chronic blood disorder originating within the bone marrow, often linked to a JAK2 gene mutation. In PV, the bone marrow autonomously overproduces red blood cells, even when EPO levels are low. In secondary polycycythemia, the bone marrow responds to high EPO signals triggered by an underlying condition.
Common Causes
Secondary polycythemia arises from conditions that stimulate red blood cell production. A frequent cause is chronic hypoxia, where the body experiences persistently low oxygen levels. Conditions such as chronic obstructive pulmonary disease (COPD), severe heart disease, and obstructive sleep apnea can lead to insufficient oxygen delivery, prompting the kidneys to release more EPO. Living at high altitudes also causes increased red blood cell production due to lower oxygen availability.
Exposure to carbon monoxide, often from heavy smoking or environmental sources, can also induce secondary polycythemia. Hemoglobin has a higher affinity for carbon monoxide than for oxygen, which reduces the blood’s oxygen-carrying capacity and triggers a compensatory increase. Certain kidney conditions (e.g., renal cysts, hydronephrosis) or tumors (e.g., renal cell carcinoma, hepatocellular carcinoma, some brain tumors) can inappropriately produce EPO. Less common causes include genetic mutations affecting oxygen sensing pathways, such as Chuvash polycythemia, and the use of exogenous EPO, testosterone, or anabolic steroids.
Recognizing the Signs
Signs of secondary polycythemia often develop gradually and are non-specific. These symptoms frequently stem from increased blood thickness, which can impede its flow through smaller blood vessels. Common manifestations include persistent headaches, dizziness, fatigue, and general weakness.
Individuals may also experience shortness of breath, blurred vision, a reddened complexion, itching, or sensations of numbness or tingling in the extremities. Chest pain or angina might also occur due to increased blood viscosity placing strain on the cardiovascular system.
Diagnosis and Management Approaches
Diagnosis of secondary polycythemia begins with routine blood tests. A complete blood count (CBC) will reveal elevated red blood cell counts, hemoglobin, and hematocrit levels. To differentiate from primary forms, measuring erythropoietin (EPO) levels is important; high EPO levels indicate a secondary cause, while low levels suggest primary polycythemia. Genetic testing for the JAK2 mutation also helps distinguish primary polycythemia, as this mutation is found in that condition but not in secondary polycythemia.
Once secondary polycythemia is suspected, investigations focus on identifying the underlying cause. These may include blood oxygen saturation tests, such as pulse oximetry or arterial blood gas analysis, to check for chronic hypoxia. Lung function tests and sleep studies can help detect respiratory conditions or sleep apnea. Imaging techniques like kidney ultrasound, CT scans of the abdomen or chest, and brain imaging identify tumors or other organ abnormalities that might be producing EPO.
Management of secondary polycythemia primarily involves addressing the underlying condition. For instance, individuals with hypoxia due to lung disease might receive oxygen therapy, while those with sleep apnea may benefit from treatments for that condition. If a tumor is producing excess EPO, surgical removal can resolve the polycythemia. Therapeutic phlebotomy, which involves removing blood to reduce the red blood cell count and blood thickness, may alleviate symptoms, especially if hematocrit levels are high or symptoms of hyperviscosity are present. While phlebotomy offers symptomatic relief, treating the root cause remains the most effective long-term strategy.