Polycythemia is a condition characterized by an abnormally high concentration of red blood cells (elevated hemoglobin and hematocrit). This increase thickens the blood, impeding its flow through small vessels and raising the risk of complications. Secondary polycythemia (erythrocytosis) is the overproduction of red blood cells resulting from an outside factor or underlying medical condition. It is a response to an external signal, not a primary problem within the bone marrow itself.
Defining Secondary Polycythemia
Secondary polycythemia is a physiological response to chronic low oxygen levels (hypoxia). When tissues lack sufficient oxygen, the body compensates by increasing the number of oxygen-carrying red blood cells. This process is governed by erythropoietin (EPO), a chemical messenger primarily produced by the kidneys.
Specialized cells in the kidneys detect changes in blood oxygen saturation. During chronic hypoxia, these cells signal for increased EPO release into the bloodstream. EPO then travels to the bone marrow, stimulating stem cells to increase red blood cell production. This mechanism is the body’s attempt to restore oxygen delivery.
The distinction between secondary polycythemia and Polycythemia Vera (PV), or primary polycythemia, is crucial. In secondary polycythemia, red blood cell overproduction is driven by an external factor causing high or normal EPO levels. PV, in contrast, is a type of blood cancer resulting from an intrinsic genetic mutation, often in the JAK2 gene. This mutation causes the bone marrow to overproduce blood cells independently of EPO. Consequently, patients with PV typically have suppressed, or very low, levels of EPO because the body senses the excess red blood cells and attempts to halt production.
Specific Conditions That Trigger It
Most cases of secondary polycythemia are triggered by chronic hypoxemia (low oxygen concentration in the blood). Chronic lung diseases are a common source. Conditions like Chronic Obstructive Pulmonary Disease (COPD) or severe emphysema impair the lungs’ ability to efficiently transfer oxygen into the bloodstream. This damaged tissue leads to a persistent state of low oxygen.
Hypoventilation syndromes also frequently cause this condition due to insufficient breathing and reduced oxygen intake. Obstructive Sleep Apnea (OSA) is a prime example, where repeated airway blockages during sleep cause intermittent oxygen drops that signal the need for more red blood cells. Similarly, living at consistently high altitudes, where atmospheric oxygen pressure is naturally lower, causes the body to increase EPO production to maintain tissue oxygenation.
Other causes involve an inappropriate increase in the EPO hormone unrelated to low oxygen. Certain tumors, particularly renal cell carcinoma in the kidney, can autonomously secrete large amounts of EPO. This hormonal overproduction bypasses the body’s normal regulatory system, leading to an unwarranted increase in red blood cell mass. Exposure to carbon monoxide, often through heavy smoking, also causes functional hypoxia because the gas binds to hemoglobin much more tightly than oxygen, triggering the compensatory response.
Recognizing Symptoms and Diagnostic Confirmation
Symptoms of secondary polycythemia result from the increased viscosity of the blood as the red blood cell mass rises. Common complaints include persistent headache, general fatigue, and dizziness. Patients may also notice a ruddy or reddish complexion, especially on the face, due to the higher concentration of red blood cells near the skin surface.
Shortness of breath and blurred vision are frequently reported, reflecting sluggish circulation through the lungs and eyes. The thickened blood can also increase the risk of thrombotic events, such as strokes or heart attacks, though this risk is often lower than in primary polycythemia. Diagnosis often begins with a Complete Blood Count (CBC), which confirms polycythemia by revealing elevated levels of hemoglobin and hematocrit.
To confirm the condition is secondary, a serum EPO level test is performed. A normal or elevated EPO level points toward secondary polycythemia, reflecting the body’s signal for red blood cell production. A blood gas test may also measure arterial oxygen saturation, helping identify chronic hypoxemia as the underlying trigger. If the cause is unclear, testing for the JAK2 gene mutation is performed; a negative result, combined with a high EPO level, solidifies the diagnosis of secondary polycythemia.
Management and Outlook
The most important management strategy for secondary polycythemia is identifying and treating the underlying cause. For individuals with Obstructive Sleep Apnea, using a Continuous Positive Airway Pressure (CPAP) machine improves oxygenation during sleep and reduces the EPO drive. Similarly, quitting tobacco use is a direct and effective way to lower the blood cell count in smoking-related polycythemia.
For chronic lung conditions, treatments like low-flow oxygen therapy can correct chronic hypoxia and reduce the stimulus for red blood cell overproduction. If the polycythemia is due to a tumor inappropriately secreting EPO, surgical removal of the mass often resolves the condition.
When the hematocrit becomes dangerously high (usually above 60% to 65%), a procedure called phlebotomy may be necessary. Phlebotomy involves the temporary removal of a unit of blood to quickly reduce blood viscosity and mitigate thrombosis risk. This is a symptomatic treatment, not a cure, as the red blood cell count will rise again unless the underlying cause is managed. The overall outlook is closely tied to the prognosis of the condition that caused it, and effective management often stabilizes or resolves the polycythemia.