Erythrocytosis describes a condition where there is an abnormally high concentration of red blood cells, hemoglobin, or hematocrit in the bloodstream. Although red blood cells transport oxygen, an excessive quantity is not beneficial. This increase in red cell mass alters the physical properties of the blood, leading to significant health issues.
Defining Erythrocytosis and Its Impact
The fundamental problem with erythrocytosis is the resulting increase in blood viscosity, which is a measure of its thickness or resistance to flow. Abnormally dense blood moves sluggishly through the circulatory system, impairing circulation, particularly in the small capillaries. This sluggish flow increases friction against vessel walls, raising the risk of blood clot formation. Paradoxically, despite the higher number of red cells, the slowed circulation decreases the overall efficiency of oxygen delivery to organs and tissues.
Distinguishing Between Primary and Secondary Causes
Erythrocytosis is categorized into two types based on its origin: primary, stemming from a defect in the bone marrow, and secondary, resulting from an external stimulus. Understanding the cause directs the diagnostic and treatment plan. Secondary erythrocytosis is significantly more common than the primary type.
Primary Erythrocytosis
Primary erythrocytosis arises from an intrinsic problem within the bone marrow, the site where blood cells are produced. The most frequent example is Polycythemia Vera (PV), a type of blood cancer where the bone marrow produces red blood cells independently of the body’s actual need. This autonomous production is often driven by a genetic defect, most commonly a mutation in the JAK2 gene. The mutated JAK2 gene continuously signals bone marrow cells to proliferate, even without the growth hormone, erythropoietin (EPO). Because the bone marrow overproduces cells without external EPO stimulation, laboratory tests typically show very low levels of circulating erythropoietin.
Secondary Erythrocytosis
Secondary erythrocytosis occurs as a physiological response to an external condition, usually chronic lack of oxygen, known as hypoxia. When tissue oxygen levels drop, the kidneys release more erythropoietin (EPO), signaling the bone marrow to increase red blood cell production to compensate. Conditions causing persistent low oxygen levels include chronic lung diseases like COPD, severe obstructive sleep apnea, and living at very high altitudes. Smoking is a common cause, as carbon monoxide binds to hemoglobin, lowering available oxygen and triggering the EPO response. Less common causes involve tumors, particularly those in the kidneys, that inappropriately secrete high levels of EPO, leading to excessive red cell production without an actual oxygen deficit.
Recognizing the Signs and Symptoms
Many individuals with erythrocytosis do not exhibit noticeable symptoms, especially in the early stages, and the condition is often detected through routine blood work. When symptoms appear, they result from increased blood viscosity and sluggish circulation. Common complaints include persistent headaches, dizziness, and fatigue. A patient may also experience plethora, a flushed, reddish appearance of the skin, especially noticeable in the face, hands, and feet. A characteristic symptom is pruritus, or intense itching, which frequently worsens after exposure to warm water, known as aquagenic pruritus. Other symptoms related to slowed blood flow include blurred vision, ringing in the ears (tinnitus), or a burning sensation in the extremities.
Diagnosis and Management Approaches
Diagnosis of erythrocytosis begins with a routine complete blood count (CBC) test, which measures the hemoglobin and hematocrit levels. If these values are elevated, the next diagnostic step is determining whether the cause is primary or secondary. This distinction is often made by measuring the serum erythropoietin (EPO) level in the blood. A suppressed or low EPO level strongly suggests a primary bone marrow disorder, such as Polycythemia Vera, prompting further investigation with genetic testing for the JAK2 gene mutation.
Conversely, a high or normal EPO level points toward a secondary cause, requiring tests like pulse oximetry to assess for chronic oxygen deprivation or imaging to check for EPO-producing tumors. Management strategies are tailored specifically to the underlying cause identified through these tests. For primary erythrocytosis, the main treatment is therapeutic phlebotomy, a procedure similar to blood donation that removes a volume of blood to rapidly reduce the red cell mass and decrease viscosity.
The goal is to maintain the hematocrit below 45% to reduce the risk of cardiovascular events, and patients are also typically prescribed low-dose aspirin to prevent clotting. Patients considered high-risk, such as those over 60 or with a history of blood clots, may also receive cytoreductive therapy using medications like hydroxyurea to suppress blood cell production in the bone marrow.
For secondary erythrocytosis, treatment focuses entirely on resolving the underlying cause of the hypoxia. This may involve quitting smoking, using a continuous positive airway pressure (CPAP) machine for sleep apnea, or optimizing treatment for heart or lung conditions. Phlebotomy is used less often for secondary causes, and only if the hematocrit is extremely high and the patient is experiencing severe symptoms related to viscosity.