Polycythemia: Introduction - The Merck Veterinary Manual

Polycythemia: Introduction

Polycythemia is a relative or absolute increase in the number of circulating RBC resulting in an increased PCV, RBC count, and hemoglobin concentration.

Relative Polycythemia:

A loss of plasma volume will result in an apparent increase in RBC numbers. Relative polycythemia can be caused by any mechanism that results in hemoconcentration, such as dehydration from vomiting or diarrhea, or in a fluid shift from the intravascular to the extravascular space due to increased vascular permeability. Transient polycythemia is a type of relative polycythemia that occurs when excitement or fear causes splenic contraction, resulting in the release of large numbers of RBC into the circulation. Transient polycythemia is characterized by an elevated PCV with normal plasma protein concentrations. In both relative and transient polycythemia, the PCV is increased but the total RBC mass is normal. Relative polycythemia is diagnosed by finding an increased PCV and plasma protein concentration. There are breed differences in normal reference ranges of PCV. For example, Greyhounds normally have a PCV of ≥60%. Treatment consists of rehydrating the animal and treating the underlying cause.

Absolute Polycythemia:

A real increase in RBC numbers results from increased production. Definitive diagnosis requires direct RBC mass determination, which usually is not clinically available. Clinical diagnosis is based on a persistently increased PCV, without concurrent splenic contraction or dehydration (the latter based on lack of response to fluids). Clinical signs include red mucous membranes, often normal hydration status, bleeding tendencies, polyuria and polydipsia, seizures or behavioral changes, ataxia, weakness, amaurosis, tortuous retinal vessels and hemorrhages, and retinal detachments and blindness.

Absolute polycythemia may be primary or secondary. Primary polycythemia, or polycythemia rubra vera, is a myeloproliferative disease of unknown cause. It has been reported in dogs, cats, and cattle. RBC production is dramatically increased, and serum erythropoietin levels are low or low normal. In secondary polycythemia, RBC production increases in response to increased erythropoietin levels. This may be seen in cases of chronic renal hypoxia from severe pulmonary disease, congestive heart failure, or right-to-left blood shunting (such as with tetralogy of Fallot, transposition of great vessels, and reversed shunting patent ductus arteriosis). Excess production of erythropoietin by the kidney has been seen with renal carcinoma, renal cyst, hydronephrosis, and pyelonephritis. Hyperadrenocorticism ( Hyperadrenocorticism) commonly causes an increase in the PCV (rarely >55-60%); the exact mechanism is unknown.

Diagnosis:

A baseline CBC, chemistry panel, and urinalysis may be helpful in distinguishing relative from absolute polycythemia (high PCV and plasma protein levels suggest relative polycythemia). An elevated absolute reticulocyte count (>49,000/µL) in an animal with an elevated PCV documents the presence of increased erythropoietic activity and an absolute polycythemia. A bone marrow biopsy is not useful in distinguishing primary from secondary polycythemia.

Increased serum erythropoietin (EPO) levels are diagnostic for secondary polycythemia; however, this test lacks sensitivity as up to half of animals with secondary polycythemia will have EPO levels within normal reference ranges. Assessment of systemic hypoxia is important in differentiating primary from secondary absolute polycythemia. Blood gas pO₂ values < 90 mm Hg or pulse oximetry oxygen saturation values <80% suggest hypoxemia as the cause of secondary absolute polycythemia. Radiographs are normal in primary polycythemia but show changes if cardiac or pulmonary disease is present. Echocardiography is necessary to evaluate the heart, and abdominal ultrasonography is used to assess the kidneys and adrenal glands. An IV pyelogram should be done to further assess the renal architecture.

Treatment:

Treatment initially consists of reduction in RBC mass and concurrent hyperviscosity using phlebotomy (5 mL/kg repeated daily until PCV is reduced <55%) and replacement of blood volume with isotonic fluids. In secondary polycythemia, the underlying disease process must be addressed; phlebotomy may be contraindicated in animals with hypoxemia. In primary polycythemia, periodic phlebotomies may be required, with or without the administration of hydroxyurea (30 mg/kg, PO, sid until PCV is below 55% and then titrated) or chlorambucil (0.2 mg/kg, PO, sid until PCV is <55% and then titrated).