Canine Polycythemia Study Guide

Overview and Clinical Importance

Polycythemia (erythrocytosis) refers to an increase in the red blood cell (RBC) count, packed cell volume (PCV), hematocrit (HCT), and hemoglobin concentration above reference intervals. While the terms polycythemia and erythrocytosis are often used interchangeably, polycythemia technically implies an increase in all blood cell lines, though this is exceptionally rare in companion animals. Clinical signs typically do not manifest until PCV exceeds 60%, with some of the highest recorded values exceeding 85%. Understanding the classification, pathophysiology, and management of polycythemia is essential for the NAVLE, as this condition represents a critical diagnostic and therapeutic challenge in small animal practice.

Definitions and Terminology

Erythrocytosis specifically refers to an increase in RBCs only, while polycythemia may imply increases in all blood cell lines (RBCs, WBCs, and platelets). However, concurrent leukocytosis and thrombocytosis along with erythrocytosis is exceptionally rare in dogs. Normal canine PCV ranges from 37-56%, with sighthounds (Greyhounds) normally having mild erythrocytosis compared to standard reference intervals. Clinical signs do not typically develop until PCV exceeds 60%.

Classification of Polycythemia

Pathophysiology

Erythropoietin Feedback Loop

Under normal conditions, decreased oxygen delivery to the kidney stimulates erythropoietin (EPO) secretion from peritubular interstitial cells. EPO then acts on erythroid progenitor cells in the bone marrow to increase RBC production. When oxygen delivery normalizes, EPO secretion decreases, completing the negative feedback loop.

Hyperviscosity Syndrome

When RBC mass increases significantly (PCV greater than 60%), blood viscosity increases exponentially. This hyperviscosity leads to: decreased blood flow in capillaries causing tissue hypoxia and sludging; increased risk of thrombosis and vessel rupture; impaired microcirculation especially in the brain and retinal vessels; and activation of coagulation cascade with platelet activation. The neurological signs commonly seen in polycythemic dogs (seizures, behavioral changes, ataxia) result from impaired cerebral blood flow and potential thrombotic events.

Clinical Signs

Dogs with polycythemia often remain asymptomatic for weeks to years and may be diagnosed incidentally on routine bloodwork before clinical signs develop. When signs occur, they are primarily attributable to hyperviscosity syndrome and typically do not manifest until PCV exceeds 60%.

Diagnostic Approach

Step 1: Confirm Polycythemia and Rule Out Relative Causes

The first step is to confirm the elevated PCV is persistent and not due to hemoconcentration or splenic contraction. Relative erythrocytosis will be accompanied by clinical signs of dehydration (tacky mucous membranes, prolonged skin tent, tachycardia), elevated total protein, and prerenal azotemia. The PCV typically does not exceed 65% with relative polycythemia and normalizes with fluid therapy.

Exam Focus: Polycythemia vera is a diagnosis of EXCLUSION. You must systematically rule out relative polycythemia (dehydration) and secondary causes (cardiac shunts, pulmonary disease, EPO-secreting tumors) before diagnosing primary erythrocytosis. Bone marrow biopsy is NOT diagnostic because all absolute causes show erythroid hyperplasia!

Secondary Appropriate Polycythemia

Tetralogy of Fallot

Tetralogy of Fallot is the most common defect producing cyanosis and secondary polycythemia. It comprises four components: pulmonic stenosis, ventricular septal defect, right ventricular hypertrophy, and overriding aorta. The right-to-left shunt causes venous admixture and systemic hypoxemia, triggering appropriate EPO release. Clinical signs include stunted growth, exercise intolerance, cyanosis, collapse, and seizures. A systolic murmur may be auscultated over the pulmonic valve area, though murmur intensity is attenuated with severe polycythemia.

Reversed Patent Ductus Arteriosus

In reversed (right-to-left) PDA, pulmonary hypertension causes blood to shunt from the pulmonary artery to the aorta distal to the brachiocephalic trunk. This results in differential cyanosis where the caudal half of the body is cyanotic while the cranial portion remains pink. The resulting systemic hypoxemia stimulates EPO production and secondary polycythemia.

Polycythemia Vera (Primary Erythrocytosis)

Polycythemia vera is a rare chronic myeloproliferative disorder resulting from autonomous clonal expansion of hematopoietic progenitor cells independent of EPO stimulation. It most commonly affects middle-aged to older dogs with no documented sex or breed predisposition. The diagnosis requires: PCV greater than 65%, low or low-normal EPO concentration, normal arterial blood oxygen, normal blood volume, and exclusion of all secondary causes.

Diagnostic criteria for polycythemia vera: PCV greater than 65%, low or low-normal serum EPO, normal arterial PaO2 (greater than 80 mmHg), no evidence of cardiac or pulmonary disease, no EPO-secreting tumors identified, erythroid hyperplasia on bone marrow (non-specific finding).

Treatment and Management

Treatment goals are to reduce blood viscosity by lowering PCV to less than 55-60% to eliminate or minimize hyperviscosity signs. The approach differs based on the type of polycythemia identified.

Phlebotomy Technique

Phlebotomy is performed via jugular venipuncture. Remove 10-20 mL/kg of blood per session, which should decrease PCV by approximately 15% for every 20 mL/kg removed. Always replace with IV crystalloids (equal or greater volume) to maintain circulating blood volume. May repeat daily until target PCV is reached. For patients requiring frequent phlebotomy, placement of a vascular access port can facilitate the procedure.

Hydroxyurea Therapy

Hydroxyurea is a myelosuppressive chemotherapeutic agent that inhibits ribonucleotide reductase, thereby decreasing DNA synthesis and RBC production. It is used when phlebotomy alone cannot adequately control PCV or when frequent phlebotomy is impractical.

Prognosis

Prognosis varies significantly depending on the underlying cause. Dogs with relative polycythemia have excellent prognosis once dehydration is corrected. For polycythemia vera, mean survival with phlebotomy alone is approximately 5 months, but survival increases to 8-33 months (with some dogs surviving over a decade) when hydroxyurea is added to the treatment protocol. Secondary inappropriate erythrocytosis may have favorable outcomes if the EPO-secreting tumor can be surgically removed. Secondary appropriate polycythemia prognosis depends on the ability to manage the underlying cardiac or pulmonary disease.