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Basics ⬇

Outline

BASICS

Definition!!navigator!!

  • An increase in the circulating RBC (erythrocyte) mass, reflected by increases in RBC count, hemoglobin concentration, and PCV
  • Increase in circulating RBC mass may be relative or absolute

Pathophysiology!!navigator!!

Relative Polycythemia

  • Caused by hemoconcentration or splenic contraction
  • Hemoconcentration results from a reduction in the plasma volume without a change in circulating RBC numbers. It is associated with external water loss or internal fluid shifts
  • Common causes of hemoconcentration include reduced water intake, diarrhea, renal failure, diuresis, or excessive sweating. Endotoxemia is a common cause of a shift in water from the plasma space
  • Splenic contraction results in a transient release of stored RBCs into the circulation. Contraction may increase the PCV by up to 50% for several hours dependent on the stimulus

Absolute Polycythemia

  • An increase in circulating RBC mass without change in the plasma volume
  • May be primary or secondary
  • Primary absolute polycythemia is a myeloproliferative disorder where increased RBC mass is associated with normal PO2 and normal or reduced EPO concentration
  • Secondary absolute polycythemia results from increased erythropoiesis due to an increased synthesis of EPO that may be appropriate (response to tissue hypoxia; low PO2) or inappropriate (excessive EPO or other hormone production and normal PaO2)

Systems Affected!!navigator!!

  • Cardiovascular
  • Hemic
  • Respiratory
  • Hepatobiliary
  • Nervous
  • Renal
  • Gastrointestinal

Genetics!!navigator!!

N/A

Incidence/Prevalence!!navigator!!

  • Relative polycythemia is common in animals with hemodynamic compromise and when blood is collected from horses that have been exercised or are excited
  • Absolute polycythemia is rare

Geographic Distribution!!navigator!!

Horses kept at altitudes >2200 m (7200 feet) are likely to have appropriate secondary absolute polycythemia.

Signalment!!navigator!!

Breed Predilections

  • There are no breed predilections
  • Reference values for RBC count, hemoglobin concentration, and PCV are greater for light horse (i.e. hot-blooded) breeds (Thoroughbreds, Standardbreds, Arabians, and Quarter Horses) than draft (i.e. cold-blooded), pony, and miniature breeds and donkeys

Mean Age and Range

Animals of any age can develop polycythemia.

Predominant Sex

N/A

Signs!!navigator!!

General Comments

  • Signs of relative polycythemia are associated with the primary disease process
  • Signs of absolute polycythemia vary with the degree of increase in RBC mass and any underlying condition

Historical Findings

  • Relative polycythemia from splenic contraction may include a history of excitement or exercise
  • Relative polycythemia from fluid shifts is dependent on the underlying disease process
  • Absolute polycythemia may include a history of weight loss, lethargy, and inappetence

Physical Examination Findings

  • Prolonged CRT, dry mucous membranes, cool extremities, and dull mentation may be observed in horses with relative polycythemia from fluid shifts
  • Mucosal hyperemia (dark red to purple in color), prolonged CRT, lethargy, epistaxis and melena may be observed in horses with absolute polycythemia
  • Abnormal mentation, tachycardia, and tachypnea may occur when PCV >60%, as increased blood viscosity impairs tissue oxygenation
  • Cardiac murmur, tachycardia, and other signs of cardiac disease may be observed with congenital cardiac defects
  • Tachypnea, abnormal bronchovesicular sounds, and dyspnea may occur with chronic pulmonary disease

Causes!!navigator!!

Relative Polycythemia

  • Inadequate water consumption—e.g. dysphagia (altered prehension or swallowing—many causes), restricted access, or altered mentation
  • Increased fluid losses—e.g. diarrhea, diuretic therapy, polyuric renal failure, diabetes insipidus, excessive sweating, anterior enteritis, peritoneal or pleural effusion, ileus, or endotoxemia
  • Internal fluid shifts—e.g. endotoxemia/SIRS, trauma, hyponatremia, hypoalbuminemia
  • Splenic contraction—exercise, excitement, administration of α1-adrenergic agonists (epinephrine, phenylephrine)

Absolute Polycythemia

  • Primary absolute polycythemia is a rare myeloproliferative disorder that occurs as a single cell disorder or as a component of polycythemia vera (concurrent thrombocytosis and leukocytosis)
  • Appropriate secondary absolute polycythemia is associated with congenital cardiac defects with right to left shunting (e.g. tetralogy of Fallot, pulmonary atresia with ventricular septal defect, persistent truncus arteriosus, tricuspid atresia), chronic pulmonary disease, and residence at high altitude
  • Inappropriate secondary absolute polycythemia is a rare paraneoplastic syndrome from inappropriate EPO, EPO-like protein, or androgenic hormone secretion (hepatocellular carcinoma, hepatoblastoma, metastatic carcinoma, or lymphoma) or administration of exogenous androgens, EPO, or cobalt. In other species, chronic nephropathies (tumors, cysts, or hydronephrosis) may result in increased EPO production

Miscellaneous

Syndrome of red cell hypervolemia in Swedish Standardbred trotters.

Risk Factors!!navigator!!

See Causes.

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Primary polycythemia is diagnosed on exclusion of relative or secondary polycythemia
  • Laboratory error—insufficient blood sample centrifugation can result in an artificially high PCV

CBC/Biochemistry/Urinalysis!!navigator!!

  • RBC count, hemoglobin concentration, and PCV greater than upper limit of laboratory reference ranges
  • Hyperproteinemia usually accompanies relative polycythemia from hemoconcentration unless protein loss occurs concurrently (e.g. diarrhea, glomerulonephritis, peritonitis, or pleuritis) where normal or reduced protein concentrations may be present
  • Mild thrombocytosis may accompany relative polycythemia from splenic contraction
  • Neutropenia, left shift, and toxic changes in neutrophils may occur in response to endotoxemia/SIRS
  • Increased hepatic enzymes may be present in cases of inappropriate secondary absolute polycythemia associated with hepatic neoplasia

Other Laboratory Tests!!navigator!!

  • PaO2 to differentiate between appropriate and inappropriate secondary absolute polycythemia
  • Blood concentrations of EPO may assist in the diagnosis of inappropriate secondary absolute polycythemia
  • α-Fetoprotein in blood is consistent with hepatic neoplasia (hepatocellular carcinoma or hepatoblastoma)

Imaging!!navigator!!

  • Echocardiography to detect congenital cardiac defects
  • Tracheal wash and aspiration, bronchoalveolar lavage, thoracic ultrasonography and radiography, and pulmonary function testing to detect lung disease
  • Renal and hepatic ultrasonography

Other Diagnostic Procedures!!navigator!!

  • Liver biopsy if evidence of hepatic disease
  • Renal biopsy if evidence of renal disease
  • Bone marrow aspirate/biopsy to detect myeloproliferative disorders

Pathologic Findings!!navigator!!

Dependent on cause of polycythemia.

Treatment ⬆ ⬇

Outline

TREATMENT

Appropriate Health Care!!navigator!!

  • The aims of treatment are resolution of polycythemia and any underlying disorder
  • Relative polycythemia from splenic contraction requires no treatment
  • Relative polycythemia from fluid shifts requires inpatient medical management
  • Absolute polycythemia requires inpatient medical management

Nursing Care!!navigator!!

  • Relative polycythemia from hypovolemia requires IV fluid therapy. If protein loss is concurrent, colloid therapy may be necessary
  • Appropriate secondary absolute polycythemia requires oxygen therapy
  • Absolute polycythemia may require phlebotomy if PCV remains >60%—10–20 mL of blood/kg can be removed and replaced by equivalent volume of polyionic crystalloid fluid. Phlebotomy is repeated every 2–3 days until PCV <50% and then as required
  • Phlebotomy is contraindicated in relative polycythemia
  • Phlebotomy should be used with caution in polycythemia associated with hypoxia as the increased RBC mass is a compensatory mechanism for tissue hypoxia

Activity!!navigator!!

In cases of absolute polycythemia, activity should be restricted as persistent polycythemia can result in hypertension, tissue hypoxia, thrombosis, and hemorrhage.

Diet!!navigator!!

N/A

Client Education!!navigator!!

Response to treatment is dependent on the underlying cause. Most cases of absolute polycythemia have a poor to hopeless prognosis.

Surgical Considerations!!navigator!!

Increased blood viscosity associated with marked absolute polycythemia increases the risk of anesthesia-associated complications.

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

  • Hydroxyurea causes reversible bone marrow suppression and has been used successfully to treat polycythemia is humans and dogs
  • The appropriate dosage of hydroxyurea in horses is unknown
  • A suggested protocol for dogs is 30 mg/kg PO once daily for 7–10 days, then 15 mg/kg PO once daily

Contraindications!!navigator!!

Exogenous EPO, androgens, and cobalt should be avoided.

Precautions!!navigator!!

The efficacy and safety of hydroxyurea in horses is unknown.

Possible Interactions!!navigator!!

N/A

Alternative Drugs!!navigator!!

N/A

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

  • Monitor PCV, total plasma protein, and clinical variables reflecting hydration status in hypovolemic animals to monitor response to fluid therapy
  • In cases of absolute polycythemia, PCV should be monitored initially every 2–3 days, particularly if phlebotomy is performed. Thereafter, measure PCV as required

Prevention/Avoidance!!navigator!!

N/A

Possible Complications!!navigator!!

  • Absolute polycythemia can increase viscosity of blood, resulting in an increased risk of tissue hypoxia, thrombosis, hemorrhage, and hypertension
  • Cobalt salt may cause severe side effects

Expected Course and Prognosis!!navigator!!

  • Relative polycythemia from splenic contraction resolves within hours
  • Most cases of relative polycythemia associated with fluid shifts resolve with appropriate IV fluid therapy; however, shock may be more refractory to treatment
  • Primary absolute polycythemia has a poor prognosis
  • Secondary absolute polycythemia has a poor to grave prognosis, except when it is the result of compensation to residency at high altitude
  • Syndrome of red cell hypervolemia in Swedish Standardbred trotters is associated with poor performance; however, horses are otherwise healthy

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

N/A

Age-Related Factors!!navigator!!

RBC count, hemoglobin concentration, and PCV in foals decrease rapidly after birth and take several months to increase to near adult values.

Zoonotic Potential!!navigator!!

N/A

Pregnancy/Fertility/Breeding!!navigator!!

RBC count, hemoglobin concentration, and PCV tend to increase during pregnancy.

Synonyms!!navigator!!

Erythrocytosis

Abbreviations!!navigator!!

  • CRT = capillary refill time
  • EPO = erythropoietin
  • PaO2 = partial pressure of oxygen in arterial blood
  • PCV = packed cell volume
  • PO2 = partial pressure of oxygen in tissues
  • RBC = red blood cell
  • SIRS = systemic inflammatory response syndrome

Suggested Reading ⬆ ⬇

Sellon DC, Wise LN. Disorders of the hematopoietic system. In: Reed SM, Bayly WM, Sellon DC, eds. Equine Internal Medicine, 3e. St. Louis, MO: WB Saunders, 2010:730–776.

Author(s) ⬆

Author: Kristopher Hughes

Consulting Editors: David Hodgson, Harold C. McKenzie, and Jennifer L. Hodgson