Erythrocytosis

Question 1

Erythrocytosis

Answer 1

AKA polycythaemia

Characterised by increase in red cell count, haemoglobin concentration and PCV/HCT, but usually with normal numbers of white blood cells and platelets

Relative or absolute

Question 2

Relative erythrocytosis

Answer 2

The total red cell mass remains normal but the decrease in plasma volume result is in an increase in PCV. No polychromasia/reticulocytosis.

Question 3

Causes of relative erythrocytosis

Answer 3

Disturbances in fluid balance resulting in dehydration (haemoconcentration.
○ Increased plasma protein too.

Splenic contraction and release of RBCs can also cause a relative erythrocytosis and would be associated with a normal plasma protein concentration.
○ Splenic contraction can be seen as an effect of adrenaline.
○ Particularly seen in dogs and horses, but transient.

Question 4

Absolute erythrocytosis

Answer 4

Increased red cell mass, can be either primary or secondary.

Question 5

Primary absolute erythrocytosis

Answer 5

Also known as polycythaemia rubra vera.

Chronic myeloproliferative disease occurring in the dog, cat and cow.

Characterised by clonal proliferation of erythroid precursor cells with maturation and differentiation into morphologically normal red blood cells.

It is not controlled by normal feedback mechanisms and therefore is not erythropoietin (EPO) dependent. EPO levels may be low or undetectable.

Question 6

Secondary erythrocytosis

Answer 6

Occurs in response to increased EPO secretion.

Increased EPO secretion may be an appropriate compensatory response to chronic hypoxia (e.g. due to chronic pulmonary disease or right to left cardiovascular shunting e.g. patent ductus arteriosus, Tetrology of Fallot) or inappropriate without systemic hypoxia, most commonly due to renal tumours.

Question 7

Consequences of erythrocytosis

Answer 7

The excess numbers of red cells leads to increased blood volume and blood viscosity.

Blood vessels are distended and there may epistaxis, haematuria and haematemesis, congested retinal vessels, seizures.

Sludging of blood also leads to poor tissue perfusion.

Question 8

Approach to erythrocytosis

Answer 8

Physical examination.
○ Is the animal dehydrated?

Check plasma protein, serum urea and creatinine concentrations.
○ Are they increased?

PCV normalises.
○ After IV fluids.
○ Without treatment – could be transient i.e. due to:
§ Splenic contraction?

If relative erythrocytosis is excluded:
○ Check for cardiac/respiratory disorders.
§ Check radiographs.
§ Echocardiography.
○ Check for renal or other tumours.
§ Ultrasound.
§ CT.
§ Radiography.
○ Measure serum [EPO]?
§ Not helpful as may be increased due to poor tissue perfusion anyway.

Question 9

Management of absolute erythrocytosis

Answer 9

In both primary and secondary absolute erythrocytosis, phlebotomisation can be used to reduce PCV and viscosity.

Chemotherapy (hydroxyurea) can be used to treat primary absolute erythrocytosis and the cause of secondary appropriate absolute erythrocytosis should also be treated (e.g. surgery for patent ductus arteriosus).