Page 1055 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

1030 CHAPTER 9

VetBooks.ir POLYCYTHAEMIA oxygenation and haemostatic disturbances may
occur with a PCV >0.60 l/l (60%) (Fig. 9.33).
Definition/overview
Polycythaemia (erythrocytosis) refers to an increase Clinical presentation
in RBC count, haemoglobin concentration or hae- Relative polycythaemia occurs in animals with
matocrit. All three of these parameters are usually clinical signs of dehydration and/or excitement.
increased concurrently. Animals with a high haematocrit from increased
production of erythrocytes may have dark red
Aetiology/pathophysiology mucous membranes. If the haematocrit is high
Dehydration results in lowered plasma volume, enough, signs attributable to sludging of viscous
producing a relative increase in RBC and protein blood may occur, including neurological and ocu-
concentrations. Relative polycythaemia can also lar signs.
develop in horses when adrenaline (epinephrine)-
induced splenic contraction occurs, such as with Differential diagnosis
excitement, resulting in a transient increase in The causes of polycythaemia need to be differenti-
RBC concentration only. Absolute polycythaemia ated, as described below.
may be observed with a neoplastic proliferation of
mature RBCs that is erythropoietin independent Diagnosis
(polycythaemia vera) or be secondary to conditions Diagnosis is made using clinical and historical
causing hypoxaemia sufficient to elevate erythro- information in combination with laboratory evi-
poietin production. Conditions in the latter cat- dence of increased RBC concentration, haemoglo-
egory include cardiopulmonary disease, living at bin and/or haematocrit. Hydration status should
high altitude and erythropoietin-producing renal be assessed clinically. Serial measurements of
tumours. Hyperviscosity, poor perfusion, decreased RBC parameters, especially following rehydration
therapy, can be used to identify relative polycy-
thaemia. Causes of absolute polycythaemia can be
9.33 further defined by measurement of erythropoietin
POLYCYTHAEMIA concentration and arterial partial pressure of oxy-
gen (PaO ). Animals with polycythaemia vera have
Absolute Relative 2
(increased RBC mass) (normal RBC mass) normal to low erythropoietin and normal PaO .
2
Animals with a secondary absolute polycythae-
Primary mia have increased erythropoietin and may have
(decreased – normal Secondary Dehydration,
erythropoietin, (increased EPO) splenic low PaO .
2
normal PaO ) contraction
2
Management
Many causes: Treatment of dehydration with appropriate fluid
cardiopulmonary
Polycythaemia vera replacement therapy will resolve dehydration-
(a myeloproliferative disease, high altitude,
hormone excess,
disease) associated relative polycythaemia. Causes of
renal masses,
neoplasia absolute polycythaemia are usually more difficult
to treat and may involve periodic blood removal
Fig. 9.33 Causes of polycythaemia in the horse. (phlebotomy). Removal of 10 ml/kg of blood every
Polycythaemia is classified as either relative 2–3 days should be performed until the PCV is
(commonly a result of dehydration) or absolute <0.50 l/l (50%). If phlebotomy is performed,
(either primary, a myeloproliferative disease or concurrent administration of a balanced electro-
secondary, because of increased erythropoietin lyte solution is preferred. Further blood removal
concentrations). should be determined based on regular monitoring