Polycythemia Flashcards
What does polycythemia mean
Polycythemia (=erythrocytosis) is the term use when the PCV exceeds the upper reference range
Plocythemia can be divided into two groups:
- relative polycthemia
- absolute polycythemia
What is relative polycythemia
Relative polycythemia is due to plasma volume depletion
- shock
- dehydration (rarely results in a PCV > 60% in cats)
- volume depletion secondary to diuretics
The PCV should return to the normal range once the animal is rehydrated with appropriate fluid therapy
What is absolute polycythemia
An absolute polycythemia is when there is an incresaed red cell mass, with normal plasma volume
Absolute polycythemia can be subdivised into two groups:
- primary polycthemia
- secondary polycythemia
What is primary polycythemia
Primary polycythemia (also known as polycythemia vera) is rare in the cat and represents a chronic myeloproliferative disease
- this represents a clonal expansion of red blood cell precursors, maturing to normal red blood cells, without a definable stimulus
- often the PCV is above 65% and other cell lines (leukocytes and thrombocytes) ca be high
- there is no documented arterial hypoxemia or elevations in EPO (i.e., EPO is low-normal or normal)
What is secondary polycythemia
Secondary polycythemia refers to a group of diseases in which the increased red blood cell mass is a result of EPO stimulus on the bone marrow RBC precursors
This may be an appropriate response to renal hypoxia
- e.g., congenital right to left shunting heart defects, chronic pulmonary disease
It may also be due to an inappropriate overproduction of EPO by a tumor
- e.g., renal carcinoma, nephroblastoma
What would be your differentials for an absolute polycythemia
Primary polycythemia vera
Secondary to increased erythropoietin production
- hypoxaemia
- paraneoplastic production of EPO, usually by a renal tumor
Overzealous EPO or blood transfusion therapy
Splenic contraction (likely to only cause mild increases in PCV)
What are the clinical signs of absolute polycythemia
Clinical signs usually only occur once the PCV is above 60%
Signs are generally due to hyperviscosity and expanded blood volume and may include:
- hyperemia of the mucous membranes (+/- cyanosis if underlying right to left cardiac shunt)
- neurological disorders (e.g., behavioral changes, seizures, lethargy)
- hemorrhage secondary to hypertension (e.g., hyphema, epistaxis)
Seiizures and mentation changes were the most common clinical sign in a recent report
Palpable bilateral or unilateral renomegaly may be appreciable if the underlying problem is renal in prigin
Signs secondary to hypoglycemia due to RBC usage of glucose
Cardiac murmur due to increased blood viscosity and the possible hypertrophic cardiomyopathy which is likely a physiological response to increased blood viscosity requiring increased ventricular pressure to maintain cardiac output
What would be your diagnostic plan for investigating polycythemia
Hematology, biochemistry and urinalysis
Thoracic radiographs
Heart and/or abdominal ultrasonography
Arterial blood gases
+/- a species-validated EPO measurement if available
NOTE: bone marrow examination is not helpful in differentiating causes of primary versus secondary polycythemia
What would be your treatment plan for a cat affected by polycythemia
If specific treatment of an underlying etiology is not possible, severely polycythemic patients may benefit from phlebotomy (20 ml/kg/session) until the PCV is < 60%
- the phlebotomy can be repeated daily until the target PCV is reached
- replacement fluid therapy is often not required as these animals may already be volume expanded
Long-term control may be achieved by repeated phlebotomy and/or chemotherapy with hydroxyurea
- 30 mg/kg once daily for one week
- then 15 mg/kg once daily until remission
- the dose is then tapered to lowest effective frequency by monitoring the hematocrit
How would you monitor a cat on hydroxyurea for polycthemia treatment
A hemogram should initially be monitored once weekly, then monthly
- cats are quite susceptible to the bone marrow toxicity effects of this drug, due to the inhibition of DNA synthesis
What are the potential side effects of hydroxyurea
Hydroxyurea will only be used if necessary as there are significant side effects, of which not all have been reported in cats:
- GI side effects (e.g., anorexia, vomiting, diarrhea)
- stomatitis
- sloughing of nails
- alopecia
- dysuria
- bone marrow depression (anemia, thrombocytopenia, leukopenia)
- pulmonary fibrosis
What treatment could be an alternative approach in the treatment of polycythemia
A novel bone marrow-sparing treatment for primary erythrocytosis has been described
- onion powder: 1/8 teaspoon once a day with wet food
