Reply To: Acute dyspnoea and small stature in a domestic shorthair cat
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Differentials for causes of an absolute polycythemia:
1. Primary (neoplasia, genetic).
(a) Appropriate (cardiac (right-to-left shunting patent ductus arteriosus, persistent truncus arteriosus, ventricular septal defect, atrial septal defect, tetralogy of Fallot), respiratory (pulmonary parenchymal disease, pulmonary vascular amyloidosis), environmental (high altitude, severe obesity)).
(b) Inappropriate (renal disease (pyelonephritis, local hypoxia), neoplasia (renal lymphoma, renal fibrosarcoma, renal cell carcinoma, nasal fibrosarcoma, Schwannoma), metabolic (hyperadrenocortisism and hyperthyroidism), iatrogenic (exogenous erythropoietin therapy)).
Primary absolute polycythemia is a neoplastic myeloproliferative disorder of the erythroid stem cells. The neoplastic stem cells produce red blood cells independant of erythropoetin. Secondary polycythemia can be either appropriate or inappropriate. Appropriate secondary polycythemia is due to systemic hypoxia causing an appropriate compensatory response increasing erythropoetin production resulting in an increase in red blood cells which increases oxygen carrying capacity of the blood. Inappropriate secondary polycythemia is the increase of erythropoetin in the absence of hypoxic stimuli. The main causes of inappropriate polycythemia have been reported to be neoplastic in origin, such as renal sarcoma.
Differentiating between primary and secondary polycythemia is difficult. It is possible to measure erythropoetin concentrations at a specialised laboratory. However, the clinical evaluation of these results is limited as there can be an overlap between erythropoetin concentration in clinically normal and polycythaemic patients. As such erythropoetin measurement, where available, is recommended to be used as an aid in confirming a diagnosis of secondary polycythemia.