Page 563 - Problem-Based Feline Medicine

25 – THE CAT WITH POLYCYTHEMIA 555

● Space-occupying renal masses including neopla-
INTRODUCTION
sia, cysts or hydronephrosis may impair renal blood
flow and stimulate secretion of inappropriate con-
MECHANISM? centrations of erythropoietin.
– Polycythemia from a renal space-occupying
Increased red cell concentration may result from a rel- mass is sometimes accompanied by signs of
ative or absolute increase in erythrocytes. renal failure, such as polydipsia/polyuria,
reduced urine specific gravity and azotemia.
A relative increase in red blood cells is caused by
dehydration. In dehydration, a reduction in plasma Primary inappropriate polycythemia is called poly-
volume causes hemoconcentration, resulting in erythro- cythemia vera, and is a neoplastic proliferation of ery-
cytosis and increased total plasma protein concentra- throid cells, associated with low or undetectable
tion. Blood oxygen saturation levels are normal. erythropoietin concentrations. Typically PCV is
greater than 60–65%.
Transient absolute polycythemia may occur with
excitement when splenic contraction releases red cells Once the PCV exceeds 65%, the ensuing hypervis-
into circulation. cosity increases peripheral resistance, decreases
cardiac output and decreases oxygen transport to
Absolute polycythemia may be an appropriate physio-
the tissues. Increased blood volume distends vessels
logical response to chronic hypoxia from cardiac or
and together with sluggish flow, predisposes to
pulmonary disease, and is mediated by increased serum
thrombosis and vessel rupture. It is hypothesized
erythropoietin levels.
that erythrocytosis impairs renal tubular concentrating
● Signs relate to the underlying cardiac cause and
mechanisms, resulting in polyuria and polydipsia.
include a cardiac murmur and cyanosis consistent
with right to left shunting of blood causing hypoxia, Clinically, polycythemia causes injected mucous
e.g. tetralogy of Fallot, atrial or ventriclar septal membranes, torturous retinal blood vessels, and
defect (ASD or VSD). there may be a history of intermittent bleeding
● Clinical signs of chronic primary lung pathology diatheses.
include coughing, dyspnea, wheezes and crackles,
Insufficient vascular perfusion of the central nervous
which accompany generalized hypoxia in more
system may result in neurological signs such as
severe cases.
seizures, ataxia, abnormal behavior and/or
Alternatively, absolute polycythemia may be caused blindness.
by increased, but physiologically inappropriate,
erythropoietin secretion, associated with a variety of
neoplastic conditions or space-occupying renal
WHERE?
masses. In these cases, blood oxygen saturation lev-
els are normal. Relative polycythemia may result from gastrointesti-
● Secondary causes of inappropriate polycythemia nal or renal disease, and rarely from extensive skin
include visceral tumors involving the kidneys, burns which cause excessive fluid loss, or severe dis-
liver, adrenal gland, female reproductive tract and ease of any organ sufficient to cause marked lethargy
central nervous system. These occasionally secrete and inadequate fluid intake.
erythropoietin or an erythropoietin-like substance
Absolute polycythemia from a physiological increase
without a hypoxic stimulus. While these tumors
in erythropoietin concentration may result from car-
have been reported to cause polycythemia in
diac or pulmonary disease.
humans and dogs, they are yet to be reported as a
cause of polycythemia in the cat. Absolute polycythemia from a pathological increase in
– Clinical signs are expected to relate to the type erythropoietin concentration may result from space-
of tumor involved, its anatomical location, and occupying renal disease or neoplasia of a wide vari-
the polycythemia. ety of organs.