Polycythemia is a term referring to excessive numbers of red blood cells (RBCs, erythrocytes) in the body. There are two forms, polycythemia vera (PV) and relative polycythemia. Polycythemia vera is also known as absolute polycythemia or absolute erythrocytosis.
In all types of polycythemia, blood becomes sluggish because there are so many RBCs in circulation that blood flow is reduced. In the dog, the percentage of a given volume of blood that should be red blood cells is 36-60%. In the cat, 29-48% should be RBCs. This test is called hematocrit or packed cell volume (PCV) and is a routine part of the Complete Blood Count (CBC). When RBCs are in the normal range blood flows unrestricted through the biggest arteries and the smallest capillaries.
In polycythemia, the hematocrit can reach 85%. In the absence of sufficient liquid (serum) in circulation, the body can no longer move red blood cells to all areas of the body. The result is poor perfusion of oxygen to tissues served by small blood vessels. All organ systems suffer, but symptoms come primarily from improper service to the brain and kidneys. Often, seizures are the first thing a pet owner notices, although astute owners may observe lethargy first.
There are two forms of polycythemia vera: primary and secondary absolute polycythemia.
The primary form results from a bone marrow defect in which the production of RBCs is uncontrolled. When feedback systems should be telling the marrow that the body has sufficient numbers of erythrocytes, the bone marrow fails to “hear” and continues its overproduction.
Secondary absolute polycythemia occurs when the kidneys produce an excess of a hormone, erythropoietin, stimulating the bone marrow to the same end effect.
The cause of primary polycythemia vera is unknown. The cause of excessive production of erythropoietin is unknown.
Relative polycythemia results when the body is dehydrated. Dehydration may occur when pets lack proper access to clean, fresh water, in illness that causes reduced intake of water, or conditions such as vomiting and diarrhea that result in excess loss of fluid. In all of these circumstances the blood becomes “thicker,” more viscous, because of the loss of fluid. Unless vomiting and/or diarrhea are associated with large amounts of blood loss, the total mass of red blood cells is unchanged. There is simply less fluid in which to dilute them. Such patients rarely have hematocrits that exceed 65%.
TREATMENT OF POLYCYTHEMIA
Relative polycythemia is treated by administration of fluid therapy, usually by the intravenous route. Of course, it is imperative that the underlying cause of the dehydration is addressed.
Phlebotomy, or removal of blood volume, is the mainstay of treatment of polycythemia vera, both primary and secondary. Blood is usually let into a large syringe or transfusion bag. Frequency of phlebotomy is determined by the individual rate of relapse and response to treatment. Likewise, the volume of blood removed is based on the same factors. Some pets may require bleeding every couple of weeks, others may go months between treatments. The goal of treatment should be to bring the PCV into the high-normal range immediately after phlebotomy.
Some patients may lead a near-normal life with regular treatment. Others may experience worsening of symptoms with time and eventually succumb to seizures, or euthanasia because of owner intolerance of the problems and/or cost.
See you Monday, Dr. Randolph.
SASHA BROWN
