Chronic renal failure (CRF) is a gradual loss of function of the kidneys over time which continues until the residual kidney function is insufficient to support life. Chronic renal failure may occur from a variety of causes but the endpoint, terminal renal failure, is the same. Chronic renal failure is frequently diagnosed in clinical veterinary practice in cats. Management of this disease is directed at long-term maintenance of the patient knowing that the patient will ultimately die from renal failure. Kidney transplantation, which is being performed with increasing frequency in cats, is one therapy that may restore renal function and improve the animals quality of life. However, consideration of a kidney transplant requires a considerable commitment from a cat owner due to the financial investment and effort required to maintain these patients after surgery.
Chronic renal failure is generally considered a geriatric disease. However, the age of cats diagnosed with chronic renal failure may vary from 9 months or less to extreme old age. In very young animals, chronic renal failure may often occur in association with congenital or inherited renal diseases.
The diagnosis of renal failure is usually based on laboratory tests which demonstrate elevated concentrations of waste products in the blood that the kidney would normally filter out and excrete in the urine. These lab tests include elevated blood urea nitrogen (BUN) and serum creatinine concentrations along with an inadequate urine-concentrating ability (dilute urine specific gravity). The chronic nature of the disease is established by historical evidence that the condition has been present for months to years or through further testing to document secondary changes, such as anemia, that eventually develop in the chronic renal failure patient.
Regardless of the cause, it must be recognized that chronic renal failure is an irreversible and progressive disease. Even if the agents responsible for the initial renal insult can be identified and eliminated, kidneys with chronic failure will continue to progressively deteriorate. There is no curative medical treatment for chronic renal failure. Maintenance treatments that are available to the practicing veterinarian are directed at reducing the clinical signs associated with renal failure and slowing the progression of renal dysfunction as much as possible. These treatments may include dietary therapy, fluid administration, management of anemia, modification of calcium and phosphorus, and management of hypertension. Depending on how far along in the progression of CRF your individual cat may be, aggressive medical management (i.e. nonsurgical) may prolong its life from months to several years. However, it is important to try to make an accurate assessment of where your cat is in this process and not just be hopeful that you will obtain a maximum duration of survival out of medical management. Waiting too long to decide on a transplant will reduce the chances of success considerably.
Kidney transplantation is the only potentially curative treatment for chronic renal failure. Cats receiving transplanted kidneys have been known to survive as long as 6 years or more after surgery. The implanted kidney functions normally and should provide a much improved quality of life for the transplant recipient. However, alleviation of chronic renal failure and elimination of the associated disease and maintenance therapy is achieved in trade for therapy directed at maintaining the transplanted kidney. Transplant patients will require daily lifelong medication to prevent rejection.
Clearly, a transplant is needed when a cat has chronic renal failure. However, it is thought that it may be advantageous to try to time the transplantation procedure to obtain maximum benefits for the kidney recipient since some successful grafts may be lost several years after transplant. The rationale for this approach is based on the fact that some kidneys will fail about 3 years after transplant due to chronic rejection. This is NOT THE CURRENT APPROACH TO TRANSPLANTATION AT THE UNIVERSITY OF WISCONSIN (see section on approach to timing of the transplant at the University of Wisconsin) but is included here since this approach may be considered by some transplant surgeons.
During the post-transplantation period, acute rejection of the transplant is prevented by administration of immunosuppressive drugs. However, long-term survivors may experience chronic rejection which is not responsive to these immunosuppressive drugs. This type of rejection results in slowly developing graft vascular disease. Graft vascular disease causes gradual blockage of the renal arteries and reduces blood flow to the transplanted kidney which will cause a slow loss of function. Thus, in some animals the transplanted kidney may stop functioning after about three years. The incidence of chronic rejection in the feline kidney transplant population is not known.
Decompensated renal failure, in simplest terms, is the point in progressive renal failure where the cat is unable to maintain its normal body condition even with adequate supportive care. In the early phases of renal failure, the cat is able to tolerate the elevated amounts of waste products in its blood and does pretty well. You may not even know the cat is ill. This is considered a compensated state of renal failure. When weight loss increases and the cat needs more and more aggressive supportive therapy then we consider it to be in a decompensated state. Eventually, this becomes an uncontrolled toxic condition (uremia).
It is common knowledge that cats with chronic renal failure may be maintained for months to years on conservative medical management. Therefore, it can be extremely difficult to predict when a cat would develop decompensated renal failure. Different animals appear to have varying tolerances to waste products which accumulate in the blood during renal failure. Serum creatinines (the best simple measure of kidney filtration function) may vary considerably before clinical signs of uremia (overt illness) are present. However, elevation of serum creatinine, when combined with observation of other physical changes such as weight loss, may suggest when to consider a transplant. Cats that are dependent on subcutaneous fluids for maintenance should have a transplant as soon as practicable. These cats have little reserve and may develop a terminal uremic crisis.
Another indication that a cat is progressing to decompensated renal failure may be the incidence of uremic crises. During the progression of renal disease, animals will commonly have several acute exacerbations of their chronic renal failure. This is when the cat becomes ill (usually becomes inappetant with possible vomiting, dehydration, malaise, lethargy, etc.) and requires intravenous fluid therapy to get back to being able to maintain itself again. Initially, these uremic crises may be resolved with intensive IV fluid management but subsequent episodes will become more difficult to reverse. A patient with chronic renal failure that has had one or two uremic crises but is still able to maintain itself should be critically evaluated for transplantation.
One very important indicator that transplant should not be delayed is weight loss which is progressive in spite of adequate dietary therapy and management of the patient. Weight loss that is unresponsive to intensified management, especially dietary management, in the chronic renal failure patient signals the change from a compensated to decompensated condition. Weight loss of 20% or more of normal body weight has been considered to carry a significantly reduced prognosis for success of the transplant surgery. Thus, a transplant should be performed as soon as progressive weight loss is identified.
Some final criteria that may be used to identify a decompensated state of renal failure is the inability of the cat to maintain certain normal metabolic processes. A good example of this is the development of anemia or the loss of the ability to maintain the levels of calcium or phosphorus within reasonable concentration ranges in the blood.
These factors may assist in identifying when your cat has gone into a state of decompensated chronic renal failure. This is a time of considerably increased risk for your cat. Our experience at the University of Wisconsin with waiting for patients to develop identifiable decompensated renal failure (in an attempt to delay the surgery as long as possible) has not been satisfactory.
Thus, our recommendation on timing of the kidney transplant at the University of Wisconsin is not to wait until decompensated renal failure occurs but to use identifiable measurements of advanced chronic renal failure as our criteria to go ahead with the surgery. These criteria are listed in the section on APPROACH TO TIMING OF FELINE KIDNEY TRANSPLANTS AT THE UNIVERSITY OF WISCONSIN.