Chronic Renal Failure

Renal failure (kidney failure) occurs when kidney function has deteriorated to such a degree that the kidneys can no longer perform their normal functions of excreting wastes, maintaining water and electrolyte balance, and producing hormones. Renal failure occurs in acute or chronic forms.

Acute renal failure is of recent onset and is potentially reversible. In contrast, chronic renal failure has been present for months to years at the time of diagnosis and is irreversible. Dogs and cats with chronic renal failure cannot be cured, but their clinical signs can often be managed successfully.

Kidneys are composed of many small functional units called nephrons (approximately 190,000 in cats and approximately 400,000 in dogs). Dogs, cats, and humans are normally born with such an abundance of nephrons that signs of kidney failure do not become apparent until more than two thirds of the nephrons have been damaged. Because of this redundant kidney tissue, it is possible to donate a kidney for transplantation and survive. On the other hand, surplus nephrons make it difficult to detect chronic kidney diseases until they are well advanced. As a consequence, chronic kidney failure is often an insidious condition that remains unrecognized until it is severe.

Because kidney disease is often quite advanced at the time of initial diagnosis, the initiating cause of chronic renal failure can rarely be established. Although chronic renal failure occurs most often in older dogs and cats, renal failure is not simply a result of aging.

The earliest signs of renal failure are typically thirst (polydipsia) and increased urine volume (polyuria). These signs result from inability of the diseased kidneys to form concentrated urine. Other common early signs include weight loss, poor haircoat, and an increasingly selective appetite. Further decline in kidney function result in progressive inability to excrete waste products, leading to retention of toxic wastes in blood and tissues in the body. This is called uremia (literally, urine in the blood).

Prominent clinical signs of uremia include loss of appetite, vomiting, ulcers in the mouth, "uremic" (foul ammonia smelling) breath, weakness, and lethargy. Other important effects of renal failure include anemia (caused by inability of failing kidneys to produce erythropoietin, the hormone responsible for making red blood cells) and high blood pressure. Anemia worsens the weakness, lethargy, and loss of appetite of dogs and cats with chronic renal failure, and high blood pressure may cause sudden blindness, strokelike signs (such as mental dullness, sudden behavioral changes, coma, or seizures), or injury to the kidneys and heart. Diagnosis of chronic renal failure is confirmed by laboratory evaluation of your pet's blood and urine. A urine test can help determine whether the kidneys can form concentrated urine and provide evidence of other urinary tract problems such as urinary tract infections.

Blood tests used to evaluate kidney function include blood urea nitrogen (BUN) and serum creatinine concentrations. Because the kidneys excrete urea and creatinine, increases in urine and creatinine concentrations in blood indicate decreased kidney function. These tests are usually done together because they provide different information. The serum creatinine concentration is the more specific test for kidney function, and treatment and other factors may influence the BUN.

Fortunately, most dogs and cats can be treated, providing a good quality of life for months or years. Treatment for chronic renal failure is tailored to the unique clinical requirements of each pet but may include a special diet (e.g., limiting protein, phosphorus, and salt intake); hydration therapy; and medications designed to control clinical signs (such as poor appetite, nausea and vomiting), acid-base and electrolyte disturbances, anemia, and hypertension.

Consumption of excess protein may make some pets ill because the waste products of protein metabolism are excreted by the kidneys and are retained in renal failure. Dehydration (abnormal depletion of body fluids) is a special threat to pets with renal failure, and they may deteriorate if episodes of vomiting, diarrhea, or inadequate water intake are not dealt with promptly. Water should never be withheld from dogs and cats with renal failure.

In humans, renal failure is most often managed by dialysis (hemodialysis or peritoneal dialysis) or renal transplantation. Chronic hemodialysis and peritoneal dialysis have thus far not proved to be satisfactory options for dogs and cats with chronic renal failure because they are expensive and fail to provide an acceptable quality of life. Renal transplantation is an expensive but potentially useful option for selected cats but has not met with similar success in dogs. Renal transplantation is best reserved for cats that can no longer be managed by standard medical therapy.

-David J. Polzin