by Jennifer Chaitman, VMD, ACVIM (Internal Medicine) and Eveline Han, ACVIM (Internal Medicine)
When the kidneys lose 75% or more of their function, and can no longer concentrate the urine and filter enough toxins from the blood, this is called chronic renal failure (CRF). Causes of CRF may be congenital, familial, or acquired. Congenital and familial causes more often occur in certain breeds and in younger animals. Acquired CRF may occur from many different causes that damage the kidneys. The most common cause of CRF in the cat is tubulointerstitial nephritis (70%). Other causes include malformations of the kidney, lymphoma, and amyloidosis.
We can gain clues to the cause of CRF by performing diagnostic tests. A radiograph will show kidney size, shape, and radio-opaque stones. An ultrasound exam will show the size, shape and architecture of the kidneys. It also shows the size of the ureters and can show stones not visible on a radiograph. Sometimes there are even signs suggesting a kidney infection, or pyelonephritis. A biopsy of the kidney may be helpful to determine if cancer or amyloidosis is present. Aspirates of urine from the center of the kidney can confirm an infection.
One of the reasons your pet feels sick from CRF is uremia. Uremia is a syndrome accompanying renal failure. It results from the kidney's inability to filter wastes from blood to deposit these wastes in urine. The buildup of toxins in blood causes decreased appetite, weight loss, nausea, vomiting, build-up of acid in the blood, and other derangements. Therapy is guided at supporting the kidneys and lessening the side effects of uremia.
Unfortunately, the damage done to the kidneys in CRF is not reversible. However, medical therapy can make your pet feel much better and at times lessen the progression of the disease.
Depending on your pet's condition, your doctor may recommend the following treatments:
Fluids: Fluids may be given subcutaneously (under the skin) or intravenously (into a vein). Intravenous fluids are only given in the hospital where nurses can monitor the catheter and fluid rate. Subcutaneous fluids may be given at home. A fluid bag is hooked up to a line of tubing which is then attached to a needle. The needle is placed under the skin, often between the shoulder blades. The fluid is absorbed into the blood, providing extra blood volume to the kidneys. The kidneys are then able to better filter the blood. Fluids are the mainstay of therapy for CRF.
Stomach protectants: Patients with CRF often have elevated gastrin levels due to the failure of the kidneys to metabolize excess levels. Gastrin is a hormone that produces stomach acid. An excess of gastrin can cause ulcers and other stomach problems, leading to nausea, vomiting, inappetance and weight loss. Other factors involved in CRF are speculated to cause stomach derangements as well. For this reason, H2 blockers (famotidine, cimetidine, ranitidine), proton pump inhibitors (omeprazole), or a drug that acts as a band-aid to ulcerated tissue (sucralfate), may be prescribed.
Blood pressure medication: CRF can lead to arterial hypertension due to abnormalities in sodium and water metabolism. Signs of high blood pressure may be difficult to recognize, but may include eye, brain, and heart injury. Some blood pressure medications include ACE inhibitors (benazapril, enalapril) for dogs and a calcium antagonist specific to blood vessels (amlodipine) for cats.
Phosphorus binders: The kidney normally places excess phosphorus in the urine. In CRF, blood phosphorus levels may rise to dangerous levels. This produces abnormalities in calcium homeostasis and can lead to calcium wasting and decreased bone density. It can also lead to hormone abnormalities that may be linked to nausea. Fortunately, Alternagel and similar products may be given orally. These medications bind phosphorus in food and prevent it from being absorbed into the body, which leads to decreased phosphorus blood concentrations.
Erythropoietin: Our kidneys make a hormone named erythropoietin. This hormone travels to the bone marrow and encourages the development of red blood cells. In pets with advanced renal failure, the kidneys fail to make this hormone, and the marrow makes less red blood cells. This results in anemia. Erythropoietin injections may be given subcutaneously to correct the anemia. Human recombinant erythropoietin was the only product available for a long time for dogs and cats. Since it is a human protein, over time your pet's body will recognize it as different from its own and destroy it, making it less effective. Another possibility is that this immune response may trigger an attack on the body's own erythropoietin. This drug may work for 3 to 4 months, so it is generally reserved for end stages of CRF.
Darbepoetin is a newer, synthetic form of erythropoietin. It is thought to be less antigenic than human recombinant erythropoietin and shows promise.
Diet: A protein restricted diet is beneficial in lessening the advancement of CRF. Since the kidneys eliminate the products of protein metabolism, decreasing dietary protein may lessen stress on the kidneys and also decrease excess protein products in the blood. Diets for patients with CRF often have decreased phosphorus, decreased sodium, increased potassium, increased B vitamins and increased ratio of omega 3 to omega 6 polyunsaturated fatty acids. The composition of the diet is different for dogs and cats since studies have shown some of these dietary components to be more helpful in certain species.
Potassium supplementation: Cats with CRF are prone to hypokalemia, or low serum potassium. Oral potassium supplementation may be given to prevent hypokalemia.
Antibiotics: Antibiotics will be prescribed if pyelonephritis (a kidney infection caused by bacteria) is suspected (while pending urine culture results) or confirmed.
Other treatments for chronic renal failure include dialysis and renal transplantation (for cats). Your doctor will discuss these options with you.