Chronic interstitial nephritis is one of the most common causes of feline chronic renal disease.
Interstitial nephritis represents the end result of several different renal diseases including chronic glomerulonephritis and chronic pyelonephritis. In most patients, however, the inciting cause cannot be determined.
The most common historical findings in cats with endstage renal disease are weight loss, anorexia, and lethargy. Polyuria, polydipsia, and vomiting are detected less commonly by owners. Physical examination may show poor hair coat, emaciation, pallor of mucous membranes, and dehydration. The kidneys are small, firm, and irregular on abdominal palpation. Laboratory findings include nonregenerative anemia, azotemia, hyperphosphatemia, metabolic acidosis, hypokalemia, and isosthenuria. Except for urine specific gravity, urinalysis findings in cats with endstage renal disease generally are unremarkable. Renal biopsy will allow identification of a specific disease process or confirmation of the diagnosis of chronic interstitial nephritis of unknown cause.
Disease that need to be considered in any differential diagnosis of chronic interstitial nephritis include:
- ureteral calculi
- Feline lower urinary tract disease (FLUTD)
- Feline upper urinary tract disease (FUUTD)
Initial treatment requires rehydration over 24–72 hours. Renal function should be evaluated after rehydration before judgment is made about the ultimate disposition of the cat.
Long term medical management is begun after the cat has been rehydrated and stabilized. This therapy includes dietary restriction of protein (3.5-4.0 g/kg q24) and phosphorus; supplementation with vitamins, taurine, and potassium; endocrine replacement therapy including calcitriol and erythropoietin; management of hypertension; and subcutaneous administration of fluids at home by the owner.
Protein restriction should be considered when moderate azotemia persists in the well-hydrated state. Effective use of a low protein diet is indicated by reduction in BUN, stable body weight, and stable serum albumin concentration on serial measurements. Fresh water should be provided ad libitum and consumption of liquids should be encouraged. Phosphorus restriction is accomplished by dietary restriction and phosphorus-binding agents if necessary (aluminium hydroxide, aluminium carbonate, calcium carbonate, or calcium acetate) at a dosage of 60–100 mg/kg/day and administered with meals. Hypokalemia occurs in 20% to 30% of cats with chronic renal failure and may be treated using oral potassium gluconate (2–4 mEq q24h).
Anabolic steroids have a low margin of safety in cats (risk of hepatotoxicity) and therefore are not routinely recommended for cats with chronic renal failure. Vomiting and uremic gastroenteritis are less common in cats with chronic renal failure than in dogs and may be managed with famotidine (5 mg orally once per day). Recently, an extremely low dosage of calcitriol (2–4 ng/kg q24h) has been used in cats with stable chronic renal failure to blunt renal secondary hyperparathyroidism. Calcitriol should not be administered until hyperphosphatemia has been controlled.
Recombinant human erythropoietin (100 U/kg SQ three times per week) has been used to correct nonregenerative anemia in cats with chronic renal failure. There is a substantial risk of antibody formation in cats treated with human erythropoietin, and the drug has not been approved for use in animals.
Hypertension usually is treated with amlodipine (0.625 mg orally once per day) or enalapril (0.5 mg/kg orally once or twice per day) in cats with glomerulonephritis that are not azotemic. The prognosis for cats with chronic renal failure is variable, and the disease appears to progress at different rates in different patients. Affected cats may live several months to years with conservative medical management.