Dr. Joe Bartges targets specific issues where practitioners can play a key role in preventing renal disease in cats and dogs by monitoring certain drug use and disease management.
Chronic renal failure is common in dogs and cats. It may be defined as renal failure that is irreversible, stable for a period of time, but ultimately progressive. By the time it is diagnosed, the cause/causes of renal failure is/are not usually present. Because of this situation, specific treatment is not possible usually and, therefore, specific preventative measures are not always available. Prevention of chronic renal failure is further complicated by the fact that the kidneys have remarkable functional reserve and regenerative capacity, and that available diagnostic testing is insensitive for detecting subtle functional and morphological changes.
A 15-year-old, spayed female, domestic shorthair cat with chronic renal failure and generalized muscle weakness due to hypokalemia.
Chronic renal failure can occur in any aged dog or cat; however, it occurs most commonly in dogs more than 7 years and in cats older than 10 years. Routine 'geriatric' health screens, including physical examination, serum biochemical analysis (and serum thyroxine concentration in cats), and urinalysis, may aid in detecting sub-clinical renal failure. Owners of dogs or cats with chronic renal failure may report a history of polyuria and/or polydipsia. Likewise, exposure to nephrotoxins or administration of potentially nephrotoxic medications may be reported. Small, irregularly shaped kidneys, evidence for a chronic disease (e.g. unkempt hair coat, weight loss, muscle wasting), severe periodontal disease and ulcerative stomatitis may be found with chronic renal failure; however, these changes often do not occur until renal failure advances in severity. In most cases of chronic renal failure, urine specific gravity is decreased before the onset of azotemia. However, there are many extra-renal influences on azotemia and urine specific gravity and urine specific gravity is influenced by water intake and hydration status; therefore, finding a low specific gravity in a single urine sample is not necessarily abnormal.
Survey lateral abdominal radiograph of a 9-year-old, castrated male, domestic shorthair cat. The closed arrows demonstrate bilateral nephroliths and a right ureterolith. The right kidney is smaller than the left kidney.
An important aspect of preventing chronic renal failure is recognizing conditions that may compromise renal function and administering prompt and adequate treatment. For example, dehydration results in decreased renal perfusion and compromise of renal function. Dehydration often occurs with gastroenteritis, pancreatitis, heat stroke and infectious diseases such as pyelonephritis, to name a few. It is likely that chronic renal failure results from repeated and probably different episodes of damage to the kidneys in many patients. Chronic renal failure may also occur as a result of acute renal failure where damage has exceeded the regenerative capacity of the kidneys. Even with successful treatment of the acute renal failure, the dog or cat may remain in renal failure. Such conditions include, but are not limited to, ethylene glycol intoxication, pyelonephritis, leptospirosis and immune-mediated renal diseases.
Once a diagnosis of chronic renal failure is made, management is directed towards minimizing excesses and deficits occurring because of chronic renal failure, and towards slowing down or stopping progression of chronic renal failure (Table 1). A part of this management is minimizing or preventing additional renal damage from occurring, especially in preventing an acute renal failure episode from being superimposed on the chronic renal failure state (so-called acute-on-chronic disease).
Table 1: 'NEPHRONS'
Chronic renal failure occurs predominantly in older dogs and cats. Because of this, extra-renal diseases may create an acute-on-chronic situation. As mentioned previously, any disease resulting in dehydration may result in worsening azotemia or progression of chronic renal failure. Older animals undergoing anesthesia for elective or necessary procedures should receive fluid support in order to minimize hypotension and hypoperfusion. Examples include dental prophylaxis and surgical removal of growths.
Administration of drugs that are nephrotoxic should be used cautiously, if at all. Non-selective, non-steroidal anti-inflammatory drugs may be administered because of osteoarthritis in older animals. These drugs may decrease production of renal vasodilatory prostaglandins, thus, increasing the risk for renal failure or for an acute-on-chronic episode. Enalapril, often used because of cardiac disease, also may decrease renal function. Catabolic drugs, such as glucocorticoids or chemotherapeutic agents, may cause protein catabolism resulting in increased azotemia. Some drugs, such as certain antibiotics, cause nausea and vomiting, which may result in dehydration. Therefore, administrating any drug to an older animal should be carefully thought out and consideration given to risks and benefits of administering the drug.
Bacterial urinary tract infections may result in decreased renal function. If the bacterial urinary tract infection becomes established in the kidneys, then renal function may be compromised or an acute-on-chronic episode may occur. Animals with chronic renal failure are at greater risk for developing bacterial urinary tract infection because of isosthenuria and alterations in other host defenses due to the renal failure. In many older animals, especially cats, bacterial urinary tract infections are often not associated with clinical signs such as hematuria, dysuria and pollakiuria. Prophylactic use of antibiotics should be avoided unless necessary because it may result in highly resistant bacteria. In addition to their catabolic actions, glucocorticoids, whether endogenously produced in excess or administered exogenously, increases risk for bacterial urinary tract infection.
Nephrolithiasis (kidney stones) are more common in older animals than in younger animals, and may cause progression of chronic renal failure. If a stone located in the renal pelvis grows, it may result in damage to renal tissue and loss of function. If nephroliths move out of the renal pelvis into the ureter and become lodged, post-renal obstruction occurs. Renal tissue and loss of function occurs due to back up of pressure, which over time may result in hydronephrosis. Although the composition of nephroliths may be of any mineral, calcium oxalate is most common and is not amenable to medical dissolution. Therefore, treatment of nephroliths and ureteroliths, if indicated, requires surgery, and surgery may induce damage to renal tissue and loss of function. Ureteral surgery is difficult and may result in ureteral stricture and obstruction. Nephrotomy to remove nephroliths results in loss of function. The decision to do surgery must be weighed against the risks of the procedure. In animals with chronic renal failure, damage to the kidney or ureter because of nephroliths or because of surgery may be detrimental. We base the decision to do surgery for nephroliths on whether the stone is increasing in size despite appropriate medical therapy, the stone is causing a ureteral obstruction, severe hematuria is present, or the stone is associated with recurrent bacterial urinary tract infections. It is important to perform an excretory urogram with nephroliths because mineralization of renal tissue also occurs in older animals, and this must be differentiated from nephroliths. Renal mineralization does not require treatment.
Systemic arterial hypertension is often missed because blood pressure monitoring is not routine in veterinary medicine. Renal failure, glomerular disease, hyperthyroidism and hyperadrenocorticism are associated with systemic arterial hypertension Systemic arterial hypertension can damage small vessels (such as retinal vessels), cause left ventricular hypertrophy, cause seizures presumably due to hemorrhage from small vessels, and cause progression of renal failure because of transmittance of the systemic hypertension to the glomerular capillaries. In animals with chronic renal failure, indirect arterial blood pressure should be a routine part of monitoring. Persistent systemic arterial hypertension should be treated in order to minimize associated consequences and possibly progression of chronic renal failure.
Hyperthyroidism often occurs in older cats and may be associated with concurrent chronic renal failure. It can adversely affect renal function because of associated systemic arterial hypertension and because of the catabolic nature of the disease. However, hyperthyroidism may protect glomerular filtration rate and renal function by the increased metabolic rate, blood pressure, and renal blood flow. In some cats with hyperthyroidism, treatment of the hyperthyroidism results in worsening renal function. Therefore, it is important to do a metabolic screen in older cats with hyperthyroidism giving particular attention to whether azotemia is present or not. In cats that are azotemic and hyperthyroid, administer methimazole and at time of re-evaluation of serum thyroxine concentration, also measure BUN and serum creatinine. If the serum thyroxine concentration is normal and the azotemia has not increased, then treatment with methimazole, surgical removal of enlarged thyroid, or iodine-131 can be undertaken. If the serum thyroxin concentration is normal and the azotemia has worsened, then methimazole dosage should be adjusted to maintain a high normal or slightly increased serum thyroxine concentration in order to minimize worsening of the renal failure.
Table 2: Guidelines for monitoring patients with CRF
Chronic renal failure is a dynamic disease; therefore, serial monitoring is important in order to minimize excesses and deficiencies associated with renal failure and to make adjustments in management (Tables 1 and 2). Preventing chronic renal failure may be difficult, but prevention of renal damage and minimizing progression of chronic renal failure is an attainable goal.