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.
Conditions affecting renal function
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.
Acute on chronic
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 infection
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.