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
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 in cats
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
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.