As cats age, just like humans, they experience a gradual loss of kidney function. They should be carefully evaluated for any signs of underlying early kidney disease, e.g. hypertension, small kidneys on radiographs, palpable changes in size or structure of kidneys, mineralization in the kidneys or hyposthenuria and hyperproteinuria. As the disease progresses, there are measurable changes in their blood levels of urea nitrogen and creatinine.
Early in renal failure, the kidneys start losing their ability to concentrate urine. This gradual process can be monitored by measuring the urine specific gravity with a refractometer. Measurements should be taken when the cat is not on a diuretic and before the administration of fluid therapy. A normal feline urine specific gravity should be greater than 1.035 mg/dL and can range to concentrations greater than 1.050. Aging alone can take its toll on the kidneys and they begin to fail, producing a more dilute urine. Thus the cat urinates more frequently and compensates by drinking more water. However, a low urine specific gravity can be caused from increased thirst secondary to hyperthyroidism and not from renal failure. Low specific gravity alone should not be used to rule out I-131 therapy.
We rely on the BUN (blood urea nitrogen), creatinine, urine analysis, kidney size, and blood pressure to evaluate and monitor feline kidney function. Urea is the end product of protein digestion and its concentration varies with the diet and with kidney function. Creatinine is produced at a fairly constant rate depending on muscle mass and renal function. As the cat’s kidneys begin to fail, they do not filter these metabolic byproducts as efficiently. Their concentration in the blood rises and the cat begins to drink more water.
Kidney disease usually goes undetected. No apparent symptoms. Usually caused by an insult to the kidney. No biochemical evidence. No azotemia.
Stage 2 kidney disease may have very subtle signs. These cats will have a creatinine level between 1.6-2.8 mg/dL. Azotemia does not preclude us from treating a patient, but we use a lower dose to try to prevent worsening the condition. Proper diet and fluid therapy remain the mainstays for renal disease. Click here for more information about feline kidney diets.
By definition, these cats are uremic with elevated BUN levels and Creatinine levels between 2.9-5.0 mg/dL. They are sometimes nauseated with vomiting or inappetance and/or lethargy.
Fluid therapy in the hospital or administered at home by the client at a frequency from once a week to every day has maintained many cats for years.
We feel the addition of essential fatty acids to the diet to reduce renal inflammation is beneficial and without any negative side effects. These are the omega-3 fatty acids.
Concurrent administration of ACE inhibitors has shown to be beneficial in some patients but is not something that we routinely recommend. When these drugs are used, the BUN and creatinine levels must be monitored closely.
These cats are in end-stage renal failure. They have Creatinine levels in excess of 5.0 mg/dL. Aggressive intravenous fluid therapy, antiemetics, and adequate caloric intake are essential.
Feeding any food at this stage is far more valuable than trying to encourage a less palatable, scientifically formulated diet. Patients in stage 4 chronic kidney disease would generally not be accepted for I-131 therapy.
Some clinicians do a Tapazole trial for a few weeks. Prescribe Tapazole at 5 mg divided b.i.d. and adjust the dose higher to reduce the thyroid hormone level to the euthyroid range, ideally between 1 and 2 mg/dL. At this time blood tests are done, including CBC, BUN, and Creatinine. The CBC should be normal and the BUN and Creatinine may be higher, but still be within normal limits. Thus if the kidney values remain stable, the cat will show a similar result when the euthyroid state is accomplished with radioactive iodine.
This is not a guarantee that the kidneys will remain healthy for a long time after the treatment, but it is a very good sign. On the other hand, if both blood tests quickly become abnormally high, e.g. BUN > 50 and Creatinine >5.0 mg/dL and these values continue to climb, this patient is not a good candidate for radioactive iodine. If the patient’s blood levels remain elevated but constant, then this patient would be an acceptable candidate based on all other factors being equal.
In addition to monitoring the Creatinine and Urea nitrogen levels, it is important to monitor blood pressure and urine protein/Creatinine levels.
We find patients with abnormal heart rhythms and murmurs due to the extremely high heart rates, sometimes exceeding 300 bpm. This tachycardia can be controlled with cardiotropic medications such as Atenolol until the radioactive iodine takes effect. Usually the heart medications can be discontinued within 3-4 weeks after treatment and the heart rate will be almost normal. Sometimes the murmurs improve or disappear, but not always.
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