The nephron, the functional unit of the kidney, can be damaged in many ways to cause renal insufficiency.

Along with all the other organs that can cause PU/PD (increased urination and drinking known as polyuria/polydipsia), the kidneys themselves are sometimes to blame. While renal disease is complex, it is important to understand some acute and chronic kidney problems that lead to PU/PD.

With acute or chronic insults to the kidneys, the ability of the kidneys to concentrate urine is lost; in most cases, the renal tubules that are responsible for drawing water back out of the urine, are damaged. The pet urinates out a higher percentage of water, thus diluting the urine and leading to possible dehydration. The pet drinks more water to counter this loss, and is officially PU/PD.

Causes: The most common causes of acute renal failure (ARF) include infections (leptospirosis, Lyme disease), shock (heat stress, trauma, hemorrhage), and toxins (antifreeze, non-steroidal anti-inflammatory drugs, lilies in cats, grape/raisins in dogs). Also, if only one kidney is functioning, an obstruction in the ureter to that kidney will cause renal failure.

The causes of chronic renal insufficiency (CRI) are more subtle and not easy to separate out. They include age (loss of renal filtering glomeruli, which cannot be replaced), hyperthyroidism/high blood pressure, chronic use of antibiotics or non-steroidal anti-inflammatory drugs (NSAIDs), chronic inflammation, and chronic urinary tract infections. Persian cats are prone to a genetic disease (polycystic kidney disease) that is quite prevalent within the breed but rare in non-Persians.

Diagnosis: Animals in acute renal failure often appear very ill; they may vomit, urinate excessively or be unable to urinate at all, appear drunk or disoriented, have apparent abdominal/back pain, be lethargic or depressed, and refuse to eat. Signs of chronic renal insufficiency depend, in part, on the degree of renal damage. Initially, if 2/3 of renal function is lost, the pet’s urine will be less concentrated and the pet may become PU/PD. There may be vomiting and bad breath as the toxins that are not filtered from the blood start to build up. There may be signs of urinary tract infection (frequent urination, blood in urine, urinating outside the box). The pet may fell less like eating and start to lose weight. He may not be able to drink enough to avoid dehydration and may begin to look shriveled or become quite lethargic; by this time, though, most pets have been diagnosed via lab work and are being treated.

Blood and urine submitted to the lab is the gold standard for diagnosing renal disease. For acute cases, the blood urea nitrogen (BUN), creatinine, and phosphorus are usually extremely high. These values are elevated because these compounds are excreted by healthy kidneys but not by failing ones. The blood count may show increased numbers of white blood cells but the red cell count should be normal. There may be blood, specific crystals, or other unusual substances in the urine.

With CRI, when 3/4 of kidney function is lost, the BUN and creatinine will start to creep up on blood work. The phosphorus may start to climb. There is often a non-regenerative anemia, due to insufficient quantities of a red cell stimulating hormone secreted by healthy kidneys. The urine will have a low specific gravity.

Treatment: In cases of ARF, it is crucial to keep the pets hydrated while ensuring that adequate urine is being produced. For antifreeze and other toxic cases that are discovered in time, it may be possible to reverse the effects of the toxin. Infections can be treated and obstructions surgically corrected. If the BUN and creatinine are elevated to a level that will cause harm, dialysis can be used to filter the blood and remove these compounds. Some renal failure pets remain on daily dialysis until it can be determined whether they are likely to survive the insult to their kidneys; some will be on periodic dialysis for the rest of their lives. Pets that are successfully treated may suffer from CRI that cannot be reversed.

For CRI, treatment is focused on helping the pet feel better and reducing progression of the disease. Subcutaneous fluids are given to prevent dehydration. Gastric acid reducers, like Pepcid, will help to reduce vomiting and encourage eating. Phosphate binders can be given with the food to reduce phosphorus. Often, a low protein diet will be recommended to manage BUN/creatinine levels and clinical signs; if a pet is losing weight on these diets, however, a normal maintenance diet is actually preferred. Some pets live a very long time after diagnosis and others succumb to the more severe health problems that can evolve.

Prognosis. The predicted outcome varies depending on cause and severity of disease on presentation, overall health of the animal, response to initial treatment, and many other factors.