There are a variety of causes to kidney failure in cats. For instance, some cats are born with poorly constructed or functioning kidneys and never reach totally optimum health. But to first understand why kidney failure occurs, you must first understand the components of the kidney.
Normal Kidney Physiology
The kidneys receive about 20 percent of the heart's blood output and play a vital role in keeping the cat in normal metabolic balance. When one or both kidneys are malfunctioning, it can lead to kidney failure. This condition can either be due to acute or chronic reasons.
The glomerular blood vessels have a large endothelial surface which allows for the active and passive transport of many chemicals into and out of the kidneys.
Normal kidney function involves the following responsibilities, among others:
Regulating the amount of fluid in the spaces surrounding the body's cell. This is called extracellular fluid volume regulation.
Regulating the amounts and types of solids in the blood in order to keep blood concentration within normal limits. This is called blood osmotic pressure regulation.
Regulating the acid-base balance of the animal through retention or elimination of specific ions in the blood. This function keeps the pH (amount of acidity) of the blood and body fluids within strict normal ranges.
Removing metabolic waste products such as uric acid and also molecular foreign substances detoxified by liver.
Reacting to Aldosterone (ADH) produced in the adrenal glands. The major target of aldosterone is the distal tubule of the kidney, where it stimulates exchange ofwater back into the blood.
Production of Erythropoetin, a chemical effecting red blood cell production.
The nephron is the structural and functional unit in the kidney. A nephron consists of a glomerulus in a capsule, proximal convoluted tubule, loop of Henle, and distal convoluted tubule which leads to a collecting duct. The collecting duct empties into the renal pelvis.
The functional unit of the kidney -- the real mechanism whereby the kidney does most of its prescribed tasks -- is called the nephron (pictured right). The nephron is a delicate, structurally complicated microscopic collection of tiny tubes (capillary beds) which are tasked with regulating the concentration of water and soluble substances like sodium salts by filtering the blood, reabosring vital components, and excreting the rest as urine.
The unit consists of the:
Glomerulus - a ball of capillaries with a large surface area at which multiple interchanges of fluids and dissolved elements occurs.
Bowman’s Capsule - the proximal end of a tubule that surrounds glomerulus.
Proximal convoluted tubule - leads to the Loop of Henle, which is situated in the medullary area of the kidney. (There is an ascending limb and a descending limb, each of which has particular and unique functions.)
Distal convoluted tubule - leads into collecting ducts.
Pelvis - is an enlargement at the distal end of the collecting ducts that provides a common area of urine collection before the urine passes down the ureter into the bladder.
The glomeruli are found in the outer area of the kidney called the cortex. Each glomerulus is surrounded by a "Bowman's Capsule". Most of the fluid that passes into the Loop of Henle in the cortex is reabsorbed in the medulla back into the blood.
The medullary area of the kidney is fed by tiny arterioles. Any damage to glomeruli affecting efferent arteriolar blood flow will also cause damage in the tubules located in the medulla. Anything that adversely impacts the blood flow through the medulla can have serious consequences for the tubular structures.
The medulla is slightly less vascular than the cortex. The renal tubules, which are responsible for water loss and coservation, make up most of the medullary tissue have high metabolic rates and therefore high nutritional requirements. Filtered water containing waste products (urine) are then passed into the renal pelvis, followed by the ureter.
In addition to waste management the renal medulla assists in regulation of blood pressure, the elimination of toxins and the production of hormones such as erythropoietin.
The renal pelvis collects the kidney filtrate and funnels the urine fluid into the ureter that leads to the bladder. The pelvic area of the kidney often is the site of kidney stones and can be a reservoir of infection once microorganisms reach this area of the kidney.
Causes of Kidney Failure
Some of the more serious causes of kidney failure include:
Hereditary and Congenital Abnormalities
These types of kidney disease are very frustrating to try to control or repair. Most cats with abnormally constructed kidneys will develop kidney failure and do not live anywhere near a normal life span.
A few hereditary conditions that lead to kidney failure include:
Polycystic kidney disease (PKD), though uncommon, creates cystic areas in the kidneys where normal function and structure are lost. Eventually, even if the cat reaches maturity, gradual increases in metabolic waste products and signs of kidney disease prevent optimum quality of life and the animal dies or is mercifully euthanized. If found, it typically occurs in Persian/Exotic cats.
Renal agenesis, also called kidney aplasia, causes the cat to be born without one or both kidneys.
Renal hypoplasia is a condition where the kidney(s) do not develop completely.
Renal cortical hypoplasia is a condition where the cortex of the kidney(s) develops incompletely.
Renal dysplasia is a condition where the kidneys develop abnormally. Renal failure develops with protein loss in urine.
Renal tubular dysfunction occurs when the filtering tubules of the kidneys do not function properly.
Infections of the urinary tract of cats are, unfortunately, very common. Generally arising from gradual spread of external bacterial organisms near the external urinary orifices, the bacteria multiply and invade the urethra, then into the bladder (causing what is termed cystitis), and occasionally further retrograde up the ureters and eventually into the kidneys.
Another less common means of kidney infection arises from a blood-borne dispersion of bacteria from a remote area such as an abscess or skin infection. Leptospirosis bacteria, for example, can have a severe effect on kidneys.
Another severe bacterial infection (Borrelia burgdorferi) may be caused by the bite of a tick. This infection causes Lyme Disease, which damages the kindey's ability to filter body waste products and transport of those waste products into the urine. Even after eliminating the bacteria with antibiotic therapy there may remain permanent structural damage to vital renal tissues -- and kidney failure ensues.
Systemic fungal infections such as Blastomycosis, Coccidioidomycosis (Valley Fever), and Histoplasmosis can attack nearly any tissue or organ in the body, including the kidneys. Most systemic fungal diseases are geographically oriented.
Trauma to Kidney
Direct trauma to the kidneys can result in kidney failure. Although rare, cats that are run over by vehicles can suffer permanent and irreparable kidney trauma. Also, sudden physical shock to the kidney tissues from being struck by vehicles, baseball bats, kicking, or falls from a height, etc. can result in suffusive bleeding into the kidney tissue and permanently impair renal function.
Blockage of Urine Flow
The most notable condition seen in cats from blockage of urine flow from the kidneys involves kidney stones or bladder stones or urethral obstructions. The obstructions caused by these mineral concretions (usually called struvite uroliths) can increase back pressure on the affected kidney, which permanently damages kidney function and causes what is termed hydronephrosis -- a kidney swollen under pressure with backed up urine.
Cats with bladderstones often obstruct when a stone passes from the bladder but cannot be voided past the os penis -- the bone present in the male feline's penis. There is an inherent lack of room for the urethra to dilate in the area of the os penis and small bladder stones often dam up the urine flow at this site. Surgical intervention is often required in these emergency urinary tract blockage cases.
Tumors, cysts, abscesses and scar tissue, if present in critical areas of the urinary tract, can create obstructive situations where the urine flow from a kidney is compromised. This can result in damage to delicate kidney tissue structures, which is often permanent. If enough tissue is destroyed or its function impaired, kidney failure will occur.
Cancer of the kidney is extremely rare in cats. If seen, it typically takes the form of secondary invasion of metastatic cancer originating in a distant tissue. In cats with leukemia disorders, the kidneys can be infiltrated with neoplastic leukemic cells which can severely compromise renal function. There is also a form of leukemia in cats that targets the kidneys and crowds out normal kidney cells.
External Toxins (Poisonings)
One of the most devastating external toxins that causes kidney failure in cats is antifreeze that contains ethylene glycol. It doesn't take much of this sweet tasting liquid to prompt crystals to form in the delicate tubules of the kidney's filtration systems. Other renal toxins include Vitamin D, thallium, turpentine, heavy metals such as lead and mercury, even parts of a Easter Lily. There is also evidence that raisins/grapes can be nephrotoxic to cats.
Endotoxins are chemicals produced within the animal that are toxic. The most common type is that group of poisons formed by certain types of bacteria. Clostridia organisms are famous for causing tetanus. Many bacteria produce toxins from their normal metabolic waste products. In others, when they die off they leave behind toxins that can have damaging effects on delicate body tissues such as kidney structures and heart valve tissues.
Endotoxins can have systemic effects as well and play a role in triggering shock in an animal where blood pressure declines, heart output diminishes and body tissues become starved for oxygen and nutrients. The resulting shock can leave irreversible damage in any organ of the body, including the kidneys.
Some types of medications can be nephrotoxic such as acetaminophen (analgesic), amphotericin B (antifungal), adriamycin (doxorubicin) in cats, kanamycin (antibiotic), neomycin (antibiotic), polymyxin B (antibiotic), cisplatin (a cancer drug), penicillamine (chelating agent/immune modulator), Cyclosporine (immunosuppressive), amikacin (antibiotic), and radiographic contrast agents.
Systemic Lupus Erythematosis (SLE), also known as the great imitator, can be difficult to diagnose since it can manifest as a disease of the skin/mucous membranes/nails, kidney and/or joints. As a consequence of the animal's adverse and abnormal immune response to its own body tissues and proteins, many organ sites can be affected, including the kidneys.
As the kidneys filter the circulating blood the abnormal immune molecules are trapped in the glomeruli and blood vessels, causing the kidney to leak protein. A condition called Glomerulonephritis is the result and all sorts of abnormal kidney function can occur due to the damaged glomeruli.
Although not proven to be a result of an autoimmune disorder, the deposition of protein called Amyloid can actually occur in any tissue of the body. The kidneys are most commonly affected and since the protein deposition destroys normal function, renal amyloidosis can be particularly serious due to the fact that kidney tissue does not repair itself.
Amyloidosis is fairly common in Abyssinian, Siamese, and Oriental Shorthaired cats.
Diagnosis of Kidney Failure
One of the first signs an animal will show when beginning to be affected by kidney failure is an increased thirst, which is known as polydipsia. Increased toxins and other metabolic waste products triggers sensors in the brain that the blood is too concentrated and through a series of chemical reactions the animal may have a sense of dehydration. Your cat, in turn, drinks more water to alleviate this sensation. Compounding this sense of dehydration is actual water loss through the kidneys above normal amounts due to the kidneys being inefficient in retaining water within the body.
The increased thirst/water intake (polydipsia) also causes an increased urine output. Known as polyuria, the increased urine output seems unintuitive if the animal is actually affected with kidney failure.
Many pet owners have been baffled when the veterinarian mentions that the patient may have early kidney failure. They often respond "How can that be, its urinating a lot more than it usually does?" What really is happening is that much more urine is being produced and eliminated however the urine is becoming more and more dilute; the urine is not bringing along all those toxins and waste products for removal from the body.
In order to make a diagnosis of renal failure your veterinarian will use two sources of data: a urine and blood sample. Checking one without the other may render an inccurate diagnosis.
The Urine Sample
In nearly all cases of kidney failure the kidneys are unable to concentrate urine. That means the Urine Specific Gravity measurement (SpG) that indicates how concentrated the urine is compared to distilled water (SpG = 1.00) will display a dilute reading ... actually, very close to distilled water.
Since the action of conserving water while allowing undesirable metabolites and toxins to remain in the urine is the job of the tubules in the kidneys, whenever the tubules are damaged water conservation is less efficient; therefore more water flows through the tubules unresorbed and washes away in the now dilute urine.
Most cases of kidney failure display a SpG of about 1.008 to 1.012. Generally, a cat's urine SpG will be about 1.025 to 1.050.
If a water deprivation test is done, where the animal has no access to water for 18 hours, the urine specific gravity goes up (i.e., the urine becomes more concentrated).
Many cases of kidney failure also show protein or sugar in the urine where in most normal animals urine protein is scarce and no glucose is present. The loss, or lack of reabsorption of protein or sugar molecules back into the blood after an initial pass into the tubular fluid, places the animal in a negative protein/energy balance. This state shows up as weight loss and muscle wasting. And since these animals have a poor appetite, the added stress of protein and energy loss in the urine really tends to make the maintenance of normal body weight nearly impossible.
Bacteria and blood may show up in the urine samples of chronic renal failure patients. Infectious agents, red and white blood cells, epithelial cells from the lining of the kidney and bladder structures, crystals, and protein plugs called casts that arise from damaged tubules all may be commonly observed in urine samples. Conversely, some patients have such dilute urine and such thirst that a urine sample may have no detectable cells or debris but simply show a low Specific Gravity and very dilute urine.
THE BLOOD SAMPLE
(See normal ranges for cat blood chemistry values here.)
Two of the most useful chemicals that veterinarian measure to see if toxins are building up in the patient's body are Blood Urea Nitrogen (BUN) and Creatinine. Normal BUN levels in cats seldom reach higher that 25 to 30 mg/dl. (Mg/dl means milligrams of material per 100 milliliters of blood.) Many patients presented in renal failure have BUN levels of 90 or higher! Similarly, Creatinine, a chemical normally present in the blood at levels less than 1.0 mg/dl, may rise to over 8 mg/dl.
Treatment for Kidney Failure
In human medicine, dialysis and kidney transplantation are the main methods of dealing with advanced kidney failure. These methods are also employed in treating cats but impose heavy financial and time burdens on the pet owner and some stress on the patient who is already stressed by the disease.
Unfortunately, once the diagnosis of kidney failure is made, most patients are so sick that response to treatment is unrewarding and slow. You may need to consider euthanasia in order to prevent the long, slow, and agonizing death that comes from complete renal shutdown.
In very extreme and special circumstances, a kidney transplant may be an animal's only hope of long term existence. Kidney transplantation is a controversial topic but the science and success rate in cats has advanced greatly in recent years.
Treating kidney failure is one of the most consistently discouraging aspects of veterinary medical practice. The difficulty stems from the fact that once a cat has lost 75 percent of total renal function, the ability to remove metabolic waste products is outweighed by the buildup of those toxins. The animal is simply not able to keep up with the "housecleaning" and as a result gradually becomes increasingly more toxic. Body chemistry swings more and more acidic, important chemicals and nutrients are lost from the body and the animal comes gradually closer and closer to a fatal uremic poisoning. In some cases, gradual kidney tissue loss can be present for years before the patient becomes critical and actual "renal failure" is diagnosed.
The goal of treatment is to allow the patient to live as close to a normal life as possible under the circumstances. Since the kidneys do not heal or regenerate new and functioning tissue, the remaining functional tissue carries the entire burden normally handled by two healthy kidneys. Intravenous and subcutaneous fluids can be administered for varying lengths of time to try to correct acid-base imbalances.
Vomiting can be controlled. Anti-ulcer medication can be given. Bicarbonate may be administered either orally or intravenously to assist in neutralizing acid buildup. B-vitamins are provided. Antibiotics are employed if there is an infection present anywhere in the body... taking into consideration that some antibiotics will also build up in the patient if renal function is compromised. Phosphate binders and Omega Fatty acids in correct amounts and proportions may be temporarily beneficial for the Chronic Renal Failure patient. High quality, low protein diets have been proven to be helpful in lessening the metabolic tasks that must be performed by the kidneys once end stage kidney disease is present.
Contrary to popular myth, there is no evidence that feeding cats diets rich in or "high" in protein actually causes kidney damage or disease (though it most certainly isn't ideal for animals already suffering from kidney issues). In fact, there is ample research and well documented studies that prove that cats thrive on diets with levels of protein consistent with a meat-eater's (carnivore) natural prey selection. Read more about protein in cat diets here.
Image: Dave Dugdale / via Flickr