CKD – a brief overview

In Symptoms
January 3, 2019
9 Min read

A brief overview

Approximately 3–5% of cats are affected by chronic kidney disease. The incidence increases with age, so by 12 years of age 50% of cats suffer from CKD. The disease is incurable, and its course is progressive. It is normally impossible to identify the factors triggering CKD. One thing they all have in common, however, is that they lead to loss of nephrons. Nephrons are the filtration units of the kidneys. They perform a variety of tasks: excretion of toxins, control of body water and electrolyte balance, formation of hormone-like substances, and regulation of blood pH.

Diseases that can lead to CKD include those listed below.

  • Inflammation of the kidneys and renal pelvis,
  • The viral disease feline infectious peritonitis (FIP),
  • Kidney tumours,
  • Other disorders of the kidney and its function (“nephropathies” such as those caused by kidney calcification),
  • Feline polycystic kidney disease,
  • Hypertension,
  • Coagulation disorders,
  • Sclerosis (alterations of connective tissue) of the kidney,
  • Kidney stones
  • Urinary stasis of the kidneys (manifested as hydronephrosis).

Diagnosis by means of clinical symptoms and laboratory tests

The disease is diagnosed on the basis of clinical symptoms : anorexia, excessive urination (polyuria (PU)), severe thirst (polydipsia (PD)), drowsiness, weight loss, vomiting, dehydration and abnormal laboratory test results. Blood tests reveal azotaemia, an increase in urea and creatinine in the blood. SDMA levels in the blood are also elevated, as is phosphate (hyperphosphataemia). At the time of first diagnosis analysis still reveals a low specific gravity of the urine and potentially increased protein in the urine (proteinuria). The cats often also exhibit high blood pressure (hypertension). Further clinical symptoms and complications emerge in later stages due to raised levels of toxins in the blood such as urea (uraemia), because the kidneys can no longer fulfil their role in removing these uraemic toxins. These complications : irritation of the stomach lining, ulcers of the mouth and stomach lining, bad breath, pancreatitis, anaemia, pneumonia, muscle twitching, compromised immune system, inflammation of the pleurae of the lungs and the peritoneum, heart muscle diseases, and comatose conditions.

A major problem for diagnosis is that, in most cases, there are no abnormalities in laboratory kidney test results until 75% of the nephrons of both kidneys have become non-functional.

Chronic kidney disease progression is unstoppable

CKD has a chronic but progressive course, meaning that the disease progressively worsens. This is down to the vicious circles that drive the disease. Cat kidneys contain approximately 200,000 filtration units (nephrons), which are not all used at the same time. “spare” nephrons exist, which are only used when existing filtration units cease to function. Over the course of life, the number of these “spare” nephrons decreases, and, in the end, no more nephrons can be replaced. However, the filtration rate of the existing nephrons can rise. Nephrons with such a raised filtration rate it for long. They also succumb, resulting in ever fewer functional nephrons.

When the number of active nephrons decreases, total kidney function drops. As the kidneys play a role in a variety of physiological processes, CKD cats suffer from a wide range of significant impairments and complications. The kidneys can be compared to sewage treatment plants; they remove metabolic breakdown products and toxins. The process goes hand in hand with control of water volume in the body; this involves elimination of excess water or its retention. If kidney function declines, too much phosphate remains in the blood (hyperphosphataemia), which, via fine regulation of the calcium-phosphate balance, leads to considerable complications mediated by the actions of parathyroid hormone and, ultimately, further nephron loss and accelerated progression of CKD.

Uraemic toxins, the breakdown products of protein metabolism, also accelerate CKD when they can no longer be sufficiently excreted. These are natural products of protein breakdown. Nevertheless, they are toxic to the kidneys, directly damaging them and leading to nephron loss and, thus, CKD progression. When they accumulate in the blood, known as uraemia, they are accompanied by clinical symptoms.

Last but not least, elevated blood pressure and the associated loss of protein (proteinuria) via the kidneys can further accelerate the disease. High blood pressure causes the pores of the blood vessels to enlarge, allowing larger molecules such as proteins to pass through into the urine. These cannot be recovered. Eventually, high blood pressure also injures the filtration units, leading to their destruction and, thus, to progression of the disease.

Staging – the four IRIS stages of CKD

Diagnosis and subsequent classification into one of the four stages is based on the guidelines of the International Renal Interest Society (IRIS), which also provides therapy recommendations for each stage. Staging is based on three factors.

  • Degree of azotaemia, measured by the creatinine blood level
  • Presence and degree of proteinuria
  • Presence and degree of hypertension

 

Staging is performed only after confirmation of a diagnosis. It is important to note that a cat can be re-assigned to a more favourable stage if treatment is successful (the ideal case).

CKD usually progresses slowly, but, despite carefully managed treatment, spontaneous decline may occur suddenly and unexpectedly, with cats experiencing a significantly reduction in their general state of health. The kidney laboratory values can also deteriorate suddenly in such a phase. Veterinary treatment, such as infusions, can help a cat get back on its feet and also lower or stabilise kidney laboratory values. The fact that such acute decline cannot be predicted in advance is problematic. If a cat is responding well to its CKD treatment and has been stable over a long period of time, some cat owners may come to believe that the cat has recovered and their current condition will persist, and potentially give up on the many measures for treatment. Unfortunately, CKD is neither curable nor can it be stopped. Correct treatment can prolong the life of CKD cats and improve their quality of life.

 

Basics of CKD treatment

Once a cat has been diagnosed with CKD, the main focus of treatment is on prolonging life (slowing progression) and improving the cat’s quality of life. In addition to treating specific clinical symptoms, the four vicious circles mentioned above must be addressed. There are two ways to reduce phosphate, both of which are related to diet. Firstly, the cat can be fed kidney-friendly food, which contains less protein and therefore less phosphate (meat contains a lot of phosphate). Secondly, the cat can potentially be administered a phosphate binder mixed in with its regular food or later with its kidney-friendly food, which absorbs phosphate from the food in the intestines and causes it to be eliminated it in the faeces. As a result, the cat absorbs less phosphate and the burden on the kidneys is reduced. According to IRIS guidelines, phosphate in the blood needs to be strictly controlled and, dependent on disease stage, maintained at a certain level. As kidney function decreases over the course of the disease, combining both measures to lower phosphate levels is also an option.

Reducing Uraemic Toxins

Reducing uraemic toxin levels involves feeding with low-protein, kidney-friendly food. However, there are limits to this approach; cats are obligatory carnivores and dependent on a high protein content in their food. If their food contains too little protein, they will start to consume their own muscles and lose strength and weight. Both are important for cats with kidney disease. Nowadays, it is possible to remove these uraemic toxins as precursors from ingested food and eliminate them harmlessly in the faeces. This relieves the kidneys, as they will then have to excrete less toxins. At the same time, the damage done by such toxins to the kidney will be reduced, and the kidneys will be better able to control uraemia, decreasing its associated clinical symptoms. Uraemic toxin binders are equivalent to dialysis, administered via the diet. Both kidney-friendly food and “oral dialysis” can be combined.

High blood pressure is controlled by suitable medication. By reducing blood pressure, protein loss via the urine (proteinuria) can also be reduced, as high blood pressure leads to more protein passing out into the urine.

Water Intake Management

There are now a large number of treatment options that can be individually adapted. Besides slowing down the four vicious circles, it is also important to control your cat’s fluid balance. CKD cats lose increasing volumes of water via their kidneys, and, although they drink a lot, they are unable to retain the water in their it is immediately lost once again in the urine. Despite drinking a lot, they become dehydrated. The damaged kidneys can no longer recover the same quantity of water that healthy kidneys can.

Proper treatment can significantly prolong a cat’s life. This is dependent on early detection of kidney disease. It is now possible to diagnose a cat with chronic kidney disease earlier, for instance with the SDMA test. Owners with cats over seven years of age should consider having their kidneys checked annually for early identification of CKD.

 

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