KIDNEY STONES (RENAL CALCULI) AND OBSTRUCTIONS
What are Kidney Stones? Back to Page Index
Renal calculi are stones that form in the kidneys, so they are commonly known as kidney stones, though the medical term is nephrolithiasis. In over 90% of cases, the stones that form are calcium oxalate stones, i.e. formed from a combination of calcium and oxalate. The stones may calcify, though some cats may develop calcified blood clots rather than actual stones. The presence of kidney stones increases the risk of kidney infections (pyelonephritis), and may cause CKD.
Some kidney stones are inactive so they do not cause any problems. A major problem arises if the cat's body attempts to pass the stones. Smaller stones, although painful, may be successfully passed, but larger stones may lodge in the ureter (the tube connecting the kidney to the bladder). This is called obstructive nephropathy or a blockage. This is very serious because it prevents waste products that would normally be excreted via the bladder from being excreted, so they build up in the kidneys. This can cause acute kidney injury.
Other stones may move into the ureter, then back into the kidney. This is less serious but may still cause kidney damage.
Causes Back to Page Index
Kidney stones may form if there is reduced urination for some reason, or if the urine contains more of the substances contained in the stones (in over 90% of cases, calcium and oxalate) than usual.
Urine that is more acidic increases the risk of developing calcium oxalate stones. The recent trend for acidifying cat food (which is done to prevent the opposite problem of struvite crystals, which develop in urine that is too alkaline) is thought to be a factor in some cases.
Many cats with calcium oxalate stones also have idiopathic hypercalcaemia (elevated calcium levels in the blood with no obvious cause). It is not yet known which comes first, the hypercalcaemia or the kidney stones.
Occasionally, calcium oxalate stones may be caused by a Vitamin B6 deficiency.
Calcium in food does not contribute to the development of kidney stones. In fact, calcium in foods can bind with oxalate and therefore help prevent the formation of kidney stones. However, calcium supplements may contribute to the problem.
Studies indicate that certain breeds appear to be more likely to develop calcium oxalate stones, including Persian, British Shorthair, Ragdolls and Scottish Fold cats.
Symptoms Back to Page Index
In some cases, there may be no obvious signs, since cats instinctively try to hide pain. In other cases the cat may be subdued and lethargic and not want to eat. Some cats will have blood in the urine (haematuria) or frequent kidney infections.
Some cats urinate more, but others exhibit reduced urination. If your cat is unable to urinate, this is a medical emergency and you need to get to a vet as quickly as possible.
Diagnosis Back to Page Index
Your vet will probably run blood tests. If a cat's kidney bloodwork suddenly becomes extremely high, a kidney stone blocking the ureter may be the cause. The cat may also have elevated phosphorus and potassium levels, and be anaemic.
Your vet will also palpate (feel) your cat's kidneys, which may be tender to the touch. The vet may also be able to feel a difference in size, in which case an ultrasound will usually be performed. X-rays may also be performed to assess the number and size of the stones.
The ultrasound may show one small kidney and one enlarged kidney. What tends to happen in such cases is that a stone moves into one ureter, blocking it, but the cat does not exhibit any symptoms. This kidney gradually ceases to function and shrinks. The remaining kidney has to take over some of its work, so it grows in size. If a stone eventually also moves from that kidney into the ureter, the cat is then in crisis, because the one working kidney is no longer able to function. This is sometimes referred to as bilateral ureteral obstruction. It is commonly known as big kidney little kidney syndrome (see renomegaly).
Some cats with big kidney little kidney syndrome will be unable to urinate, which is a medical emergency and you need to get to a vet as quickly as possible. See below for treatment options.
Treatments Back to Page Index
Inactive kidney stones are not normally treated in cats. The cat should be monitored via urinalysis (to make sure the cat has not developed pyelonephritis) and x-rays every few months (to see if the stones have moved).
Ureteral dilemma: non-surgical management of ureteroliths (2007) Adams LG Hill's Symposium on Lower Urinary Tract Disease 2007 pp14-21 discusses non-surgical treatments in particular.
Ureteral obstructions in dogs and cats: a review of traditional and new interventional diagnostic and therapeutic options (2011) Berent AC Journal of Veterinary Emergency and Critical Care 21(2) pp 86–103 has an overview of the various treatment options.
Calcium oxalate stones cannot be dissolved through a change in diet.
If you are feeding an acidified food (these foods are often labelled "for urinary tract health"), stop immediately. These foods are intended to treat the opposite problem to calcium oxalate stones, and can therefore make the problem worse by making the urine even more acidic. You should not give your cat anything containing cranberries for the same reason. Acidified foods are not normally appropriate for CKD cats in any event.
As it happens, foods formulated for CKD, such as Hill's k/d, are also suitable for cats with calcium oxalate stones, whether or not they have CKD.
Pet food safety: sodium in pet foods (2008) Chandler ML Topics in Companion Animal Medicine 23(3) pp148-53 states that "Increased dietary sodium increases urine output and may decrease the risk of forming calcium oxalate uroliths due to the decrease in relative supersaturation of solutes. However, caution should be used in increasing the sodium intake of patients with renal disease as increased dietary sodium may have a negative effect on the kidneys independent of any effect on blood pressure." Do not increase your cat's sodium intake without your vet's approval.
If the cat has developed obstructive nephropathy, there are a number of possible treatments, which are listed below in order of invasiveness.
This means that the vet tries to flush out the stones. The cat will be hospitalised for 2-3 days on intravenous fluids. How successful this is depends upon a variety of factors, including the size of the stone and how long it has been in the ureter. Calcified blood clots can often be dislodged, but fewer than 10% of kidney stones are passed.
Sometimes diuretics such as Lasix or mannitol are used to increase the flushing effect and help force the stone out.
Medications may also be given to relax the ureter in the hope that this will help the stone to pass through. Commonly used medications for this purpose are amitriptyline and prazosin, a muscle relaxant. One cat who was given prazosin for six weeks managed to pass the stones and lived for a further 20 months, see Success Stories.
Amitriptyline eliminates calculi through urinary tract smooth muscle relaxation (2003) Achar E, Achar RAN, Paiva TB, Campos AH & Schor N Kidney International 64 pp1356–1364 discusses the use of amitriptyline in this manner in cats.
Mar Vista Vet has some information about amitriptyline.
Extracorporeal shock wave lithotripsy uses shock waves to smash the stones so they can be passed. This works well in humans, but can be difficult to perform in cats because of their small size. However, a small number of facilities in the USA do now offer this treatment. It is only suitable if the stones are large enough to be seen on ultrasound.
Michigan State University College of Veterinary Medicine has more information about the treatment and where it is available in the USA.
Ureteral dilemma: non-surgical management of ureteroliths (2007) Adams LG Hill's Symposium on Lower Urinary Tract Disease 2007 pp14-21 discusses lithotripsy on page 80 (6 of 8).
In one or two centres in the USA, a stent (a tube which by-passes the ureter) can be inserted. The stones can then be passed through the stent. The success rate is approximately 94%, and the stent can remain in place for years.
In about 5% of cases, the stent moves, but it can usually be repositioned.
Ureteral obstructions in dogs and cats: a review of traditional and new interventional diagnostic and therapeutic options (2011) Berent AC Journal of Veterinary Emergency and Critical Care 21(2) pp 86–103 has detailed information on the use of stents.
Subcutaneous Ureteral Bypass
This is a relatively new microsurgery which was pioneered at the Animal Medical Center in New York City. It consists of a tube within a tube surrounded by mesh. Although the tube may fill up with kidney stones, the urine should still be able to flow through the mesh. The tube also contains an access point in the abdomen which can be easily accessed later under sedation only. If the cat survives the initial surgery to fit the tube, the prognosis is good.
The surgery is now available at a small number of centres in the USA, Canada, UK, France, Switzerland, Italy and France. One member of Tanya's CKD Support Group had this surgery performed on her cat in the USA in early 2012. She paid US$2000 for the tube. Surgery costs are an additional US$4000 - 6000.
A number of possible surgeries are available, such as opening up the kidney and removing the stones. This is very expensive (it costs several thousand dollars) and invasive. It has a relatively high mortality rate, and the stones often recur. In almost all cases, a stent or a subcutaneous ureteral bypass would be a better choice.
Renalof is a dietary supplement for humans made from activated couch grass (Agropyron repens). It is said to help dissolve calcium oxalate stones. It has apparently undergone trials in humans in Cuba and Romania. It is not approved for cats, though I did hear from one lady whose vet decided to use it as a last ditch attempt to help her cat. As always, do not use this product without your vet's approval.
Prevention Back to Page Index
Prevention is not easy, but you can take a few steps to reduce the risks:
Links Back to Page Index
The University of Minnesota Urolith Center will analyse feline stones free of charge.
Mar Vista Vet gives a good overview of calcium oxalate stones.
Management and outcome of cats with ureteral calculi: 153 cases (1984 - 2002) (2005) Kyles AE, Hardie EM, Wooden BG, Adin CA, Stone EA, Gregory CR, Mathews KG, Cowgill LD, Vaden S, Nyland TG, Ling GV. Journal of the American Veterinary Medical Association 226(6) pp937-44 discusses a large number of cases over a period of eighteen years, and states that even after successful treatment, many cats will have impaired renal function.
Oxalate degradation by intestinal lactic acid bacteria in dogs and cats (2004) Weese JS, Weese HE, Yuricek L & Rousseau J Veterinary Microbiology 101(3) pp161-6 mentions that the use of prebiotics, including fructooligosaccharide products (FOS), a type of fermentable fibre, may help reduce the formation of calcium oxalate stones. However, fermentable fibre may lead to elevated calcium levels in the blood (hypercalcaemia).
Some cats have the opposite problem of struvite crystals, which are formed when the cat's urine is too alkaline. Harpsie's Website has some information about this condition, which is often known as Feline Lower Urinary Tract Disease or FLUTD.
This page last updated: 17 April 2013
Links on this page last checked: 18 April 2012