Feline Oxalate Bladder Stones

oxalate stones

The cat with bladder stones tends to have the classical symptoms of feline idiopathic cystitis: straining to urinate, bloody urine, urinating in unusual places, genital licking. In the course of testing to pursue these symptoms, a radiograph is taken and a stone or group of stones is seen in the urinary bladder. Alternatively, the cat might have no symptoms of any kind and the stone is discovered when a bladder infection fails to resolve after appropriate antibiotics. The stone may be found incidentally when the cat has radiographs taken for some other reason. At the point where we see the stone on the radiograph or on an ultrasound image, we do not know what kind of stones we are up against. To find out we must:

  • Surgically remove the stones and analyze them.
     
  • Retrieve a stone by getting the cat to pass one, even a small one (generally only possible in female cats).
     
  • Retrieve a stone with a cystoscope (small gadget that enters the bladder) and submit it for analysis.
    (Again, generally only possible in female cats).
     
  • Look at a urinalysis for clues as to the stone type. (Urine pH, crystals present, presence or absence of infection can help us make an educated guess).

Unlike Struvite stones, Calcium Oxalate stones cannot be dissolved with special diets; surgery is needed to remove any stones too large to pass.

WHY DO CATS DEVELOP CALCIUM OXALATE BLADDER STONES?

In older times (25 years or so ago), cats virtually never developed calcium oxalate bladder stones. Cat bladder stones could reliably be assumed to be made of struvite (a matrix of ammonium-magnesium-phosphate). In those days, feline lower urinary tract symptoms were generally thought to be caused by struvite crystals in urine and feline lower urinary tract symptoms were extremely common. The pet food industry responded by acidifying cat foods to prevent the development of crystals. In a way it worked. Feline lower urinary tract symptoms declined. Male cats with struvite urinary blockages became far less common. The trade off was that calcium oxalate bladder stones began to develop. Acidifying the body leads to an acid urine pH and more calcium loss into the urine, both factors in the development of a calcium oxalate stone. Currently most bladder stones formed by cats are calcium oxalate stones.

Burmese head

Burmese and Himalayan cats appear genetically predisposed
to the development of calcium oxalate bladder stones.

Most calcium oxalate stones develop in cats
between ages 5 and 14 years.

35% of cats with calcium oxalate bladder stones
have elevated blood calcium (hypercalcemia).
 

himalayan head

Cats with calcium oxalate bladder stones tend not to have crystals in their urine
(while those with struvite stones do tend to have struvite crystals in their urine).

Cats with calcium oxalate stones tend not to have bladder infections
and tend to have acid urine pH on their urinalysis.

 

oxalate xray

HOW TO GET RID OF THE STONES

    CYSTOTOMY (SURGICAL REMOVAL)
    The fastest way to resolve a bladder stone issue is to remove the stones surgically. To accomplish this, the cat is anesthetized and an incision made through the belly. The bladder is lifted into view, opened, and stones are removed. Cultures to rule out infection are obtained if not done previously. The bladder is closed in several layers. The belly is closed and the patient is awakened. Pain medication and antibiotics are routinely used after surgery. The patient usually remains hospitalized for a day or two to observe urination. The stones themselves will be sent to the lab for analysis. It is normal for some blood to be evident in the urine for several days after surgery.

    CYSTOSCOPY
    A less invasive method involves using a cystoscope, a long skinny instrument to remove stones from the bladder using a small basket-like retrieval accessory. This can only be done with small stones and can only be done in female cats. For larger stones, laser lithotripsy can be used to break the stone into smaller pieces which can be removed or passed. Laser lithotripsy requires the cystoscope laser to be in contact with the stone so, again, the cat must be female; the male cat's urethra is too small for a cystoscope.

    VOIDING UROHYDROPROPULSION
    This technique can work if the stones are small enough to pass through the patient's urethra. The patient is sedated, the bladder is distended with fluid, agitated, and manually expressed under pressure. By positioning the sedated patient vertically, gravity "loads" the stones in the neck of the bladder, positioned for expulsion. When the bladder is expressed, often stones can be passed that might otherwise have stayed in the bladder. Larger stones cannot be passed using this technique.

USING DIET TO DISSOLVE A CALCIUM OXALATE STONE IS NOT POSSIBLE.

ONCE A STONE HAS BEEN RETRIEVED,
IT CAN BE SUBMITTED TO THE LABORATORY FOR ANALYSIS.
AFTER IT IS CONFIRMED AS CALCIUM OXALATE,
THE GOAL IS PREVENTION OF FUTURE STONES.

STONE PREVENTION

Retrieving the stones is generally the easy part of calcium oxalate stone management. Prevention of future stones is more challenging. If the patient is one of the 35% with an elevated blood calcium then steps to control the calcium level and determine why it is high should be taken. (See Hypercalcemia). If blood calcium levels are normal, the following step by step regimen is recommended:

STEP ONE: FEED A NON-ACIDIFYING DIET THAT MINIMIZES CALCIUM OXALATES IN URINE

Such diets use a normal calcium content, a moderate magnesium content, and citrate to bind urinary calcium. Hills c/d multicare diet, Royal Canin S/O, Iams Eukanuba Moderate pH/O, and Purina UR st/ox are all appropriate foods. Canned diet is preferred over dry food due to the high water content of canned foods. Part of the goal is to create a dilute urine and the extra water consumption is helpful. Meal feeding rather than free feeding also may be helpful in maintaining the desired urinary pH.

Avoid supplementation with vitamin C. Vitamin C is converted to Oxalic acid which modifies into oxalate. Be careful of pet vitamin supplements.

In 2-4 weeks, a urinalysis is performed to see if there are calcium oxalate crystals present (there should not be), if the urine is dilute (the specific gravity of the urine should be less than 1.020), and if the urine pH is alkaline (it should be 6.8-7.5).

oxalate crystals

Oxalate crystals are classically marked with an X (the X is naturally present in the crystals when viewed under a microscope)

STEP TWO: CORRECTING PROBLEMS IN THE FIRST URINALYSIS

If the urine specific gravity is > 1.020, this means that the urine is not adequately diluted. The cat will need to drink more water. This is best accomplished by increasing the percentage of canned food in the diet.

If the urinary pH is <6.5, the urine is too acidic and potassium citrate must be given as a supplement, either as a chewable tablet, capsule or oral liquid.

Another urinalysis is performed in 2-4 weeks.

STEP THREE: IF OXALATE CRYSTALS ARE PRESENT, THE URINE IS NOT DILUTE, OR IF THE PH OF THE URINE IS ACID (pH < 7.5) THE FOLLOWING STEPS ARE TAKEN:

A thiazide diuretic is added to dilute the urine and correct the necessary electrolyte balance in the urine. Vitamin B6 is supplemented. A population of cats has been identified for which a B-6 deficiency leads to oxalate stone development. This may or may not be helpful but is worth trying. The vitamin B-6 deficiency leads to an increase in blood oxalic acid which in turn leads to an increase in urine oxalates. A different food may need to be selected

Once a urinalysis with the appropriate values is obtained, the patient is rechecked every 3-6 months with both a urinalysis and radiographs. If the patient is female, stones may be identified when they are still small enough to be induced to pass naturally. A male cat will require surgery to remove stones as the male tract is invariably too small for the passage of stones.

Page last updated: 9/6/2012