Feline Oxalate Bladder Stones

• 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).
• 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 (20 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 caused by struvite crystals in urine (or at least this was the assumption). Also in those days, 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 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).
• Cats with calcium oxalate bladder stones tend not to have crystals in their urine (while those with struvite stones 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.
   

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.

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 should be taken to control the calcium level and determine why it is high. If blood calcium levels are normal, the following regiment is recommended:

Step One: Feed a Protein-Restricted Alkalinizing Diet Such foods are high in fiber, not restricted in phosphorus, and mildly restricted in protein. Hills c/d diet, Royal Canin (formerly Waltham) S/O, Iams Eukanuba Moderate pH pH/O 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 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 that modifies into oxalate. Be careful of pet vitamin supplements.

In 2 to 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 1.030), and if the urine pH is alkaline (it should be 6.8-7.5).


Oxalate crystals  when viewed under a microscope

Step Two: If Oxalate Crystals Are Present, the Urine is Not Dilute, or if the pH of the Urine is Acid, a Greater Percentage of Canned Food Should Be Used and a Potassium Citrate Supplement Should Be Added.  Potassium citrate is available in chewable form, capsules or liquid. It is a natural stone inhibitor. The goal urine pH is now 7.5.
In 2 to 4 weeks, another urine sample is performed.

Step Three: If the Urine is Still Having Crystals or Other Undesired Properties, a Vitamin B-6 Supplement is Introduced.
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.
Once a urinalysis with the appropriate values is obtained, the patient is rechecked every 3 to 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.