CALCIUM OXALATE BLADDER STONES
OXALATE BLADDER STONES IN THE DOG
HOW DO WE DETERMINE WHETHER MY DOG'S STONES ARE CALCIUM OXALATE STONES OR SOME OTHER KIND OF STONE?
Although a urinalysis can provide a clue, the only way to know for sure that a dog’s bladder stone is an oxalate stone is to retrieve a stone and have a laboratory analyze it. If the stones are very small, flushing the urinary bladder and forcefully expressing it may produce a stone sample for testing. The only other ways to obtain sample stones are by surgically removing the stones from the bladder (the traditional method), retrieving them by sending a cystoscope up the urethra, or by using laparoscopy to send a mechanical arm through a small incision into the abdomen and then into the bladder to retrieve them. Calcium oxalate stones cannot be made to dissolve over time by changing to a special diet (as can be done with struvite or uric acid bladder stones).
WHY WOULD MY DOG FORM CALCIUM OXALATE STONES?
It should not be too surprising that there is a strong hereditary component to the formation of oxalate bladder stones. This is also true in humans. There is a substance (called “nephrocalcin”) in urine which naturally inhibits the formation of calcium oxalate stones. This substance is defective in both humans and dogs who form calcium oxalate bladder stones. The production of defective nephrocalcin may be a genetic problem.
In humans, the genetic predisposition for stone formation is coupled with dietary issues (problem foods include: spinach, peanuts, chocolate, dairy products, calcium supplements, vitamin C supplements, and tea). Dogs are more likely to eat a single commercial brand of dog food without nearly the dietary variety that humans experience. This means that diet can be used to manage the problem in the dog but, unless the dog is receiving an unusual treat supply, genetics (not diet) is the cause.
There are some metabolic diseases that might predispose a dog to creating a calcium oxalate stone. For example, a dog with Cushing's disease will be over-producing cortisol (commonly known as "cortisone"). This hormone increases calcium excretion in urine. The extra calcium present in the urine will promote a stone. Some animals may have an elevated blood calcium level ("hypercalcemia") for any number of reasons and this could lead to excess calcium in the urine as the body attempts to restore calcium balance. Again, extra calcium in the urine promotes a stone. Screening for such medical conditions is important and is generally part of the medical work up once stones are detected on a radiograph. If any such metabolic problems are uncovered, they will need to be treated separately and as these problems are resolved, no further stone prevention measures should be needed.
WHY CAN’T THE STONES STAY WHERE THEY ARE?
The most immediate concern for the dog with bladder stones is that the urinary opening may obstruct as the dog attempts to pass the stones. This is largely a male dog problem but the results can be life-threatening uremic poisoning. In such cases, the veterinarian will try to dislodge the stone, flushing it back into the bladder to restore the patency of the urinary opening. If the stone cannot be dislodged, a new urinary opening may have to be surgically created. The urethra (the narrow tube connecting the urinary bladder to the outside world) is a difficult place to perform surgery so it is preferable to move the stone back into the bladder for removal rather than attempting removal from the urethra.
Bladder stones are irritating to the bladder simply by rubbing on the tender bladder lining. Bleeding typically results and, of course, the chance of developing chronic bladder infections is markedly increased with the presence of bladder stones.
BECAUSE CALCIUM OXALATE STONES
STUDIES HAVE SHOWN THAT 50% OF DOGS
THE FOLLOWING THERAPEUTIC PLAN HAS BEEN DEVISED
STEP ONE: DIET
While special diets cannot dissolve existing stones, they do help prevent the development of new stones. Prevention centers on creating a urinary environment with minimal calcium and minimal oxalate as well as creating a urine pH that is not conducive to calcium oxalate formation. Further, high protein, high sodium diets should not be used and diets low in phosphorus may stimulate vitamin D production and could increase calcium absorption.
Diets currently recommended by the University of Minnesota Urolith laboratory are:
For each of the above diets, the canned form is preferable to dry so as to increase water consumption and thus help dilute the urine. It is especially important to avoid table scraps when caring for an oxalate stone forming dog. Still, no one wants their dog to live life without treats. The following treats are acceptable for oxalate stone forming dogs:
Selecting a diet that is acceptable for stone prevention but also desirable to the picky pet may be challenging. It may be possible to have a veterinary nutritionist provide a recipe for an appropriate home cooked diet. Please visit www.balanceit.com or www.petdiets.com for details.
There are some medications which can increase the risk of calcium oxalate stone development and these should be avoided. Prednisone and other cortisone-type medications (commonly used for itchy skin, arthritis, inflammatory bowel disease, and other inflammatory conditions) should be avoided if possible. Furosemide (brand name: Lasix or Salix) is a diuretic mostly used in the treatment of heart failure. Both these medications lead to excess calcium in urine. Should a diuretic be needed for a patient at risk for calcium oxalate stones, one from the thiazide class (see later) would be a better choice. Supplementation with Vitamins D or C also increase risk of forming oxalate stones; such supplementation should be discontinued.
STEP TWO: URINALYSIS AND RADIOGRAPHS
Two to four weeks after surgery, a urine sample is checked and the bladder is radiographed. If the urine is not adequately dilute (meaning the specific gravity is less than 1.020), water consumption will have to be increased (either by adding water to the food or increasing the amount of canned food).
If urinary pH is less than 6.5 or if oxalate crystals are seen, it is time to go to Step Three. If all is well, a urinalysis should be performed every 3-6 months or so for the rest of the dog’s life (University of Minnesota’s current recommendation). Radiographs are also taken every 6-12 months so that any new stones will still be small enough to be retrieved from the bladder by flushing. If one skips this monitoring, one may find stones present have developed to a large size and surgery is again needed to remove them.
STEP THREE: POTASSIUM CITRATE
By taking potassium citrate orally, citrate levels increase in the urine. Calcium binds to citrate instead of to oxalate. This is a desirable event since calcium citrate tends to stay dissolved whereas calcium oxalate tends to precipitate out as mineral deposit. Potassium citrate also helps create an alkaline urine (in which calcium oxalate stones have difficulty forming). Potassium citrate supplements are typically given twice a day.
STEP FOUR: URINALYSIS AGAIN
Two to four weeks after potassium citrate is added to the therapeutic diet, a urine sample is checked. If the urine is not adequately dilute, water consumption will have to be increased (either by adding water to the food or increasing the amount of canned food).
If urinary pH is less than 6.5 or if oxalate crystals are seen, it is time to go to Step Five. If all is well, a urinalysis and radiographs should be performed every 6-12 months or so for the rest of the dog’s life (University of Minnesota’s current recommendation). In other words, if the urinalysis shows all the desired parameters and radiographs show no new stones forming, then therapy is working and the pet can go on a regular monitoring schedule. If testing shows the patient is still predisposed to stone formation, it is necessary to proceed down this list until control is achieved. Again, radiographs should be taken every 6 - 12 months.
STEP FIVE: THIAZIDE DIURETICS
A diuretic is a medication designed to increase the amount of urine produced. The thiazide class of diuretics does this in a way that reduces the calcium content of the urine. This medication is typically given twice a day and electrolytes are generally measured via blood test within the first two weeks of this treatment.
There will always be some dogs who continue to form stones no matter what is done. With radiography monitoring, it is hoped that new stones can be detected while they are still small. As research continues, it is possible that new developments will arise that will not require such frequent monitoring. Until then, these are the guidelines for oxalate prevention as proposed by the specialists at the University of Minnesota.
To view the Guidelines Directly visit:
PREVENTION OF MALE DOG OBSTRUCTION
In the male dog, a bone called the "os penis" is located in the penis, surrounding the urethra. Urine passes through this hollow bone like water through a pipe. Because this area is made of bone, it cannot stretch or expand to accommodate a stone trying to pass. A stone blockage of this type leads to an emergency and death if it goes untreated.
In a perfect world, controlling the bladder stones controls the risk of obstructions, but controlling the bladder stones is not always easy and treating a dog over and over for obstruction becomes expensive. A surgical procedure called a urethrostomy can be performed to prevent obstruction. This procedure involves creating a new urinary opening in the area of the scrotum. This allows urine to be expelled earlier in its course so that passage through the os penis does not occur. The flexibility of the non-bony part of the urethra plus the surgically enlarged urinary opening allows for smaller stones to pass rather than stick in the os penis.
In order to perform this surgery, the male dog must be neutered (which can be done at the time of the urethrostomy). The creation of the new opening usually requires removal of the empty scrotum.
While this is not as good an outcome as preventing stone recurrence at least the emergency obstruction is prevented.
A new technique of stone removal involves the use of a laser to blast the stone into pieces small enough to pass. This option (called "laser lithotripsy") is especially useful when the number of stones present is small and/or when a stone is lodged in the urethra and cannot be flushed into the bladder. It will not be helpful for a large number of stones as the process would be too time consuming relative to a more conventional approach but, for the right patient, it is less invasive than other procedures. For more information on this technique click here.
Calcium oxalate bladder stones can be very frustrating. Not only do they tend to recur but following monitoring recommendations involves a visit to the veterinarian's office every 3-6 months. It is up to every owner how rigid he or she wishes to be with guidelines but keep in mind that the trouble and expense of a stone surgery weighs against the much less trouble and expense of monitoring.
Page last updated: 1/13/2023