What are oxalate bladder stones?
Oxalate bladder stones are composed of a mineral called calcium oxalate. While a small amount of calcium oxalate crystals in the urine can be a normal finding, some cats have very high numbers of these crystals. Under certain conditions, these crystals can combine into stones within the bladder or other areas of the urinary tract.
Oxalate bladder stones constitute a growing percentage of the bladder stones found in cats. While these stones were once uncommon, accounting for less than 10% of feline bladder stones, their incidence has increased over the last 40 years and they now constitute over 40% of bladder stones in cats.
What causes oxalate bladder stones?
The exact cause of oxalate bladder stones is unknown. A number of risk factors have been identified. Certain purebred cats are predisposed to developing oxalate bladder stones. These breeds include Burmese, Himalayans, Persians, and Siamese. Oxalate stones are more likely to form in males, obese cats, and in middle-aged to older cats.
Cats are more likely to develop oxalate stones when their urine contains high levels of calcium and oxalate. In some cases, this is also associated with high blood calcium levels.
Additionally, a low urine pH (acidic urine) promotes the formation of oxalate stones. This factor likely has contributed to the increasing incidence of oxalate stones in recent years. Historically, cats were much more likely to develop another type of bladder stone, known as struvite stones. Pet food manufacturers began creating more acidic diets in order to reduce the formation of struvite stones, but this has led to a rise in oxalate stones in cats.
What are the clinical signs of oxalate bladder stones?
Bladder stones can cause significant inflammation and irritation of the bladder wall. Therefore, they cause signs similar to those of a urinary tract infection or any inflammatory bladder disease.
Signs may include frequent urination, straining to urinate, blood in the urine, and urinating outside of the litterbox. Some cats may also show nonspecific signs of discomfort, such as lethargy and decreased appetite.
"Signs may include frequent urination, straining to urinate, blood in the urine, and urinating outside of the litterbox."
In some cats, small oxalate bladder stones may be asymptomatic. These stones may be detected as part of the workup of another condition.
How will my veterinarian diagnose oxalate bladder stones?
If your cat presents to the veterinarian for urinary signs, your veterinarian will first perform a urinalysis. This test involves obtaining a small sample of urine for biochemical analysis and examination under the microscope. If your cat has oxalate bladder stones, the urinalysis will likely show the presence of a low urine pH, red blood cells (due to bladder trauma), white blood cells (consistent with inflammation), and increased numbers of oxalate crystals in the urine.
Your veterinarian will also likely perform blood tests, including a complete blood cell count and serum biochemistry profile. These tests will assess your cat’s overall health and rule out other medical conditions that may be contributing to your cat’s urinary signs. Some cats with oxalate stones have high blood calcium levels, which can be detected on bloodwork.
Finally, your veterinarian will likely recommend abdominal radiographs (X-rays). Many types of bladder stones, including oxalate bladder stones, are visible on radiographs. Oxalate stones often have a spiculated (spiny) appearance within the bladder, similar to a sandspur (burr grass). Ultrasound may also be used for imaging.
What is the treatment for oxalate bladder stones?
Treatment of oxalate stones usually requires surgical removal, known as a cystotomy. This surgery is performed under general anesthesia. Your veterinarian will make an incision into your cat’s abdomen and then open the bladder to remove the stones. The stones are then sent to a laboratory to confirm their chemical composition. Cats typically are uncomfortable and have blood in their urine for several days after surgery; they also must have their activity restricted for 1-2 weeks to allow their incision to heal.
"Treatment of oxalate stones usually requires surgical removal, known as a cystotomy."
Less commonly, bladder stones may be removed via a process known as cystoscopy. This involves inserting a small camera into the bladder, with a basket or retrieval device that can be used to remove the stones. Other techniques have also been described to remove bladder stones, but they are less commonly utilized.
Will oxalate bladder stones recur after treatment?
Unless the conditions that led to the formation of oxalate bladder stones are corrected, the stones are likely to recur. Therefore, your cat will require ongoing management.
The first component of treatment is to feed a prescription diet. This diet is intended to be low in calcium and oxalate, while also increasing the urine pH. By altering the composition of the urine, this diet will make oxalate stones less likely to develop. Your veterinarian will assess your cat’s urinalysis after some time on this diet, in order to determine whether the diet is effective.
"Unless the conditions that led to the formation of oxalate bladder stones are corrected, the stones are likely to recur."
It is also important to increase your cat’s water intake. In most cases, feeding a canned food is effective at leading to the formation of dilute urine. If your cat’s urine is still concentrated on a canned diet however, you may need to consider additional measures, such as providing running water fountains, flavored water, etc.
If these measures alone are not effective at preventing the formation of oxalate crystals, your veterinarian may recommend additional medications to further modify the urine.
Your veterinarian will recommend regular rechecks over the remainder of your cat’s life. Regular rechecks will ensure that your cat’s urine is staying within desired parameters and allow early detection of any future recurrence.