What are calcium oxalate bladder stones?
Calcium oxalate bladder stones are composed of a mineral called calcium oxalate. While a small amount of calcium oxalate crystals in the urine can be normal, 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. Several risk factors have been identified. Certain purebred cats are predisposed to developing oxalate bladder stones. These breeds include Burmese, Himalayan, Persian, and Siamese. Oxalate stones are more likely to form in male, obese, and middle-aged to older cats.
"The exact cause of oxalate bladder stones is unknown."
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 has likely 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 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 non-specific signs of discomfort, such as lethargy and decreased appetite. In some cats, small oxalate bladder stones may be asymptomatic. These stones may be detected as part of the workup of another condition.
Large stones may act almost like a valve or stopcock, causing an intermittent or partial obstruction at the point where the bladder attaches to the urethra. Small stones may flow with the urine into the urethra where they can become lodged and cause an obstruction. If an obstruction occurs, the bladder cannot be emptied fully; if the obstruction is complete, the cat will be unable to urinate at all. If the obstruction is not relieved, the bladder may rupture or the kidneys may fail.
A complete obstruction is potentially life threatening and requires immediate emergency treatment. Complete obstruction is more common in male cats due to their longer, narrower urethras.
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.
"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."
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 X-rays of your cat’s abdomen. Many types of bladder stones, including oxalate bladder stones, are visible on X-rays. Oxalate stones often have a spiculated (spiny) appearance within the bladder. Ultrasound may also be used for imaging, especially for tiny stones.
What is the treatment for oxalate bladder stones?
Treatment of oxalate stones usually involves surgical removal, known as a cystotomy. This surgery is performed under general anesthesia. Your veterinarian makes an incision into your cat’s abdomen and then opens the bladder to remove the stones. The stones are then sent to a laboratory to confirm their chemical composition. Cats often have blood in their urine for several days after surgery. They must have their activity restricted for 1-2 weeks to allow their incision to heal. Pain medication will be required.
"Treatment of oxalate stones usually involves surgical removal, known as a cystotomy."
Less commonly, bladder stones may be removed via a process known as cystoscopy, in which a small camera is inserted into the bladder, along 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?
The stones are likely to recur unless the conditions that led to their formation are corrected. Therefore, your cat will require ongoing management.
The first component of treatment is to feed a specific prescription diet. These diets are designed to be lower in calcium and oxalate, while also maintaining a more neutral pH range. By altering the composition of the urine, a prescription diet will make oxalate stones less likely to develop. Your veterinarian will assess your cat’s urinalysis after some time on this diet, to determine whether the diet is effective.
It is also important to increase your cat’s water intake. In most cases, feeding prescription canned food leads to the formation of more diluted 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 and flavored water.
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 stays within desired parameters and allows early detection of any future recurrence.
