Calcium oxalate (CaOx) stones (uroliths) are rock-like formations composed of crystalloids and a small amount of organic matrix, which stick together and form stones that increase in size and number over time. The most common location is the bladder, and kidneys (less often). The etiology isn’t known, but we do know that supersaturated urine promotes the formation of crystals which combine to form stones. The incidence of calcium oxalate stones has increased in the past 35 years. In 1981, calcium oxalate made up less than 10% of stones compared to 47% now.
The role of magnesium restricted diets to prevent struvite stones has been suggested as a possible factor, as magnesium has been reported to be an inhibitor of calcium oxalate crystallisation. Previously, struvite stones were the most common crystal or stone, which formed in urine with high alkalinity. To combat this, pet food manufacturers began to acidify diets to promote acidic urine, and cats with a urine pH between 6 and 6.2 are three times more likely to produce calcium oxalate uroliths compared to diets which produce a urine pH of 6.5 to 6.9.
Risk factors include excess urinary calcium (hypercalciuria), excess urinary oxalate (hyperoxaluria), low urinary citrate (hypocitraturia), low serum magnesium (hypomagnesemia) and increased blood acidity (acidosis). Increased urinary calcium excretion can develop due to increased intestinal absorption of calcium due to excess dietary calcium and vitamin D or low blood phosphate (hypophosphatemia), certain medications (furosemide and corticosteroids), increased movement of calcium from the bones due to hyperparathyroidism, hyperthyroidism and excessive vitamin D intake).
There is a higher incidence in Burmese, Himalayan, Ragdolls and Persian breeds and a lower incidence in Siamese and Abyssinian cats. Calcium oxalate stones typically occur in older cats, with a peak risk of 8-12 years.
Urolithiasis may affect up to 25% of cats with lower urinary tract disease.
Symptoms depend on the size and number of stones and where they are located in the urinary tract. Calcium oxalate stones can irritate the bladder wall and can lodge in the urethra, causing a partial or complete urinary obstruction. Male cats are at increased risk due to their narrower urethra. A cat who cannot urinate requires immediate veterinary attention, as toxins build up in the bloodstream and can quickly become fatal.
Common symptoms of calcium oxalate stones include:
- Blood in the urine
- Straining to urinate
- Crying in the litter tray
- Frequent need to urinate (pollakiuria) which may result in only a small amount of urine being passed
- Weak flow of urine
- Painful urination (stranguria)
- Urinating outside the litter box
- Genital licking
- Complete absence of urination
- Loss of appetite
- Abdominal pain
- Rupture of the bladder
The veterinarian will perform a complete physical examination and obtain a medical history from you. Upon examination, the veterinarian will palpate the bladder to evaluate the size, shape, thickness of the bladder wall, a grating sensation may be noted as well as pain.
Questions they may ask:
- What symptoms does the cat have?
- How long have the symptoms been present?
- What diet is the cat on?
- Do you give the cat any supplements?
- Does the cat go to the toilet in a litter tray or outside?
- How many litter trays do you have?
- How often are the litter trays cleaned?
- Does the cat have any underlying medical condition?
It will be necessary to run a number of tests to evaluate the overall health of the cat, and look for the presence of stones within the kidney, bladder or lodged in the urethra.
- Urinalysis and sediment exam: Crystals are uncommon in the urine of cats with oxalate stones, but a urinalysis may reveal high calcium and acid levels.
- Radiographs: To look for the presence of radiopaque calcium oxalate stones in the kidneys, bladder or urethra.
- Complete blood count and biochemical profile: If the cat has developed a urinary blockage, these blood tests can evaluate how the kidneys are functioning and look for electrolyte derangements, in particular, hyperkalemia (increased blood potassium) due to reduced urinary excretion and hypercalcemia.
- Cystoscopy: A thin tube with a camera is inserted into the urethra to obtain biopsy tissue from the lower urinary tract.
- Echocardiogram: An ultrasound of the heart will be performed on any cat with a urinary obstruction as hyperkalemia can affect the heart’s ability to function properly.
A definitive diagnosis of calcium oxalate stones requires laboratory analysis of a sample of the stone.
Unfortunately, stone dissolving diets don’t work for calcium oxalate stones, and manual removal of the stones is necessary.
Non-surgical bladder stones
- Cystoscopy: This less invasive treatment may be attempted in female cats with smaller stones. A cystoscope with a stone-extracting basket is used to remove small stones from the bladder. Larger stones may be broken up with laser lithotripsy before the cystoscopy can be performed.
- Voiding urohydropropulsion: This method can be used for cats whose stones are small enough to pass through the urethra. The cat is placed under general anesthesia and a catheter is passed through the urethra and into the bladder. Sterile saline is infused to distend the bladder. The catheter is removed, and the cat is held upright and the bladder agitated to move the stones into the trigone. Expression of the bladder is then performed with manual pressure push the stones out of the bladder.
Surgical bladder stones
- Cystotomy: Surgical removal is required for cats with urethral obstructions and large stones. The cat is placed under anesthesia and an incision is made in the abdomen, the bladder is carefully lifted, opened and the stones are removed and the bladder flushed before it is closed with surgical stitches.
An obstructed cat is a medical emergency, and the first priority is to correct electrolyte derangements and stabilise the cat.
- Decrease potassium levels: Administer calcium gluconate which antagonises the effect of hyperkalemia on the heart. Insulin/dextrose can help to shift potassium into the cells and sodium bicarbonate raises blood pH, which also drives potassium into the cells.
- Catheterisation: The veterinarian inserts a catheter through the urethra and into the bladder to dislodge the blockage and allow urine to pass. Heavy sedation is necessary to catheterise the cat, which involves the insertion of a thin tube into the penis and up to the bladder. A sterile solution is flushed through the tube to push the obstruction back into the bladder, where it will dissolve or be surgically removed.
- Once the obstruction has been removed, the kidneys are able to resume their job of urine production, and excretion to remove toxins from the blood. Urine production may be increased for a few days which is known as post-obstructive-diuresis), as the kidneys do their job of cleaning the blood.
- Intravenous fluids will be continued to prevent dehydration and hypokalemia due to increased urine output.
Kidney (nephroliths) and ureter stones
- Extracorporeal shock wave lithotripsy (ESWL): A noninvasive procedure to break up the stone
with the use of energy or sound waves. The stone fragments pass down the ureter and into the bladder. This procedure is not as effective in cats.
- Surgery: The veterinarian makes a small incision into the kidneys to remove the stones. The incision is then closed and sutured.
Cats who have had calcium oxalate crystals are at increased risk of a recurrence; protocols must be put in place to decrease the incidence after initial treatment. The cornerstone of treatment is to increase water consumption to dilute the urine and promote urine specific gravity of less than 1.020 and urine pH of 6.8-7.5.
Feed a wet diet which has a higher water content than dry food. The veterinarian may recommend a therapeutic diet which has reduced quantities of protein, calcium, oxalate, vitamin C (is converted to oxalic acid and modifies into oxalate), and vitamin D (which promotes intestinal absorption of calcium). Therapeutic brands of food for cats with calcium oxalate stones include Purina UR st/ox, Hills c/d Multicare, Hills w/d, Royal Canin S/O and Iams Eukanuba Moderate pH/O.
Avoid excess dietary calcium and dietary oxalate.
Potassium citrate increases urinary excretion of citric acid and therefore, the formation of calcium citrate.
Ensure a constant supply of fresh and clean drinking water, and encourage water intake with the provision of multiple water stations or a water fountain.
Diligent litter tray maintenance is essential as cats may hold on to urine until they can get to a clean litter tray. Remove solids twice a day and empty, wash and refill with fresh litter once a week.
Male cats who have had repeat bouts of urinary obstruction can benefit from a perineal urethrostomy (PU). The urethra which passes from the bladder to the tip of the penis narrows towards the penis end. The perineal urethrostomy removes a large portion of the penis, leaving a wider opening in the remaining portion under the anus. This surgery can potentially prevent urethral obstructions in 90% of male cats. Calcium oxalate stones can still develop, but there is less risk of a life-threatening blockage developing.