Dog and cat sharing pet bed

Photo courtesy of Dr. Teri Ann Oursler

Bladder stones are solid mineral deposits that form inside the bladder of dogs and cats and are quite common. Stones start out as crystals that form in the urine. Crystals form when a combination of events take place, such as urine pH change (pH is a measure of acidity and alkalinity in urine), increased urine concentration, and changes in the mineral makeup of the urine produced by the body. Over time, the crystals combine and layer together to form bladder stones. The process of forming bladder stones is called urolithiasis or cystolithiasis, 

The number of bladder stones produced can range from one or two stones to hundreds. Some bladder stones are small and grit-like, while others can grow to be larger than two inches in diameter. Kidney stones, more common in humans, occur much less often in dogs and cats. Only 2% of stones found in the urinary tract of our pets are found in the kidney.

The two most common types of bladder stones are those made of calcium oxalate and those made of struvite (also known as magnesium ammonium phosphate). Urate, cystine, xanthine, calcium phosphate, and silica are other mineral types that can form bladder stones but are rare.

Types of Bladder Stones 

Struvite bladder stones can occur with bladder infections. Certain bacteria will change the urine’s pH to reproduce faster. This pH change causes the urine to be more alkaline, causing struvite crystals to form. Struvite stones can also form without an infection, which is seen more commonly in cats than dogs.

Less is known about why struvite stones form without an infection. They often play a role in idiopathic cystitis in cats (also known as feline lower urinary tract disease), a condition associated with stress and straining to urinate.

Calcium oxalate stones tend to form with a more acidic pH and are rarely caused by bacteria. Other types of stones can occur with toxins, such as antifreeze poisoning, or can be breed-related, as occurs in Dalmatians.

Clinical Signs

Signs related to bladder stones include:

•       straining to urinate
•       urinating small amounts more frequently
•       dribbling urine
•       urinating in unusual places
•       vocalizing or crying when urinating
•       licking the vulva or penis frequently
•       urine may have a strong odor or show mucus or blood

Diagnosis

To diagnose bladder stones, your veterinarian will perform a thorough physical examination. Sometimes large stones can be felt by your veterinarian during an exam when they feel the belly (also called abdominal palpation) to check for normal organs within the abdomen. Urine can be obtained to look for crystals, pH changes, and evidence of infection. Radiographs, also called X-rays, are almost always necessary to confirm bladder stones. Unfortunately, some stones do not show up well on X-rays, so your veterinarian will also take into account any signs present during a physical exam and examination of your pet’s urine.

Ultrasound is sometimes used to look for bladder stones, especially those invisible on X-rays. It can also be used to view any damage to the urinary tract from bladder stones.

Determining the type of mineral in a stone is difficult when looking at its shape or appearance. Urine pH can provide clues, but this is not very accurate. To determine the stone’s composition, it must be sent to a laboratory that does urolith analysis, and results can take several weeks.

Treatment Options

Any infection will need to be treated. Depending on the type of stone, removing bacteria that change the pH can prevent new stones from forming. Very small stones may dissolve with normalized pH. Unfortunately, large stones create a perfect environment for bacteria to live, so antibiotics may not be able to completely kill off all the bacteria. Some stones are made of different layers of various minerals, so adjusting the urine pH by getting rid of bacteria may not have a big impact on these "combination" bladder stones. Also of concern, giving antibiotics in situations where all bacteria cannot be killed can potentially cause antibiotic-resistant bacteria. That can make infection harder to control, even after bladder stones are gone.

In some cases, usually with small stones and in combination with antibiotics for bacteria, struvite stones can be dissolved by feeding a therapeutic diet recommended by your veterinarian. This diet acidifies urine pH and restricts certain minerals such as magnesium and phosphorus. Dissolving the stones can take between one week to two months, depending on many factors. Calcium oxalate crystals cannot be dissolved with diet, but specific therapeutic diets can change the urine environment such that enlargement or new stone formation is less likely.  

Sometimes, bladder stones migrate along the urinary tract and become lodged in the urethra, which is the tube that leads from the bladder to outside your pet’s body (to the hole from which they urinate). In such cases, your pet is at serious risk of a urethral blockage, which prevents your pet from urinating. When this occurs, a method called retropulsion (also called retrograde hydropulsion or hydropropulsion) may be attempted. A urinary catheter is inserted into the urethra, and sterile saline (and sometimes lubricant) is injected into the catheter to try to push the urethral stones into the bladder to allow for easier removal.

Sometimes veterinarians will use retropulsion to try to remove small stones from the bladder itself (in which case it may be called voiding hydropulsion or hydropropulsion). The increased pressure of additional fluid in the bladder allows for small stones to pass easily and quickly out of the bladder and through the urethra with the extra fluid once the catheter is removed.

If hydropulsion does not work and diet change is not an appropriate option for your pet, surgery is needed to remove the bladder stones. This type of surgery is called a cystotomy. Your veterinarian will surgically open the abdomen and bladder to physically remove the stones. Sutures, also called stitches, or staples are used to close surgical openings.

After surgery, recovery can take two to four weeks. Pets will often need pain medication for at least one week. Pets are usually given antibiotics after surgery if they had a urinary tract infection prior to surgery. Remember, more stones may form if the infection is not cured. Your pet will need to be on restricted activity (this means no exercise, no running in the house, going outside only on a leash, even to use the bathroom) for one to two weeks following the surgery so they don’t damage the surgical sites or break internal sutures. Urine may be blood-tinged for several days following surgery. Straining to urinate should improve by two to three weeks after the surgery.

Lithotripsy, a method to fragment stones into a smaller size so they can be passed or removed through the urinary tract, is extremely uncommon in veterinary medicine. The procedure is only available at a few referral institutions and veterinary schools.

Increasing water intake by providing more water or adding canned food can also help with bladder stones. Increased water allows for increased flushing of the bladder and dilution of minerals within the urine.

Consequences

If left untreated, bladder stones can grow to the point that urinating is difficult or impossible. This difficulty is especially problematic if stones become stuck in the urethra. The inability to urinate is a life-threatening situation. Other issues associated with bladder stones are chronic pain and an increased risk for urinary tract infections.

Prevention

Once your pet has had bladder stones, therapeutic diets selected for that specific stone can help prevent recurrence. It is crucial that only the therapeutic food be given to your pet. No additional treats (unless specific to the diet), bones, or flavored chew toys can be given. Even a small change in the diet can change the pH and mineral content of the urine and lead to the stones reforming. Increase water consumption as much as possible to help dilute the urine to further decrease the chances of bladder stone formation.