Aaron Animal Clinic & Emergency Hospital

Ultrasound

Composition

There are many different types of bladder stones. Some of the most common ones include:

Struvite

This urolith is diagnosed in almost half the cases of bladder stones. It is also called MAP (Magnesium-ammonium-Phosphate), or triple phosphate. It is the predominant stone type in female dogs of all breeds except Dalmatians.

Several factors predispose dogs to getting struvite uroliths:

• Excess struvite crystals in the urine that set the stage for the formation of the urolith. Some breeds, notably Miniature Schnauzers, have a genetic predisposition to forming excess amounts of these crystals in the urine.

• Urinary Tract Infection (UTI)- some bacteria produce a byproduct called urease. Urease will increase the pH of the urine and promote ammonium in the urine.

• The formation of an alkaline (pH greater that 7.0) urine from the diet or urease producing bacteria will cause the struvite crystals to precipitate out of solution and begin the formation of a urolith.

• Urine that stays in the bladder longer than usual gives the struvite crystals further opportunity to precipitate out of solution and form a urolith.

  Struvite uroliths come in many different shapes and sizes, are radiodense, and form in an alkaline urine. Miniature Schnauzers are the most commonly affected breed.

Calcium Oxalate

This is the second most prevalent type of urolith after struvite, making up around 30-50% of the uroliths we diagnose, especially in male dogs of all species except Dalmatians. They come in two versions; the monohydrate and the dihydrate. Sometimes the two are found together, sometimes they are found with other uroliths like calcium phosphate, Strive, or ammonia urate.

Feline

These tend to occur in neutered middle aged cats, especially Burmese, Himalayan, and Persian breeds. One of the predisposing conditions might be an increased calcium level in the bloodstream. These uroliths tend to form in an acidic urine.

Canine

This is a problem most commonly in older male dogs. It is suspected that there is a correlation with hormone changes that occur as a pet ages. Several breeds are prone:

• Schnauzer
• Lhasa apso
• Yorkshire terrier
• Bichon Frise
• Shih Tzu
• Miniature poodle

Several predisposing factors work together to increase the chance of this urolith forming:

Urate and ammonium Urate

This is a common urolith found in Dalmatians around middle age, with males affected much more often than females. This is because they metabolize protein differently in the liver, with the end result being uric acid buildup in the urine. In addition to their high prevalence in Dalmatians, they are found in Bulldogs and Yorkshire terriers.

There does not seem to be a connection with a urinary tract infection, and they tend to form in an acidic urine.

Urate stones are radiolucent. If they get large or covered with other minerals they might become radiopaque. Urate calculi tend to be small and occur as several stones. These stones usually form in the bladder, and when passed through the urethra, can become lodged.

Ammonium urate uroliths are sometimes formed in pets with PSS (liver disease) due to improper metabolism of ammonia to urea. This will cause excess uric acid levels in the bloodstream. The kidneys filter out this excess uric acid in the production of urine, thus increasing the level of uric acid in the bladder. The excess ammonia that is in the bloodstream from the liver problem also builds up in the urine in the bladder. These two compounds combine to form the ammonium urate bladder stone.

Dogs with ammonium urate bladder stones might have ammonium urate crystals in their urine and a low specific gravity (dilute urine). These stones might not be seen on a radiograph because they are radiolucent. This same radiograph might also show a small liver, an indication of PSS. This small liver is due to the diverted blood flow to the liver. Dogs with PSS will commonly have abnormalities in the blood sample to give us further clues.

Compound Uroliths

Most bladder stones are caused predominantly by one type of mineral. The more common ones have been described above. In a small percent of cases, the bladder stone is caused by a combination of minerals in similar quantities. These stones are called mixed uroliths. Some bladder stones consist of a core mineral surrounded by a lesser amount of a different mineral in a different layer. These are called compound uroliths. Why some minerals form mixed uroliths and others form compound uroliths is not understood.

Compound uroliths form when the factors that predispose to one type of stone formation have now changed to factors that favor a different type of stone formation. If a struvite stone is treated with antibiotics and urinary acidifiers the problem tends to resolve. The change in urine pH might promote excess calcium in the urine, resulting in a shell of calcium oxalate formation around the core struvite stone. The opposite can occur also- a struvite stone can form over a calcium oxalate stone.

In general, these stones are removed surgically and an effort is made to medically prevent the mineral that is at the core of the stone.

Miscellaneous Uroliths

There are other uroliths that occur, although they are relatively uncommon. They include cystine, silica, calcium phosphate, and miscellaneous minerals.

Medical Treatment

Struvite

Struvite bladder stones can literally be treated medically with a food called Hills S/D. The mechanism involves creating a urine that is undersaturated with the crystals that caused the struvite urolith to form in the first place. This undersaturation literally cause the urolith to dissolve in the urine, and then get urinated out.

S/D has several modifications in its ingredients to set up this undersaturated urine. Its reduced in protein, so there is less ammonia buildup in the bladder from bacteria. Magnesium and phosphorous are restricted also. With less contents of the minerals that form the struvite urolith (magnesium, ammonia, and phosphorous- MAP) the urolith starts dissolving.

S/D also has an increased amount of salt (sodium chloride). This promotes drinking and urination and literally helps flush the struvite crystals out of the bladder. It also changes the pH to a more acidic state, which further makes the struvite stone dissolve.

S/D must be the only food fed for it to work. We can monitor whether or not an owner is doing this by looking at the pH of the urine along with the specific gravity of the urine. also, the BUN (blood urea nitrogen) of a pet on S/D should be lower than normal.

We have clues from other diagnostic tests to help decide if a urolith found in the urinary bladder on a radiograph is truly struvite. The urinalysis gives us an idea of the composition of the urolith by looking at the crystals in the sediment. The pH of the urine will be alkaline. Also, the presence of bacteria on a culture in a breed that is prone to struvite uroliths is also a strong indication.

If bacteria are found on a urine culture (rare in cats) then antibiotics must be used simultaneously while a pet is on S/D. It must be the correct antibiotic, so the importance of the urine culture is obvious. Both S/D and antibiotics are used for one month after the stone is no longer visible on a radiograph. If the urolith is still present after 2 months of S/D and antibiotics, then surgery should be performed. Most pets need to be fed S/D for 4-6 months for complete resolution.

S/D is restricted in protein, so it is not a complete diet for long term use in dogs. While on S/D your pet's blood should be monitored to ensure there are no side effects of the restricted protein. Feline S/D is not protein restricted, so it can be used for the rest of your cat's life.

Calcium Oxalate

The only treatment for these uroliths is surgical removal.

Urate

If there is no PSS these uroliths can sometimes be handled medically also. A food called Hills U/D should be used. It is low in purines and has restricted protein. a medication called allopurinol is given which helps minimize the amount of uric acid produced in the urine. Potassium citrate is also used to make the urine less acidic, although this might occur with the use of U/D alone. Adding salt to the diet or mixing water with the food will also promote urination of the urate crystals. Recurrence is common, so this diet needs to be used for the life of your pet, especially in Dalmatians. We will talk more about this later in the prevention section.