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UROLITHIASIS (URINARY STONES)
Overview
Urolithiasis (urinary stones) is a common condition responsible for lower urinary tract disease in dogs and cats. The formation of bladder stones is associated with precipitation and crystal formation of a variety of minerals (magnesium ammonium phosphate hexahydrate, calcium oxalate, urates, and others).
Causes and Risk Factors
What causes urinary stones? Several factors are responsible for the formation of urinary stones. The understanding of these processes is important for the treatment and prevention of urinary stones. In general, conditions that contribute to stone formation include:
a high concentration of salts in urine retention of these salts and crystals for a certain period of time in the urinary tract an optimal pH that favors salt crystallization a scaffold for crystal formation a decrease in the body's natural inhibitors of crystal formation.
The sequence of events that triggers stone formation is not fully understood. High dietary intake of minerals and protein in association with highly concentrated urine may contribute to increased saturation of salts in the urine. Disease conditions such as bacterial infections in the urinary tract can also increase urine salt concentration.
What are the consequences of urinary stones? Most bladder stones are located in the urinary bladder or urethra and only a small percentage are lodged in the kidneys or ureters. Urinary stones can damage the lining of the urinary tract causing inflammation. This inflammatory reaction may predispose the animal to bacterial urinary tract infection (UTI). Urinary stones may physically block the urine flow causing urinary obstruction that requires immediate emergency treatment. Small urinary stones may become lodged in the urethra, particularly in male dogs, causing an obstruction that requires urgent treatment. Stones may also become lodged in the ureter (the portion of the urinary tract carrying urine from the kidney to the urinary bladder) also causing an obstruction that results in serious kidney damage.
Diagnosing Urinary Stones
How are the urinary stones diagnosed? The diagnosis of urinary stones is based on the manifestation of clinical signs:
- the owner observes hematuria (blood in the urine)
- low back or abdominal discomfort
- depression
- anorexia (loss of appetite)
- vomiting
- difficulties in urination (dysuria or stranguria)
- frequent urination (pollakiuria)
- cloudy urine.
Diagnosis can also be made based on abnormalities such as blood or inflammatory cells (hematuria or pyuria) seen on analysis of the urine (urinalysis). Other systemic changes may include blood chemistry imbalances or heart rate and rhythm abnormalities seen on ECG. Identification of urinary tract infection associated with urinary stones requires culture not only of the urine but also of the bladder lining and the urolith (bladder stone). A prospective study comparing different sampling methods showed that bacteria were cultured in almost 20% of bladder mucosal biopsies and uroliths even when urine cultures were negative.
Imaging techniques: Several diagnostic imaging modalities can be performed to assess morphology and functionality of the urinary tract. X-rays (radiography) and ultrasound are the most commonly performed imaging techniques. Radiographs (Fig 1) can be plain, with positive contrast, negative contrast and double contrasted. Special radiographic techniques are:
- excretory urography - where a contrast agent is injected into the blood stream and is then cleared by the kidneys, outlining the kidney, ureters and urinary bladder
- renal angiography - where contrast agents are injected into the renal artery to assess the blood flow to the kidney
- retrograde urethrocystography - where contrast agent is injected into the urethra to outline the urethra and urinary bladder
Figure 1. Lateral radiographs of a canine abdomen showing urinary stones in the bladder (Arrow).
Ultrasound examination can be very useful in evaluation of the kidneys or bladder, especially when renal blood flow and renal function are depressed limiting the use of radiographic techniques. Another technique that has been used more recently is nuclear scintigraphy, which provides a non invasive method for critical analysis of renal blood flow and function.
What are the Types of Urinary Stones? The stone type is named after its mineral composition. The most common stones are struvite (magnesium ammonium phosphate), calcium oxalate, urate, cystine and silica.
Struvite Stones
The most common mineral type found in dogs is magnesium ammonium phosphate hexahydrate (Struvite). This type of urinary stone accounts for 50% of all canine urinary stones. The prevalence in cats is around 30%. Miniature Schnauzer, Miniature Poodles, Bichon Frise, and Cocker Spaniels are the most affected breeds. Urinary tract infection is an important factor in the formation of struvite stones. The enzymatic action of some bacteria on urinary excreted urea increases the pH (makes the urine more alkaline) which decreases the solubility of Struvite crystals. Cystitis increases the amount of organic debris in the urine providing a surface for crystallization.
Figure 2. Photograph of struvite stones
Figure 2a. 100% magnesium ammonium phosphate hexahydrate (stone). Note the miliimeter marks at the bottom. This stone measures 35mm long (1.5 inches).
Figure 2b. Struvite stones from a cat's urethra.
Figure 2c. This is a mixed stone consisting of 60% ammonium acid urate and 40% magnesium ammonium phosphate hexahydrate (struvite).
Calcium Oxalate Uroliths
In dogs, calcium oxalate stones account for about 35% of all stones, while they account for 50-70% of feline stones. Stones from the kidney or ureter of cats have been diagnosed as calcium oxalate in 70% of the cases submitted. Breeds that are most affected in dogs include Miniature and Standard Schnauzers, Miniature Poodles, Bichon Frise, Lhasa Apso, Yorkshire Terrier and Shi Tzu. Burmese, Persian and Himalayan cats are the feline breeds most commonly affected. Older male castrated cats and dogs are more affected by calcium oxalate calculi. They may also be found more frequently in the kidney than struvite stones.
The cascade of events leading to calcium oxalate stone formation is completely unknown but there is some indication that normal postprandial (after a meal) increases in urinary calcium concentration could be involved in the stone formation. In cats, magnesium ammonium phosphate (struvite) stones used to be more common than they are today, and oxalate stones much less fequently seen. Results of epidemiologic studies support the hypothesis that special diets developed to reduce the formation of struvite stones may have inadvertently increased the incidence of oxalate stones. Decreased urine concentration of natural body crystal formation inhibitors, and increased dietary intake of oxalate may also play a role in the calcium oxalate stone formation.
The physical appearance of Oxalate stones varies widely depending on exactly which form of oxalate has precipitated. The following figures show the wide variety of oxalate stones which can be found. Figures 3a-3e are all 100% calcium oxalate monohydrate. Figure 3f is 100% calcium oxalate dihydrate and Figures 3g-3j are composed of various mixtures of calcium oxalate monohydrate and dehydrate.
Figure 3. Photograph of calcium oxalate stones
Figure 3a.
Figure 3b.
Figure 3c.
Figure 3d.
Figure 3e.
Figure 3f. This stone is comprosed of 100% calcium oxalate dehydrate.
Figure 3g. This stone is composed of 60% calcium oxalate monohydrate and 40% calcium oxalate dehydrate. These stones were removed from the same patient at Figure 3d about 16 months later.
Figure 3h. These stones are composed of 80% calcium oxalate monohydrate, 10% calcium oxalate dehydrate and 10% calcium phosphate.
Figure 3i. These stones are composed of 70% calcium oxalate dehydrate and 30% calcium oxalate monohydrate.
Figure 3j. These stones are composed of 75% calcium oxalate monohydrate and 25% calcium oxalate dehydrate.
Urate Stones
Urate stone formation in dogs may result from two different mechanisms. One is related to the high excretion of ammonium biurate crystal in cases of porto-systemic shunts. See the related web site article on Porto-systemic shunts for a discussion of this problem. Dalmatian dogs, which have a defective hepatic membrane transport of uric acid, will also frequently form urate stones.
A retrospective study to assess clinical signs and response to surgical treatment in Dalmatians with urate stones showed that blood was found in the urine in 85% of the dogs and crystals were found in the urine in 54% of the dogs. The majority of dogs affected were males. Contrast radiographs and ultrasound were the most useful tools to diagnose urate stones.
Figure 4. Photograph of urate stones
Figure 4a. This stone is composed of 100% ammonium acid urate.
Figure 4b.These stones are composed of 100% ammonium acid urate.
Cystine Stones
Excessive elimination of cystine in the urine is an inherited disorder of renal tubular transport that is thought to be the primary cause of cystine stones. High concentrations of cystine in an acid environment (low pH) can form stones. The breed commonly affected is the male Dachshund and the most common age is 3 to 6 years old.
Figure 5. Photograph of cystine stones
Figure 5b. This stone is composed of 100% cystine
Silicate Stones
The mechanism of formation of silicate stones is completely unknown; however, there may be a relationship between this type of stones and the dietary intake of silicates, silica acid and magnesium silicate. The formation of these stones has been linked to the consumption of large amounts of corn gluten and soy bean hulls which are high in silicates. Male German Shepards, Old English Sheepdogs and Golden and Labrador Retrievers are the most affected breeds. The average age of diagnosis of silicate stones is between 6 and 8 years of age.
Figure 6. Photograph of silica stones
Medical Management
Calcium oxalate, urate, cystine and silicate stones CANNOT be dissolved and require surgical treatment. Most of these calculi have a high recurrence rate. Struvite stones can occasionally be dissolved by using a commercially prepared diet (S/D) which is specifically formulated for this purpose.
Prevention
Struvite stones: Prevention of Struvite stones depends on restricting protein, calcium, phosphorous and magnesium in the diet, and using a relatively high salt content which will tend to make the urine more acid. The treatment of any urinary tract infections is essential for the success of preventive measures.
Calcium oxalate stones: Feeding a protein and sodium-restricted, alkalinizing diet may delay recurrence of oxalate stones. Potassium citrate may be given to help achieve a neutral or slightly alkaline urine. Other treatments that are occasionally used are the addition of vitamin B6 and hydrochlorothiazide (a diuretic).
Cystine stones: Feeding low protein, alkalinizing diet is highly successful in preventing the formation of Cystine stones. The pH of the urine should be kept above 7.5. Potassium citrate, 2-mercaptopropioglycine, and D-Penicillamine are occasionally used to help achieve the alkaline urine.
Urate stones: Urate stones can be prevented in Dalmation by feeding a low protein alkalinizing diet and Allopurinol, a drug that helps prevent the formation of uric acid, is also occasionally used in the prevention of urate stones. Allopurinol should not be given to cats. Urate stones can be prevented in 80% of dogs and 95% of cats.
Silicate uroliths: Urine alkalinization and diets with low amounts of silicates should be used to prevent the formation of Silicate stones.
Medical Management of Urinary Obstruction
Urinary obstruction caused by stones lodged in the urethra is an urgent or emergency situation. The urinary obstruction must either be relieved to allow the bladder to empty, or the bladder must be drained by cystocentesis (a procedure where a needle is placed into the distended bladder through the abdominal wall, and the urine is removed with a syringe).
Stones lodged in the urethra can often be dislodged and forced back into the urinary bladder with a technique called retrograde urohydropulsion.
Figure 7. Schematic drawing illustrating urinary retrograde urohydropulsion
The obstruction may also be relieved with a technique known as voiding urohydropulsion (Figure 8). Once the obstructing stones are removed, stones remaining in the urinary bladder are removed surgically.
Figure 8 – Schematic drawing illustrating voiding urinary hydropulsion
Surgical Treatments
The procedure for the surgical removal of urinary stones depends on where they are located in the urinary tract. The procedure for removing urinary in the urinary bladder is called a cystotomy. When stones are in the urethra, the procedure is called a urethrotomy. Occasionally a permanent opening is made to allow any further stones to pass without causing an obstruction. This procedure is called a urethrostomy. A common procedure in male cats that become obstructed is called a perineal urethrostomy, or P.U.
Stones that develop in the kidney may be removed by a nephrotomy. Most veterinarians will perform a cystotomy, however, many prefer to refer animals in need of a urethrotomy urethrostomy, or nephrotomy to a surgical specialist. To find an ACVS veterinary surgeon in your area, click here: Search for a Surgeon
—Alexander El Warrak, DMV, MS
Small Animal Resident
Editor: William Daly, DVM
Diplomate ACVS
Posted 1/12/2006
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