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Overview : 

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 : 

Causes 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.

What are the consequences of urinary stones? : 

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.

How are the urinary stones diagnosed? : 

How are the urinary stones diagnosed? 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.

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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 : 

Imaging techniques 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

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Figure 1. Lateral radiographs of a canine abdomen showing urinary stones in the bladder (Arrow).

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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.

Types of Urinary Stones : 

Types of Urinary Stones 1. 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.

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Figure 2.  Photograph of struvite stones

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Figure 2a. 100% magnesium ammonium phosphate hexahydrate (stone).  Note the miliimeter marks at the bottom.  This stone measures 35mm long (1.5 inches).

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Figure 2b. Struvite stones from a cat's urethra.

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Figure 2c. This is a mixed stone consisting of 60% ammonium acid urate and 40% magnesium ammonium phosphate hexahydrate (struvite).

2. Calcium Oxalate Uroliths : 

2. 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.

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Figure 3. Photograph of calcium oxalate stones

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Figure 3a.

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Figure 3b.

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Figure 3c.

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Figure 3d.

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Figure 3e.

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Figure 3f. This stone is comprosed of 100% calcium oxalate dehydrate.

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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.

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Figure 3h. These stones are composed of 80% calcium oxalate monohydrate, 10% calcium oxalate dehydrate and 10% calcium phosphate.

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Figure 3i. These stones are composed of 70% calcium oxalate dehydrate and 30% calcium oxalate monohydrate.

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Figure 3j. These stones are composed of 75% calcium oxalate monohydrate and 25% calcium oxalate dehydrate.

3. Urate Stones : 

3. 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.

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Figure 4. Photograph of urate stones

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Figure 4a. This stone is composed of 100% ammonium acid urate.

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Figure 4b.These stones are composed of 100% ammonium acid urate.

4. Cystine Stones : 

4. 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.

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Figure 5. Photograph of cystine stones

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Figure 5b. This stone is composed of 100% cystine

5. Silicate Stones : 

5. 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.

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Figure 6. Photograph of silica stones

Medical Management : 

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 : 

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 : 

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).

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Cystocentesis can be performed with the animal in dorsal recumbency, lateral recumbancy, standing or being held standing on its hindlimbs by elevating its forequarters. The position of the animal is primarily based on personal preference. Cystocentesis is usually performed with a 25-22 gauge needle.

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The bladder is palpated and immobilized. The bladder should not be squeezed tightly as the puncture is being made as this can cause urine to leak from the puncture site into the abdominal cavity.

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The needle should be inserted at a 450 angle, a short distance cranial to the junction of bladder and urethra (a) If the needle is inserted at the apex of the bladder (b), as urine is removed, the bladder gets smaller and moves away from the needle. The needle should be inserted into the bladder while creating negative pressure by pulling back on the plunger of the syringe. The needle should not be redirected if urine is not obtained, due to the risk of penetrating a bowel loop and subsequently taking the contaminated needle into the bladder. If a sample is not obtained on the first attempt, change the needle before making another attempt. If a sample is not obtained on 3 attempts, the bladder is probably small and in the pelvic canal. If a sample is obtained, the syringe plunger is released, and the needle is removed from the abdomen.

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If you cannot palpate the bladder, you can perform "blind" cystocentesis with the dog in dorsal recumbency. Pick a point on the midline, midway between the umbilicus (u) and brim of the pelvis (p) . Notice when you are preparing the puncture site, this is the same point at which the antiseptic solution will pool when the patient is in dorsal recumbancy. This is the starting point of cystocentesis. If urine is not obtained with the first puncture, two additional punctures can be attempted from 1 to 2 centimeters cranial and 1 to 2 centimeters caudal to the initial puncture site. If urine is not obtained on the first attempt, change the needle before making another attempt. If a sample is not obtained on 3 attempts, the bladder is probably small and in the pelvic canal. Try again later to collect a sample.

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Cystocentesis can be performed on the midline in male dogs by retracting the penis and prepuce off the midline.

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Stones lodged in the urethra can often be dislodged and forced back into the urinary bladder with a technique called retrograde urohydropulsion.

Retrograde Urohydropulsion : 

Retrograde Urohydropulsion

7 steps to perform retrograde urohydropropulsion : 

7 steps to perform retrograde urohydropropulsion Step 1: Verification and localization of urethroliths Perform appropriate diagnostic procedures to localize the sites of urethrolith(s). Evaluate their number, size, radiodensity and surface characteristics. Since uroliths rarely form in the urethra but migrate from the bladder lumen into the urethra, be sure to include the urinary bladder in the evaluation. Palpation of the posterior urethra (including palpation of the urethra per rectum) followed by appropriate survey or contrast radio-graphy should be performed to establish the site(s) and cause(s) of outflow obstruction.

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Figure 1: Removal of urethrolith in a male dog by urohydropulsion. Dilation of the urethral lumen is achieved by injecting fluid with considerable pressure. Digital pressure applied to the external urethral orifice and the pelvic urethra has created a closed system.

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Ultrasonography may be of value in evaluation of the urinary bladder. If the urethroliths become lodged in an unusual or unexpected location in context of the caudal aspect of the os penis, use contrast urethrography to rule out mural and/or extramural urethral lesions that are contributing to outflow obstruction. If the patient has signs of systemic illness, or if the history suggests prolonged outflow obstruction, pretreatment urine and blood samples should be obtained to assess renal function, systemic fluid, electrolyte and acid-base status.

Step 2: Decompressive cystocentesis : 

Step 2: Decompressive cystocentesis If obstruction to urine outflow already has resulted in over-distension of the bladder lumen, it should be decompressed by cystocentesis. To prevent iatrogenic over-distension of the bladder, decompressive cystocentesis generally should be performed prior to urohydropropulsion.

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Figure 2: Sudden release of digital pressure at the pelvic urethra and subsequent movement of fluid propelling the urethrolith toward the bladder lumen.

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Technique of decompressive cystocentesis : A 22-gauge needle attached to a flexible intravenous extension set which in turn is attached to a large-capacity syringe is recommended. After the needle is inserted into the bladder lumen, one individual should digitally immobilize the urinary bladder containing the tip of the 22-gauge needle, while another aspirates urine from the bladder lumen through flexible intravenous tubing into a large-capacity syringe. Gentle agitation of the distended bladder in an up-and-down motion prior to cystocentesis may disperse particulate matter or crystals throughout the urine, and thus facilitate their aspiration into the collection system.

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Step 3: Lubrication of urethroliths Step 4: Restraint and anesthesia

Step 5: Technique of retrograde urohydropropulsion (Figures 1 and 2). : 

Step 5: Technique of retrograde urohydropropulsion (Figures 1 and 2). 1. Inject a liberal quantity of a mixture of sterilized saline solution and aqueous lubricant through a flexible catheter into the urethral lumen adjacent to the uroliths.   2. Next, an assistant familiar with this procedure should gently insert a lubricated gloved index finger into the rectum. Firmly occlude the lumen of the pelvic urethra by applying digital pressure against the ischium through the ventral wall of the rectum. 3. A flexible catheter with an attached 20-ml to 35-ml syringe filled with sterilized saline should then be inserted into the lumen of the penile urethra via the external urethral orifice and advanced to the site of the urethroliths. The penile urethra should be compressed around the shaft of the catheter by firm digital pressure. As a result of these steps, a portion of the urethra from the external urethral orifice to the bony pelvis becomes a closed system. Failure to properly occlude the pelvic and/or the distal-most portion urethral lumen will result in impaired ability to flush the urethroliths into the urinary bladder (Table 1). 4. Saline should be injected into the urethra until a marked increase in the diameter of the pelvic urethra is perceived by the assistant. Confirmation that the urethra has been markedly distended is important because distention of the urethra to its maximum capacity must be achieved before a sufficient degree of pressure can be created within the urethral lumen to advance the uroliths. Failure to create sufficient pressure in the urethral lumen often results in inability to flush the urethroliths back the urinary bladder (Table 1). The likelihood of rupture of the urethral lumen as a result of intraluminal pressure generated by this technique is remote because the path of least resistance for fluid is through the urethra into the bladder lumen and/or out the external urethral orifice. However, caution must be used not to rupture the urinary bladder by over-distending the lumen with the flushing solution. Therefore, the size of the bladder should be monitored at appropriate intervals by abdominal palpation. If and when it becomes full, decompressive cystocentesis should be repeated.

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5. At this point, the lumen of all portions of the isolated urethra, except that located in the ventral groove of the os penis, will be markedly dilated (Figure 1, p. 6S ). Dilation of the lumen of the segment of the urethra located in the ventral groove of the os penis is limited to that caused by stretching of the ventral portion of the urethral wall. 6. Next, digital pressure applied to the pelvic urethra (but not the penile urethra) should be rapidly released. Pressure should be maintained in the urethral lumen by continuing to inject saline by pushing the syringe barrel over the syringe plunger after the assistant has released digital pressure applied through the rectal wall. This step requires coordination between the two individuals performing the technique (Table 1). When properly coordinated, this step will propel the saline mixture and the urethroliths toward (or into) the bladder lumen (Figure 2, p. 7S). Often, especially in cases where the uroliths have recently passed into the urethra, the urethroliths are easily flushed into the bladder lumen during the first attempt. However, in some situations, the uroliths do not move, or move only a short distance, before the pressure in the urethral lumen has dissipated. If this occurs, it may be necessary to repeat the procedure several times before all the uroliths reach the bladder lumen. The position of the urolith(s) may be monitored either by means of: a) digital palpation of the perineal and pelvic urethra, b) a catheter carefully advanced through the urethra, and/or c) by means of radiography. If it is necessary to repeat the technique, accumulation of large amounts of saline and urine in the lumen of the bladder will necessitate repeating decompressive cystocentesis.

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Step 6: Minimize catheter-induced trauma to the urinary tract, and iatrogenic urinary-tract infection Step 7: Consider an appropriate technique to manage the urocystoliths

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The obstruction may also be relieved with a technique known as voiding urohydropulsion. Once the obstructing stones are removed, stones remaining in the urinary bladder are removed surgically.

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Schematic drawing illustrating voiding urinary urohydropulsion

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Recently, a nonsurgical technique to remove urocystoliths called voiding urohydropropulsion was developed. By taking advantage of the effect of gravity on urolith position in the urinary bladder and dilation of the urethral lumen during the voiding phase of micturition, this technique allows small urocystoliths to be flushed out of the urinary tract.

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The success of voiding urohydropropulsion is not dependent on the gender of patients. Rather, it is dependent on whether the size of uroliths will allow them to pass through urethral lumens. Because the diameter of the urethra in male dogs is smaller than female dogs, and because the os penis in male dogs restricts expansion of a portion of the urethral lumen, larger uroliths can be voided from female dogs compared to male dogs of equal size.

Technique of Voiding Urohydropropulsion : 

Technique of Voiding Urohydropropulsion Step 1--Anesthetize the patient.  Step 2--Distend the bladder with a sterile physiologic solution injected through a transurethral catheter.  Step 3--Remove the catheter. If fluid begins to pass through the urethra prematurely, occlude the distal urethra by digital compression of the vulva or penis.  Step 4--Position the patient so that the patient's vertebral column is approximately vertical.  Step 5--Gently agitate the bladder by palpation to promote gravitational movement of uroliths into the bladder neck.  Step 6--Apply steady digital pressure to the bladder to induce voiding. Once voiding begins, sustain bladder compression.

Surgical Treatments : 

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.


CYSTOTOMY Surgical incision or puncture into a URINARY BLADDER. Cystotomy may be used to remove URINARY CALCULI, or to perform tissue repair and reconstruction. Common names: Cystotomy; Cystotomies; Vesicotomy; Vesicotomies

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Figure 1. Portal locations for the optic (1) and the grasping forceps (2) and location of parainguinal incision of the abdominal wall (3)

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Figure 2. The urinary bladder was grasped at its cranial part for mobilization under endoscopic control. A = grasping forceps, B = cecum, C = urinary bladder, D = ventral abdominal wall, E = small intestine

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Figure 3. The urinary bladder was exteriorized through an incision in the ventral abdominal wall. A = grasping forceps, B = cecum, C = urinary bladder, D = ventral abdominal wall

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Figure 4. The bladder was held in situ with 2 traction 3-0 monofilament glycomer sutures for the opening, rinsing and extracorporeal suturing

Postoperative Care: : 

Postoperative Care: Figure 5. The open end of the urinary catheter was covered with an open protection sheet from plastic so that the catheter was allowed to drain spontaneously

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