Effective management of urolithiasis always involves two components: removal and prevention. Once uroliths are removed, preventative therapy should be initiated to reduce recurrence. Prevention should be based on quantitative urolith analysis performed by a veterinary urolith center. Removal of bladder and urethral stones is usually performed by open surgical procedures, such as cystotomy and urethrotomy; however, many uroliths can be managed by less invasive options including medical dissolution, voiding urohydropropulsion, laser lithotripsy and laparoscopic-assisted cystotomy.

Medical Dissolution

Medical dissolution is effective for some urolith locations and types. For dissolution to occur, uroliths must be surrounded by undersaturated urine. Urocystoliths and nephroliths are amenable to dissolution, whereas ureteroliths and urethroliths are not. Urolith types that may be medically dissolved include struvite, urates, and cystine. Calcium oxalate and silica uroliths cannot be medically dissolved. Medical dissolution of urocystoliths in male animals is associated with risk of urethral obstruction.

Struvite uroliths in dogs are usually infection-induced from infection by urease-producing bacteria.¹ Over 90% of struvite stones in dogs are caused by UTI with Staphylococcus or Proteus. Other urease-producing organisms that infrequently cause struvite uroliths include Pseudomonas, Klebsiella, and Corynebacterium urealyticum.¹ Urease-producing mycoplasmas, such as Ureaplasma urealyticum, may also cause struvite uroliths. While cats also develop infection-induced struvite uroliths, most struvite uroliths in cats are sterile and are considered a dietary-induced urolith. Medical dissolution of infection-induced struvite urocystoliths requires a combination of appropriate antimicrobial and calculolytic dietary therapy. Antimicrobial therapy must be given throughout the entire dissolution period. If antibiotic therapy is discontinued, then the infection rapidly recurs because viable bacteria are contained within struvite uroliths, and dissolution will not be successful. For sterile struvite uroliths in cats, antimicrobial therapy is not necessary. Antimicrobial and dietary therapy should continue approximately 1 month beyond radiographic resolution or until resolution of all uroliths on ultrasonography. Urine should be obtained by cystocentesis for urinalysis and culture 5 days after initiation of antimicrobial therapy. Urinalysis should reveal decrease of the urine pH to < 7.0 along with a negative urine culture. If the UTI is persistent, antimicrobial therapy should be changed on the basis of urine culture and sensitivity testing. Monitoring of dissolution therapy should include abdominal radiographs and urinalyses every 4 weeks. If bacteriuria, pyuria or inappropriate alkaline urine pH are present, the urine should be recultured. Prevention and treatment of recurrent UTI is critical for prevention of infection-induced struvite uroliths.

Voiding Urohydropropulsion

Voiding urohydropropulsion (VUH) is the removal of smaller cystoliths by inducing voiding, while the dog is positioned vertically so that cystoliths are voided.² General anesthesia facilitates complete urethral relaxation, preventing development of high intravesicular pressure that could cause iatrogenic trauma to the bladder wall. If the bladder is not distended with urine, it is distended with sterile saline via cystoscopy or urethral catheterization. The dog is positioned so that the spine is roughly 25 degrees caudal to a line perpendicular to the effects of gravity, such that a line drawn through the urethra into the bladder is approximately vertical. The bladder is agitated, and the urocystoliths settle in the trigone by gravity. The bladder is palpated and the intravesicular pressure is gradually increased by manual compression of the bladder to initiate a detrusor contraction. Once voiding begins, the bladder is compressed more firmly to attempt to maintain maximum urine flow to flush out the urocystoliths. The bladder is refilled with sterile saline through the cystoscope or a urinary catheter and the process is repeated until no urocystoliths are passed with the expelled fluid. Then, postprocedural radiographs are performed to confirm complete removal of the urocystoliths.

Retrograde Urohydropropulsion

Retrograde urohydropropulsion or retropulsion of urethral stones is preferred over attempts to push urethral stones back into the bladder using a urinary catheter. General anesthesia is usually required for retropulsion. If the bladder is overdistended, decompressive cystocentesis (using a 22-gauge needle connected to a 60-ml syringe by extension tubing and a three-way valve) is performed and fluid therapy should be administered to correct acid-base and electrolyte imbalances before anesthesia. The largest urinary catheter possible is gently passed into the distal urethra. An assistant compresses the urethra against the pubis via rectal palpation. A 2:1 mixture of sterile saline and sterile aqueous lubricant is rapidly flushed into the urethra, while compressing the urethral orifice around the catheter to prevent fluid leakage from the penis. An estimate of the total volume of flush required is 5 ml/kg body weight or 60 ml, whichever is less. Once the urethra begins to dilate, the rectal compression is released while flushing continues. If the urethroliths pass retrograde, the catheter is gently advanced to the trigone. If the calculus does not move retrograde, the degree of bladder distention should be assessed prior to repeating the procedure. Subsequent retrograde flushes are done with sterile saline alone, because the urethra is well lubricated. Also the use of smaller diameter syringes generate more force for dilating the urethra, so changing to a 20-ml syringe may improve success with subsequent flush attempts. Successful retrograde movement of the urethroliths should be confirmed by cystoscopy or radiography. The aim of retrograde urohydropropulsion is to flush the uroliths into the bladder, not to push them with the urinary catheter. Pushing against urethroliths with rigid polyethylene catheters may result in mucosal trauma, urethral perforation, or the catheter being passed round the urolith.

Laser Lithotripsy

Laser lithotripsy using the holmium YAG laser has replaced older techniques for fragmentation of bladder stones. The holmium laser energy is absorbed in 0.5 ml of fluid, allowing it to safely fragment uroliths within the urethra or bladder without damage to adjacent mucosa. Results with laser lithotripsy have been excellent with higher success rates in female dogs compared to male dogs.^3,4 Laser lithotripsy may be preferred over urethrotomy for removal of urethroliths.^3,4 Most dogs are clinically normal within 12 to 24 hours of the procedure. Male dogs with large stone burdens are not good candidates for removal of cystoliths by laser lithotripsy.^3,4 Likewise male dogs smaller than 5 kg are often too small for this technique. Therefore, we often utilize laparoscopic-assisted cystotomy for male dogs.

Management of Urolith Recurrence

Calcium oxalate uroliths tend to recur in most dogs, with recurrence rates of up to 50% within 3 years of initial diagnosis. Therefore, preventative measures, such as diet and medications, should be utilized to reduce the risk of recurrence. Dietary changes should be attempted, and the clinician should add medications if there is persistence of calcium oxalate crystalluria or recurrence of calcium oxalate urolithiasis. Increased water intake through feeding a canned diet or by adding water to the diet may be the most important recommendation to help prevent recurrence of calcium oxalate urolithiasis in dogs.

The commercial diets recommended to reduce the risk of calcium oxalate urolithiasis include Royal Canin Canine Urinary S/O diet and Hill's Prescription diets u/d® and g/d®. Alternative diets suggested for dogs with a history of pancreatitis, obesity, diabetes mellitus or hyperlipidemia are Royal Canin Canine Urinary S/O Moderate Calorie diet and Hill's Prescription diet w/d® with supplementation of oral potassium citrate to achieve a urine pH of 6.5–7.0.

Dietary therapy alone will not always prevent calcium oxalate urolith recurrence. Hydrochlorothiazide (2 mg/kg PO q12h) should be considered in dogs that have persistence of calcium oxalate crystalluria or recurrence of calcium oxalate urolithiasis despite diet therapy. Thiazide diuretics cause subclinical volume depletion resulting in increased proximal tubular reabsorption of sodium and calcium. Once dietary and hydrochlorothiazide therapy have been implemented, if calcium oxalate crystalluria is persistent or calcium oxalate uroliths recur, potassium citrate should also be given to effect to achieve a urine pH of 6.5–7.0 using a starting dose of 50–75 mg/kg PO q12h. The serum potassium should initially be monitored monthly during potassium citrate supplementation and the dose should be reduced if hyperkalemia occurs.

Because calcium oxalate uroliths commonly recur, appropriate radiographic surveillance should be scheduled every 6 months to document recurrences before the uroliths become too large for removal by VUH. If small recurrent urocystoliths are diagnosed, many recurrences may be managed by VUH. Although some dogs with recurrent calcium oxalate urocystoliths may be asymptomatic for extended periods of time, removal of urocystoliths is recommended to prevent urethral obstruction.

References

1.  Adams LG, Syme HM. Canine ureteral and lower urinary tract diseases. In: Textbook of Veterinary Internal Medicine. Ettinger SJ and Feldman BF (eds.), Elsevier, St. Louis; 2010:2086–2115.

2.  Lulich JP, et al. Voiding urohydropropulsion. Lessons from 5 years of experience. Vet Clin North Am Small Anim Pract. 1999;29:283–291.

3.  Adams LG, Berent AC, et al. Use of laser lithotripsy for fragmentation of uroliths in dogs: 73 cases (2005–2006). J Am Vet Med Assoc. 2008;232:1680–1687.

4.  Bevan JM, Lulich JP. Comparison of laser lithotripsy and cystotomy for the management of dogs with urolithiasis. J Am Vet Med Assoc. 2009;234:1286–1294.