Although nephroliths and ureteroliths are much less common than uroliths in the lower urinary tract, they are associated with potentially serious complications including obstruction, recurrent pyelonephritis and progressive chronic kidney disease (CKD). Calcium oxalate is the most common composition of nephroliths and ureteroliths in dogs and cats.^1,2 The second most common nephrolith composition in cats is dried solidified blood calculi.³ The second most common nephrolith composition in dogs is struvite, which occurs as an infection-induced urolith secondary to pyelonephritis with urease-producing bacteria such as Staphylococcus.² Nephroliths and ureteroliths are much more common than 20 years ago, especially calcium-containing upper-tract uroliths in cats. Nephroliths may obstruct the renal pelvis or ureter, predispose to pyelonephritis, or result in compressive injury of the renal parenchyma leading to progressive CKD. Nephroliths are considered "inactive" if they are not causing any of these complications. Inactive nephroliths do not require removal, but they should be monitored periodically by urinalysis, urine culture, radiography and ultrasonography. Indications for removal of nephroliths in dogs include obstruction, recurrent infection, progressive nephrolith enlargement, symptomatic nephroliths, and patients with nephroliths in a solitary functional kidney.⁴ The most common indication for removal of upper-tract uroliths in cats is obstructive ureteroliths.⁵

Diagnosis of Nephroliths and Ureteroliths

Diagnostic evaluation of patients with nephroliths and ureteroliths should include CBC, serum biochemistry profile, urinalysis, urine culture, systemic blood pressure, abdominal radiography and ultrasonography. If obstructive ureteroliths are suspected based on presence of renal pelvic and ureteric dilation, careful attempts to trace the ureter using ultrasonography may allow confirmation of ureteroliths. If ultrasonography fails to identify obstructive ureteroliths, the presence or absence of the ureteroliths can be confirmed by performing computerised tomography (CT) scans before and after the administration of contrast. If the renal pelvis is dilated to a minimum of 5 mm, then nephropyelocentesis with contrast injection with fluoroscopic monitoring may also be used for confirmation of obstructive ureteroliths. Nephropyelocentesis is most useful as the initial step in interventional procedures, such as ureteral stent placement, rather than simple diagnostic confirmation.

Treatment of Nephroliths and Ureteroliths

The treatment approach to nephroliths and ureteroliths is different for dogs and cats. Infection-induced struvite nephroliths are amendable to medical dissolution with dietary therapy and concurrent antibiotic treatment for the entire dissolution period. Struvite nephroliths are mainly seen in dogs and are currently uncommon in cats. Because most nephroliths and ureteroliths are composed of calcium oxalate, medical dissolution is not possible. Therefore, these uroliths must be removed by mechanical means, such as surgical removal or shock wave lithotripsy, or obstructive ureteroliths may be bypassed using ureteral stents. Surgical removal of nephroliths or ureteroliths by nephrotomy and ureterotomy respectively is associated with potential for many complications.⁵ This manuscript will not address these surgical techniques.

Extracorporeal Shock Wave Lithotripsy (ESWL)

The goal of ESWL treatment of nephroliths and ureteroliths is to achieve fragmentation of uroliths into small urolith fragments, which are small enough to pass down the ureter and then be voided spontaneously. Shock-wave lithotripsy treatment of nephroliths results in small, "passable" stone fragments in approximately 85% of dogs, whereas ESWL does not work effectively in cats. Given the small diameter of the feline ureteral lumen (0.4 mm), ESWL fragmentation usually results in urolith fragments that are too large to safely pass down the ureter into the urinary bladder. Furthermore, feline nephroliths are relatively resistant to fragmentation in comparison to similar canine nephroliths.⁶ Therefore, the author does not recommend ESWL for treatment of nephroliths and ureteroliths in cats.

Treatment with ESWL requires general anesthesia and accurate patient imaging to allow targeting of the uroliths. Patient positioning for targeting of ESWL therapy is guided by fluoroscopy. The shock wave generator must be effectively coupled to the patient to transmit the shock waves from the generator into the patient. The Dornier HM-3 is a water-bath lithotripter that has very good fragmentation efficiency. This lithotripter relies upon transmission of shock waves through a water-bath medium, requiring the patient to be partially submerged during treatment. The author has treated over 175 dogs using this lithotripter. High-energy shock waves are repeatedly passed through the region of the affected nephrolith or ureterolith. Dry lithotripters may also be used for ESWL treatment of dogs. Careful coupling of the SWL cushion to the patient's body is required to transmit the shock waves from the generator into the patient.

Several complications are possible from SWL treatment in dogs. The most common complication is transient ureteroliths that partially obstruct the ureter in approximately 10% of dogs treated for nephroliths. If the nephrolith fragments occlude the ureter for more than 24–48 hours, then ureteral stent placement may be required or a second ESWL treatment may be used to further fragment the uroliths into passable fragments. Lithotripsy treatment of the right kidney in small dogs often results in mild asymptomatic increases in amylase, lipase, and cPLI suggestive of pancreatic injury. Approximately 2% of dogs develop clinical acute pancreatitis following SWL treatment of the right kidney.

Ureteral Stents

Ureteral stents are effective for relief of ureteral obstruction by ureteroliths in both dogs and cats.⁵ Ureteral stents are soft polyurethane-type catheters that have a double pigtail design with multiple fenestrations along the length of the stent. Ureteral stents may be used as temporary indwelling stents to bypass ureteral obstructions, and then the stent may be removed cystoscopically once it is no longer needed. Ureteral stents are designed to allow urine to pass through the lumen of the stent. Stents also promote ureteral dilatation around the stent to allow urine to pass around the stent in most animals. In cats, ureteral stents may be left in place long-term to relieve ureteral obstruction by ureteroliths. Ureteral stent placement may be performed non-surgically via retrograde placement up the ureter during rigid cystoscopy in female dogs. The ureteral orifice is visualized within the urinary bladder trigone. An appropriate sized flexible tipped urologic guide wire is passed retrograde up the ureter under fluoroscopic guidance until the wire coils within the renal pelvis. The ureteral stent is then passed over the guide wire through the working channel of the cystoscope until the proximal end coils in the renal pelvis. Next, the guide wire is slowly withdrawn allowing the proximal end of the stent to reform the pigtail confirmation within the renal pelvis and the distal end to coil within the urinary bladder. Cystoscopic placement is much more difficult in male dogs and female cats. In larger male dogs, a temporary perineal urethrostomy approach to provide access for rigid cystoscopy may be used to facilitate ureteral stent placement using the same technique as described above for female dogs.

In cats, most ureteral stents are placed during open surgery through an antegrade technique. Less commonly, the stent may be passed retrograde up the ureter during cystotomy using the technique described for female dogs. Nephropyelocentesis is performed using a 22-gauge over-the-needle catheter to obtain access to the renal pelvis for antegrade stent placement. Then a 0.018"-urologic guide wire is inserted through the catheter into the renal pelvis and down the ureter to the urinary bladder with fluoroscopic monitoring. A 2.5-French dilator is then passed down the guide wire to dilate the ureter. A 2.5-French feline ureteral stent is loaded onto the guide wire and passed down the ureter immediately behind the ureteral dilator. Once the distal end of the stent is positioned within the urinary bladder, then the guide wire is removed and reinserted such that the soft flexible tip is passed up into the renal pelvis to allow the proximal end to be coiled to form a loop within the renal pelvis. Positioning of the proximal coil within the renal pelvis is confirmed by fluoroscopy. The guide wire is withdrawn allowing the ureteral stent to form a distal coil within the urinary bladder lumen.

Ureteral stents may be used long term to bypass obstructive ureteroliths in cats as an alternative to removal of the urolith. If renal function declines, indwelling ureteral stents should be monitored for migration and for stent encrustation with stone material, which can cause stent failure and recurrence of ureteral obstruction. Ascending infection may also occur resulting in lower UTI ascending into the kidneys with resultant pyelonephritis. Therefore, serial urinalyses and urine cultures are indicated in animals with a history of UTI.

Conclusions

Treatment of nephroliths and ureteroliths in dogs using ESWL has resulted in successful fragmentation of uroliths eliminating the need for open surgical procedures. Recent developments in placement of ureteral stents in cats have provided a new minimally invasive method of treatment of cats with obstructive ureteroliths.

References

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2.  Low WW, et al. Evaluation of trends in urolith composition and characteristics of dogs with urolithiasis: 25,499 cases (1985–2006). J Am Vet Med Assoc. 2010;236:193–200.

3.  Westropp JL, et al. Dried solidified blood calculi in the urinary tract of cats. J Vet Intern Med. 2006;20:828–834.

4.  Adams LG, Goldman CK. Extracorporeal shock wave lithotripsy. In: Nephrology and Urology of Small Animals. Polzin DJ, Bartges J (eds.). Blackwell Publishing, Ames, Iowa; 2011:340–348.

5.  Berent AC. Ureteral obstructions in dogs and cats: a review of traditional and new interventional diagnostic and therapeutic options. J Vet Emerg Crit Care. 2011;21:86–103.

6.  Adams LG, et al. In vitro evaluation of canine and feline urolith fragility by shock wave lithotripsy. Am J Vet Res. 2005;66:1651–1654.