Overview of Urolithiasis in Small Animals

Mineral solutes contained in the urine can precipitate to form crystals. These crystals can aggregate and grow to macroscopic size, at which point they are known as uroliths (see Samoyed and Collie renal calculus images).

Renal calculus, Samoyed

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Courtesy of Ontario Veterinary College.

Renal calculus, Collie

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Courtesy of Ontario Veterinary College.

The mineral content of the urine relative to the urine concentration, the presence of stone promoters or inhibitors, and urine pH can all affect how likely it is for a particular stone type to develop.

Regardless of the underlying mechanisms, uroliths are not produced unless the urine concentrations of urolith-forming constituents are sufficiently high and the transit time of crystals in the urinary tract is prolonged. For selected stones (eg, struvite, cystine, urate), other favorable conditions for crystallization (eg, proper pH) must also exist. These criteria can be affected by urinary tract infection, diet, intestinal absorption of stone components, urine volume, frequency of urination, therapeutic agents, and genetics.

Many urinary stones contain only one mineral type; others are mixed or compound uroliths. Compound uroliths have a core corresponding to urine characteristics that were present when the urolith originally formed, and the outer layers correspond to more recent conditions. 

Minerals found in uroliths have a chemical name and often a mineral or crystal name (see the table Urolith Types).

Clinical signs associated with urolithiasis develop because macroscopic uroliths that form in the lower urinary tract interfere with urine flow and/or irritate the mucosal surface. Signs can include dysuria, hematuria, and stranguria. By themselves, microscopic crystals seldom cause clinical signs.

Mechanical obstruction of the urethra (ie, obstructive uropathy) can occur as a result of urethroliths. Urethral obstruction is an emergency.

Nephroliths are often not associated with clinical signs unless pyelonephritis exists concurrently or they pass into the ureter. 

Ureteral obstruction can produce vomiting, lethargy, and/or flank and renal pain, particularly if there is acute total obstruction with distention of the renal capsule (see Acute Kidney Injury in Dogs and Cats). The only clinical sign associated with unilateral ureteroliths might be pain, which can be difficult to detect in dogs and cats. If these initial signs of ureteral obstruction do not lead to a diagnosis, unilateral ureteral obstruction can result in hydronephrosis with loss of function of the ipsilateral kidney. 

Ureteroliths can also precipitate a uremic crisis in cats with previously compensated chronic kidney disease. Because clinical signs of renal dysfunction are generally not apparent until two-thirds or more of total functional renal parenchyma is lost, clinical signs might not be evident except in the following situations: both ureters are obstructed, there is contralateral chronic kidney disease, or a renal infection develops. 

• Radiography

• Urinalysis

• Quantitative stone analysis

Unilateral ureteroliths might be identified incidentally during abdominal imaging studies or surgery. 

Uroliths are not typically identified on physical examination; however, an enlarged, firm, and painful kidney suggests a ureteral obstruction. Rectal palpation might identify stones located in the pelvic urethra. Large or numerous cystoliths can sometimes be appreciated on abdominal palpation.

Because multiple uroliths can be present throughout the urinary tract, a complete radiographic examination of the tract is indicated, taking care to include the entire urethra in the images. Radiodense calculi > 3 mm in diameter are usually visible on radiographs (see struvite bladder stones and renal calculi images). Urate, and occasionally cystine, uroliths might be radiolucent, requiring contrast radiography or ultrasonography to confirm their presence. 

Struvite bladder stones, radiograph, Dachshund

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Courtesy of Ontario Veterinary College.

Renal calculi, radiograph, dog

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Courtesy of Ontario Veterinary College.

Urinalysis is an important part of the diagnostic evaluation of any animal with confirmed or suspected uroliths. It can help identify other causes of lower urinary tract signs and/or the presence of complicating bacterial infections. Characteristics of the urine, such as pH and urine specific gravity, can give an idea of the type of urolith present or enable monitoring of dissolution or prevention protocols. 

Crystalluria can also help with the determination of urolith type. However, uroliths can be present without crystalluria, and the presence of crystalluria alone does not indicate urolithiasis. Furthermore, fresh urine samples should be examined, because changes in temperature and pH over time after urine collection can result in precipitation of crystals that are of no clinical importance.

Determination of urinary stone composition is important when assessing the potential for medical dissolution and for preventing or discouraging recurrence after stone removal. Quantitative mineral analysis can determine the content of a stone, if available (obtained via surgery, voiding urohydropropulsion, or cystoscopy or otherwise passed).

• Medical stone dissolution

• Mechanical removal

• Lithotripsy

Multiple urolith types (struvite, urate, cystine) can be medically dissolved by altering the composition of the urine to make it unfavorable for precipitation of stone components. Ideal circumstances for dissolution include the patient's tolerance of the diet and/or medications needed for dissolution of the specific stone type, excellent client compliance, and location of the stones where they can be continually bathed in the altered urine environment (eg, cystoliths). 

Ureteroliths, nephroliths, and urethroliths must be removed via means other than dissolution, with a few exceptions. Mechanical removal of uroliths can be accomplished via multiple mechanisms, including surgery and minimally invasive techniques.

Cystoliths can be removed surgically (cystotomy), via less invasive surgically assisted methods (such as percutaneous cystolithotomy, intracorporeal laser lithotripsy, or cystoscopic-assisted retrieval methods), or via voiding urohydropropulsion (VUH).

Nonsurgical techniques for removing cystoliths are preferred when possible, because of the risk of suture-induced urolith recurrence. However, minimally invasive techniques such as laser lithotripsy or cystoscopic retrieval methods require specialized equipment and expertise and might not be accessible or appropriate in all cases. 

VUH can be used to remove cystoliths that are small enough to easily pass through the urethra, and it requires no specialized equipment. In general, stones < 5 mm in dogs > 10 kg, 1–3 mm in small male dogs, and < 5 mm in female cats are likely to be removed successfully using VUH. VUH might also be successful for voiding larger stones (5–10 mm) in large-breed female dogs. Male cats are uncommonly candidates for VUH because of their small urethral diameter; however, stones < 1 mm can be successfully voided.

VUH is performed as follows:

1. The animal is placed under general anesthesia, and a urinary catheter is passed into the urinary bladder using sterile technique. 

2. The bladder is drained of urine and subsequently filled with sterile saline solution (0.9% NaCl) to a volume at which the bladder is maximally distended, as determined by palpation.

3. The animal is then held upright, and the catheter is removed, taking care not to allow urine to escape the bladder. The opening to the vulva or prepuce is occluded digitally to prevent the escape of urine.

4. The bladder is palpated and vigorously ballotted to suspend the small stones, and then it is forcibly expressed to capture the expressed urine and retrieve the stones.

Imaging should be repeated after the procedure to confirm that the stones have been successfully voided. 

VUH is a safe procedure when performed correctly; however, risks include transient hematuria, urethral obstruction, and, rarely, bladder rupture. If urethral obstruction occurs during VUH, stones should be retropulsed into the bladder and removed via alternative means.

For urethroliths, laser lithotripsy or cystoscopic retrieval methods (using grasper or stone basket) are ideal. When this is not possible (there is no access and/or the patient is a male cat or a very small dog with a urethra too small for cystoscopy), retrograde flushing of the ureteroliths into the urinary bladder for removal via cystotomy is preferred as an alternative to surgery directly on the urethra (urethrotomy/urethrostomy).

Ureteroliths can be removed surgically. However, the rate of complications associated with ureteral surgery and the high incidence of obstruction recurrence due to additional stones or stricture, means that placement of a ureteral stent or subcutaneous ureteral bypass system might be more appropriate. 

Removal of nephroliths is not commonly indicated. However, when nephroliths are problematic (causing recurrent infection, pain, etc), minimally invasive removal—via extracorporeal shock wave lithotripsy—is recommended.

• Lulich JP, Berent AC, Adams LG, Westropp JL, Bartges JW, Osborne CA. ACVIM Small Animal Consensus Recommendations on the Treatment and Prevention of Uroliths in Dogs and Cats. J Vet Intern Med. 2016;30(5):1564-1574.

• Bartges JW, Callens AJ. Urolithiasis. Vet Clin North Am Small Anim Pract. 2015;45(4):747-768.

• Lulich JP, Osborne CA, Sanderson SL, et al. Voiding urohydropropulsion: lessons from 5 years of experience. Vet Clin North Am Small Anim Pract. 1999;29(1):283-291, xiv.

• Also see pet owner content regarding urinary stones in dogs and cats.