Ureteral Stones

Urinary stones (urolithiasis) are a frequent cause of lower urinary tract disease in dogs and cats. Bladder stones (calculi) form when different minerals precipitate and crystals develop. Stone formation is influenced by multiple factors, and understanding these mechanisms matters for both treatment and prevention. In general, factors that promote stone development include:

• high concentrations of salts in the urine
• retention of these salts and crystals within the urinary tract for a period of time
• a urine pH that supports crystallization of salts
• a “framework” or scaffold on which crystals can form
• reduced levels of the body’s natural inhibitors of crystallization

The exact initiating sequence is not fully defined. High dietary intake of minerals and protein, together with very concentrated urine, may increase salt saturation in the urine. Certain diseases—such as bacterial infections of the urinary tract—may also raise urine salt concentrations.

Clinical signs vary depending on where stones are located. Most urinary stones are found in the urinary bladder or urethra; only a small proportion lodge in the kidneys or ureters. Stones can injure the urinary tract lining, causing inflammation, which may increase the risk of bacterial urinary tract infection (UTI).

Signs of bladder stones can include:

• blood in the urine
• straining to urinate
• frequent urination of small volumes
• abdominal discomfort
• urinary “accidents”

Stones can physically obstruct urine flow, producing a urinary blockage that needs immediate emergency care.

Signs of urethral stones can include:

• dribbling urine
• straining or adopting a urination posture with no urine produced

If your pet shows signs consistent with urinary obstruction, veterinary care should be sought immediately.

Stones can also become stuck in the ureter (the tube carrying urine from kidney to bladder), creating an obstruction that can cause severe kidney damage.

Signs of ureteral stones can include:

• abdominal discomfort
• decreased appetite
• lethargy
• vomiting
• blood in the urine

Figure 1
Your primary care veterinarian will usually recommend evaluation of blood and urine. Urinary obstruction can lead to heart rate and rhythm disturbances that may be detected on an ECG. When urinary stones are associated with infection, identifying the bacteria requires culture not only of urine, but also of the bladder lining and/or the urolith itself.

Imaging is used to evaluate the urinary tract. The most common tests are radiographs (x-rays) and ultrasound. Most—but not all—stones are visible on radiographs (Figure 1). If stones are poorly seen on plain x-rays, they may be detected by adding contrast material and/or gas into the urinary tract, typically via a urinary catheter.

Ultrasound is often very helpful for assessing kidneys, ureters, and bladder, but it has limited capacity for evaluating the urethra. A more recently used technique is nuclear scintigraphy, which provides a non-invasive assessment of renal blood flow and function.

Types of Urinary Stones

Stone types are defined by mineral composition. The most common types are struvite (magnesium ammonium phosphate), calcium oxalate, urate, cystine, and silica.

Struvite stones

Figure 2
In dogs, the most common mineral type is magnesium ammonium phosphate hexahydrate (struvite; Figure 2), accounting for about 50% of canine urinary stones. In cats, prevalence is around 30%. Breeds most often affected include Miniature Schnauzer, Miniature Poodle, Bichon Frise, and Cocker Spaniel. Urinary tract infection is a key contributor to struvite stone formation. Some bacteria enzymatically act on urea, raising urine pH and reducing struvite crystal solubility. Inflammation of the bladder lining increases organic debris in the urine, creating surfaces that facilitate crystallization.

Calcium oxalate stones

Figure 3
In dogs, calcium oxalate stones (Figure 3) make up about 35% of all stones; in cats, they account for about 50–70%. In cats, 70% of stones originating from the kidneys or ureters are diagnosed as calcium oxalate. Dog breeds most affected include Miniature and Standard Schnauzer, Miniature Poodle, Bichon Frise, Lhasa Apso, Yorkshire Terrier, and Shih Tzu. Commonly affected cat breeds include Burmese, Persian, and Himalayan.

The pathway leading to calcium oxalate stones is largely unclear, though there is some suggestion that normal post-feeding rises in urinary calcium could contribute. Reduced concentrations of natural inhibitors of crystal formation and higher dietary oxalate intake may also be involved.

Urate stones

Figure 4
Urate stones (Figure 4) in dogs may develop through two mechanisms. One involves increased excretion of ammonium biurate crystals in cases of portosystemic shunts. Dalmatians also commonly develop urate stones due to defective hepatic membrane transport of uric acid. These stones may be difficult to detect on plain x-rays but are readily identified with ultrasound.

Cystine stones

Figure 5
Excessive cystine excretion in urine is an inherited renal tubular transport disorder and is believed to be the main cause of cystine stones (Figure 5). When cystine concentration is high and urine is acidic (low pH), stones can form. Male Dachshunds aged 3–6 years are most commonly affected. These stones may be faintly visible on x-rays but are best seen with ultrasound.

Silicate stones

Figure 6
How silicate stones form is unknown; however, they may be linked to dietary intake of silicates, silicic acid, and magnesium silicate. Formation has been associated with diets high in corn gluten and soybean hulls, which contain substantial silicates. German Shepherds, Old English Sheepdogs, and Golden and Labrador Retrievers are most commonly affected.

Figure 7

Medical management

Calcium oxalate, urate, cystine, and silicate stones cannot be dissolved and therefore require surgical removal. Struvite stones can sometimes be dissolved using a commercially prepared diet (for example, Hill’s S/D) formulated specifically for dissolution; this diet is not intended for long-term feeding.

Medical management of urinary obstruction

When stones lodge in the urethra and obstruct urine flow, this is an urgent emergency. The blockage must be relieved to allow bladder emptying, or the bladder must be decompressed by cystocentesis (placing a needle through the abdominal wall into a distended bladder and removing urine with a syringe).

Urethral stones can often be dislodged and pushed back into the bladder by flushing through a urinary catheter—this method is called retrograde urohydropulsion (Figure 7).

Surgical treatments

The surgical approach depends on stone location:

• Cystotomy: removal of stones from the urinary bladder
• Urethrotomy: removal of stones from the urethra
• Urethrostomy: creation of a permanent opening to allow stones to pass without causing obstruction; in male cats, a common version is perineal urethrostomy (P.U.)

Laser lithotripsy is a minimally invasive stone-removal method that has been used successfully, but it requires advanced laser and endoscopic equipment. Sometimes it is performed through the urethra; in other cases, a small incision is made into the bladder, and an endoscope plus laser fiber are introduced through that port into the bladder and upper urethra. Laser lithotripsy is most useful for urethral stones or small numbers of bladder stones.

Kidney stones may be removed via nephrotomy, though many do not require surgical removal. If ureteral stones obstruct urine flow, they may be removed by ureterotomy. As an alternative, ureteral stenting may be performed so urine can pass from kidney to bladder while bypassing the obstruction; subcutaneous ureteral bypass (SUB) systems are increasingly used for this purpose. Many primary veterinarians perform cystotomy, but cases requiring urethrotomy, urethrostomy, ureterotomy, ureteral stenting, or nephrotomy are often referred to an ACVS board-certified veterinary surgeon.

Postoperative care depends on the stone location and the procedure performed. In general, possible complications include infection, stone recurrence, stricture formation (especially after urethral or ureter surgery), and blood in the urine. An E-collar may be needed to prevent self-trauma at the surgical site. After surgery, monitor for normal urination as well as return of appetite and energy. A small amount of blood in the urine is common for the first one to two weeks after urinary surgery.

Many urinary stones recur at high rates.

Prevention

• Struvite stones: diet guidance and frequent urine monitoring are important; treating any UTI is essential for prevention.
• Calcium oxalate stones: diet guidance and frequent urine monitoring are recommended.
• Cystine stones: diet guidance and frequent urine monitoring are recommended; urine pH should be maintained above 7.5. A dietary additive to raise urine pH may be suggested.
• Urate stones: diet guidance and frequent urine monitoring are recommended; pets with portosystemic shunts should have that condition managed medically or surgically. Dalmatians may benefit from a medication that alters hepatic metabolism. Urate stone prevention is possible in 80% of dogs and 95% of cats.
• Silicate uroliths: diet guidance and frequent urine monitoring are recommended; diets excessively high in silicates should be avoided.