Definition

Formation of calcium oxalate (CaOx) uroliths within the urinary tract and associated clinical conditions (e.g., urinary obstruction, idiopathic hypercalcemia, chronic kidney disease).

Pathophysiology

Presence of hypercalciuria, hyperoxaluria, hypocitraturia, and defective crystal growth inhibitors.

Systems Affected

Renal/Urologic

Incidence/Prevalence

In dogs, calcium oxalate accounts for approximately 42% of the uroliths removed from the lower urinary tract and 45% of those removed from the upper urinary tract. In cats, calcium oxalate accounts for approximately 42% of the uroliths removed from the lower urinary tract and 90% of those retrieved from the upper urinary tract.

Signalment

Signs

Causes

See “Pathophysiology”

Risk Factors

• Oral calcium supplements given independent of meals.
• Feeding acidifying foods that promote formation of acidic urine (less than a pH of 6.6 in dogs and 6.25 in cats) was associated with CaOx urolithiasis. In normal cats alkaline urine was associated with the lowest saturation for CaOx.
• Excessive dietary protein, sodium (greater than 1.2% DMB or 350 mg/100 kcal) and vitamin D promote hypercalciuria.
• Additional dietary oxalate (e.g., chocolate and peanuts) and ascorbic acid promote hyperoxaluria.
• Exogenous or endogenous exposure to a high concentration of glucocorticoids and furosemide promote hypercalciuria
• Pyridoxine (vitamin B₆)-deficient diets (e.g., homemade) promote hyperoxaluria.
• Consumption of dry diets is associated with a higher risk for calcium oxalate urolith formation than consumption of high-moisture canned diets.

Differential Diagnosis

• Other common causes of hematuria, dysuria, and pollakiuria, with or without urethral obstruction, include urinary tract infection, urinary tract neoplasia, and idiopathic feline lower urinary tract disease.
• Other common radiodense uroliths include those composed of magnesium ammonium phosphate, calcium phosphate, cystine, and silica.

CBC/Biochemistry/Urinalysis

• Urinary tract mineralization may resemble uroliths.
• Results usually unremarkable.
• Urinary sediment evaluation may reveal calcium oxalate crystals, but absence of crystalluria does not exclude uroliths as a possibility.
• Hypercalcemia (rare in dogs, more common in cats) should be further evaluated to determine its cause and contribution to urolith formation.

Other Laboratory Tests

Quantitative mineral analysis of uroliths

Imaging

• Calcium oxalate uroliths 2 mm in diameter are radio-opaque and easily detected by survey radiography.
• Intravenous urography, contrast pyelography, or ultrasonography is required to verify ureteral obstruction.

Appropriate Health Care

• CaOx uroliths are not amenable to medical dissolution.
• Small stones that can pass through the urethra (<3 mm in most dogs >5 kg) should be removed by voiding urohydropropulsion or basket retrieval. Percutaneous cystolithotomy or routine cystotomy can be used to remove stones in smaller dogs and male cats.
• Consider laser lithotripsy, percutaneous cystolithotomy, or routine cystotomy to remove larger stones from the urinary bladder.
• Urethral surgery is discouraged. Retrograde urohydropropulsion is an effective procedure to flush urethral stones back into the urinary bladder prior to their removal.
• Persistently obstructed ureters require urgent intervention to minimize progressive kidney damage. If fluid therapy, -adrenergic blocking agents, and diuretics do not relieve the obstruction in 1-3 days, consider ureteral stents in dogs and subcutaneous ureteral access devices in cats to preserve kidney function. When performed by trained surgeons, ureterotomy may be considered for dilated ureters containing very few stones.
• Removal of unobstructing nephroliths is usually not necessary.

Activity

Reduce during the period of tissue repair after surgery.

Diet

• No reports of dissolution of CaOx uroliths with therapeutic foods. Epidemiologic studies support feeding high moisture foods that promote formation of less acidic urine (pH >6.3-6.6) to minimize formation of calcium oxalate.
• Hypercalcemia in cats without evidence of hyperparathyroidism or malignancy is sometimes minimized by feeding Hill's Prescription Diet Feline w/d. Management of idiopathic hypercalcemia with bisphosphonates and other drugs that inhibit osteoclast function are controversial; there use should be determined on a case-by-case basis weighing their risks and advantages.

Client Education

• Urolith removal does not alter the factors responsible for urolith formation; eliminating or minimizing risk factors is necessary to minimize recurrence.
• Approximately 50% of dogs reform uroliths within 2 years; a third of cats reform uroliths in 2 years.
• Patients with hypercalcemia typically reform uroliths at a faster rate.

Surgical Considerations

• Consider surgical removal of lower tract uroliths that cannot be removed by minimally invasive procedures (e.g., voiding urohydropropulsion, basket retrieval, intracorporeal laser lithotripsy, percutaneous cystolithotomy).
• Avoid performing disfiguring urethrostomies and urethrotomies by using retrograde urohydropropulsion to flush urethroliths into the bladder or using lithotripsy to fragment urethroliths.
• Shock wave lithotripsy is an alternative to surgery for removal of nephroliths and ureteroliths in dogs.
• To minimize urolith reformation over suture nidus, minimize surgical procedures to remove uroliths and use absorbable suture and suture patterns that minimize suture exposure in the lumen of the urinary bladder.
• Only surgeons trained in ureteral surgery should attempt ureterolith removal. However, in lieu of surgery consider ureteral stents in dogs and subcutaneous ureteral access devices in cats.
• Consider parathyroidectomy for patients with primary hyperparathyroidism and hypercalcemia.

Drug(s) Of Choice

No available drugs effectively dissolve calcium oxalate uroliths.

Precautions

Steroids and furosemide promote calciuria.

Patient Monitoring

• Post-surgical radiographs are essential to verify complete urolith removal.
• To prevent the need for repeated surgery, evaluate abdominal radiography every 3–5 months to detect urolith recurrence early. Small uroliths are easily removed by voiding urohydropropulsion or stone basket retrieval.

Prevention/Avoidance

• Even with appropriate contemporary therapy, CaOx urolith recurrence is common (up to 50% in dogs and 33% in cats in 2 years). Therefore, regular monitoring and compliance check-ins are essential to adjust therapy to extend the interval between recurrences.
• Only recommend high moisture foods (e.g., can, loaf, gravies). Feeding dry foods, combining dry and wet foods, or adding water to dry food is usually ineffective in maintaining consistent low urinary concentrations (specific gravity <1.020 in dogs and <1.030 in cats) of calculogenic minerals.
• Feeding high-sodium foods ( 350 mg/100 kcal) should not be recommended as a substitute for feeding high-moisture foods. Their efficacy to promote low urine specific gravity appears to be short-lived (3-6 months) and their use is contraindicated in dogs and cats with kidney disease.
• Avoid feeding diets that promote urine acidification. Diets that promote formation of acidic urine in dogs (pH <6.6) and cats (pH <6.25) were associated with CaOx uroliths. A linear increase in urine pH was associated with a linear decrease in urine CaOx saturation in normal cats.
• Commercially manufactured diets have been designed to prevent CaOx recurrence, but they may not be an ideal for all patients.
• Hills Prescription diet u/d for dogs has been shown to decrease calcium and oxalate excretion in dogs with calcium oxalate urolithiasis. This food has lower levels of sodium and protein, and promotes a neutral to alkaline urine. Although it has passed AFCO feeding trials, some consider that the protein quantity is too low. If this is the case consider feeding &half u/d and &half of a canned moderate protein senior food that does not acidify the urine (e.g., Prescription diet g/d). Because u/d is higher in fat, dogs with hereditary hyperlipidemia (e.g., some miniature schnauzers) may benefit from a similar feeding mixture.
• Hills Prescription diet w/d has been recommended for dogs with CaOx urolithiasis and fat/lipid intolerance or fat/lipid responsive diseases (e.g., dogs with a history of pancreatitis). Because this diet promotes formation of acidic urine, administer potassium citrate to promote a more favorable urine pH (>6.5).
• Royal Canin SO has been shown to decrease CaOx relative supersaturation in dogs with calcium oxalate uroliths, but the urine samples were collected by owners in this study, raising the suspicion of incomplete sample collection. Because this diet promotes acidic urine, concomitant administration of potassium citrate is necessary to achieve a more favorable urine pH (>6.5). We do not recommend dry foods to prevent CaOx urolith.
• Avoid supplements with vitamins C and D.
• Reevaluate patients in 2–4 weeks after initiation of diet therapy to verify appropriate reduction in specific gravity (<1.020 for dogs and <1.030 for cats), appropriate urine pH ( 6.5), and amelioration of crystalluria. Do not use inappropriately collected or stored urine samples (e.g., urine collected by owners, refrigerated, or contaminated with debris) to monitor therapeutic efficacy. To promote less-concentrated urine strongly recommend canned or gravy formulations of food or add additional water to all types of food. If urine is acidic, consider administration of potassium citrate (75 mg/kg PO q12h); adjust dosage to achieve a pH between 6.5 and 7.5. Potassium citrate medications formulated with cranberry are not recommended because cranberry is a source of vitamin C; vitamin C supplementation should be discouraged because of its ability to increase urine oxalate.
• Vitamin B₆ (2–4 mg/kg PO q24–48h) may help minimize oxalate excretion, especially for animals fed homemade or pyridoxine-deficient diets.
• If dietary changes are inadequate at slowing the rate of recurrence, consider hydrochlorothiazide diuretics (dog, 2 mg/kg and cat, 1 mg/kg, q12-24h).
• If the patient is hypercalcemic, correct underlying cause if possible. Consider either Prescription Diet w/d or c/d multicare for cats with idiopathic hypercalciuria; bisphosphonates should be used cautiously.
• Oxalobacter formigenes is an intestinal bacterium that ingests oxalate as its sole nutrient. By metabolizing dietary oxalate in the intestine, less oxalic acid is available for absorption and less is excreted in urine. To preserve healthy populations of intestinal Oxalobacter, avoid indiscriminant or prolonged use of antimicrobics.

Possible Complications

• Urocystoliths can pass into and obstruct the urethra in male dogs and cats, especially if the patient is dysuric.
• Dogs that do not consume their daily requirement of some urolith prevention foods may develop various degrees of protein calorie malnutrition.
• Diet-associated hyperlipidemia develops in some patients consuming foods with higher fat content. Miniature schnauzers with hereditary hyperlipidemia are predisposed to pancreatitis when consuming some prevention foods; Hill's Prescription Diet Canine w/d can be used as an alternative. This diet should be supplemented with potassium citrate as needed to maintain a urine pH between 6.5 and 7.5.

Expected Course and Prognosis

• Approximately 50% of dogs and 33% of cats reform uroliths in 2 years. Treatment to minimize recurrence is helpful.
• Patients with persistent hypercalcemia typically reform uroliths at a faster rate.

Associated Conditions

Conditions predisposing to hypercalciuria (e.g., hyperadrenocorticism, acidemia, hypervitaminosis D, and hyperparathyroidism) or hyperoxaluria (e.g., vitamin B₆ deficiency, hereditary hyperoxaluria, and ingestion of chocolate and peanuts).

Age-Related Factors

Rare in young (<1 year old) animals

Pregnancy/Fertility/Breeding

Diets used to prevent calcium oxalate uroliths are not appropriate.

Synonyms

Oxalate urolithiasis

See Also

Crystalluria

Client Education Handout Available Online