Definition

Formation of polycrystalline concretions (i.e., uroliths, calculi, or stones) composed of MAP (a.k.a. as struvite) in the urinary tract.

Pathophysiology

Systems Affected

Renal/Urologic

Genetics

• The high incidence of struvite uroliths in some breeds of dogs such as miniature schnauzers suggests a familial tendency. We hypothesize that susceptible miniature schnauzers inherit some abnormality of local host defenses of the urinary tract that increases their susceptibility to urinary tract infection.
• Sterile struvite uroliths were found in a family of English cocker spaniels.

Incidence/Prevalence

Struvite uroliths account for approximately 40% of stones affecting the canine lower urinary tract and 33% of stones affecting the upper urinary tract.

Geographic Distribution

Ubiquitous

Signalment

Signs

Causes

• Urinary tract disorders that predispose to infections with urease-producing bacteria, fungal pathogens, or ureaplasma in patients whose urine contains a large quantity of urea.
• Specific causes of sterile struvite uroliths are unknown.

Risk Factors

• Exogenous or endogenous exposure to high concentrations of glucocorticoids predispose to bacterial urinary tract infection.
• Abnormal retention of urine.
• Alkaline urine decreases the solubility of struvite.

Differential Diagnosis

• Uroliths mimic other causes of pollakiuria, dysuria, hematuria, and/or outflow obstruction.
• Differentiate from other types of uroliths by signalment, rectal exam, urinalysis, urine culture, radiography, and quantitative analysis of voided or retrieved uroliths.

CBC/Biochemistry/Urinalysis

• Complete outflow obstruction can cause post-renal azotemia (e.g., high BUN, creatinine, and phosphorus).
• Magnesium ammonium phosphate crystals typically appear as colorless, orthorhombic (having three unequal axes intersecting at right angles), coffin-like prisms. They may have three to six or more sides and often have oblique ends.

Other Laboratory Tests

• Quantitative bacterial culture of urine, preferably collected by cystocentesis.
• Bacterial culture of inner portions of infection-induced struvite uroliths.
• Quantitative mineral analysis of uroliths retrieved during voiding, by voiding urohydropropulsion, by aspiration into a urinary catheter, or by cystoscopy.

Imaging

• Struvite uroliths are radiodense and may be detected by survey radiography.
• Ultrasonography-can detect uroliths, but provides no information about their density or shape.
• Determine precise location, size, and number of uroliths; the size and number are not a reliable index of probable efficacy of dissolution therapy.

Appropriate Health Care

• Retrograde urohydropropulsion to move urethral stones and reestablish urethral patency, voiding urohydropropulsion to eliminate bladder stones.
• Shock-wave lithotripsy and/or surgery require short periods of hospitalization.
• Medical dissolution of struvite uroliths is an outpatient strategy.

Diet

• Infection-induced and sterile struvite urocystoliths and nephroliths may be dissolved by feeding a calculolytic food (Hill's Prescription Diet Canine s/d).
• Continue calculolytic diet therapy for 1 month beyond survey radiographic evidence of urolith dissolution.
• Avoid use of the protein-restricted diet in patients with protein-calorie malnutrition. The calculolytic diet is designed for short-term ( weeks to months) dissolution therapy, rather than long-term (months to years) prophylactic therapy. If used, monitor the patient for evidence of protein malnutrition. Avoid prolonged feeding of the calculolytic diet to immature dogs.

Client Education

• If dietary management is used, limit access to other foods and treats.
• Short-term treatment with a calculolytic food and administration of antibiotics has been effective in dissolving struvite uroliths.
• Comply with dosage schedule for antibiotic and diet therapy.

Surgical Considerations

• Ureteroliths cannot be dissolved; consider surgery or shock-wave (ESWL) lithotripsy for persistent ureteroliths associated with morbidity.
• Urethroliths cannot be medically dissolved; consider voiding urohydropropulsion if the urethroliths are likely to pass through the entire length of the urethra. Alternatively, consider lithotripsy or move urethroliths into the bladder by retrograde urohydropropulsion.
• Immovable urethroliths may require urethrotomy or urethrostomy.
• Nephroliths causing outflow obstruction or associated with non-functioning kidneys cannot be dissolved medically.
• Consider surgical correction if uroliths are obstructing urine outflow and/or if correctable abnormalities predisposing to recurrent urinary tract infection are identified by radiography or other means.

Drug(s)

• Dietary dissolution of infection-induced urocystoliths or nephroliths requires oral administration of appropriate antibiotics, chosen on the basis of quantitative bacterial culture and antimicrobial susceptibility tests. Give antibiotics at therapeutic dosages until there is no radiographic evidence of uroliths and there is laboratory confirmation of eradication of urinary tract infection.
• Patients with infection-induced struvite urocystoliths associated with persistent bacterial infection with urease-producing bacteria and refractory to dietary and antibiotic dissolution may be given AHA (Lithostat, Mission Pharmacal, 12.5 mg/kg PO q12h). AHA is a urease inhibitor that blocks hydrolysis of urea to ammonia.

Contraindications

AHA is teratogenic and should not be given to pregnant dogs.

Precautions

• Diet-induced polyuria will reduce the concentration of antimicrobial drugs in urine; consider this fact when calculating antimicrobic dosages.
• Prolonged administration of AHA at higher doses induces abnormalities in bilirubin metabolism in some dogs.
• Higher doses of AHA may induce a reversible hemolytic anemia.

Patient Monitoring

Monitor rate of urolith dissolution at monthly intervals by urinalysis, urine culture, ultrasonography, and/or survey or contrast radiography.

Prevention/Avoidance

• Infection-induced struvite urolithiasis may be prevented by eradicating and controlling infections by urease-producing bacteria.
• Recurrent sterile struvite uroliths may be prevented by use of acidifying, magnesium-restricted diets (Hill's Prescription Diet Canine c/d) or urine acidifiers.
• Monitor patients whose urine has been acidified for calcium oxalate crystalluria. Change management protocol if persistent calcium oxalate crystalluria develops.
• In patients at risk for both struvite and calcium oxalate crystalluria, focus on prevention of calcium oxalate uroliths-struvite uroliths can be medically dissolved if they recur; recurrent calcium oxalate uroliths cannot be dissolved.

Possible Complications

• Benefits and risks are associated with feeding struvitolytic diets. Not all patients qualify for dietary medical management, including those with (1) abnormal fluid accumulation, (2) azotemic primary renal failure, and (3) predispositions to pancreatitis (especially miniature schnauzers with hyperlipidemia).
• Urocystoliths may pass into and obstruct the urethra of male dogs, especially if the patient is persistently dysuric. Urethral obstruction may be managed by retrograde urohydropropulsion or lithotripsy.
• Dysuria may be minimized by antimicrobic treatment of bacterial urinary tract infections and oral administration of anticholinergic drugs.
• Dogs that do not consume their daily requirement of the calculolytic diet may develop varying degrees of protein calorie malnutrition. This can be prevented by proper calculation of the daily dietary requirement and adjustment in the quantity of food fed on the basis of serial physical examination.
• Diet-associated polyuria will result in voiding increased urine volume. This may be associated with varying degrees of urinary incontinence in neutered female dogs with a predisposition to estrogen-responsive incontinence.

Expected Course and Prognosis

• In our hospital, the mean time for dissolution of infection-induced urocystoliths in dogs was approximately 3 months (range 2 weeks–7 months). The mean time for dissolution of infection-induced struvite nephroliths in dogs was 6 months (range 2–10 months). The mean time for dissolution of sterile struvite urocystoliths in dogs was 6 weeks (range 4–12 weeks).
• Compliance with dietary recommendations is suggested by a reduced concentration of urea in serum (approximately 10 mg/dL), and a low urine specific gravity (1.004–1.014).
• If uroliths increase in size during dietary management or do not begin to decrease in size after approximately 4–8 weeks of appropriate medical management, alternative methods should be considered. Difficulty in inducing complete dissolution of uroliths by creating urine under-saturated with struvite should prompt consideration that (1) the wrong mineral component was identified, (2) the nucleus of the uroliths has a different mineral composition than other portions of the urolith, and (3) the owner is not complying with therapeutic recommendations.

Associated Conditions

Any disease that predisposes to bacterial urinary tract infection.

Age-Related Factors

Infection-induced struvite is the most common form of urolith in immature dogs. The uroliths develop as a result of microbial UTI.

Pregnancy/Fertility/Breeding

• AHA is teratogenic.
• The calculolytic food is not designed to sustain pregnancy.

Synonyms

• Phosphate calculi
• Infection stones
• Urease stones
• Triple-phosphate stones

Abbreviations

• AHA = acetohydroxamic acid
• ESWL = extracorporeal shock wave lithotripsy
• MAP = magnesium ammonium phosphate
• UTI = urinary tract infection

Client Education Handout Available Online