Definition

Struvite uroliths and struvite urethral plugs have physical and etiopathogenic differences; thus, these terms should not be used as synonyms. Struvite uroliths are polycrystalline concretions composed primarily of magnesium ammonium phosphate and small quantities of matrix. Struvite feline urethral plugs commonly are composed of large quantities of matrix mixed with crystals (especially magnesium ammonium phosphate). Some urethral plugs are composed primarily of organic matrix, sloughed tissue, blood, and/or inflammatory reactants.

Pathophysiology

• See Urolithiasis, Struvite-Dogs.
• The most commonly encountered form of naturally occurring feline urethral plugs contains relatively large quantities of matrix in addition to minerals, especially struvite. Risk factors associated with formation of MAP crystals contained in urethral plugs are similar to those associated with formation of struvite uroliths. Prevention or control of these risk factors should minimize the recurrence of the struvite component of urethral plugs. Specific causes and composition of urethral plug matrix have not yet been classified. One hypothesis is that plug matrix follows the onset of urinary tract infections or other inflammatory disorders such as idiopathic cystitis.

Systems Affected

Renal/Urologic-upper and lower urinary tract

Incidence/Prevalence

• The prevalence of feline struvite uroliths submitted to the Minnesota Urolith Center declined from 78% in 1981 to 33% in 2002, but then increased to 48% in 2005. Struvite uroliths comprised 46% of feline uroliths in 2013. In comparison, uroliths comprised primarily of calcium oxalate increased from approximately 2% in 1981 to 55% in 2002, but then decreased to 40% in 2005. Calcium oxalate comprised 41% of feline uroliths in 2013. These dramatic changes in the frequency of occurrence of the mineral composition of feline uroliths parallel changes in the composition of manufactured diets.
• Currently, struvite makes up approximately 50% of all types of uroliths in the feline lower urinary tract. Of these, 95% are sterile.
• Struvite has been detected in approximately 8% of feline nephroliths.
• Since 1981, struvite has remained the most common (∼90%) mineral in matrix-crystalline urethral plugs.

Signalment

Signs

Causes

See “Pathophysiology”

Risk Factors

• For formation of sterile struvite uroliths, risk factors include mineral composition, energy content, and moisture content of diets; urine-alkalinizing metabolites in diets; quantity of diet consumed; ad libitum versus meal-feeding schedules; formation of concentrated urine; and retention of urine.
• Probable risk factors for infection-induced struvite urolithiasis-include urinary tract infection with urease-producing microbial pathogens, abnormalities in local host defenses that allow bacterial urinary tract infections (including perineal urethrostomies), and the quantity of urea (the substrate of urease) excreted in urine.
• The normal small diameter of the distal urethra of male cats predisposes them to obstruction with plugs and urethroliths.

Differential Diagnosis

• Uroliths mimic other causes of pollakiuria, dysuria, periuria, hematuria, and/or outflow obstruction.
• Differentiate struvite uroliths and urethral plugs from other types of uroliths by signalment, urinalysis, urine culture, radiography, ultrasonography, cystoscopy, and quantitative analysis of voided or retrieved uroliths or plugs.

CBC/Biochemistry/Urinalysis

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

Other Laboratory Tests

• Pretreatment quantitative bacterial urine cultures (preferably with specimen obtained by cystocentesis) yield bacterial urinary tract infections in only ∼1–3% of affected 2- to 7-year-old patients.
• Quantitative mineral analysis is the accepted standard of practice for uroliths and urethral plugs retrieved during voiding, by voiding urohydropropulsion, by aspiration into a urinary catheter, or by cystoscopy.
• Bacterial culture of inner portions of uroliths retrieved from patients with urease-positive microbes cultured from urine may be of value.

Imaging

Diagnostic Procedures

Cystoscopy reveals location, number, size, and shape of urethroliths and urocystoliths.

Pathologic Findings

Urethral plugs may contain red blood cells, white cells, transitional epithelial cells, bacteria, and/or viruses in addition to matrix and minerals.

Appropriate Health Care

• Retrograde urohydropropulsion to eliminate urethral stones, lavage to remove urethral plugs.
• Voiding urohydropropulsion to eliminate bladder and urethral stones, and/or surgery require short periods of hospitalization.
• Medical dissolution of struvite uroliths is an outpatient strategy.

Diet

• Treatment of sterile struvite uroliths with an appropriate diet (prototype diet is Hill's Prescription Diet Feline s/d) typically results in dissolution within 2–4 weeks of therapy. It has become the treatment standard of practice.
• Infection-induced struvite urocystoliths may be dissolved by feeding a calculolytic diet and an appropriate antimicrobic.
• Continue diet therapy for 1 month after survey radiographic evidence of urolith dissolution.
• Struvite crystalluria may be minimized by feeding magnesium-restricted urine-acidifying diets.
• Canned (moist) foods help to reduce urine concentration of calculogenic metabolites and promote increased frequency of normal voiding.

Client Education

• If dietary management is used, limit access to other foods and treats.
• Short-term ( weeks to months) treatment with a calculolytic diet (Hill's Feline s/d) and antibiotics as needed is effective in dissolving infection-induced struvite uroliths.
• Owners of cats with infection-induced struvite urocystoliths must comply with dosage schedule for antibiotic therapy.
• Avoid feeding calculolytic diets to immature cats.

Surgical Considerations

• Ureteroliths cannot be dissolved. Consider surgery for persistent ureteroliths associated with morbidity.
• Urethroliths cannot be medically dissolved. Consider voiding urohydropropulsion to remove urethroliths or urethral plugs. Alternatively, move urethroliths into the bladder by retrograde urohydropropulsion.
• Immovable urethroliths, recurrent urethral plugs, or strictures of the distal urethra may require perineal urethrostomy.
• Consider laser lithotripsy for urocystoliths and/or urethroliths.
• 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.
• Uroliths and urethral plugs should be localized before considering surgical correction.
• Radiographs should be obtained immediately following surgery to verify that all uroliths were removed.

Drug(s)

• Dietary dissolution of infection-induced urocystoliths or requires oral administration of appropriate antibiotics, chosen on the basis of bacterial culture and antimicrobial susceptibility tests. Give antibiotics at therapeutic dosages until the urinary tract infection is eradicated and there is no radiographic evidence of uroliths.
• Buprenorphine may be considered to alleviate clinical signs of discomfort; suggested empirical dose is 15 µg/kg via buccal transmucosal administration q8–12h as needed. Tolteridine may be considered as an anticholinergic and antispasmodic to minimize hyperactivity of the bladder detrusor muscle and urge incontinence; the suggested empirical dose is 0.05 mg /kg PO q12h.

Contraindications

Do not give urine acidifiers to azotemic patients or immature cats.

Precautions

Azotemic patients are at increased risk for adverse drug events.

Patient Monitoring

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

Prevention/Avoidance

• Recurrent sterile struvite uroliths in cats may be prevented by using acidifying, magnesium-restricted diets or urine acidifiers. Do not administer urine acidifiers with acidifying diets.
• Consider use of a modified high-moisture, magnesium-restricted, acidifying diet that has not been supplemented with sodium (Prescription Diet c/d Multicare Feline) to minimize recurrence of struvite and/or calcium oxalate crystalluria and uroliths.
• For patients whose urine has been acidified, carefully monitor them 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 preventing calcium oxalate uroliths. Struvite uroliths can be medically dissolved; recurrent calcium oxalate uroliths cannot be dissolved.
• Infection-induced struvite urolithiasis can be prevented by eradicating and controlling urinary tract infections. Use of magnesium-restricted, acidifying diets is not required if the urease-positive microbes can be eradicated.

Possible Complications

• Urocystoliths may pass into and obstruct the urethra of male cats, especially if the patient is persistently dysuric. Urethral obstruction may be managed by retrograde urohydropropulsion.
• An indwelling transurethral catheter increases the risk for iatrogenic bacterial urinary tract infection and/or urethral stricture.

Expected Course and Prognosis

In our hospital, the mean time for dissolution of feline sterile urocystoliths was 1 month (range, 2 weeks–5 months). The mean time for dissolution of infection-induced struvite urocystoliths was 10 weeks (range, 9–12 weeks).

Associated Conditions

Any disease that predisposes to bacterial urinary tract infection.

Age-Related Factors

Infection-induced struvite is the most common urolith in immature cats. Sterile struvite is rare in immature cats.

Synonyms

• Feline lower urinary tract disease
• Feline urologic disease
• FUS

See Also

• Lower Urinary Tract Infection chapters
• Nephrolithiasis
• Urolithiasis, Struvite-Dogs

Abbreviations

• FUS = feline urologic syndrome
• MAP = magnesium ammonium phosphate

Client Education Handout Available Online