Concretions composed primarily of highly organized crystalloids and a small organic matrix. Uroliths usually named according to mineral composition. Calcium oxalate and struvite (magnesium ammonium phosphate) are most common types of feline uroliths.
Systems. Feline uroliths most commonly found in urinary bladder and urethra but may occur in kidneys/ureters.
Genetic implications. Burmese, Himalayan, and Persian are predisposed to calcium oxalate uroliths.
Incidence/Prevalence. Up to 25% of cats with lower urinary tract disorders. Nephroliths and urocystoliths may be present without clinical signs.
Geographic distribution. Worldwide.
Breed predilection. See genetic implications, above.
Age and range. 3 months to > 20 years; average is 7 years. Peak incidence of struvite and calcium oxalate uroliths appears to be 4 and older than 9 years, respectively. Sterile struvite uroliths are more common in young adult cats, and calcium oxalate uroliths are more common in middle-aged and older cats. Infection-induced struvite uroliths are more common in kittens and in older cats.
Gender. At least one epidemiologic study found that male cats are more prone to calcium oxalate uroliths, whereas females are more prone to struvite uroliths.1-3 No gender predisposition has been observed for cystine and urate uroliths.
Feline urine is a complex solution in which salts (e.g., calcium oxalate and magnesium ammonium phosphate) can remain in solution under conditions of supersaturation. Supersaturated urine does, however, have a potential energy for precipitation, or the tendency to form crystals from dissolved salts. Uroliths form when crystals aggregate and grow into calculi.
Struvite uroliths usually form in sterile urine; however, pathophysiology is poorly understood. Dietary/metabolic factors that result in alkaline urine/increased concentrations of magnesium, ammonium, and phosphate in urine have been implicated. Diets high in magnesium, phosphorus, calcium, chloride, and fiber with moderate protein content have been associated with increased risk. Struvite uroliths may also be associated with UTI, especially when caused by bacteria that produce urease (Staphylococcus and Proteus species). In such cases, urease increases ammonium concentration in urine, resulting in increased urine pH and change in ionization state of phosphorus. Struvite solubility decreases in alkaline urine. Risk for struvite uroliths seems greatest in 1- to 2-year-old female cats.
Calcium oxalate uroliths are associated with hypercalcemia/hypercalciuria, but in most cases cause is unknown. Diets low in sodium/potassium or formulated to maximize urine acidity are associated with increased risk for calcium oxalate uroliths. Calcium oxalate solubility decreases in acidic urine. Other risk factors include feeding dry cat foods ad libitum, excessive vitamins C and D, and lack of dietary variety. Several studies suggest long-term use of struvite prevention diets has contributed to increased incidence of calcium oxalate uroliths over the past 10 to 15 years. Middle-aged to older cats, cats housed exclusively indoors, and overweight cats have increased incidence. Calcium oxalate uroliths occur most commonly in male cats, especially neutered males. Finally, calcium oxalate uroliths occur more frequently in the kidneys than do other uroliths.
Ammonium urate and uric acid uroliths associated with portovascular anomalies; however, cause often remains unknown. Diets high in purines (e.g., liver) may be risk factor. Ammonium solubility decreases in alkaline urine; uric acid solubility decreases in acidic urine.
Cystine uroliths in cats are usually caused by a renal tubular reabsorptive defect that results in increased concentrations of cystine and other amino acids in urine. Cystine solubility decreases in acidic urine.
Depends on number of uroliths, location, and physical characteristics. Solitary, smooth uroliths in bladder/renal pelvis may not significantly inflame tissue. Irregular/sharp borders and multiple uroliths are more likely to cause inflammation. Uroliths associated with bacterial UTI may induce more inflammation.
Nephroliths are often asymptomatic but can be associated with microscopic or gross hematuria (for differential diagnoses of hematuria, see page 32). All nephro liths can damage renal tissue/cause chronic inflammation, which may decrease normal host defense mechanisms/increase risk for bacterial pyelo nephritis in cats with lower UTIs. Acute bacterial pyelonephritis may cause lethargy, anorexia, fever, and bacteriuria; however, chronic bacterial pyelonephritis may be associated with vague, mild signs or be asymptomatic. Larger nephroliths can cause hydronephrosis associated with pelvic and/or ureteral obstruction. Smaller nephroliths may pass through ureters asymptomatically; however, hydroureter/hydro nephrosis are possible sequelae to ureteral obstruction.
Cystoliths may be asymptomatic or may irritate the uroepithelium, resulting in hematuria, pollakiuria, and dysuria/stranguria. Struvite cystoliths can form as a result of urease-producing bacterial UTI; however, any urocystolith can compromise host defense mechanisms and predispose to complicated bacterial UTI.
Urethral uroliths are common causes of partial/complete urethral obstruction in males, leading to altered/absent urine stream, hematuria, and/or dysuria/stranguria. In complete urethral obstruction, postrenal azotemia and uremia develop within 24 to 36 hours.
History. Cats with urocystoliths may present with pollakiuria, hematuria, dysuria/stranguria; however, uroliths may be asymptomatic. Nephroliths may be associated with abdominal pain or hematuria.
Physical examination. Urocystoliths may be detected by abdominal palpation, but inability to palpate uroliths does not exclude them.
Imaging. Number, mineral composition, size, and location in the urinary tract affect radiographic/ultrasonographic appearance of uroliths. Ammonium urate uroliths are relatively radiolucent compared with calcium oxalate, struvite, and cystine uroliths. Small uroliths (< 3 mm) of any composition may be difficult to visualize on radiographs. Double-contrast cystography affords increased sensitivity for detecting small or radiolucent uroliths. Ultrasonography may also be used for detection of radiolucent uroliths as well as obstructive uropathy resulting in renal pelvic or ureteral dilatation. Intravenous urography and ultrasonography may be used to detect radiolucent nephroliths and differentiate mineralized renal tissue from actual nephroliths.
Laboratory findings. Urine pH, crystalluria, and presence of urease-producing bacteria may aid in presumptive identification of urolith type. Serum calcium concentrations should be assessed in suspected calcium oxalate uroliths to rule out hypercalcemia. Liver function should be assessed via BUN/pre- and postprandial bile acids in ammonium urate or uric acid uroliths to rule out portovascular anomalies.
Definitive diagnosis. Requires quantitative mineral analysis of uroliths that have been voided or recovered.
Includes relief of urethral obstruction/bladder decompression, if necessary. This can usually be accomplished by passage of a small-bore catheter, cystocentesis, dislodgment through hydropropulsion, or in rare cases emergency urethrotomy/urethrostomy. Fluid therapy should be initiated to restore water and electrolyte balance if postrenal azotemia exists. Hyperkalemia is potentially life-threatening in cats with postrenal azotemia caused by urethral obstruction/ rupture of urinary bladder or urethra. Serum potassium concentration, BUN, and creatinine concentrations should be measured in cats with suspected urethral obstruction. Alternatively, bradycardia and electrocardiographic findings of flattened P waves, prolonged PR interval, widened QRS complexes, and tall or spiked T waves suggest hyperkalemia and indicate aggressive treatment to lower serum potassium concentration before sedation/anesthesia for urethral catheterization.
Medical dissolution of feline struvite uroliths has been shown to be effective; however, whether to remove surgically or dissolve medically is not always clear (see Surgical vs. Medical Treatment). Voiding urohydropropulsion or catheter urolith retrieval are other nonsurgical methods for small urocystoliths.
Struvite uroliths. Feeding Prescription Diet Feline s/d (Hill's)-a magnesium-reduced, acidifying diet-usually dissolves struvite uroliths. Because of its acidifying nature, the diet should not be fed to immature cats/those with postrenal azotemia/uremia. In addition, supplemental sodium chloride/urinary acidifiers are not recommended. Diet should be fed for at least 30 days after uroliths are no longer visible radiographically. Rate of dissolution is proportional to surface area exposed to undersaturated urine. Sterile struvite uroliths usually dissolve in 2 to 4 weeks. Diet does not dissolve nonstruvite uroliths and is not usually effective for persistent UTI or if the cat is fed anything in addition to the s/d diet.
In addition to decreasing crystalloids in urine, eliminating bacterial UTI is essential in struvite urolithiasis. If infection is present at start of treatment, antibiotics should be continued throughout course of medical dissolution to destroy bacteria liberated from dissolving uroliths. Antibiotics are selected based on urine culture/sensitivity. Dissolution time for infection-induced struvite uroliths is usually 2 to 3 months.
Measures to prevent recurrence include preventing/controlling UTIs, maintaining acidic urine, and decreasing dietary magnesium. Numerous struvite prevention maintenance diets are available commercially.
Calcium oxalate uroliths. Dissolution diets for oxalate urolithiasis are not yet available. Small urocystoliths may be removed by catheter retrieval or voiding hydropulsion; surgery is the usual means for removal of large calcium oxalate uroliths. Some calcium oxalate uroliths, especially nephroliths, may remain clinically quiescent for months to years. Surgery is not always recommended due to nephron destruction associated with nephrotomy. One clear indication for surgery is obstructive uropathy.
Several commercially available diets have been specifically formulated to prevent recurrence of calcium oxalate uroliths in cats. In addition, oral supplementation of potassium citrate may help prevent recurrence, because citrate combines with calcium, thereby forming a relatively soluble calcium citrate complex. Potassium citrate also results in mild urine alkalinization, increasing the solubility of calcium oxalate. Preventive effects of thiazide diuretics and supplemental vitamin B are unproven in cats.
Ammonium urate and uric acid uroliths. Correction of portovascular anomalies may result in spontaneous dissolution of these uroliths. Moistened renal failure diets (low in purine precursors like liver tissue) may help prevent recurrence in cats with idiopathic urate uroliths.
Cystine uroliths. Medical dissolution protocols are not available. Moistened renal failure diets have been recommended to prevent recurrence.
RELATIVE COST OF TREATMENT*
Emergency care/stabilization of a male cat with a urethral obstruction, $750 to $1000. Includes emergency call fees, fluid therapy, electrocardiographic monitoring, sedatives or short-acting anesthetics, laboratory costs, radiographs, and 2 to 3 days in critical care.
Initial work-up for urolithiasis, $350 to $400. Includes examination, minimum database, abdominal radiographs/ultrasonography, urine culture and sensitivity.
Surgery for cystotomy, additional $400 to $600. Includes preoperative medications, anesthesia monitoring, surgery fees, urolith analysis, postoperative radiographs, and postoperative management.
* Actual costs will vary according to region.
• Radiographs/ultrasonography to ensure complete removal of uroliths
• Frequent urinalysis (every 2 to 4 weeks) to monitor urine pH and crystalluria
• In cases of new or continued hematuria, pyuria, or bacteriuria: urine culture/sensitivity and survey abdominal radiographs
• Radiographs/ultrasonography at 6 months to check for recurrence
Prognosis and Course. Recurrence rates have been reported to be 19% to 37%. Uroliths can reform within weeks to several months after complete removal. Pseudo-recurrence, or incomplete removal, may be as high as 20%.
As we learn more about the pathophysiology of various types of uroliths, it is likely that additional diets and/or medications will be developed for dissolution and prevention.
Surgical VS. Medical Treatment
Surgical • Advantages
• Allows definitive diagnosis of urolith type via quantitative analysis
• Allows surgeon to correct any concurrent or predisposing anatomical abnormalities (e.g., urachal remnants, urinary bladder polyps)
• Enables collection of urinary bladder mucosal samples for bacterial culture if urine yields no growth on culture
Surgical • Disadvantages
• Requires anesthesia
• Surgery is invasive with potential for complications
• Possibility of incomplete urolith removal
• Persistence of underlying causes does not preclude recurrence; thus medical management is still necessary
Medical • Advantages
• Lower initial costs
• Lower complication rate
Medical • Disadvantages
• Total cost is similar to that of surgical treatment because of an involved follow-up protocol
• Considerable owner compliance for several weeks to months during follow-up, which frequently involves multiple urinalyses, bacterial cultures if uroliths are associated with a bacterial UTI, and radiographs
• Cannot be used for cats with urolith-induced obstructive uropathy or in cases of nonstruvite urolithiasis
TX... at a glance
1. If hyperkalemia detected through electrocardiography, aggressively lower serum potassium concentration before sedation/anesthesia for urethral catheterization
2. Initiate fluid therapy in postrenal azotemia
In urethral obstruction > 6 to 12 hours, measure serum potassium concentration, BUN, and creatinine concentrations
3. Relieve urethral obstruction.
Bladder decompression, if necessary, by one of following:
• passing small-bore catheter
• dislodging urethral calculi by hydropulsion
• cystocentesis (use small bore needle)
• in rare cases, emergency urethrotomy/urethrostomy