Definition

Uroliths composed of uric acid, sodium urate, or ammonium urate

Pathophysiology

• Impaired conversion of uric acid to allantoin causes high concentration of uric acid in serum and urine.
• Patients with portosystemic shunts may develop ammonium urate uroliths because of impaired hepatic metabolism of uric acid and ammonia.

Genetics

Dalmatians have a breed predisposition to forming urate urolithiasis.

Incidence/Prevalence

Approximately 5–8% of uroliths retrieved from dogs and cats

Signalment

Signs

Causes

Rule out portosystemic shunt

Risk Factors

• High purine intake (glandular meat)
• Persistent aciduria in a predisposed animal

Differential Diagnosis

Other causes of lower or upper urinary tract disease

CBC/Biochemistry/Urinalysis

• Aciduria, urate crystalluria, azotemia in patients with urinary outflow obstruction.

• Low BUN and microcytosis in patients with portosystemic shunts; reduced production of urea by the liver may mask hypoglycemia, hypoalbuminemia, and increased hepatic enzyme activities in patients with more severe hepatic dysfunction.

Other Laboratory Tests

Liver function tests such as bile acids have abnormal results in patients with portosystemic shunts.

Imaging

• Urate uroliths may be radiolucent; may need intravenous pyelogram to detect nephroliths or double contrast cystography to detect urocystoliths. Microhepatica in patients with portosystemic shunts.
• Ultrasonography may reveal small uroliths and a portosystemic shunt.

Diagnostic Procedures

Liver biopsy; bile acids, blood ammonia

Pathologic Findings

In patients with portosystemic shunts, liver biopsy may reveal hepatic atrophy and/or dysplasia.

Appropriate Health Care

Urethral or ureteral obstruction may require inpatient treatment. Urate uroliths can be dissolved on outpatient basis.

Nursing Care

Fluid therapy to correct dehydration

Activity

Usually not restricted, except after surgery

Diet

For dissolution and prevention, a high moisture, low-purine, urine-alkalinizing diet.

Client Education

Recurrence of uroliths is possible. Therefore a plan to minimize recurrence should be developed.

Surgical Considerations

• Cystotomy, urethrotomy, nephrotomy, percutaneous cystolithotomy, or cystoscopy and retrieval or laser lithotripsy to remove uroliths.
• Portosystemic shunt ligation.

Drug(s)

Allopurinol (15 mg/kg PO q12h), a xanthine oxidase inhibitor, for dissolution (see Figure 2).

Contraindications

Glucocorticoids and other immunosuppressive drugs may promote hyperuricosuria.

Precautions

Allopurinol is contraindicated in animals with renal failure and is not effective in animals with portosystemic shunts.

Possible Interactions

Skin eruption with use of allopurinol and ampicillin.

Patient Monitoring

See Figure 3.

Prevention/Avoidance

High-moisture, low-purine, urine-alkalinizing diet

Possible Complications

• Urethral obstruction
• Uroliths likely to recur if no preventive measures

Expected Course and Prognosis

• Medical dissolution takes an average of 4 weeks, if there is good compliance.
• Medical dissolution usually not successful with portosystemic shunt.

Associated Conditions

Portosystemic shunt

Pregnancy/Fertility/Breeding

Low-protein/purine diet is not recommended for pregnant or lactating animals.

Client Education Handout Available Online