Definition

• Nephroliths-uroliths located in the renal pelvis or collecting diverticula of the kidney(s).
• Nephroliths or nephrolith fragments may pass into the ureters (ureteroliths).
• Nephroliths that are not infected, not causing obstruction or clinical signs, and not progressively enlarging are termed inactive.

Pathophysiology

Nephroliths can obstruct the renal pelvis or ureter, predispose to pyelonephritis, and result in compressive injury of the renal parenchyma leading to renal failure; see chapters on the different urolith types for pathophysiology of urolithiasis; in cats, nephroliths composed of blood clots that may be mineralized with calcium phosphate or calcium oxalate can form secondarily to chronic renal hematuria.

Systems Affected

• Renal/urologic-affects the urinary tract, with potential for obstruction, recurrent urinary tract infections, or renal failure.
• Obstruction of the renal pelvis or ureter in an animal with pyelonephritis may result in septicemia and thus affect any body system.

Genetics

Refer to chapters describing genetics related to different types of uroliths.

Incidence/Prevalence

• Nephroliths retrieved from dogs and cats compose 1–1.5% of uroliths submitted to the Minnesota Urolith Center for analysis. The true incidence of nephroliths is likely much higher. Many animals with nephroliths are asymptomatic, or are not treated by methods that encompass retrieval of uroliths that can be quantitatively analyzed.
• Mineral compositions of canine nephroliths submitted for analysis, in descending frequency-calcium oxalate (38%), struvite (27%), compound (13%), purines (e.g., ammonium urate, sodium urate, uric acid; (12%), mixed (4%), calcium phosphate (3%), cystine (1.5%), and silica (0.6%). Mineral compositions of nephroliths in cats submitted for analysis, in descending frequency-calcium oxalate (76%), struvite (7%), non-crystalline matrix (including dried blood clots) (6%), compound (5.5%), calcium phosphate (2.5%),and purines (<1%).

Signalment

Signs

Causes

• For an extensive listing of causes, see chapters on each urolith type. Oversaturation of the urine with calculogenic minerals is a risk factor for urolithiasis.
• Calcium oxalate urolithiasis-hypercalciuria, hypercalcemia, hypocitraturia, hyperoxaluria, primary hyperparathyroidism, excess dietary calcium.
• Calcium phosphate urolithiasis-chronic renal bleeding (cats), hypercalcemia, hyperparathyroidism, excess dietary calcium and phosphorus, renal tubular acidosis.
• Cystine urolithiasis-cystinuria.
• Infection-induced struvite urolithiasis-urinary tract infection with urease-producing microbes. High-protein diets that produce a large quantity of urea that is excreted in urine are also an integral part of the etiopathogenesis of infection-induced struvite uroliths.
• Urate urolithiasis-genetic urate transporter defect in conversion of uric acid to allantoin (Dalmatians, bulldogs, and others), portosystemic shunt.
• Xanthine urolithiasis-allopurinol administration and high dietary purine in dogs predisposed to urate urolithiasis. Allopurinol administration in the treatment of leishmaniasis. Apparently an inborn error of purine metabolism in cats.

Risk Factors

• Alkaline urine-struvite and calcium phosphate uroliths.
• Acid urine-calcium oxalate, cystine, urate, and xanthine uroliths.
• Urine retention and formation of highly concentrated urine.
• Lower urinary tract infection-ascending infection and pyelonephritis.
• Conditions that predispose to urinary tract infection (e.g., perineal urethrostomy, ectopic ureters, vesicoureteral reflux, and exogenous steroid administration or hyperadrenocorticism [calcium oxalate uroliths]).

Differential Diagnosis

• Consider nephroliths in any patient with renal failure, recurrent urinary tract infection, acute vomiting (acute pancreatitis, acute gastroenteritis, intestinal or gastric obstruction, etc.), or abdominal or lumbar pain (e.g., intervertebral disc protrusion, peritonitis).
• Nephroliths are usually confirmed by radiographs or ultrasonography; differentiate mineralization of the renal pelvis or collecting diverticula from true nephrolithiasis.

CBC/Biochemistry/Urinalysis

• CBC results-usually normal unless the patient has pyelonephritis; patients with pyelonephritis may have leukocytosis and immature neutrophilia.
• Serum biochemistry analysis-usually normal unless bilateral obstruction, pyelonephritis, or compressive renal injury leads to renal failure (azotemia with an inappropriate urine specific gravity, hyperphosphatemia); hypercalcemia may contribute to formation of calcium oxalate or calcium phosphate nephroliths.
• Urinalysis-may reveal hematuria and crystalluria; crystal type may indicate mineral composition; pyuria, proteinuria, and bacteriuria may also be seen in animals with urinary tract infection.

Other Laboratory Tests

• Submit all retrieved nephroliths or nephrolith fragments for quantitative analysis. Although definitive identification of nephrolith type requires quantitative analysis, mineral composition can frequently be predicted on the basis of signalment, radiographic appearance, and urinalysis findings.
• Results of bacterial culture of urine may confirm urinary tract infection in animals with concurrent pyelonephritis.

Imaging

• Can detect radiopaque nephroliths (e.g., calcium phosphate, calcium oxalate, struvite) by survey radiography; cystine and silica are slightly radiopaque.
• Purines (ammonium urate, sodium urate, uric acid, xanthine etc.) are usually radiolucent unless they contain a mixture of radiodense biogenic minerals.
• Use ultrasonography or excretory urography to confirm, size and number of nephroliths or ureteroliths regardless of radiographic density.

Diagnostic Procedures

After ESWL, nephrolith fragments can be retrieved for quantitative analysis by voiding, cystoscopy, catheter-assisted retrieval, or voiding urohydropropulsion.

Appropriate Health Care

Manage patients with inactive nephroliths as outpatients. When appropriate, medical dissolution protocols can be administered to outpatients. Removal of nephroliths by surgery or ESWL requires hospitalization.

Diet

Medical dissolution of nephroliths requires a diet appropriate for the specific nephrolith type. See “Medications.”

Client Education

• Inactive nephroliths-may not require removal but should be monitored periodically by urinalysis, urine culture, and radiography. Nephroliths can potentially cause obstruction at any time, which can result in hydronephrosis without clinical signs. Conservative management and monitoring carries a slight risk of undetected and potentially irreversible renal damage, which must be weighed against the potential renal damage from nephrotomy.
• Nephroliths (especially metabolic uroliths) tend to recur after removal; monitor the patient every 3–6 months.

Surgical Considerations

• Indications for removal of nephroliths-obstruction, recurrent infection, symptomatic nephroliths, progressive nephrolith enlargement, and a non-functional contralateral kidney.
• Treatment options for nephroliths-medical dissolution, surgery, and ESWL. Calcium oxalate is the most common mineral detected in nephroliths retrieved from dogs and cats, and not yet amenable to medical dissolution. Ureteroliths or nephroliths causing complete obstruction are also not amenable to medical dissolution.
• Surgical options-nephrotomy or pyelolithotomy. Because the nephroliths are surrounded by renal tissue, nephrotomy is required in most dogs and cats. Nephrolith removal by percutaneous nephrolithotomy has been reported in dogs.
• ESWL-safe and effective method of treating canine nephroliths and ureteroliths; nephrolith fragments pass down the ureter into the bladder and are voided with urine.
• ESWL-not as effective for treatment of nephroliths and ureteroliths in cats compared with dogs.

Drug(s) Of Choice

• Antibiotics selected on the basis of urine culture and sensitivity testing as needed; periprocedural antibiotics are recommended when infected nephroliths are treated by ESWL or surgical removal.
• Medical dissolution protocols are limited to struvite, purine, and cystine uroliths.
• When feasible, consumption of water in high moisture (canned) foods should be incorporated into treatment protocol. Try to increase excretion of urine with a specific gravity <1.020 (dogs), or <1.025 (cats).
• Medical dissolution protocols for struvite nephroliths include a calculolytic food (Hill's Prescription Diet s/d) and appropriate antibiotic therapy (i.e., if patient has a urinary tract infection) for the duration of treatment.
• Medical dissolution of canine purine nephroliths can be attempted by a protein and purine restricted, alkalinizing food (Hill's Prescription Diet Canine u/d), allopurinol (15 mg/kg PO q12h), and supplemental potassium citrate as needed to maintain urine pH ∼7.0.
• Medical dissolution of canine cystine nephrolithiasis can be attempted using a protein-restricted, alkalinizing diet (Hill's Prescription Diet Canine u/d), 2-MPG or tiopronin (Thiola, 15 mg/kg PO q12h), and supplemental potassium citrate as needed to maintain urine pH ∼7.5.

Contraindications

• Do not use allopurinol without dietary purine restriction because this combination may cause xanthine nephrolithiasis in dogs predisposed to urate urolithiasis.
• Do not give acidifying diets to azotemic patients unless blood pH and total CO₂ are monitored for development of metabolic acidosis.

Patient Monitoring

Abdominal radiographs (ultrasonography for radiolucent uroliths), urinalysis, and urine culture every 3–6 months to detect nephrolith recurrence. Dogs treated with ESWL-check every 2–4 weeks by radiographs and ultrasonography until nephrolith fragments have passed through the excretory system.

Prevention/Avoidance

Eliminate factors predisposing to individual urolith type, augment urine volume, and correct factors contributing to urine retention.

Possible Complications

Hydronephrosis, renal failure, recurrent urinary tract infection, and pyelonephritis.

Expected Course and Prognosis

• Highly variable; depends on nephrolith type, location, and size, secondary complications (e.g., obstruction, infection, renal failure), and owner compliance with treatment and prevention protocol.
• Inactive nephroliths may remain inactive for years, resulting in an excellent prognosis.
• Excellent results have been reported using ESWL to treat dogs with nephroliths-return to normal health and an excellent prognosis.
• The prognosis for patients with renal failure caused by nephrolithiasis depends on the severity and rate of progression of renal failure.
• Nephroliths causing outflow obstruction or associated with non-functioning kidneys cannot be dissolved medically.

Associated Conditions

Hyperadrenocorticism and chronic glucocorticoid administration are associated with calcium oxalate uroliths, and urinary tract infection resulting in struvite urolithiasis.

Pregnancy/Fertility/Breeding

• Contraindication to ESWL
• Acetohydroxamic acid, a urease inhibitor, is teratogenic

Synonyms

Kidney stones, renal calculi, renoliths, kidney calculi

See Also

• Hydronephrosis
• Pyelonephritis
• Renal Failure, Chronic
• Urinary Tract Obstruction
• Urolithiasis, Calcium Oxalate
• Urolithiasis, Calcium Phosphate
• Urolithiasis, Cystine
• Urolithiasis, Struvite-Cats
• Urolithiasis, Struvite-Dogs
• Urolithiasis, Urate
• Urolithiasis, Xanthine

Abbreviations

• ESWL = extracorporeal shock wave lithotripsy

Internet Resources

www.vet.utk.edu/clinical/sacs/lithotripsy

Authors Carl A. Osborne, Larry G. Adams, Jody P. Lulich, and Lori A. Koehler

Consulting Editor Carl A. Osborne

Client Education Handout Available Online