Urolithiasis

46 Urolithiasis



Mala Erickson, Stanley I. Rubin



1. What are clinical signs of urolithiasis?
• Clinical signs that may be seen with urolithiasis include hematuria, dysuria, pollakiuria, and inappropriate urination.

• Animals with nephroliths may not show signs or may show lethargy, pyrexia, pain over the lumbar area, anorexia, or a stiff gait.

• Oliguria or anuria may be present with bilateral ureterolithiasis. Small uroliths may pass into the urethra during voiding causing urethral obstruction.

2. Is crystalluria significant?
• Crystals can be present in urine of dogs and may not be clinically significant.

• Factors that can affect crystalluria include age, diet, urine pH and volume, length of time between voiding, temperature and specific gravity of urine, and concurrent disease.

• Struvite, amorphous phosphates, and oxalate crystals can be found in normal urine samples.

• Bilirubin crystals may be found in concentrated urine samples.

• Urate crystals are commonly seen in urine from Dalmatians and may be seen in animals with liver disease or portosystemic shunts.

• Although crystalluria may indicate a higher risk of developing calculi, it can occur with or without urolithiasis and crystal type does not necessarily indicate urolith type. Crystals can also form in urine when it cools, which should be differentiated from in vivo crystalluria.

3. How do uroliths form?
• Oversaturation of one or more substances eliminated in urine can result in their precipitation and growth. A crystal nidus forms to begin the initiation phase of urolith formation. As urine becomes supersaturated with calculogenic material, nucleation occurs.

• The degree of saturation is affected by the amount of crystalloids excreted by the kidneys, pH of the urine, urine concentration, and presence of substances in the urine that promote or inhibit crystallization. Other substances that can influence formation include the presence of proteinaceous matrix substances with Tamm-Horsfall mucoprotein, red blood cells, white blood cells, epithelial cells, and amorphous material.

• The duration and degree of urine supersaturation, the crystal nidus remaining in the urinary tract lumen, and physical characteristics of the nidus itself are also factors affecting growth of the urolith.

• Once formed, a crystal nidus may be voided or grow within the urinary tract.

• Growth of one crystal type on the surface of another type can also occur giving a urolith of mixed composition.

4. What are the main types of uroliths?

Struvite uroliths are composed mainly of magnesium ammonium phosphate with small amounts of matrix. Most struvite stones in dogs are infection-induced with urea-splitting bacteria such as Staphylococcus spp. being most commonly implicated. Proteus spp. and Ureaplasma are less commonly involved. The enzyme urease hydrolyzes urea in the presence of water to produce a high concentration of ammonia and carbonate ions. Ammonium ions result when ammonia combines with water or hydrogen ions, which leads to an elevated urinary pH, thereby reducing the solubility of magnesium ammonium phosphate and favoring struvite crystal precipitation. Struvite uroliths can form within 2 weeks of induction of a staphylococcal urinary tract infection in dogs. Foreign bodies may also act as a nidus for infection-induced struvite uroliths.


An alkaline urine, diet factors, and genetic predisposition have also been associated with formation of struvite uroliths.


Risk factors for the formation of calcium oxalate uroliths include altered balance between calculogenic minerals such as calcium and oxalate in the urine and inhibitors of crystallization such as citrate, phosphorus, magnesium, potassium, uric acid, Tamm-Horsfall mucoprotein, and nephrocalcin. Most dogs have a urine pH <6.5 at time of diagnosis, and infection does not seem to contribute to oxalate stone formation. Consumption of diets with reduced magnesium that promote formation of acidic urine has also been identified as a risk factor. Excess gastrointestinal calcium absorption as a cause of hypercalciuria and increased levels of oxalic acid from foods such as spinach, rhubarb, peanuts, chocolate, and tea has also been proposed as factors. Phosphorus has a role in minimizing renal production of calcitriol and enhancing excretion of pyrophosphate, an inhibitor of calcium oxalate salts so dietary phosphorus restriction is not desired. Use of urine acidifying medications and hypercalcemia has also been identified as risk factors for calcium oxalate urolith formation.


Ammonium urate uroliths are considered the third most common urolith. Urate calculi are usually small (<1 mm to 1 cm) and are more common in the bladder. These uroliths may contain a mixture of purines including uric acid, sodium urate, or ammonium urate. In dogs, abnormal conversion of uric acid to allantoin will lead to high concentrations of uric acid in serum and urine. Portovascular anomalies and renal tubular reabsorptive deficits have been identified as contributing factors. Urine that is highly acidic with a high urine-specific gravity associated with a diet high in purine precursors are risk factors for urate urolithiasis. Other risk factors include increased concentration of uric acid and increased renal excretion, renal production, urine retention, or production of ammonium by microbial urease.


Cystine uroliths are linked to a hereditary condition affecting renal tubular transport of cystine, a nonessential sulfur-containing amino acid. Cystine is normally freely filtered at the glomerulus and is 99% to 100% actively reabsorbed in the proximal tubule. Solubility of cystine is pH dependent; it is relatively insoluble in acidic urine and becomes more soluble in alkaline urine. Cystinuria and a previous history of cystine urolithiasis are considered predisposing factors. Female Great Danes and Bull Mastiffs have a relatively high occurrence of ammonium urate and cystine stones but a relative lack of struvite uroliths. Other breeds reported with cystine uroliths include Dachshunds, Basset Hounds, English Bulldogs, and Rottweilers with the age at diagnosis ranging from 2 to 7 years.


Calcium phosphate uroliths have been reported in various breeds, including the previously mentioned breeds at risk for calcium oxalate uroliths. They may contain hydroxyapatite and carbonate apatite or as minor components in struvite or calcium oxalate stones. No gender or age trends have been recognized.


Silica uroliths are quite uncommon but have been reported in the German Shepherd and Old English Sheepdog and have been linked to diets high in cereal grains containing silicates (corn gluten, soybean hulls).


Xanthine uroliths are reported after the administration of a xanthine oxidase inhibitor (e.g., allopurinol) for the management of urate uroliths. Xanthinuria has been reported in Cavalier King Charles Spaniels and in Dachshunds and is thought to be due to an inborn error of purine metabolism.


5. What imaging techniques may be used to visualize uroliths?
• Diagnostic imaging should be performed to determine the presence of uroliths in an animal with supportive clinical signs.

• The location, shape, number, and density of any uroliths present can be determined.

• Radiographic and ultrasonographic examinations can be performed to evaluate the animal. Although radiopacity of uroliths can be determined, this will not reliably identify the mineral composition of the stone.

• Contrast techniques can also be used to thoroughly evaluate the urethra for uroliths. Excretory urography involves taking abdominal radiographs after intravenous administration of an iodinated contrast agent. Radiographs are taken at intervals to evaluate renal shape, size, and location to evaluate filling defects in the renal pelvis or ureters. Excretory urography should not be performed in dehydrated animals because of the risk of adverse effects on renal function. Contrast techniques will also allow evaluation of the mucosal surface of the bladder.

• Struvite uroliths are radiopaque and can be visualized on survey radiographs (if they are large enough) as round or disk-shaped structures with a smooth or rough surface.

• Calcium oxalate uroliths are radiopaque although ultrasonography or an intravenous pyelogram may be helpful to visualize renal or urethral calculi.

• Urate and cystine calculi are usually radiolucent, so contrast techniques or ultrasonography may be needed to identify them.

• Retrospective studies have shown that most radiodense stones in dogs are infection-induced struvite uroliths with uroliths >10 mm in diameter being >92% likely to consist of struvite; uroliths >15 mm in any dimension are considered rare.

6. What diagnostics are indicated in an animal with clinical signs of urolithiasis?
• A complete blood cell count and serum chemistry panel should be completed to evaluate the dog for evidence of inflammation or other systemic disease.
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Jul 31, 2016 | Posted by in INTERNAL MEDICINE | Comments Off on Urolithiasis

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