Xanthine Urolithiasis in Dogs
Xanthine, a by-product of purine metabolism, naturally occurs in the body. Typically, it is converted into uric acid, a waste product of blood proteins, through the enzyme xanthine oxidase, and eliminated via urine. However, due to its low solubility among purines excreted in urine, excessive xanthine levels can lead to the formation of xanthine uroliths (stones). When xanthine oxidase function is impaired, it results in hyperxanthinemia (elevated blood xanthine levels) and xanthinuria (xanthine excretion in urine). This impairment can either be congenital or acquired, such as through the use of allopurinol, a medication.
Naturally occurring xanthinuria often stems from a familial or congenital defect in xanthine oxidase activity. In Cavalier King Charles spaniels, it is suggested to follow an autosomal recessive mode of inheritance.
Acquired xanthinuria frequently occurs in dogs undergoing treatment with allopurinol for urate urinary tract stones or leishmaniasis (a parasitic infection). Additionally, diets high in purines (such as those rich in protein) can elevate the risk of xanthinuria in patients receiving allopurinol treatment.
Symptoms and Types
- Some cases may show no symptoms
- Urine may appear mustard-colored
- Bladder stones may cause:
- Frequent urination (polyuria)
- Difficulty urinating
- Bloody urine (hematuria)
- Stones in the urethra may lead to:
- Frequent urination
- Difficulty urinating
- Bloody urine
- Possible urethral blockage
- Kidney stones (nephroliths) may present as:
- Asymptomatic cases
- Hydronephrosis, characterized by kidney swelling due to a blocked ureter (the tube connecting the kidney to the bladder)
- Kidney disease
Causes
- Presence of xanthines in the urine can contribute to stone formation
- Genetic predisposition observed in Cavalier King Charles spaniels
- Excessive use of allopurinol medication combined with a high purine diet
Factors related to urine chemistry include:
- Acidic urine pH
- Highly concentrated urine
- Incomplete and infrequent urination
Diagnose
Your veterinarian will conduct a comprehensive physical examination of your dog, considering the history of symptoms and any previous conditions that could have contributed to the current situation. A thorough blood profile will be performed, including a chemical blood profile, complete blood count, urinalysis, and electrolyte panel. Xanthine crystals will be evident in the urine sediment during the urinalysis.
However, these crystals cannot be distinguished solely through light microscopy. For an accurate diagnosis, urine should be analyzed using infrared spectroscopy, which can differentiate xanthine uroliths (urinary tract stones) from other types of uroliths. Additionally, high-pressure liquid chromatography of urine can detect xanthine, hypoxanthine, and other purine metabolites.
Ultrasonography, double-contrast cystography, and intravenous urography are additional diagnostic tools that can help identify uroliths and their originating locations. Uroliths typically do not appear on standard X-rays.
Xanthine uroliths in the urethra and bladder stones near the urethra may be detected through urethrocystoscopy. This procedure utilizes a small flexible tube equipped with a camera, which can be inserted into confined spaces, such as the urethral passage. Small uroliths can be retrieved for analysis by removing fluid using a transurethral catheter or through a technique known as voiding urohydropulsion. In the latter method, the bladder is filled while the patient is under anesthesia, and then emptied after attempting to dislodge stones into the urethra, facilitating their collection.
Treatment
Voiding urohydropulsion proves effective for eliminating smaller xanthine uroliths that can easily pass through the urethra. However, surgery remains the optimal approach for removing larger uroliths from the lower urinary tract. Perineal urethrostomy surgery may be recommended to minimize recurrent urethral obstruction in male dogs.
To prevent the formation of xanthine uroliths, urine pH may be increased, and a low-purine diet supplemented with ample water intake can be provided to enhance urine output. Depending on your dog’s overall health and the severity of the condition, your veterinarian may suggest a diet tailored for dogs with renal failure. The objective is to reduce purine ingestion, decrease the acidity of urine, and increase bladder voiding to keep the passages free from stone-forming chemicals.
Living and Management
Your veterinarian will arrange monthly follow-up appointments for your dog to perform urinalysis, contrast X-rays, or ultrasonography exams. The treatment plan for your dog may be modified based on its response to the initial treatment and the progression of its health.