Calcium oxalate urolithiasis in the canine: Surgical management and preventative strategies

Abstract

A 4-year-old, neutered male Chihuahua was presented with inappetance, lethargy, and a tense abdomen attributed to pain. Abdominal radiographs revealed a large cystic calculus and small urethral calculi. Urethral hydropulsion followed by cystotomy were performed successfully. Dietary management was initiated as the primary means of preventing recurrence.

A 4-year-old, neutered male Chihuahua with a 2-day history of inappetance, lethargy, and apparent abdominal pain, was admitted into the hospital for evaluation.

Unfortunately, a detailed history of the current problem was not obtained from the owner at the time of presentation. The dog was receiving no medications other than preventive heartworm medication and was up-to-date on all vaccinations. There was a cat living in the same household and the owner reported that the dog occasionally ate the cat’s food. The dog’s regular diet was not specified. An episode of acute vomiting and inappetance had occurred 1 mo prior to presentation. At that time, stress-induced respiratory distress precluded blood collection for diagnostic purposes, and a tentative diagnosis of intermittent collapsing trachea and esophagitis was made. The dog was prescribed an H₂ antagonist (Ranitidine; Glaxo Wellcome, Mississauga, Ontario), 0.5 mg/kg bodyweight (BW), PO, q12h, and sent home.

On the day of presentation (day 1), all vital parameters were within normal limits. During the examination, the dog exhibited increased respiratory noise and distress when handled, as had previously been noted. Mucous membranes were pink and slightly tacky. Abdominal palpation was resented, which made evaluation difficult. No other clinical abnormalities were detected.

A plain lateral abdominal radiograph revealed a 2.5-cm, spiculated, radiopaque cystic calculus. As well, 2 small (<1 cm in diameter) calculi were visible in the urethra. When taken to an outdoor site, the dog spontaneously urinated without straining and the volume voided was judged normal. Thus, placement of an indwelling urinary catheter was not indicated. The owner agreed to urohydropulsion and cystotomy being carried out on the following morning. The dog was treated with IV fluids at a rate of 2 ml/kg BW/h, an amount equivalent to 1.5 × the daily water maintenance requirements (1). Pain was managed with butorphanol (Torbugesic; Ayerst, Guelph, Ontario), 0.3 mg/kg BW, SC, and meloxicam (Metacam; Boehringer Ingelheim, Burlington, Ontario), 0.2 mg/kg BW, SC. The dog was fasted overnight in preparation for surgery on day 2.

The following morning (day 2), the dog was bright and responsive, and all vital parameters were within normal limits. The dog had urinated overnight indicating a patent urethra. The dog was premedicated with a previously prepared mixture of butorphanol, acepromazine (Atravet; Ayerst, Guelph, Ontario), and atropine (Atropine sulphate; Bimeda-MTC, Cambridge, Ontario), 0.04 mg/kg BW, SC.

Twenty minutes later, urohydropulsion was performed to flush the smaller urethral calculi into the bladder to facilitate their removal via cystotomy. A lubricated 5 French male urinary catheter was passed into the penile urethra until some resistance was felt, approximately at the level of the pelvis. A 12-mL syringe filled with sterile saline was attached to the catheter and continuous flushing was used to help in advancing the catheter into the bladder. The procedure was successful, and with the catheter seated in the bladder, a urine sample was collected. The sample was centrifuged and the sediment viewed microscopically; a mild pyuria, mild hematuria, a small number of transitional epithelial cells, and a few coccioid bacteria were observed. There was no evidence of crystalluria. The urinary catheter was removed and ampicillin (Ampicillin; Novopharm, Toronto, Ontario), 22 mg/kg BW, IV, was administered. A 2nd plain lateral abdominal radiograph confirmed that all uroliths were in the bladder.

Anesthesia was induced with thiopental (Thiopental 2.5% solution; Abbott, Montreal, Quebec), 11 mg/kg BW, IV, followed by isoflurane (Isoflurane; Bimeda-MTC) administered via a Bain system to maintain anesthesia. Once the dog had been anesthetized, blood for a complete blood (cell) count (CBC) and serum biochemical profile was collected from the left jugular vein.

The bladder was approached through a ventral midline incision and isolated from the remaining viscera. Sterile, saline-moistened, gauze squares (4 cm × 4 cm) were packed around the bladder to prevent urine spillage into the abdominal cavity. A 20-gauge needle attached to a 12-mL syringe was inserted through the bladder wall to facilitate drainage of the bladder prior to incising it. Two stay sutures were placed in cranial and caudal positions through the serosal and submucosal muscular layers of the ventral bladder wall. Tension was applied to the stay sutures while the surgeon, using a #10 scalpel blade, incised the ventral bladder wall. The exposed mucosa appeared slightly thickened and hyperemic. A large 2.5-cm, tan, spiculated, hard calculus, and 2 similar, but much smaller (< 1 cm), calculi were removed. The bladder lumen was flushed with 5 mL of ampicillin. The bladder wall was closed by using 4.0 polydioxanone (PDS11; Ethicon, Sommerville, New Jersey, USA) in a 2 layer inverting (Cushing) pattern. The gauze squares were removed and the abdominal cavity was irrigated with 5 mL of ampicillin before closure. The dog was treated prophylactically for pain with butorphanol, 0.4 mg/kg BW, IV, and maintained on IV fluids and ampicillin, 22 mg/kg BW, IV, for the rest of day 2. Postoperative radiographs were not taken to confirm removal of all calculi. The uroliths were submitted to the Canadian Veterinary Urolith Centre (Guelph, Ontario) for composition analysis.

On day 3, the dog appeared bright and vital signs were normal. There was evidence of urination in the cage, and when the dog was walked outdoors, it voided without obvious effort. The CBC revealed a mild leukopenia, mild neutropenia, and a mild decrease in the erythrocyte count. Serum biochemical abnormalities included a mild hypoproteinemia, a mild decrease in urea, and a mild decrease in amylase. The serum calcium level was normal. The dog was sent home to be treated with amoxicillin (Amoxil; Pfizer, London, Ontario), 10 mg/kg BW, PO, q2h for 14 d.

At the time of suture removal (day 12), the dog appeared to be recovering well and the incision had healed. The uroliths were reported to be 100% calcium oxalate monohydrate with a surface composition of 95% calcium oxalate monohydrate and 5% calcium oxalate dihydrate. Calcium oxalate urolithiasis was the definitive diagnosis. The owner chose a preventive diet (Waltham S/O Lower Urinary Tract Support; Effem Food, Bolton, Ontario) and agreed to return in 3 mo for a follow-up urinalysis.

Calcium oxalate calculi are most commonly found in the bladder and less often in the upper urinary tract; they are more common in adult male, small breed dogs (2). In humans, it has been suggested that the presence of calcium oxalate dihydrate on the surface of the calculus is an indication of recent formation (2). Calcium oxalate urolithiasis (COU) is not usually associated with a urinary tract infection. However, in this case, the dog demonstrated evidence of cystitis and was treated with antibiotics. Several conditions are associated with an increased risk of COU, including hyperparathyroidism, hyperadrenocorticism, hypervitaminosis D, and paraneoplastic hypercalcemia (2). The etiology of COU in dogs is thought to be multifactorial. Urethral hydropulsion followed by a cystotomy is the only therapeutic approach for successful removal of calcium oxalate uroliths.

Placement of stay sutures prior to incision into the bladder, results in easier tissue handling and less tissue trauma; however, this requires the assistance of a 2nd surgeon (3). The mucosa of the bladder wall may be thickened and hyperemic, usually attributed to irritation caused by the presence of the cystic calculi. The bladder wall should be biopsied for histologic examination. The main complication following cystotomy is leakage of the bladder wall, usually occurring only when the bladder is severely diseased preoperatively (3). The recurrence rate 1 y following surgery is about 36% (4). With such a high recurrence rate, medical management, dietary modification, and constant monitoring are all necessary objectives of postoperative care.

Postoperative management of dogs with COU begins by obtaining radiographs immediately after surgery. This helps to differentiate surgical failure to remove all of the calculi from recurrence of calculi, if calculi are discovered at a later date (4). A baseline urinalysis should also be performed, including sediment analysis for the presence of bacteria, crystals, or both, and pH measurement to permit objective postsurgical monitoring (4).

The main goals of dietary modification are to decrease calcium concentration in the urine, to decrease oxalate concentration in the urine, to promote high concentration of crystal formation inhibitors in the urine, and to decrease urine concentration. Dietary protein should be restricted to 10% to 18% on a dry matter basis (DM). Higher levels of protein intake have been shown to significantly increase urinary calcium and oxalate excretion (2,5). Dietary sodium should be restricted to < 0.3% DM, because urinary sodium excretion is directly correlated with urinary calcium excretion, such that increasing the excretion of one leads to an increase in excretion of the other (2,5). This includes avoiding table scraps and commercial pet treats that tend to be high in sodium. Dietary calcium should be restricted to between 0.3% and 0.6% DM (2,5). Reducing dietary calcium to these levels reduces the chance of excessive absorption and excretion of calcium (2). Vitamin D and vitamin C supplements should be avoided, since the former will enhance intestinal absorption of calcium and the latter serves as a precursor for oxalate (2,4). Diets that promote relatively alkaline urine (pH 6.8 to 7.0) are encouraged to minimize oxalate crystal formation. Lastly, water should be provided ad libitum, and the dog should be encouraged to drink. Ideally, urine specific gravity should be maintained at < 1.020 (4,5). The chance of crystal formation and precipitation increases as the urine becomes more saturated with solutes. If following these recommendations does not prevent crystal formation, the addition of crystal formation inhibitors may be necessary. The most suitable inhibitor for COU is citrate, because it forms a soluble salt with calcium and decreases precipitation (5).

Currently, 3 commercial diets that aid in the management of COU are available (Hill’s u/d diet; Hill’s Pet Nutrition, Mississauga, Ontario; Waltham S/O Lower Urinary Tract Support; and Medi-Cal Reduced Protein; Veterinary Medical Diets, Guelph, Ontario). It is suggested that a 50:50 mix of wet and dry products be fed to promote increased water intake (5). Although spontaneous dissolution does not occur with any of these diets, preventive dieting remains the most useful means of reducing the recurrence rate in dogs (4). Successful treatment and prevention of canine calcium oxalate urolithiasis relies in part on owner compliance. Therefore, the importance of client education in the management of this disease cannot be over emphasized.