Struvite urolithiasis with eosinophilic polypoid cystitis in a shih tzu dog

Abstract

A 7-year-old female spayed shih tzu dog was presented with hematuria of 4 weeks’ duration. Radiographs revealed 1 cystic calculus. A polypoid mass was found incidentally during cystotomy and was removed by partial cystectomy. Histopathology revealed eosinophilic polypoid cystitis and urolith analysis reported struvite. A urinary tract infection was treated.

A 7-year-old female spayed shih tzu dog was presented to the Alpine Veterinary Medical Centre in Whitehorse, Yukon, with clinical signs of hematuria of 4 weeks’ duration. On presentation, the dog was bright, alert, responsive, and had a body condition score of 5/9. Physical examination revealed a small palpable mass in the caudal abdomen and a grade 1 luxating patella in the left hind limb. The remainder of the physical examination was unremarkable. The patient was up-to-date on rabies, canine distemper virus, canine adenovirus type-2, canine parvovirus, and canine parainfluenza virus (DA2PP) vaccinations. A complete blood cell count (IDEXX Procyte Dx; IDEXX Laboratories Canada, Burlington, Ontario) revealed a moderate thrombocytosis [587 × 10⁹/L; reference interval (RI): 148 to 484 × 10⁹/L], and elevated plateletcrit (0.60%; RI: 0.14% to 0.46%). Biochemistry (IDEXX Catalyst Dx; IDEXX Laboratories Canada) revealed high normal total protein (81 g/L; RI: 52 to 82 g/L), albumin (39 g/L; RI: 23 to 40 g/L), and globulin (42 g/L; RI: 25 to 45 g/L).

A plain right lateral abdominal radiograph revealed one 18-mm round radiopaque cystic calculus. A cystotomy was recommended for removal of the urolith. The patient was pre-medicated with hydromorphone (Hydromorphone hydrochloride; Summit Veterinary Pharmacy, Aurora, Ontario), 0.1 mg/kg body weight (BW), SC, and acepromazine (Atravet; Boehringer Ingleheim, Burlington, Ontario), 0.03 mg/kg BW, SC, induced with ketamine (Ketalean; Bimeda-MTC Animal Health, Cambridge, Ontario), 10 mg/kg BW, IV, and diazepam (Diazepam, Sandoz Canada, Boucherville, Quebec), 0.25 mg/kg BW, IV, and maintained with isoflurane inhalant and oxygen on a Bain system (fresh gas flow rate: 150 mL/kg BW per min). Cefazolin (Cefazolin Sodium; Hospira, St. Laurent, Quebec), 22 mg/kg BW, IV, was given at induction.

The patient was placed in dorsal recumbency and the ventral abdomen was clipped and prepared from xiphoid to pubis. A ventral midline celiotomy was performed from just caudal to the umbilicus to the cranial brim of the pubis. The urinary bladder was exteriorized and the abdomen was packed with moistened gauze. Two stay sutures were placed in cranial and caudal positions through the serosal and submucosal layers. Tension was placed on the stay sutures and a ventral midline incision was made. One 22-mm brown/beige, smooth, irregular stone was removed from the bladder. Palpation of the bladder identified a 15-mm polypoid mass in the cranial dorsal aspect of the bladder mucosa. At this time, the owners were contacted and they agreed to removal of the mass without knowing its composition. The cystotomy incision was closed with 3-0 PDS (Ethicon; Johnson & Johnson, Markham, Ontario) in a continuous pattern and a partial cystectomy was performed around the stump of the mass for en bloc removal. The incision was closed with 3-0 PDS (Ethicon; Johnson & Johnson) in a continuous pattern. The patient’s abdomen was closed using a routine 3-layer closure. Intravenous fluids (0.9% NaCl, 3 mL/kg BW per hour) were continued after surgery and meloxicam (Metacam; Boehringer Ingleheim, Burlington), 0.1 mg/kg BW, SC, was given. A postoperative right lateral abdominal radiograph was taken and confirmed removal of all calculi. The patient was discharged that night and prescribed meloxicam (Metacam; Boehringer Ingleheim), 0.1 mg/kg BW, PO, q24h, for 4 d and amoxicillin (Amoxil; Pfizer, Kirkland, Quebec), 22 mg/kg BW, PO, q12h for 14 d.

The urolith was sent to the Canadian Veterinary Urolith Centre, Guelph, Ontario, for analysis of its composition. The urolith was reported to be 70% magnesium ammonium phosphate hexahydrate (struvite) and 30% calcium phosphate (carbonate form) with a 100% struvite shell.

The fixed mass was sent to the Animal Health Laboratory (AHL), Guelph, Ontario, for histopathology. Histopathology revealed 1 large, raised polypoid mass composed of plump spindle to stellate stromal cells in an edematous and fibrillar matrix with extensive inflammation. The inflammation site was characterized by many eosinophils, along with lymphocytes, plasma cells, a few macrophages, and neutrophils. These findings were consistent with a diagnosis of eosinophilic polypoid cystitis.

The patient was returned to the clinic 2 wk after surgery for a urinalysis (IDEXX SediVue Dx; IDEXX Laboratories Canada). The owners reported no further hematuria or abnormal behavior since surgery. A urine sample obtained via cystocentesis was cloudy, dark yellow, with a urine-specific gravity (USG) of 1.030, > 50 WBC/high power field (HPF), and cocci. The patient was treated empirically with enrofloxacin (Baytril; Bayer, Mississauga, Ontario), 11 mg/kg BW, PO, q24h, for 7d and the owners were advised to increase the patient’s daily water intake.

The patient was returned to the clinic the following week for a repeat urinalysis. A urine sample obtained via cystocentesis was clear, pale yellow, with a USG of 1.023, 1 to 5 WBC/HPF, and no-to-rare cocci. A urine culture was negative for bacterial growth.

Discussion

Eosinophilic polypoid cystitis (EPC) is an uncommon disease of the urinary bladder of dogs characterized by inflammation, epithelial proliferation, and infiltration of a high number of eosinophils (1–6). Polypoid cystitis is well-documented in humans and is most commonly a complication following mucosal irritation from urethral catheterization (7).

The etiology of EPC in dogs is not clear; however, it is suspected to be associated with chronic irritation of the bladder mucosa due to urinary tract infection or urolithiasis (1–6,8,9). Lesions occur most commonly in the cranioventral bladder and vary from solitary to multiple, polypoid, pedunculated, or nodular masses arising from the mucosa or submucosa (1,2,6). Chronic hematuria is often the presenting complaint and is suspected to be a result of irritation and ulceration of the mass(es); however, when present with concurrent UTI or urolithiasis, there may be multiple sources of hemorrhage (1,3–5).

Differentials for a urinary bladder mass include: transitional cell carcinoma, leiomyoma, fibrosarcoma, myxoid sarcoma, and polyp (4,9). Diagnosis of EPC is based on histopathologic examination which reveals fibrosis of mucosa and muscularis, mucosal ulceration, Bunn’s nests, and presence of inflammatory infiltrate with numerous eosinophils involving the mucosa and submucosa (2,4).

Treatment of eosinophilic polypoid cystitis requires the removal of the inciting cause of inflammation, reduction of inflammation, and possible removal of polyps (1,4,5). Partial cystectomy for full thickness removal of polyps, removal of uroliths if present, and adjunctive treatment of infection have been reported as an effective treatment (1,3,5). Mucosal or submucosal excision has been described for cases of diffuse polypoid cystitis even if as much as 75% of the bladder needs to be removed, in cases where the trigone is intact (5).

Concurrent struvite urolithiasis and a UTI were identified in this case. Struvite urolithiasis is a common disorder in dogs, occurring most frequently in female miniature schnauzer, shih tzu, bichon frise, Yorkshire terrier, Lhasa Apso, pug, and cocker spaniel breeds (10–12). Predisposition in small breeds has been suggested to be a result of lower urine volume, lower frequency of micturition, and a higher USG (13).

Urinary tract infection with bacteria that produce urease is the most common cause of struvite urolithiasis (12,14). The urease enzyme hydrolyzes urea to ammonia, which buffers the hydrogen ions forming ammonium ions, increasing the urine pH, and increasing dissolved ionic phosphate which may precipitate to form a urolith in the presence of magnesium (14).

Treatment of infection-induced struvite urolithiasis involves increasing water intake, appropriate antimicrobial therapy, and feeding a urine-acidifying diet with a relative super saturation for struvite (11,14). In cases in which urine outflow is obstructed, a cystotomy for removal of uroliths is recommended (11).

In this case, the patient’s clinical signs resolved following partial cystectomy, removal of the urolith, and antimicrobial therapy. This report describes a rare form of cystitis characterized by ulcerative polypoid masses and eosinophilic infiltrate. Eosinophilic polypoid cystitis is an important differential diagnosis for urinary bladder masses as not all masses are neoplastic and treatments and prognoses are different. Further study is warranted in order to determine the exact pathogenesis of EPC and its potential for malignant transformation.