23 Chloe Wormser, Jeffrey J. Runge, and Clarence A. Rawlings Despite advances in the prevention and management of urinary calculi in dogs and cats, urolithiasis remains a commonly encountered problem in companion animal medicine (Figures 23.1 and 23.2). Furthermore, changes in calculus management over the past decade have resulted in an increased percentage of uroliths that are difficult to manage medically.1 Given that persistent urolithiasis can lead to cystitis, urinary tract infection (UTI), hematuria, or urethral obstruction, timely diagnosis and intervention are important to minimize morbidity and the perpetuation of lower urinary tract disease. Figure 23.1 Abdominal radiograph of multiple uroliths in a dog. Figure 23.2 Abdominal radiograph of multiple uroliths in a cat. Bladder and urethral calculi account for 5% of human urinary stones in the Western world and usually occur in patients with underlying urologic disorders.2,3 The most prevalent cause is bladder outlet obstruction due to benign prostatic hyperplasia, but UTI, neurogenic bladder disorders, urethral strictures, foreign bodies, and calculi migration from the upper urinary tract have also been described.3 Conversely, bladder stones are considered endemic among children in developing countries, representing up to 30% of diagnosed urinary calculi in this subpopulation.4-7 Whereas the main component of stones seen in developed countries is struvite, the majority of stones in developing countries are composed of ammonium urate.8 Although the pathogenesis and biochemical explanation for endemic stone formation remain unclear, there seems to be a correlation between urolithiasis and malnutrition, low animal protein intake, and vitamin A deficiency.8-10 In both children and adults, open surgery for removal of lower urinary tract stones has been almost entirely replaced by minimally invasive techniques. The transition to a minimally invasive approach has been prompted by the high number of complications reported with traditional open surgery for stone removal, including recurrent calculus formation, the need for serial surgeries, suture-induced stone formation, strictures, adhesions, bleeding, uroabdomen, pain, and other life-threatening conditions.11-13 Indeed, open surgical removal of stones is currently described in only 0.3% to 4% of human patients with urolithiasis.11 Many stones can be removed using entirely noninvasive methods such as intra- or extracorporeal lithotripsy.11,14,15 In stones not amenable to these techniques, transurethral disintegration and retrieval of bladder and urethral calculi is the current standard of care in adult urologic patients.14-16 The narrow diameter of the urethra in pediatric patients precludes routine use of this technique, and instead percutaneous cystolithotomy is preferable.14,17-19 This procedure is also used in adult patients with large solid stones and in circumstances that do not allow a transurethral approach, such as in patients with anatomic abnormalities.19 In small animals, surgical removal of uroliths by cystotomy or urethrotomy has been the traditional method of choice, and these procedures are still common (Figure 23.3). However, several studies have elucidated insufficiencies in the open cystotomy technique. In a 1992 study, calculi remained in the bladder after cystotomy in 10% of dogs and 20% of cats.20 In a larger, more recent study, removal of uroliths was incomplete in 20% of dogs after cystotomy.21 Furthermore, a 2008 study reported that 9.4% of recurrent stones in dogs were suture induced, indicating that cystotomy could, in fact, increase the risk of stone formation.22 Recently, complications associated with traditional surgical cystotomy, regardless of closure method, were reported in 37% to 50% of cases, with a mean duration of hospitalization of 4 days.23 Figure 23.3 A, Open cystotomy on a male dog with a large cystic calculi. B, Urethrostomy in a male dog that had urethroliths. The above listed shortcomings of the open cystotomy procedure make less invasive alternatives with fewer complications, fewer long-term stone recurrences, more efficient stone removal, and shorter hospitalization times desirable. In small animals, minimally invasive treatment options for lower urinary tract stones include voiding urohydropulsion, intracorporeal lithotripsy, extracorporeal shock-wave lithotripsy, transurethral cystoscopic stone retrieval, laparoscopic-assisted cystotomy, and percutaneous cystolithotomy. Uroliths are organized concretions of primarily organic or inorganic crystalloid (the ionic component of crystals) and a much smaller amount of organic matrix2423.1. Urolith formation is dependent on a multitude of interrelated factors within the urine, including supersaturation with crystals, organic molecule contributions such as protein inhibitors or promoters of crystallization, urine pH, and presence of urine stasis. Not only does the interplay of these factors lead to urolith formation, but it also dictates urolith composition.24,25 It is important to note that the presence of crystals within the urinary tract does not necessarily mean that the patient is at risk for urolithiasis.24 Crystalluria in itself is not a disease, and no particular treatment is necessary unless concurrent urolithiasis is present or the patient has a history of past urolith formation.24 Approximately 89% to 96% of all cats’ uroliths submitted to urolith centers are composed of magnesium ammonium phosphate (struvite) or calcium oxalate.26,27 Currently, struvite and oxalate stones occur with similar frequency in the feline lower urinary tract, although 75% to 87% of upper urinary tract stones (nephroliths and ureteroliths) are calcium oxalate.26,27 An increase in oxalate urolithiasis from 1985 to 1994 is thought to be the result of widespread dietary acidification.26 Changes in dietary management, with less emphasis on urinary acidification, may have affected urolith composition in recent years.26 Less common uroliths recovered from cats include urate (3%–10%); calcium phosphate (0.1%–6%); and dried, solidified blood calculi (1%).26,27 The signalment may help predict the composition of bladder calculi in feline patients. Specifically, male cats are more likely to have oxalate than struvite uroliths (1.6:1).26,27 In contrast, female cats are slightly more likely to have struvite than oxalate uroliths (1.2:1).26,27 Younger cats (younger than 4 years of age) have a higher frequency of struvite uroliths, middle-aged cats (4–7 years) have an equal proportion of struvite and oxalate uroliths, and cats older than 7 years of age are more likely to have oxalate uroliths. However, these differences are not dramatic.26 Breed predilections have also been recognized, with oxalate uroliths twice as common as struvite uroliths in Persians and Himalayans.26,27 Urate uroliths are more commonly recognized in Siamese cats than in other breeds.26,27 Between 80% and 91% of all uroliths submitted from dogs are struvite or calcium oxalate.28-30 Similar to cats, the frequency of oxalate uroliths has increased compared with struvite uroliths over recent years.28 However, the frequency of struvite uroliths is still slightly higher than that of calcium oxalate uroliths (44%–45% vs. 35%–42%).29,30 Whereas female dogs are 12 to 15 times more likely to have struvite uroliths compared with oxalate uroliths, male dogs are three times more likely to have oxalate uroliths.28,29 Younger dogs are more likely to have struvite as opposed to oxalate uroliths.28 Certain breeds are predisposed to developing uroliths, although the interactions among breed, sex, and age complicate prediction of composition. In contrast to cats, in which most upper urinary tract stones contain calcium, upper urinary tract uroliths in dogs are evenly distributed among struvite, calcium oxalate, and other urolith types.1,31,32 The lower urinary tract in male and female dogs can be generally divided into two regions, the bladder and the urethra. The urinary bladder can vary greatly in both size and location within the abdomen depending on the amount of urine it contains. The blood supply to the bladder comes from the cranial and caudal vesical arteries. The urinary bladder has sympathetic innervation from the hypogastric nerves and parasympathetic innervation from the pelvic nerves. The pudendal nerve provides somatic innervation to the external bladder sphincter and the striated musculature of the urethra. Patient evaluation should be aimed at determining renal function, the presence of UTI, and systemic organ function. With regard to uroliths, the number, size, and location of calculi should be determined. Clinical signs associated with bladder calculi are similar to those seen with other diseases of the lower urinary tract and include hematuria, pollakiuria, stranguria, and dysuria. Uroliths and masses can cause a partial or complete urethral obstruction, which may result in bladder distension, abdominal pain, paradoxical incontinence, stranguria, and signs of postrenal azotemia (anorexia, vomiting, depression). Occasionally, bladder rupture and uroabdomen ensue. With cystic calculi, the bladder wall may be thickened, and uroliths may be palpable. With urethral obstruction, the bladder is distended on abdominal palpation. Rectal examination may reveal palpable urethral calculi as well as a distended urethra when obstruction is present. A complete blood count and biochemical profile should be performed in any patient with history or clinical signs compatible with urolithiasis. Abnormalities may suggest a certain urolith type, such as the presence of hypercalcemia in patients with calcium oxalate or calcium phosphate urolithiasis. In cases of lower urinary tract obstruction, azotemia may be present. Azotemia should be addressed before calculus removal in all but the most urgent clinical cases. Confirmed renal dysfunction may require modification of the plan for calculus removal. Uroliths of both the upper and lower urinary tracts may cause or be associated with infection. Leukocytosis may be seen with pyelonephritis in some cases but is not typically associated with simple cystitis.33 A urinalysis should also be evaluated in patients with urinary disorders. Crystal solubility is affected by urine pH. Specifically, struvite uroliths are more likely to form in alkaline urine; calcium phosphate in alkaline to neutral urine; calcium oxalate and silica in neutral to acidic urine; and urate, xanthine, cystine, and brushite in acidic urine.34 In patients without urinary tract disease, calcium oxalate and struvite crystals may form in urine samples that have been refrigerated or analyzed more than 4 to 6 hours after collection. However, in patients with uroliths, crystalluria in a fresh urine sample (<60 minutes) may provide some insight into urolith composition. Uroliths are often associated with UTI. Therefore, urine sediment evaluation is of importance and may reveal pyuria or bacteriuria. Urine culture is indicated in all cases of urolithiasis. Infection has been documented in approximately 75% of dogs with cystic calculi when the results of urine, bladder mucosal biopsy, and urolith culture are combined.35 Ideally, all stone retrieval procedures should be performed after a negative urine culture result has been documented or the patient has been receiving an appropriate antibiotic for at least 24 hours before intervention, although this is not always possible or completely necessary. It is the authors’ opinion that surgery timing in relation to antibiotic treatment is determined on a patient-by-patient basis. Further testing may be indicated for specific urolith types depending on clinical findings (e.g., tests for hyperadrenocorticism in patients with calcium oxalate urolithiasis), which is beyond the scope of this text. Calcium oxalate and struvite uroliths are generally radiopaque; as such, they are usually visible on survey radiographs.36,37 However, 1.7% to 5.2% of these uroliths are not radiographically apparent. These undetected stones are typically small (<1 mm).37 Urate, cysteine, and calcium phosphate calculi are variably radiopaque, and approximately 25% of survey radiographs are interpreted as negative for these uroliths.36 The incidence of false-negative results with survey radiography is 13% for all urolith types combined.37 Radiographic views must include the entire urethra such that urethral calculi are not overlooked. A false-positive radiographic diagnosis can occur because of end-on vessels, nipples, and other structures mistaken for uroliths. Thus, orthogonal radiographic projections should always be taken and compared. Negative-contrast (air) cystography is more sensitive than survey radiography for detection of calculi within the lower urinary tract, with a false-negative rate of 6.5%.37 Double-contrast radiography further improves diagnostic accuracy, with a false-negative rate of 4.5%.37 These techniques have been described in detail elsewhere.38 Briefly, it is recommended that a pool of 200 mg/mL of contrast agent (1 part contrast to 1 part sterile saline) 5 mm deep (∼1–5 mL) provides the best accuracy for determining whether calculi are present or absent.37,39 Although double-contrast radiography is the most sensitive method for counting calculi, an accurate count is reported in only 53% of cases.37 Abdominal ultrasonography is useful for the detection of both radiopaque and nonradiopaque calculi within the urinary bladder. Ultrasonography also helps obtain more information about renal structure and function. Urethral calculi, however, are difficult to visualize with ultrasonography unless they are lodged near the neck of the bladder. The false-negative rate of ultrasonography for detecting uroliths is 3.5%, making it more accurate than survey radiography and comparable to double-contrast radiography.37 Computed tomography (CT) is commonly used for detection of urinary tract stones in human urologic patients. A recent study showed that noncontrast CT could be used to predict stone composition in dogs on the basis of radiodensity, measured in Hounsfield units.40 Cystoscopy is increasingly used and constitutes a reliable method for urolith detection; it also allows for evaluation of the inner architecture of the bladder and urethra for inflammatory and neoplastic lesions. Cystoscopy is typically combined with minimally invasive therapies, including stone retrieval and biopsy.41 Case selection is of critical importance when choosing between open surgery when uroliths are very large (Figure 23.4) or with minimally invasive urolith retrieval techniques when the urolith size does not require the need for conversion to an open cystotomy (Figure 23.5). Surgical failure and complications can often be avoided when careful attention is paid to both patient factors (signalment and concurrent disease) and stone characteristics (location, size). Figure 23.4 Radiograph of male dog with numerous large cystic and urethral calculi. Figure 23.5 Radiograph of a female dog with multiple small cystic calculi. Nearly all calculi in female dogs and cats can be removed by transurethral cystoscopy, laparoscopic-assisted cystotomy, or percutaneous cystolithotomy.41 Most male dogs can be treated with laparoscopic-assisted cystotomy or percutaneous cystolithotomy. Transurethral cystoscopy is preferred for female cats and dogs because it is less invasive than laparoscopic-assisted techniques. However, the calculi must be small enough to be exteriorized by the transurethral route. Size criteria are continually being modified, but in general, in female cats and dogs, calculi can be removed that are twice the diameter of the largest cystoscope appropriate for the patient. In male dogs, transurethral removal is limited to smaller calculi because the stones must pass the os penis region of the urethra. Calculi in male cats can be removed by laparoscopic-assisted cystoscopy and percutaneous cystolithotomy, but the urethra is likely too small for current transurethral cystoscopy techniques. Transurethral cystoscopic calculus removal in female dogs has been enhanced in some specialty hospitals by cystoscopic lithotripsy.42-46
Laparoscopic-Assisted Cystoscopy for Urolith Removal and Mass Resection
Preoperative Considerations
Introduction to Urolith Management
Surgical Management of Urolithiasis: Comparative Aspects
Veterinary Surgical Management of Urolithiasis
Pathophysiology of Uroliths in Dogs and Cats
Epidemiology
Cats
Dogs
Surgical Anatomy
Diagnostic Workup and Imaging
History and Clinical Signs
Physical Examination Findings
Laboratory Testing
Diagnostic Imaging
Survey Radiography
Contrast Radiography
Abdominal Ultrasonography
Computed Tomography
Cystoscopy
Patient and Technique Selection
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