Radiography

Although a clinical suspicion may be present based on physical examination findings, definitive diagnosis is obtained via imaging. Abdominal radiography generally is the first-line diagnostic imaging modality used to evaluate the urinary tract for radiopaque uroliths. With radiography, the exact location and number of uroliths often can be determined, whereas these sometimes can be difficult to distinguish with other forms of imaging. On the other hand, radiography has the distinct limitation of being able to identify only radiopaque uroliths, and thus dried solidified blood calculi, mucus plugs, and radiolucent uroliths potentially can be missed. Furthermore, overlapping visceral organs occasionally can make diagnosis difficult; abdominal preparation with enemas is necessary if fecal material obscures the ureteral paths. The sensitivity of abdominal radiography for the diagnosis of ureteroliths was 81% in one feline study and 88% in one canine study (Kyles et al, 2005a; Snyder et al, 2005).

Ultrasonography

Parenchymal hyperechogenicity, perirenal effusion, and ureteral wall thickening may be observed on ultrasonographic examination. Although dried solidified blood calculi, mucus plugs, and radiolucent uroliths can be challenging to visualize via ultrasonography, renal pelvic and ureteral dilation would support a diagnosis of ureteral obstruction. Marked hydronephrosis or hydroureter may be noted with prolonged or complete obstruction. The sensitivity of abdominal ultrasonography was 77% for the detection of feline ureteroliths in one retrospective series, whereas in one canine study, sensitivity reached 100% (Kyles et al, 2005a; Snyder et al, 2005).

Abdominal ultrasonography and radiography are best used as complementary imaging modalities. When ultrasonography and radiography were evaluated in tandem, sensitivity for detection of uroliths was 90% in cats with ureteral obstruction (Kyles et al, 2005a). In dogs, ureteroliths and nephroliths were identified during ultrasonographic examination that were missed on abdominal radiographs (Snyder et al, 2005).

Other Modalities

When the location and degree of ureteral obstruction are difficult to document definitively, advanced techniques occasionally must be used to aid diagnosis and planning. One such technique is antegrade pyelography, in which a contrast agent is injected directly into the renal pelvis with ultrasonographic guidance, which minimizes systemic absorption. Fluoroscopy or serial radiography follows the flow toward the urinary bladder. Antegrade pyelography has a high specificity for ureteral obstruction; in a retrospective study of 11 cats with 18 obstructed kidneys, antegrade pyelography allowed correct identification of the anatomic location of the ureteral obstruction in 13 (72%) of the ureters (Adin et al, 2003). However, leakage of contrast material developed in 8 of 18 kidneys (44%) and prevented diagnostic interpretation in 5 of 18 studies (28%). Although highly useful, antegrade pyelography requires general anesthesia and a skilled sonographer. Hemorrhage also is a possible complication of the technique.

Conservative Management of Nephroliths

Most nephroliths initially are managed conservatively and never require further intervention. The majority of nephroliths in dogs are asymptomatic. Nephroliths have not been associated with an increase in disease progression or poorer outcome in cats with concurrent kidney disease and nephrolithiasis compared with cats with chronic kidney disease alone (Ross et al, 2007).

Nephroliths can become damaging to renal function and patient comfort, however, when they contribute to recurrent urinary tract infection or obstruction. In 2006, Dalby and colleagues described seven cases of spontaneous retrograde movement of ureteroliths in two dogs and five cats. Based on this observational study, the authors theorized that some mobile nephroliths may contribute to continued kidney injury by intermittent obstruction of the ureter. The location of the nephrolith may influence the risk of obstruction; nephroliths that are located within the renal interstitium certainly pose less threat of intermittent obstruction than nephroliths located within the renal pelvis. Indications and options for intervention in cases of problematic nephroliths are discussed further in Chapter 195.

Management of Concurrent Urinary Tract Infection

In two studies of dogs with upper urinary tract urolithiasis, 65% to 70% of dogs had a concurrent urinary tract infection (Ling et al, 1998; Snyder et al, 2005). The most commonly observed bacteria were Staphylococcus intermedius, Escherichia coli, Proteus mirabilis, and Streptococcus spp, and infection was recognized more commonly in female than in male dogs. Cats with urolithiasis less commonly have an associated concurrent infection than do dogs, although approximately one third of cats have a bacterial infection (Ling et al, 1998). Antibiotic therapy should be guided by urine or urolith culture results and should be continued for a minimum of 4 to 6 weeks, with repeat urine cultures performed 1 to 2 weeks after the initiation of antibiotic therapy and 1 week after the completion of therapy (Weese et al, 2011). If the patient has concurrent azotemia, then the possibility of pyelonephritis should be considered and the duration of antibiotic therapy should be 6 to 8 weeks, with cultures performed every 2 weeks during therapy and 1 to 2 weeks after the termination of treatment. In dogs, in which 50% of nephroliths are struvite (infection-induced) uroliths, antibiotics may need to be administered for as long as 2 to 3 months before there is complete dissolution. Some dogs have had documented negative results on cultures of lower urinary tract specimens, but positive results on cultures of samples from the upper urinary tract, particularly when the infection is associated with urolithiasis or antibiotics were administered before culture (Snyder et al, 2005).