Feline nephroliths and ureteroliths (Proceedings)
Nephroliths are uroliths (calculi) located in the renal pelvis and/or collecting diverticula of the kidney and ureteroliths are calculi located in the ureter. Although only 5 to 7% of all feline uroliths submitted to stone centers for analysis are nephroliths, the true incidence of nephroliths may be higher because many animals with nephroliths are asymptomatic. Also because removal of uroliths from the upper urinary tract is inherently more difficult, nephroliths and ureteroliths are less likely to be submitted to stone centers than are lower tract uroliths. Furthermore, nephroliths are an important clinical problem because of their potential complications. Nephroliths may obstruct the renal pelvis or ureter, may predispose to pyelonephritis, and may result in compressive injury of the renal parenchyma leading to renal failure. Nephroliths may be considered "inactive" if they are not causing any complications. Inactive nephroliths may not require removal, but they should be monitored periodically by urinalysis, urine culture, and radiography. Indications for removal of nephroliths in cats include obstruction and recurrent infection. We have observed spontaneous retrograde movement of ureteroliths five cats. Furthermore, we have also documented uroliths moving from the renal pelvis into the proximal ureter, and back into the renal pelvis in some animals. This observation indicates that even so-called 'inactive' nephroliths may cause intermittent obstruction and subsequent renal injury.
Calcium oxalate was the most common composition (70.5%) of feline nephroliths and ureteroliths submitted to the Minnesota Urolith Center, followed by matrix (8%), struvite (8%), calcium phosphate (4.5%), compound (5%), purine (2%), and mixed (2%).7 Feline ureteroliths are much more common than 20 years ago. One study showed a 50-fold increase in the frequency of calcium oxalate uroliths in the upper urinary tract of cats from 1981 to 2000.
Clinical SignsMany dogs and cats with nephroliths are asymptomatic and the nephroliths are diagnosed during work up of other problems. Clinical signs noted in some patients include hematuria, recurrent urinary tract infection (UTI), vomiting, abdominal or lumbar pain (uncommon), and uremia due to either bilateral ureteral obstruction or progressive renal injury resulting in renal failure. Cats with ureteroliths may also present with acute or chronic renal failure. Ureteroliths may be bilateral at the time of presentation, or one kidney may be non-functional from prior ureteral obstruction when a subsequent ureterolith obstructs the contralateral kidney.
Radiodense nephroliths and ureteroliths are usually diagnosed by abdominal radiography. Ultrasonography or excretory urography may be used to confirm the presence, size, and number of nephroliths and ureteroliths; however, ultrasonographic confirmation of ureteroliths is not always possible. Cats with ureteral obstruction may have poor excretion of dye during excretory urography and identification of the cause of ureteral obstruction may be facilitated by antegrade pyelography via nephropyelocentesis. In cats, mineralization of the renal pelvis and/or collecting diverticula must be differentiated from true nephrolithiasis. Sometimes this distinction is difficult to definitively determine. Definitive identification of nephrolith mineral type requires quantitative analysis of nephroliths or nephrolith fragments,although nephrolith composition can frequently be predicted on the basis of signalment, radiographic appearance, and urinalysis findings.
Urinalysis results from cats with nephroliths or ureteroliths may reveal hematuria and crystalluria. Crystal identification and urine pH may be helpful in predicting nephrolith composition. Pyuria and bacteriuria may be noted in patients with concomitant UTI. Urine culture should be performed on urine obtained by cystocentesis from all cats with nephrolithiasis or ureteroliths. Serum chemistry profile and CBC may indicate systemic abnormalities that have resulted from or contributed to nephrolithiasis. An inflammatory leukogram may be noted with concurrent pyelonephritis especially if ureteral obstruction is also present. Azotemia may be present with bilateral renal disease and/or obstruction. Hypercalcemia is occasionally noted as a predisposing factor to calcium oxalate urolithiasis.