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.
Many 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.