Page 1195 - Clinical Small Animal Internal Medicine

123 Urolithiasis in Small Animals 1133

that is, there are concurrent clinical signs of infection. example of this is a calcium oxalate stone forming in an
VetBooks.ir Urinary tract infections can be the primary cause of uro­ at‐risk breed of dog after an incidence of struvite uro­
lithiasis due to bacterial infection.
lith formation (e.g., struvite uroliths in dogs), or they can
be secondary to urolith‐induced irritation of the urinary
tract. If the urine culture is negative despite a high suspi­ Stone Layers
cion of UTI in a patient (i.e., a dog with struvite stones), When a urolith is analyzed, it is named according to the
culture of the bladder wall and stone nidus, at the time of constituents making up the majority of the “stone”
stone removal, is strongly advised in order to accurately (Figure 123.10). These layers consist of the following.
direct any needed antimicrobial therapy.
● Nidus (center of the urolith): this may or may not be
composed of crystalline material and/or a foreign
Bloodwork body, such as suture material.
Stone: the stone may or may not be composed of the
Complete blood count (CBC) is usually normal in ● same minerals as the nidus.
patients with urolithiasis. A left shift may (or may not) be Shell: the layer(s) that surround the stone. The shell is
present with pyelonephritis or pyonephrosis. A serum ● the outermost complete layer on the urolith.
biochemistry may show evidence of liver dysfunction in Surface crystals: these may or may not be present on
a dog with hepatic vascular abnormalities (portosys­ ● the outermost surface of the urolith. They do not com­
temic shunt [PSS]), microvascular dysplasia (portal pletely encase the urolith (incomplete layer).
hypoplasia), or severe hepatic disease. The lab changes
can include increased alanine aminotransferase (ALT) Ideally, all four layers of the urolith should be analyzed
and alkaline phosphatase (ALP), and with hepatic dys­ separately and their respective mineral amounts listed.
function there can be low albumin, urea, and glucose. Only quantitative analysis ensures that all urolith layers
Bile acid (pre‐ and postprandial) measurement is are examined and provides an estimate of the mineral
strongly recommended if a young dog or cat presents content of each visible layer. This is important since con­
with bladder stones and clinical signs compatible with fusion over urolith content and therapy and prevention
PSS (i.e., signs of hepatoencephalopathy, polyuria/poly­ can occur when only the outer layer is analyzed. This type
dipsia [PU/PD], poor growth) or increased liver enzymes. of confusion can also occur when using the qualitative
Dogs with portovascular abnormalities are more likely to
have urate uroliths than those with other types of hepatic
dysfunction. Middle‐aged, small‐breed dogs with a por­ CANADIAN VETERINARY
tovascular anomaly and good portal perfusion can have UROLITH CENTRE
normal chemistry panels with urate stones, making bile Terminology for Urolith Reporting*
acid testing indicated.
In cases of ureteral calculi resulting in urinary tract
obstruction, blood urea nitrogen (BUN) and creatinine
may or may not be increased. This will depend on the
laterality (unilateral or bilateral obstruction) and under­ Shell
lying intrinsic renal damage due to the calculi. Azotemia Stone
will not be present if the contralateral (nonobstructed) Surface
kidney is normal. Most cats (88%) with unilateral ure­ Crystals
teral calculi are azotemic (>90%), indicating the presence Nidus
of intrinsic renal disease in the contralateral kidney.

Stone Analysis
A common pitfall for clinicians dealing with urolithiasis
in small animals is attempting to guess the type of
stone(s) noted on imaging based on signalment of the
patient, diet, urine pH, urine culture results, and radio­
graphic density of the stone. For this reason (and if pos­ * These terms will be used
sible), it is always recommended to submit uroliths for in all reports from the
Canadian Veterinary
mineral analysis. It is important to note that even if there Urolith Centre.
is recurrence of urolithiasis in the same patient, the uro­ Figure 123.10 Stone layers description. Source: Courtesy of
lith type may be different from the prior episode. An Andrew Moore, Canadian Veterinary Urolith Centre.