Page 1197 - Clinical Small Animal Internal Medicine

123 Urolithiasis in Small Animals 1135

transducer, and found that ultrasonography may be
VetBooks.ir more sensitive than survey and contrast cystography
for detection of urocystoliths, with false‐negative rates
of 6%.
Ultrasonography allows simultaneous evaluation of
the upper urinary tract for nephroliths and ureteroliths.
Therefore, radiography combined with ultrasonogra­
phy are preferred over contrast radiography alone.
Ultrasound can detect upper urinary tract obstruction
that is characterized by renal pelvis dilation >13 mm.
However, pelvis dilation can be iatrogenic (diuresis) or
secondary to different diseases such as chronic kidney
disease or pyelonephritis. Although a renal pelvis
>13 mm may indicate ureteral obstruction, the absence
of dilated renal pelvis does not rule out an upper urinary
tract obstruction. The combination of proximal ureteral
dilation and normal kidney pelvis diameter can be asso­
Figure 123.12 Abdominal radiographs (lateral view) showing ciated with ureteral obstruction in cats. In a recent study,
bladder and urethral stones in a dog. Source: Courtesy of 25% of cats with confirmed ureteral obstruction had a
Stephanie Nykamp, Radiologist, Ontario Veterinary College.
renal pelvis <13 mm but a proximal ureteral dilation. In
25% of cats with ureteral obstruction, ureteropyelog­
raphy is necessary to determine the cause of the
obstruction.

Computed Tomography
Determination of urolith composition using computed
tomographic (CT) peak attenuation measurement has
been described in vitro. In humans, this technique is
preferentially used in order to detect uroliths and also to
predict their composition. CT may be more specific for
ureteral calculi than either radiography and/or ultra­
sonography in cats.
Figure 123.13 Double‐contrast cystography (lateral view) in a
dog showing multiple, various sized filling defects centrally in the Educated Guess
contrast pool indicative of bladder stones. Source: Courtesy of
Dr Stephanie Nykamp, Radiologist, Ontario Veterinary College. Determining the type of urolith based on imaging stud­
ies alone is very risky. This type of “guesstimate” of uro­
Ultrasonography lith composition based on the radiolucency of a stone
Ultrasonography is a cost‐effective tool in the diagno­ and its shape can lead the clinician to misdiagnosis
sis of urolithiasis but it may overlook some important (Figure 123.14). In certain scenarios the chemical com­
aspects of the stones that can help the clinician predict position of uroliths may be predicted based on patient
the nature of the uroliths, such as surface contour and signalment, in combination with the results of urinalysis,
shape. One limitation of ultrasound is that it often urine culture, and radiographs. However, it must be
overestimates urolith size, so radiographs should be understood that this technique can lead to error and is
used for determining size when possible. Ultrasound is not advised as the best method of urolith identification.
also less reliable than radiography for predicting the In situations where uroliths are not available for analysis
number of stones and their location. Finally, ultra­ or are pending analysis, the guesstimate may need to be
sound only provides evaluation of the most proximal used; however, communication to clients that errors may
portion of the urethra and therefore cannot replace be made with this technique and verification of stone
radiography. type (when practical and possible) are ideal. Unfortunately,
Ultrasonographic examination can detect radiopaque this verification cannot be made until stone analysis
and radiolucent uroliths. One study compared survey occurs. For example, the clinician can make an educated
and contrast radiographs, pneumocystography, double‐ guess as to the likelihood that a given stone is com­
contrast cystography, and ultrasound using a 7.5 MHz posed of struvite based on finding a urease‐positive