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on the metal resulted from shock waves generated
by the collision. Shock waves traveling through fluids create damage by creating a strong tensile force on
the interface of solution and solid material. Struvite is most easily fragmented, followed by calcium oxalate, then urate, then cystine. When upper urinary tract uroliths fragment, the small fragments pass into the urinary bladder over several weeks; their passage is
not instantaneous. Animals that have undergone ESWL should have periodic examinations including serum biochemical analysis, urinalysis, and radiographs to monitor for successful passage of fragments and to detect any compromise of renal function due to the lithotripsy procedure. Acceptable canine candidates
are those with nephroliths smaller than 2-3 cm in
their greatest dimension or ureteroliths. With bilateral nephroliths, both kidneys are treated at the same time, unless there is concern about compromising renal function further. More than 100 dogs with nephroliths
or ureteroliths have been treated at 3 institutions. Most
of the uroliths were composed of calcium oxalate and most nephroliths fragment with 1 or 2 treatments. Feline nephroureteroliths appear to be more difficult to fragment with ESWL than in dogs, and renal function is more
likely to be compromised. Successful fragmentation
has been reported to occur in < 20-25% of feline
cases. Although renal function was normal in 4 healthy cats undergoing lithotripsy, lithotripsy of clinical cases of upper urinary tract urolithiasis in cats suggests that many cats, particularly those with pre-existing renal disease, experience renal function compromise or worsening of their renal failure. While lithotripsy is considered safer and less invasive than surgical removal of upper urinary tract uroliths, there are risks. Abdominal pain, hemorrhage, and bruising of the kidneys occur, and hematuria may
be observed immediately after the procedure. More significant hemorrhage within or around the kidney
may occur in some cases. Residual stone fragments often take several weeks to move from the kidney into the urinary bladder. Transient or permanent ureteral obstruction can occur. If permanent and progressive ureteral obstruction occurs, it requires re-treatment
by lithotripsy or surgical intervention. Uncommon complications include pancreatitis, bowel irritation, hemolysis, and systemic hypertension.
Intracorporeal laser lithotripsy is not often possible
due to the inability to access the ureter via the urinary bladder in cats and most dogs. The smallest flexible ureteronephroscope is 2.2mm in diameter and feline ureters are approximately 0.5 mm in diameter and canine ureters are approximately 0.5 to 1.5mm
in diameter. One option is to perform endoscopic nephrolithotomy. Placement of a ureteral stent (described below) results in ureteral dilation over time and ureters may dilate to approximately 4mm around a ureteral stent, which could allow for passage of a flexible
An Urban Experience
ureteronephroscope through the urinary bladder and into the ureter if the scope can be inserted through the ureterovesical junction.
Endoscopic nephrolithotomy. A rigid scope can
be inserted through the renal parenchyma into the dilated renal pelvis. The nephrolith is visualized and either retrieved or fragmented using laser lithotripsy or fragments retrieved. Usually a rigid cystoscope (1.9mm or 2.7mm) is used as human nephroscopes are too large to use in cats and dogs. There are no large studies of efficacy and complications in dogs and cats and this technique requires a surgical approach to the kidney.
Ureteral stent.(6) In patients where nephroureteroliths cannot be managed surgically, urinary diversion may be accomplished by placing a ureteral stent. Usually
a double pig-tailed stent is placed surgically, fluoroscopically, or via cystoscopy. One of the pig- tails is placed so that it is within the dilated renal pelvis and the other pig-tail is placed so that it is within the urinary bladder. The body of the stent connects the 2 pigtails and provides diversion of urine flow around the obstructive ureteroliths.
Subcutaneous ureteral bypass device (SUB).(7) A subcutaneous ureteral bypass device is used to divert urine from renal pelvis to urinary bladder bypassing the ureter. It is similar to a nephrostomy tube; however, it can be used long term and implanted subcutaneously. A locking pig-tail catheter is inserted surgically into the renal pelvis and the kidney is sutured to the body wall (nephropexy). The tube is tunneled subcutaneously to
a metallic port that is implanted subcutaneously just off of ventral midline. The metallic port is used for collecting a urine sample using a special needle (Huber needle).
A tube exits the other side of the port and re-enters the abdomen and is inserted near the apex of the urinary bladder, which may be sutured to the ventral abdominal wall (cystopexy).
Comparison of ureteral stent with SUB. Ureteral
stenting is often performed in cats and more so than in dogs. There is a reported mortality of < 2% for dogs and approximately 8% for cats. Success is highly dependent on the training and ability of the individual placing the ureteral stent. Dysuria from the urinary bladder pig tail occurs in approximately 40% of cats and less than 2% of dogs. Often this can be managed with anti-inflammatory drugs and anti-spasmodic. Re-occlusion may occur in up to 10% of dogs and 25% of cats. The advantage of a ureteral stent is that no addition management is required; however, stent encrustation may occur necessitating removal and possible replacement. The ureter often dilates around the ureteral stent and ureteroliths may subsequently pass into the urinary bladder. In many dogs, ureteral stents may be placed using interventional radiology whereas in cats usually stents require surgical placement. SUBs are more often placed in cats than
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