Page 1199 - Clinical Small Animal Internal Medicine

123 Urolithiasis in Small Animals 1137

of water increase strategies is to obtain a urine specific Since the rapidity of dissolution is related to the size and
VetBooks.ir gravity of <1.020. surface area of the urolith being exposed to urine, one
large urolith will take longer to dissolve than multiple
small uroliths. An analogy for this is that one large ice
Indications for (and Limitations of) Medical
Urolith Dissolution cube will dissolve more slowly than an equal volume of
crushed ice. The rate of dissolution is influenced by the
Urolith Type surface area of the urolith exposed (“bathed”) in under­
Struvite, urate, and cystine stones in dogs may be amena­ saturated urine.
ble to medical dissolution; however, the uroliths must be
located in undersaturated urine for this to occur. This Presence of Obstruction
requires intervention with diet and urine dilution strate­ Medical dissolution is contraindicated in patients with
gies, along with appropriate therapy of concurrent bacte­ obstruction of the urinary tract. Relieving urinary tract
rial infections. Dissolution success is much lower for obstruction in those cases is mandatory in order to avoid
urate and cystine stones than struvite, and in cats, only consequences such as postrenal azotemia, irreversible
struvite uroliths are responsive to medical dissolution. In renal damage, bladder rupture, and death.
cats, low‐protein diet and allopurinol has been reported
as effective in urate stone dissolution; however, until fur­ Risk of Obstruction in Males
ther studies looking into safety and efficacy of this treat­ Medical dissolution of urocystoliths in male dogs is asso­
ment are performed, mechanical removal of bladder or ciated with a risk of urethral obstruction once the uro­
urethral urate uroliths is advised. Mechanical urolith cystoliths are small enough to pass into the urethra. The
removal is recommended for bladder or urethral cystine risks and clinical signs of urethral obstruction should be
stones in cats, in part because 2‐MPG (Thiola®, tiopronin) explained to the owner if medical dissolution is elected
is toxic in cats, and is only effective about 60% of the time in male dogs, and regular monitoring of urination is rec­
in dogs.
ommended. Medical dissolution should only be tried in
male dogs after clear owner communication and assur­
Urolith Location ance of owner compliance and ability to observe and act
Medical dissolution for specific stone types can be suc­ on clinical evidence of obstruction.
cessful, but is more efficient when uroliths are located in
an area where they are in constant contact with urine,
such as the bladder. Suspected struvite nephroliths or Infection‐Induced Urolith
ureteroliths should be medically dissolved unless they If the urease‐producing bacterium is isolated from uro­
are too large to be effectively bathed in urine. lith culture, appropriate antibiotic therapy based on
If uroliths are obstructive, relief of obstruction with a urine culture and susceptibility must be administered
minimally invasive approach and rapid control of infec­ during the entire urolith dissolution period. Otherwise,
tion should preserve kidney function. the persistence of infection with a urease‐producing
Obstructive ureteroliths cannot be dissolved by medi­ organism will confound dissolution of the stones. Despite
cal or dietary protocols unless a ureteral stent is placed diet change, concurrent urinary tract infection will
concurrently. Urethroliths are typically not amenable to maintain alkaline urine and promote urolith formation
dissolution, although they may be flushed into the blad­ and growth.
der by retrograde hydropulsion and then dissolution can
be attempted. Dietary Factors
Calcium oxalate uroliths are not amenable to medical Limitations of medical dissolution are inherent to the
dissolution regardless of location, and must be removed diet and medical therapy needed for the given urolith; for
from the urinary tract mechanically. In cats, because example, an acidifying diet with low protein and caloric
more than 90% of feline upper urinary tract stones are content should not be fed to a pregnant or lactating
composed of calcium oxalate, dissolution should not be bitch. Similarly, thoughtful consideration of nutritional
attempted with obstructive nephroliths or ureteroliths. growth requirements in young dogs and kittens needs to
Silica uroliths are also thought to be unresponsive to occur when a diet lower in purines (protein) is indicated
medical dissolution. for medical urolith dissolution (e.g., urate uroliths).
Similarly, a diet with a high salt or fat content should be
Size and Number of Uroliths avoided in a patient with congestive heart failure or pan­
The size and number of the stones will influence rapidity creatitis, respectively, as these patients may require vary­
of dissolution but do not dictate the likelihood of ther­ ing levels of sodium or fat restriction for their concurrent
apy success in stones that are amenable to dissolution. medical condition.