Urinary Stones: Cause, Treatment, Prevention

Does your dog or cat have urinary stones? Perhaps you know them
by the name kidney stones or bladder stones. They form in the
urinary tract of all kinds of animals and in humans. Urinary
stones are rock-hard structures. Stones in the urinary tract
cause problems because this system is designed to eliminate body
wastes in liquid form, whereas the intestinal tract is designed
to eliminate wastes in solid form.

In animals and in humans, urinary stones are often called
“calculi,” from the Latin word for stone. It is the same Latin
word used in the mathematical term calculus, as stones were at
one time used for counting. The word “lith” as in “urolith” is
from the Greek word for stone. The prefix “uro-” is a Greek term
referring to the urinary tract. Thus, a urolith is a urinary
tract stone that may be located in the kidneys (nephrolith;
nephro is Greek for kidney), ureters (ureterolith), urinary
bladder (urocystolith), or urethra (urethrolith).

Have you ever wondered why stones form in the urinary tract? The
process is a complicated one, but basic stones form because
certain waste products present in urine increase in
concentration to a point at which they precipitate as
microscopic crystals. If these crystals remain in the urinary
tract and grow, they become large enough to see with the unaided
eye. With time, the stones may fill the space in the urinary
tract normally occupied by urine.

All urinary stones contain two major components-minerals (which
typically constitute about 95 per cent of a stone) and
nonmineral matrix (typically about 5 per cent of a stone). The
matrix can be thought of as a kind of mortar that may help to
cement minerals together. Because stones are composed primarily
of minerals, their number, location, and size can usually be
detected by x-ray studies or ultrasound studies.

Do all urinary stones have the same mineral composition? The
answer is no. The most common types of minerals in urinary
stones formed by dogs and cats are magnesium ammonium phosphate
(also called struvite), calcium oxalate, calcium phosphate,
ammonium urate, cystine, and silica. Sometimes stones contain
more than one type of mineral. On occasion, the center of a
urolith may be composed of one type of mineral (for example,
calcium oxalate), whereas outer layers are composed of a
different mineral (especially struvite).

Determining the types of minerals in stones is important because
different mineral types occur as a result of fundamentally
different causes. Therefore, urinary stones should not be
considered a single disease but rather a potential consequence
of several underlying risk factors. Treatment and/or prevention
of stone formation depends on identifying their mineral
composition. In addition, evaluation of your pet’s diet, blood
analysis, and urine composition analysis are important steps in
formulating recommendations for stone treatment and prevention.

How can urinary stones be effectively treated? Options include
various types of surgery and various types of nonsurgical
therapy designed to dissolve stones in the urinary tract. Which
treatment is best? The risks and benefits of medical versus
surgical therapy must be considered for each pet. Complete
obstruction of the flow of urine through the urinary tract
should be regarded as an emergency.

Although surgical removal is an effective method that may
immediately eliminate uroliths, surgery alone is associated with
several limitations, including (1) persistence of the underlying
causes of stones and therefore a high rate of recurrence of
uroliths after surgery, (2) risks inherent in general anesthesia
and the type of surgery performed, and (3) inability to remove
all uroliths during surgery. For these and other reasons (that
is, the urolith is asymptomatic), medical dissolution of some
types of uroliths may be considered.

The objectives of medical dissolution of uroliths are to stop
further stone growth and/or to promote stone dissolution by
correcting or controlling underlying abnormalities. For medical
dissolution therapy to be effective, it must reduce the urine
concentration of minerals that have precipitated to form the
stone. This usually involves a change in diet and in addition
often includes administration of specific drug.

The size and number of uroliths as such do not dictate the
likelihood of response to dissolution therapy. There has been
success in dissolving uroliths that are small and large, single
and multiple. However, the rate of dissolution is related to the
size and surface area of the urolith exposed to urine. Just as
one large ice cube dissolves more slowly than an equal volume of
crushed ice, one large urolith dissolves more slowly than an
equal volume of many smaller uroliths.

Uroliths tend to recur. Prevention of recurrent uroliths that
reduces the need for medical therapy and/or surgery is therefore
cost effective. In general, prevention strategies are designed
to eliminate or control the underlying causes of various types
of uroliths. When causes cannot be identified, preventive
therapy is usually designed to minimize risk factors associated
with formation of all stones. Recommendations commonly include
dietary modifications and sometimes administration of drugs.

The above is general veterinary information. Do not begin
any course of treatment without consulting your regular
veterinarian. All animals should be examined at least once every
12 months.