New York Urological Associates, PC
R. Ernest Sosa, MD
880 5th Ave.
New York, NY 10021
Tel (212) 570-6800 Fax (212) 861-7964
drsosa@nyurological.com
Prevention of Renal Stones
Approximately 12% of the American population will suffer from a kidney stone at
some point in their lives. Kidney stones are more common in men than in women,
occurring in men twice as often as in women. Caucasians are more likely to suffer
from kidney stones than African-Americans. The prevalence of urinary calculi
increases with age, peaking in the 35 to 60 year range. Once you form a urinary
stone, there is, on average, a 10% risk per year of forming a second stone. In five
years, on average a stone former has a 50-50 chance of having a new stone. There
are patients that may be even more prone to form stones due to their life style, their
diet, or due to an underlying medical condition that afflicts them. Kidney stones are
painful and they are costly.
Prevention of renal stones is possible in most patients. For this, it is important to
investigate the potential underlying factors that could cause a stone to form in an
individual and correct any anomalies that may be detected. In the United States, a
large number of patients form stones due to dietary abundance and a lack of
adequate water intake.
For the patient that is found to have a stone in the urinary tract, there are often
several options for treatment. Conservative management with hydration and pain
medication will often support of patient as they pass their stone spontaneously. Over
half of the symptomatic stones that are diagnosed can pass without intervention. For
those patients with stones too large to pass on their own, there are a variety of
treatment options, many of which are minimally invasive and can be rendered in the
outpatient setting.
Prevention of Kidney Stones
Kidney stones can form for a variety of reasons. Whatever the mechanism at play,
for a stone to form, urine has to become too concentrated with a certain chemical to
keep that substance in solution. In the body, the chemical elements most commonly
found in kidney stones are calcium, oxalate, phosphates, uric acid and cystine.
A stone can form because there is too much of one these elements in the urine. For
example, salt (which is made of sodium chloride) dissolves in water as long as there
is enough water present. If the water begins to evaporate, salt crystals begin to form
that we can see as a powdery solid. In the urinary tract it is calcium containing or
uric acid crystals that form. First a few crystals precipitate. If the urine remains
concentrated than more crystals are deposited and a small stone begins to take
shape. On this small nidus (of calcium or uric acid) additional crystals are deposited
and the stone grows larger and larger. One can see that drinking plenty of water
could help prevent a stone from forming or from growing.
Calcium Stones
The most common stones that form in the kidney are made of calcium and oxalate.
When calcium and oxalate join, they bind very tightly and are difficult if not
impossible to dissolve. In the past, advice to patients who suffered form calcium
containing kidney stones was to decrease the calcium in their diet. A growing body
of evidence, however, suggests that calcium restriction is inappropriate in
patients with calcium stones. The reason for this surprising finding has to do with
several other factors responsible for stone formation. One of these factors is
oxalate. Oxalate in the urine is a very powerful stone former. It is a more powerful
stone former than is calcium! Unattached oxalate, (see appendix 1 for foods rich in
oxalate), is normally absorbed into the blood stream from the intestines. Calcium
and oxalate, once bonded together are not easily absorbed from the gut. By
maintaining an adequate amount of calcium in the diet, one may decrease the
oxalate absorption from the gastrointestinal tract and thus decrease the urinary
excretion of oxalate. Moreover, calcium intake is a very important factor for the
prevention of bone loss as we age.
Dietary manipulation is one of the most important tools for preventing new calcium
oxalate stone formation. However, adequate fluid intake is the single most
important factor in the prevention of urinary stones. How much water should
one drink to prevent stones? It is advisable to drink enough fluids to make about two
liters of urine a day (a little over 2 quarts). For most people this means emptying a
full bladder 7 times a day. On a cold day one would need to drink less water to
achieve this volume of urine than on a hot day. The endpoint of fluid intake,
therefore, should be determined by the volume of urine made. Colas, coffee, tea,
and beer do not help prevent stones. Cranberry juice, which may have a role in
preventing urinary infections, does not help prevent stones. Cranberry, like many
other berries, metabolizes to form oxalates that are an ingredient for stone
formation. It is important to drink water for most of the fluid volume one consumes.
The second most important factor in the prevention of stone formation is to
keep the animal protein intake to less than 6 ounces a day. Animal proteins
are metabolized into purines, which increase the risk for uric acid stone formation.
Animal proteins also make the urine more acid, which makes uric acid more likely to
crystallize and also lowers the production of citrate by the kidneys. Citrate is a
naturally occurring stone inhibitor that serves to prevent the formation of uric acid
and calcium stones. Finally, glycine, an amino acid found in proteins is metabolized
to oxalate; methionine, another amino acid, drives more calcium into the urine.
Reducing the animal protein content of one’s diet has an impact on
decreasing stone recurrence, while helping to prevent bone loss.
Beyond adequate fluid intake, avoidance of high animal protein intake, what else can
be done to prevent calcium stone formation?
There are several mechanisms for calcium oxalate stone formation. The role of diet
in calcium stone formation has been outlined above. Many patients with calcium
stones can prevent further stone formation by drinking plenty of fluids and eating
small portions of animal protein. Lowering one’s salt intake is also important in
decreasing stone formation. The higher the sodium contents of a diet, the greater
the renal excretion of calcium into the urine, even if the calcium intake stays the
same.
A number of folks will form calcium oxalate stones because they absorb more
calcium from their diet than is normal. These “hyperabsorbers” of calcium are the
only ones that need to watch their calcium intake carefully. Another subgroup of
patients will leak calcium into their urine. The urinary calcium level is always
elevated, regardless how little calcium may be found in their diet. In fact, lowering
calcium in the diet of these patients will result in leaching of calcium from their
bones, producing osteoporosis. Management of these patients involves the use of
medications to diminish the renal calcium leak. Finally, less than 1% of calcium stone
formers will suffer from oversecretion of parathyroid hormone. The parathyroid
glands produce this hormone, which is important for the regulation of calcium
metabolism. An adenoma may form on one of the parathyroid glands resulting in
overproduction of parathyroid hormone. The treatment is the identification of the
adenoma and its removal.
About 15% percent of calcium oxalate stones form on a uric acid nidus. In these
patients, keeping the urinary uric acid level under control is therefore important for
stone prevention (see next section).
Uric Acid Stones
Uric acid stones account for approximately 10% of all urinary stones. They affect
men four times more often than they affect women. Although patients with gout
suffer from uric acid elevation, the majority of patients with uric acid stones do not
have gout. The typical patient who presents with a uric acid stone will fit the
following scenario:
The afflicted patient will generally be a male, over forty, who works hard all
day not stopping to drink water, eats lightly because of lack of time. He
arrives home after work feeling ravenous; he consumes a large dinner, drinks
very little water and is more inclined to have wine or a cocktail. The total
calorie intake of this patient may not be excessive, but he is presenting most
of the calories to his body at one time. The fluid intake is abysmal, as this
man has been dehydrated all day. He is a prime candidate for uric acid stone
formation.
Preventing future stones for this patient will call for a more even distribution of
calories during the day, a much greater consumption of fluids. Animal proteins
(chicken, beef, pork, and fish) contribute to an increase in uric acid stone formation
(see low purine diet for other foods that contribute to uric acid stone formation).
Eating large amounts of salt will also increase the risk for uric acid stones.
Other factors, besides diet and gout that may contribute to a high uric acid excretion
in the urine include myeloproliferative disorders being treated with chemotherapy,
weight loss on a high calorie diet or due to low caloric intake. Acid urine also
increases the risk of uric acid stone formation. At a pH of <5.5 uric acid crystallizes
in the urine. This may occur with high protein intake, with distal small bowel disease
or resection (regional enteritis) where there is loss of base, and with terminal
ileostomies.
Unlike the calcium renal stones, uric acid stones can be dissolved by adding base to
the urine to raise the urine pH to a range of 6-7. Of course, plenty of water is sure to
help a great deal. If a large amount of uric acid is found in the urine, medical
treatment to prevent uric acid formation will be necessary to complement hydration,
dietary manipulation and alkalinization of the urine. Allopurinol will decrease uric acid
formation.
Of note, uric acid stones are not visible on a standard x-ray like calcium stones are.
To diagnose uric acid stones an unenhanced helical CT of the abdomen and pelvis
will permit visualization of the entire urinary system for uric acid stones.
Alternatively, a renal sonogram will allow inspection of the kidney, but leave the
ureters unexplored. Once a uric acid stone is diagnosed, if there is no blockage to
the urinary tract, medical management can be started to try to dissolve the uric acid
stone. For this, potassium citrate is given by mouth to raise the urine pH of to a 6-7
range, which increases the solubility of uric acid. A low purine diet is observed (see
appendix B), salt intake is kept low (less than 150 mEq/day) and hydration to make
2 liters of urine a day is maintained.
Cystine Stones
Cystine stones make up less than 5% of all urinary calculi. Cystine stone formation
results from the impaired reabsorption of the amino acids cystine, lysine, ornithine
and arginine from the renal tubules. Cystine is the only amino acid of this group that
is poorly soluble and therefore results in urinary stone formation. Cystinuria, an
autosomal recessive disease, results from excessive excretion of the four basic
amino acids, cystine, ornithine, lysine, and arginine (COLA) into the urine. Cystine is
relatively insoluble in acid urine. Patients with cystinuria (>250mg/24 hr.) produce
supersaturated urine and are at risk for crystallization and stone formation. Cystine
is not very radiodense and can be difficult to find on a flat plate of the abdomen.
Moreover, it is resistant to Shock Wave Lithotripsy and must be fragmented by
ultrasonic, laser or electrohydrualic lithotripsy.
Prevention and medical management of cystine stones depends on decreasing the
total urinary cystine concentration by fluid hydration; increasing the solubility of
cystine by alkalinizing the urine to a pH of 7.5 or higher using potassium citrate and
by binding cystine with penicillamine to form a much more soluble cystine - S penicillamine complex. This latter molecule is 50 times more soluble than cystine in
urine. Another agent that can be substituted for penicillamine is Thiola.
Struvite Stones:
Struvite stones, commonly called infectious stones, are composed of magnesium,
ammonium and phosphate, mixed with carbonate. Struvite stones form as a result of
the effects of certain bacteria on the urine. Species of bacteria, such as Proteus and
Klebsiella, produce an enzyme called urease. The urea in the urine is split by urease
to cause an increase in the pH of the urine to a value of 7.2 or higher.
Prevention of Struvite stones depends on maintaining sterile urine, and of course, a
good state of hydration. Management of Struvite stones already present requires a
multimodal therapy. First, the stone must be removed as completely as possible
from the kidney. After the stone has been removed from the kidney, medication,
called Renacidin is instilled into the kidney to dissolve any residual stone fragments.
Medical management with antimicrobial therapy, chemolysis with Renacidin, and
urinary acidification helps to prevent stone recurrence.
Patient Evaluation:
A urinary calculus that obstructs the flow of urine will produce a renal or ureteric
colic. The pain can be very severe, often starting in the flank with radiation to the
groin of the same side. Patients with renal colic find it impossible to be still. No relief
from the pain is found in any position. The colic may be associated with blood in the
urine. The amount of blood may be small and not visible to the naked eye, its
detection requiring use of a microscope to study the urine. The urinalysis can also
tell a physician about the urine pH (a measure of the acid-base content of the urine),
the presence of bacteria and infection, and can help detect crystals to suggest the
composition of the stone.
The most effective test to document the presence of a urinary stone is the
Unenhanced Helical CT. This test is rapid, requires no contrast to be injected so it
is safe for everyone and it is the most reliable imaging modality for stone disease.
Sonography can be helpful but it is less reliable in the kidney than the Unenhanced
CT. Moreover, Sonography cannot image a ureteral stone. Intravenous pyelography
is another useful test, less reliable than the Unenhanced CT and it requires contrast.
Once the diagnosis of a urinary stone is known, plans for treatment and prevention
of further stones are initiated.
Treatment options:
For the patient in acute renal colic, the first order of business is to make the
diagnosis, provide medication to control the pain, hydrate the patient and treat the
stone if it is not likely to pass spontaneously. Treatment options to render the patient
stone free are discussed below.
Dietary
For the patient afflicted with stones, a high water intake, a low salt and animal
protein intake (see above) is important to decrease recurrent stones. For patients
with uric acid stones, a low Purine diet is recommended (see appendix B); for
patients with oxalate stones, a low oxalate diet is appropriate (see appendix A).
Medications
Patients with uric acid stones benefit from alkalinization of the urine. For this,
potassium citrate, to maintain the urine pH in the 6-7 ranges, is important.
Allopurinol decreases the uric acid production.
Patients with calcium stones with associated high calcium levels in the urine benefit
from a low sodium diet and hydrochlorothiazide. This medication lowers the
calcium content of the urine, moving it into the blood stream where it can be utilized
by the body.
In cystine stones, the use of urinary alkalinization with potassium citrate as well as
increasing the solubility of cystine with medications such as penicillamine and
Thiola tends to decrease the risk for new stone formation.
Infection stones are treated in part by antibiotics to kill urea-splitting bacteria. Use of
medications to inhibit the splitting of urea, such as acetohydraximic acid
(Lithostat) is useful but side effects, like the formation of blood clots in the legs are
undesirable.
Interventional Treatments to Remove Renal and Ureteral Stones
Shock-wave lithotripsy (SWL) is a non-invasive technique that fragments urinary
stones using high-energy waves that are focused on a targeted stone. For most renal
stones of less than 2 cm in size, SWL is the preferred treatment. The overall success
rate is about 80 % (see figure 5). For stones less than one centimeter in size, found
in the ureter the success rate is about 85%, with a retreatment rate of 10-15%. SWL
can be performed under monitored anesthesia care in an outpatient setting.
SWL does not work well for:
 Stones > 2 cm stones
 Stones in a calyceal diverticulum
 Stones is a horshoe kidney
 Stones in a patient who is bed-ridden
 Kidneys or ureters with obstruction distal to the stone
 Stones impacted into the wall of the ureter
 Stones composed of calcium oxalate monohydrate and cystine stones.
These problem stones that cannot be broken up by SWL are fragmented and
removed by percutaneous nephrolithotomy or sometimes, by ureteroscopy (see
below).
For patients with uric acid stones, a stent must be placed in the kidney to allow
injection of contrast so as to visualize the stone with fluoroscopy.
Ureteroscopy is a minimally invasive technique that involves passage of a small
caliber rigid or flexible videoscope into the ureter or into the kidney to break up a
stone using a laser, ultrasound or electrohydraulic energy. After the procedure, a
small stent is left in the ureter, running from the kidney to the bladder for a few
days. The stent, called a Double-J stent is pulled out in the office. Ureteroscopy has
better than 90% success rate for the treatment of ureteral stones.
Percutaneous nephrolithotomy is an operation where a needle is introduced into
the kidney harboring a stone under x-ray guidance. The patient is under anesthesia
for this operation. The needle tract is dilated with a balloon and a sleeve is left in
place going from outside the skin in the back to the inside of the kidney. A videotelescopic instrument called a nephroscope is introduced to see the stone, fragment
it and remove the fragments rendering the patient stone free (see figures 1-4). A
small catheter is left through the tract into the kidney for a few days to give the
kidney and tissues of the back a chance to heal. This tract allows access for a second
examination of the kidney if additional work has to be done. The catheter is removed
in the office and the opening closes in 24 hours with a bandage alone.
All patients who have undergone treatment for kidney stones require careful follow
up to assure that the stone fragments pass. Moreover, an aggressive effort must be
initiated and maintained to prevent future stone formation.
Figure 1. Stone in the kidney is found
Figure 2. Stone in kidney being fragmented
Figure 3. Fragments of stone in kidney
Figure 4. Removal of stone fragments
Figure 5. Success of SWL for treatment of 1 cm stones in different part of the kidney
Success with SWL
~85%
~85%
~90%8
5%90
20-70%