Urinary Stone Disease
Presentation
Most urinary stones pass through the collecting system in an
uneventful fashion. When a ureteral stone becomes obstructed it
can result in severe pain with associated nausea and vomiting.
Stones less than 5 mm in size have a 50% chance of spontaneous
passage within a six-week period of time. If a patient has uncontrollable
pain, persistent vomiting or a fever intervention is indicated.
The imaging modality of choice is the non-contrast CT scan. All
stone types except protease inhibitors (used in immuno-compromised
patients) will be visualized. It eliminates the need for intravenous
contrast, a skilled technician, and has the added benefit of imaging
the entire abdomen and pelvis.
Intervention
If conservative observation while awaiting spontaneous passage
is unsuccessful intervention is required. Ureteral stones can
be approached in a variety of ways. The least invasive is with
extracorporeal shock wave lithotripsy. Limitations include difficulty
targeting small stones including those overlying the sacro-iliac
joint. Additionally, after shock wave lithotripsy, patients have
to pass stone fragments through the collecting system. If patients
are away from immediate medical facilities, a double-J stent can
be placed to assure urinary drainage. Successful treatment with
shock wave lithotripsy is in the 60-80% range.
Distal ureteral stones in women of child-bearing age are best
approached in an ureteroscopic fashion limiting potential damage
(from SWL) to the ovaries. Stones can be retrieved with baskets
or fragmented with a variety of lithotrites including ultrasonic,
pneumatic, electrohydraulic, or laser lithotripsy. Smaller ureteroscopes
have limited the need for routine balloon dilation of the ureteral
orifice. Flexible ureteroscopes can be utilized to treat stones
in the collecting system. Large stone burdens are best approached
via a percutaneous route.
Percutaneous nephrolithotomy allows access to the entire upper
tract. Urologists or interventional radiologists can successfully
obtain percutaneous renal access. Lower pole renal access decreases
postoperative morbidity but may increase the likelihood of a rare
colonic injury, especially if the percutaneous puncture site is
more than 1-2-finger breadths later to the paraspinous musculature.
Superior pole punctures ease access into the ureter but typically
require access above the costal margin, thus increasing potential
injury to the pleura. The tract can be dilated with high pressure
balloons, sequential Amplatz fascial dilators, or co-axial metal
dilators. Contemporary series have reported blood transfusion
rates less than 5-7%.
Rare anterior calyceal stones may best be approached in a laparoscopic
fashion.
Metabolic Evaluation
After appropriate diagnosis and treatment a full metabolic evaluation
is indicated to help reduce the recurrence of urinary stones.
Eighty-five per cent of all stones are calcium based and are radio-dense
as seen on plain abdominal radiographs. This excludes:
Magnesium-ammonium-phosphate stones
Magnesium-ammonium-phosphate stones, also referred to as infectious
calculi, or struvite stones. These stones are found most frequently
in women who present with recurrent urinary tract infections due
to bacteria other than E. coli and typically grow urease-producing
organisms such as Proteus, Providencia, or Pseudomonas. These
urease-producing bacteria increase the urinary pH from a normal
value of 5.8 to more than 7.2. Struvite stones will only precipitate
in urine with a pH greater than 7.2. Struvite stones rarely present
with ureteral colic.
Uric acid stones
Uric acid stones are relatively radiolucent on KUB radiographs.
They typically occur in while males with a high recurrence rate.
Uric acid stone will form in urine with an acidic melieu, < 5.5.
Uric acid stones can be dissolved medically by increasing the
urinary pH to 6.0-6.5 range with potassium citrate or potassium
bicarbonate. Stones typically dissolve at a rate of one cm, as
seen on KUB, per month with compliant urinary alkalization. Chronic
urinary alkalization will drastically limit recurrent uric acid
stones.
Cystine stones
Cystine stones are caused by abnormalities in amino acid pumps
that effect cystine, ornithine, lysine and arginine (COLA). Patients
frequently present with recurrent stones that have a ground-glass
appearance on KUB. Intervention should be based on clinical symptoms
or evidence of progressive obstruction. One should limit open
surgical extraction as the vast majority of patients will develop
many recurrent stones despite our best attempts to prevent them
with disulfide binders (Thiola or Penicillamine) and urinary alkalization
with potassium citrate. There are no known urinary inhibitors
of cystine and thus patients should be encouraged to increase
fluid intake (during the day and night) to decrease urinary supersaturation
of cystine.
Calcium based stones
Calcium based stones can be evaluated based on complete 24-hour
urine collections and can be divided into hypercalciuria, hyperuricosuria,
hyperoxaluria and hypocitraturia. Patients may have one or multiple
abnormalities.
Hypercalciuria is defined as > 4mg/kg of calcium in a 24-hour
collection, or > 250-300 mg. There are three types of absorptive
hypercalciuria. Type I absorptive hypercalciuria is dietary independent.
Patients have elevated urinary calcium levels with a high or low
calcium diet. One should not routinely recommend a reduced calcium
diet for patients with calcium based stones. Such patients can
be treated with calcium binders such a cellulose phosphate or
alternatively with potassium citrate. Potassium citrate will be
effective for a limited period of time (4-6 years) and as such
repeat 24-hour urine collections are required to reassess values
and ensure patient compliance. These patients, like other calcium
stone formers need to be encouraged to have a low sodium diet.
Where sodium goes, so does calcium. Additionally, patients are
encouraged to reduce animal protein consumption by approximately
30%. That does not mean they need to be vegetarians. This reduced
animal protein consumption will limit the post-prandial acid flux
that will result in calcium being leached from bone. Finally,
a generalized recommendation is to have patients drink adequate
volumes so that they will void 1.5-2.0 liters per day. One should
emphasize to the patient that it is the voided volume rather than
the absolute volume consumed that is important. Type II absorptive
hypercalciuria is dietary dependent and patients will have elevated
urinary calcium levels only while consuming a high calcium diet.
It is these patients that should be encouraged to reduce their
calcium intake by about 50%. Type III absorptive hypercalciuria
is secondary to a phosphate renal leak. The decreased phosphate
results in a secondarily increased parathyroid hormone level and
an increase in vitamin D production. The increased vitamin D results
in increased phosphate AND increased calcium absorption, thus
the name absorptive hypercalciuria. These patients can be successfully
treated with phosphate supplements that are bio-available, such
as with neutraphos.
Resorptive hypercalciuria is due to primary hyperparathyroidism.
Such patients have elevated serum calcium and PTH levels, and
the PTH values will not decrease after a one week HCTZ load; this
an easy way to differentiate primary from secondary hyperparathyroidism
in urinary stone patients. Such patients can not be effectively
treated with medications and as such should undergo parathyroid
surgery. Urinary and serum values will return to normal in 7-10
days.
Renal hypercalciuria is due to abnormal renal calcium pumps,
resulting in increased urinary calcium and secondarily increased
PTH. Unlike patients with primary hyperparathyroidism these patients
have normal serum calcium levels. These patients can be effectively
treated with potassium citrate.
Hyperuricosuria is the second major etiology of calcium-based
stones. The most common etiology is dietary dependent and can
be effectively treated by limiting purine intake. Those unresponsive
to dietary recommendations should be treated with Allopurinol.
Alternatively, they can be treated with potassium citrate.
Hyperoxaluria is most commonly found in those with gastrointestinal
disorders such as short-gut syndrome or inflammatory bowel disease
resulting in chronic diarrhea. The chronic diarrhea results in
increased fat and bile contents in the stool. The fat and bile
will bind with the intra-luminal calcium forming a soap-like compound.
The oxalate in the gut is therefore unbound, as the calcium has
been "consumed" and is freely available for absorption and subsequent
increased urinary oxalate levels. The diarrheal states should
be optimally treated. Typically, the diarrhea continues and these
patients should be treated with oral calcium in the form of milk
products or TUMS. This oral calcium will successfully bind the
free oxalate. These medications must be taken with meals to bind
to the dietary oxalate. There is a rare type of endogenous hyperoxaluria
that results in oxalate precipitating in solid organs and frequently
results in renal and liver failure. These rare patients can be
successfully treated with a combined liver/kidney transplant.
Hypocitraturia is the last main subdivision of patients with
calcium based stones. Hypocitraturia is defined as a 24-hour urinary
citrate level < 320 mg. It is typically found in patients with
a metabolic acidosis. Renal tubular acidosis (type I, distal)
is a classic example. Other common causes include chronic diarrhea,
chronic diuretic use, and chronic dehydration. These patients
are best treated with potassium citrate supplementation. Potassium
citrate can be delivered in a wax matrix tablet. Patients should
be warned that these tablets can be seen in the stool, yet are
typically well absorbed. Alternatively, potassium citrate crystals
can be given when mixed in a glass of fluid. Oral formulations
are cheap, yet may not be as convenient as the crystal and tablet
formulations. Lemonade has been shown to increase quantitative
urinary citrate levels by approximately 150 mg/day. In patients
with mild hypocitraturia and intolerant or unwilling to take pharmacologic
supplements, lemonade is a reasonable alternative.
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