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Kidney Stones
Kidney Stone Formation
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Kidney stones (calculi) are hardened mineral deposits that
form in the kidney. They originate as microscopic particles
and develop into stones over time. The medical term for this
condition is nephrolithiasis, or renal stone disease.
The kidneys filter waste products from the blood and add them
to the urine that the kidneys produce. When waste materials in
the urine do not dissolve completely, crystals and kidney
stones are likely to form.
Small stones can cause some discomfort as they pass out of the
body. Regardless of size, stones may pass out of the kidney,
become lodged in the tube that carries urine from the kidney
to the bladder (ureter), and cause severe pain that begins in
the lower back and radiates to the side or groin. A lodged
stone can block the flow of urine, causing pressure to build
in the affected ureter and kidney. Increased pressure results
in stretching and spasm, which cause severe pain.
Kidney Stone Formation
Kidney stones form when there is a high level of calcium (hypercalciuria),
oxalate (hyperoxaluria), or uric acid (hyperuricosuria) in the
urine; a lack of citrate in the urine; or insufficient water
in the kidneys to dissolve waste products. The kidneys must
maintain an adequate amount of water in the body to remove
waste products. If dehydration occurs, high levels of
substances that do not dissolve completely (e.g., calcium,
oxalate, uric acid) may form crystals that slowly build up
into kidney stones.
Urine normally contains chemicals— citrate, magnesium,
pyrophosphate— that prevent the formation of crystals. Low
levels of these inhibitors can contribute to the formation of
kidney stones. Of these, citrate is thought to be the most
important.
Types of Kidney Stones
The chemical composition of stones depends on the chemical
imbalance in the urine. The four most common types of stones
are comprised of calcium, uric acid, struvite, and cystine.
Calcium Stones
Approximately 85% of stones are composed predominantly of
calcium compounds. The most common cause of calcium stone
production is excess calcium in the urine (hypercalciuria).
Excess calcium is normally removed from the blood by the
kidneys and excreted in the urine. In hypercalciuria, excess
calcium builds up in the kidneys and urine, where it combines
with other waste products to form stones. Low levels of
citrate, high levels of oxalate and uric acid, and inadequate
urinary volume may also cause calcium stone formation.
Calcium stones are composed of calcium that is chemically
bound to oxalate (calcium oxalate) or phosphate (calcium
phosphate). Of these, calcium oxalate is more common. Calcium
phosphate stones typically occur in patients with metabolic or
hormonal disorders such as hyperparathyroidism and renal
tubular acidosis.
Increased intestinal absorption of calcium (absorptive
hypercalciuria), excessive hormone levels
(hyperparathyroidism), and renal calcium leak (kidney defect
that causes excessive calcium to enter the urine) can cause
hypercalciuria. Prolonged inactivity also increases urinary
calcium and may cause stones.
Renal tubular acidosis (inherited condition in which the
kidneys are unable to excrete acid) significantly reduces
urinary citrate and total acid levels and can lead to stone
formation, usually calcium phosphate.
Uric Acid Stones
Digestion produces uric acid. If the acid level in the urine
is high or too much acid is excreted, the uric acid may not
dissolve and uric acid stones may form. Genetics may play a
role in the development of uric acid stones, which are more
common in men. Approximately 10% of patients with kidney stone
disease develop this type of stone.
Struvite Stones
This type of stone, also called an infection stone, develops
when a urinary tract infection (e.g., cystitis) affects the
chemical balance of the urine. Bacteria in the urinary tract
release chemicals that neutralize acid in the urine, which
enables bacteria to grow more quickly and promotes struvite
stone development.
Struvite stones are more common in women because they have
urinary tract infections more often. The stones usually
develop as jagged structures called "staghorns" and can grow
to be quite large.
Cystine Stones
Cystine is an amino acid in protein that does not dissolve
well. Some people inherit a rare, congenital (i.e., present at
birth) condition that results in large amounts of cystine in
the urine. This condition (called cystinuria) causes cystine
stones that are difficult to treat and requires life-long
therapy.
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