| Kidney Stones
Overview
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
ureter (tube that carries urine from the kidney to the bladder), 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.
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
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 - The digestion of protein 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 (present from 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.
Incidence and Prevalence
People who live near large bodies of water (e.g., Great Lakes, Gulf of
Mexico), those who live in "soft" water areas, and those who have a sibling
or parent with the condition experience a higher incidence of renal stone
disease. According to the U.S. National Institutes of Health, 1 person in 10
develops kidney stones during their lifetime and renal stone disease
accounts for 7–10 of every 1000 hospital admissions. Kidney stones are most
prevalent in patients between the ages of 30 and 45, and the incidence
declines after age 50.
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