Kidney stones
Kidney stones form either because of
excess salts or inefficiency of the body's natural clearing mechanism.
Many
people have stones that are asymptomatic (no symptoms) and cause no pain.
Drinking lots of water helps prevent
stone recurrence.
Drinking
excessive amounts of water does not help stones to clear.
Some
stones can be reduced and dissolved by alkalising the urine.
Description
Urine is an ionic solution containing many dissolved salts and other
products. Sometimes these salts crystallise and the crystals aggregate to form
stones. Most urinary tract calculi (kidney stones) consist of a crystal
component and a matrix component (protein and debris). Kidney stones are usually
named according to the crystal component of the stone. When a stone is found in
the bladder it is called a bladder stone. Stones may be asymptomatic or severely
symptomatic. Pain is usually caused by obstruction, especially when a stone
blocks the ureter (the tube that drains urine from the kidney into the bladder).
Stones can also cause infection and bleeding.
Cause
The
medical name for the process of stone formation is called urolithiasis (renal
lithiasis or nephrolithiasis). Stones may form because:
the
urine becomes too saturated with salts that can form stones or
the
urine lacks the normal inhibitors of stone formation.
obstruction
causes urine stasis and subsequent stone formation
stones can form on foreign bodies in
the urinary tract such as indwelling catheters and stents.
Types of stones and relative incidence
Calciumoxalate:
75%
Uric
acid stones: 8%
Struvite (Infection) stones:
15%
Calcium, ammonium, magnesium phoshate
Caciumphosphate:
rare
Cystine:
rare
Calciumoxalate stones are by far the most common. Stones containing calcium are
clearly seen on X-rays. Cystine stones are poorly visible on routine X-rays and
uric acid stones are radiolucent.
Struvite
stones – a mixture of magnesium, ammonium, and phosphate – are also called
infection stones, because they form only in infected urine
Stones
vary in size from too small to be seen with the eye alone to 1 inch or more in
diameter. A large staghorn calculus (stone) may be shaped by the renal pelvis,
and may fill it and the tubes that drain into it.
Cystine
stones form due to a hereditary defect in the metabolism of the amino acid
cystine. People with cystinuria excrete large amounts of cystine in the urine.
Cystine is poorly soluble in urine and crystallises to form stones.
Symptoms
Stones may not cause any symptoms.
Stones in the bladder may cause pain
in the lower abdomen.
Stones
that obstruct the ureter or renal pelvis or any of its drainage tubes may cause
back pain or a severe colicky pain (renal colic). Renal colic is characterised
by an excruciating intermittent pain, usually in the flank, that spreads across
the abdomen, often to the genital area and inner thigh.
Other
symptoms include nausea and vomiting, abdominal distention, chills, fever, and
blood in the urine.
A person may need to urinate
frequently, particularly as a stone passes down the ureter.
Stones
may cause a urinary tract infection. When stones block the flow of urine,
bacteria become trapped in urine that pools above the blockage, leading to an
infection. When stones block the urinary tract for a long time, urine backs up
in the tubes inside the kidney, producing pressure that can distend the kidney (hydronephrosis)
and eventually damage it.
The
combination of obstruction and infection is an emergency because the kidney can
be permanently damaged in 24 to 36 hours.
Diagnosis
Stones
that cause no symptoms may be discovered by chance during a routine microscopic
analysis of the urine (urinalysis). Stones that cause pain are generally
diagnosed on the basis of the symptoms of renal colic. This is characterised by
severe colicky intermittent pain in the loin that radiates to the groin or
genitals. Despite the severe pain, physical examination is usually inconclusive.
Microscopic analysis of the urine may disclose blood or pus in the urine as well
as small stone crystals.
When
someone presents with renal colic the diagnosis is usually confirmed by
intravenous urography. In intravenous urography, a radio- opaque substance,
which is visible on x-rays, is injected into a vein and travels to the kidneys
where it outlines uric acid stones so they can be seen on x-rays. Intravenous
urography will not only confirm the diagnosis, but will also indicate the
position of the stone and whether any obstruction is present or not. More
recently, spiral CT scanning has emerged as an alternative initial imaging
investigation in patients with renal colic. Spiral
CT scanners are available in some but not all hospitals in
South Africa. Spiral
CT scanning is as accurate as intravenous urography to make the
diagnosis and does not involve the use of intravenous injection of contrast
medium.
Additional tests that help make the diagnosis involve collecting 24-hour urine
samples and blood samples, which are analyzed for levels of calcium, cystine,
uric acid, and other substances known to produce stones.
Treatment
Small
stones which aren't causing symptoms, obstruction, or an infection usually are
not treated.
Drinking
plenty of fluids increases urine production and helps wash out some stones. Once
a stone is passed during urination, no other immediate treatment is needed.
The pain
of renal colic may be relieved with narcotic analgesics
Sometimes a stone in the renal pelvis or uppermost part of the ureter that's 1cm
or less in diameter can be broken up by ultrasound waves (extracorporeal shock
wave lithotripsy). The pieces of stone are then passed in the urine. Certain
stones are removed through a small incision in the skin (percutaneous
nephrolithotomy), followed by ultrasound treatment.
Small
stones in the lower part of the ureter may be removed by an endoscope (a small,
flexible tube) inserted into the urethra and through the bladder.
Uric
acid stones are sometimes dissolved gradually by making the urine more alkaline
(for example, with potassium citrate), but other types of stones can't be
removed this way.
Rarely,
larger stones that are causing an obstruction may need to be removed surgically.
Prevention
Measures
to prevent the formation of new stones vary, depending on the composition of the
existing stones. Most people with calcium stones have a condition called
hypercalciuria, in which excess calcium is excreted in the urine.
Thiazide diuretics such as
trichlormethiazide reduce new stone formation in such people.
Drinking large amounts of fluids – 8
to 10 glasses a day – is recommenced.
A
normal calcium intake is recommended.
Paradoxically, a low calcium intake
may increase the risk of stone formation due to increased oxalate absorption
from the gut.
Taking
sodium cellulose phosphate, a resin, may help.
Potassium
citrate may be given to increase a low urine level of citrate, a substance that
inhibits calcium stone formation.
A
high level of oxalate in the urine, which contributes to calcium stone
formation, may result from excess consumption of foods high in oxalate, such as
rhubarb, spinach, cocoa, nuts, pepper, and tea, or from certain intestinal
disorders. A change in diet may help, and the underlying disorder is treated.
Rarely,
calcium stones result from another disorder, such as hyperparathyroidism,
sarcoidosis, vitamin D toxicity, renal tubular acidosis, or cancer. In such
cases, the underlying disorder is treated.
For
stones that contain uric acid, a diet low in meat, fish, and poultry is
recommended, because these foods increase the level of uric acid in the urine.
Allopurinol
may be given to reduce the production of uric acid.
Potassium
citrate may be given to make the urine alkaline, because uric acid stones form
when urine acidity increases. Drinking large amounts of fluids also helps.
If there are struvite stones,
which always indicates a urinary tract infection, antibiotics are given.
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