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KIDNEY STONE
Kidney stones (calculi) are hardened mineral deposits that form in the kidney. They originate as microscopic particles and crystals 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. These stones may stay in the kidney for a long time or they may pass out of the kidney and become lodged in the tube that carries urine from the kidney to the bladder (ureter). If left untreated, the kidney may swell and lose its function. |
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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.
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).
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.
- 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 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.
Several factors increase the risk for developing kidney stones, including:
- Inadequate fluid intake and dehydration
- Reduced urinary flow and volume
- Certain chemical levels in the urine that are too high (e.g., calcium, oxalate, uric acid) or too low (e.g., citrate)
- Medical conditions that block or reduce the flow of urine (e.g. urinary obstruction, genetic abnormality) also increases the risk.
- Urinary tract infections
- Diet - A diet high in sodium, fats, meat, and sugar, and low in fiber, vegetable protein, and unrefined carbohydrates increases the risk for renal stone disease. High doses of vitamin C (more than 500 mg per day) can result in high levels of oxalate in the urine (hyperoxaluria) and increase the risk for kidney stones. Oxalate is found in berries, vegetables (e.g., green beans, beets, spinach, squash, tomatoes), nuts, chocolate, and tea. Stone formers should limit their intake of cranberries, which contain a moderate amount of oxalate.
Small, smooth kidney stones may remain in the kidney or pass out without causing pain (called "silent" stones).
Larger stones may cause symptoms including:
- Loin to groin pain - Stones that lodge in the tube that carries urine from the kidneys to the bladder (ureter) result in severe pain that begins in the loin and lower back and radiates to the groin or front of the lower abdomen.
- Blood in the urine (hematuria)
- Increased frequency of urination
- Nausea and vomiting
- Pain during urination (stinging, burning)
- Tenderness in the abdomen and kidney region
- Urinary tract infection (fever, chills, loss of appetite)
Diagnosis of renal stone disease involves a medical history, physical examination, laboratory evaluation, and imaging tests.
- Urinalysis to detect the presence of blood (hematuria) and bacteria (bacteriuria) in the urine.
- Blood tests for Creatinine (to evaluate kidney function), Urea & Electrolytes (to detect dehydration), Calcium (to detect hyperparathyroidism), and a Full Blood Count (to detect infection).
Imaging tests used to diagnose kidney stones include:
- Ultrasound #
This test uses high-frequency sound waves to produce pictures of internal structures (e.g., organs, kidney stones). Ultrasound can detect a dilated (stretched) upper urinary tract and kidney caused by a stone lodged in the ureter, but may not be able to detect small stones, especially those located outside the kidney. It is the preferred imaging method for kidney stone patients who are pregnant.
- IVU (Intravenous Urogram)#
This test involves taking a series of x-rays after injecting a contrast agent (dye) into a vein. The contrast agent flows through the veins, is excreted by the kidneys, and improves the x-ray images of the kidneys and ureters. If a kidney stone is blocking a ureter, the contrast agent builds up in the affected kidney and is excreted more slowly. Most kidney stones (e.g., calcium stones) can be precisely located using this procedure. There is a slight risk for an allergic reaction to the contrast agent during this procedure and overall kidney function must be normal. IVU can take a very long time if the blockage to the kidney is severe.
- CT scan (Computer tomography)#
This test uses a scanner and a computer to create images of the urinary system. The non-contrast CT scan is the most common imaging test used to evaluate possible kidney stone. CT scan can also help identify medical conditions (e.g., ruptured appendix, bowel obstruction) that cause symptoms similar to kidney stones.
If any stones are found, a plain abdominal x-ray is also taken to determine their size, shape, and orientation. X-rays are used for follow-up studies to monitor the stones' progress.
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Small stones may move through the ureter and be passed out naturally with adequate fluid intake.
If a kidney stone does not move through the ureter on its own, surgery is considered. There are several options available, including:
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This procedure requires general anesthesia. An incision is made in the patient's back and the stone is extracted through an incision in the ureter or kidney. Most patients require prolonged hospitalization and recovery takes several weeks. This procedure is now rarely used for kidney stones.
If a kidney stone does not move through the ureter within 30 days, surgery is considered. Urologists use several procedures to break up, remove, or bypass kidney stones.
- Ureteroscopy #
This procedure can be used to remove or break up (fragment) stones located in the lower third of the ureter. A fiberoptic instrument resembling a long, thin telescope (ureteroscope) is inserted through the urethra and passed through the bladder to the stone. Once the stone is located, the urologist either removes it with a small basket inserted through the ureteroscope (called basket extraction) or breaks the stone with a laser or similar device. The fragments are then passed by the patient. Ureteroscopy is performed under general or local anesthesia on an outpatient basis.
- Lithotripsy #
This procedure is effective for stones in the kidney or upper ureter. It uses an instrument, machine, or probe to break the stone into tiny particles that can pass naturally. Lithotripsy is not appropriate for patients with very large stones or other medical conditions.
- Ultrasonic lithotripsy uses high frequency sound waves delivered through an electronic probe inserted into the ureter to break up the kidney stone. The fragments are passed by the patient or removed surgically.
- Electrohydraulic lithotripsy (EHL) uses a flexible probe to break up small stones with shock waves generated by electricity. The probe is positioned close to the stone through a flexible ureteroscope. Fragments can be passed by the patient or extracted. EHL requires general anesthesia and can be used to break stones anywhere in the urinary system.
- Extracorporeal shock wave lithotripsy (ESWL) uses highly focused impulses projected from outside the body to pulverize kidney stones anywhere in the urinary system. The stone usually is reduced to sand-like granules that can be passed in the patient's urine. Large stones may require several ESWL treatments. The procedure should not be used for struvite stones, stones over 1 inch in diameter, or in pregnant women.
Patients undergoing lithotripsy are given a sedative and general or local anesthesia, and the procedure takes over an hour. More than one treatment may be required.
- Percutaneous Nephrostolithotomy (PCN) #
This surgical procedure is performed under local anesthesia and intravenous sedation. Percutaneous (i.e., through the skin) removal of kidney stones (lithotomy) is accomplished through the most direct route to stones through the kidney. A needle and guidewire are used to access the stones. The surgeon then threads various catheters over the guidewire and into the kidney and manipulates surgical instruments through the catheters to fragment and remove kidney stones. This procedure achieves a better stone-free outcome in the treatment of medium and large stones than shock wave lithrotripsy. This procedure usually requires hospitalization, and most patients resume normal activity within 2 weeks.
- Ureteroscopic Stone Removal #
This procedure is performed under general anesthesia to treat stones located in the middle and lower ureter. A small, fiberoptic instrument (ureteroscope) is passed through the urethra and bladder and into the ureter. Small stones are removed and large stones are fragmented using a laser or similar device. A small tube (or stent) may be left in the ureter for a few days after treatment to promote healing and prevent blockage from swelling or spasm.
- Open Surgery #
This procedure requires general anesthesia. An incision is made in the patient's back and the stone is extracted through an incision in the ureter or kidney. Most patients require prolonged hospitalization and recovery takes several weeks. This procedure is now rarely used for kidney stones.
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Prevention of renal stone disease depends on the type of stone produced, underlying urinary chemical risk factors, and the patient's willingness to undergo a long-term prevention plan. The patient may be asked to make lifestyle modifications such as increased fluid intake and changes in diet.
Lemonade with real lemon juice is a good source of citrate and may be recommended as an alternative to water. Limiting meat, salt, and foods high in oxalate (e.g., green leafy vegetables, chocolate, nuts) in the diet may also be recommended. Medication may be prescribed and treatment for an underlying condition that causes renal stone disease may be necessary.
24-Hour Urine Test Effective preventative measures are based on the patient's chemical risk factors, which can often be uncovered with a 24-hour urine test and a blood test.
- The patient strains their urine to collect stones for chemical analysis.
- The physician performs a blood test to evaluate the serum calcium, uric acid, phosphate, electrolytes, and bicarbonate content.
- Urine is collected during a 24-hour period and analyzed for calcium, citrate, uric acid, magnesium, phosphate, sodium, oxalate, pH (acid level), and total volume.
The physician evaluates the data and recommends dietary modifications, supplements, and medications to minimize the risk for developing kidney stones. The 24-hour urine test may be repeated several months after treatment has begun to determine the success of the therapy and any adjustments that should be made. Long-term strict compliance and periodic retesting may substantially reduce the risk for future stone formation.
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.
- 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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Several factors increase the risk for developing kidney stones, including inadequate fluid intake and dehydration, reduced urinary flow and volume, certain chemical levels in the urine that are too high (e.g., calcium, oxalate, uric acid) or too low (e.g., citrate), and several medical conditions. Anything that blocks or reduces the flow of urine (e.g., urinary obstruction, genetic abnormality) also increases the risk.
Chemical risk factors include high levels of the following in the urine:
- Calcium (hypercalciuria)
- Cystine (cystinuria; caused by a genetic disorder)
- Oxalate (hyperoxaluria)
- Uric acid (hyperuricosuria)
- Sodium (hypernatremia)
A low level of citrate is a risk factor for hypocitraturia.
The following medical conditions are also risk factors:
- Congenital kidney defect that may increase urinary calcium loss and stone formation (medullary sponge kidney)
- Excessive parathyroid hormone, which causes calcium loss (hyperparathyroidism)
- Gout (caused by excessive uric acid in the blood)
- High blood pressure (hypertension)
- Inflammation of the colon that causes chronic diarrhea, dehydration, and chemical imbalances (colitis)
- Inherited condition in which the kidneys are unable to excrete acid (renal tubular acidosis)
- Intestinal disorder that causes chronic diarrhea, dehydration, and low citrate (Crohn's disease)
- Painful joint inflammation (arthritis)
- Urinary tract infections (affect kidney function)
Diet plays an important role in the development of kidney stones, especially in patients who are predisposed to the condition. A diet high in sodium, fats, meat, and sugar, and low in fiber, vegetable protein, and unrefined carbohydrates increases the risk for renal stone disease. Recurrent kidney stones may form in patients who are sensitive to the chemical byproducts of animal protein and who consume large amounts of meat.
High doses of vitamin C (i.e., more than 500 mg per day) can result in high levels of oxalate in the urine (hyperoxaluria) and increase the risk for kidney stones. Oxalate is found in berries, vegetables (e.g., green beans, beets, spinach, squash, tomatoes), nuts, chocolate, and tea. Stone formers should limit their intake of cranberries, which contain a moderate amount of oxalate.
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