Kidney stones
Overview
Fortunately, most stones pass out of the body without any intervention. If you are not so lucky, the following information should help you and your doctor could address the causes, symptoms and possible complications created by your kidney stone disease.
Kidney stones (calculi) are hardened mineral deposits that form in the kidney. They originate as microscopic particles or 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 may form.
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, excrutiating pain that begins in the lower back and radiates to the 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 the severe pain.
Stone Formation
Causes and Risk Factors
Several factors increase the risk for developing kidney stones, including inadequate fluid intake and dehydration, reduced urinary volume, certain chemical levels in the urine that are too high (e.g., calcium, oxalate, uric acid) or too low (magnesium, citrate), and several medical conditions such as reflux, medullary sponge kidney, renal tubular acidosis and urinary tract infections (UTIs). Anything that blocks or reduces the flow of urine (e.g., urinary obstruction, genetic abnormalities) 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 (hypernaturesis)
Urine normally contains chemicals-citrate, magnesium, pyrophosphate-that help prevent the formation of crystals and stones. Low levels of these inhibitors can contribute to the formation of kidney stones. Of these, citrate is the most important
The following medical conditions are also risk factors for kidney stone disease:
- Colitis (inflammation of the colon that causes chronic diarrhea, dehydration, and chemical imbalances)
- Gout (caused by excessive uric acid in the blood) which leads to high urinary uric acid levels
- Hyperparathyroidism (excessive parathyroid hormone, which causes calcium loss from bone)
- Mdullary sponge kidney (MSK; a congenital kidney defect associated with urinary tract infections, low urinary citrate levels, and increased urinary calcium loss)
- Renal tubular acidosis (inherited condition in which the kidneys are unable to excrete acid)
- Urinary tract infections (UTIs; affect kidney function and chemistry)
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 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 by products of animal protein and who consume large amounts of meat.
Oxalate is found in, Red meat (beef, pork, mutton), vegetables (e.g., greens, spinach,), nuts, chocolate, coffee and strong tea. Stone formers should limit their intake of these items, which contain a moderate amount of oxalate.
Types of Kidney Stones
The chemical composition of stones depends on the chemical imbalance in the urine. The five most common types of stones are comprised of calcium-calcium oxalate and calcium phosphate, 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). Extra 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 kidney stones. Low levels of urinary citrate and magnesium, high levels of oxalate, sodium 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 much 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, hypervitaminosis D), and renal calcium leak (kidney defect that causes excessive calcium to enter the urine) can all cause hypercalciuria. Prolonged inactivity also increases urinary calcium and may cause stones. (This is why astronauts in space tend to lose bone mass and form kidney stones.)
Renal tubular acidosis (inherited condition in which the kidneys are unable to excrete acid) significantly reduces urinary citrate (a natural urinary antacid) as well as total urinary acid levels and can lead to stone formation, usually calcium phosphate.
Uric Acid Stones
Digestion and metabolism of meat protein produces uric acid. If the acid level in the urine is high, the uric acid may not dissolve and uric acid stones will form.
Genetics may play a role in the development of uric acid stones. Approximately 10% of patients with kidney stone disease develop this type of stone. This is the only type of stone that can be dissolved just with medication. Alkalinizing medicines like potassium citrate and sodium bicarbonate can allow uric acid stones to dissolve. To do this, the urinary pH (a measurement of urinary antacid activity) must be maintained between 6.5 and 7.0. pH can be measured with special dipsticks and the dosage of the alkalinizing medications adjusted accordingly.
Struvite Stones
This type of stone, also called an infection stone or triple phosphate stone, develops when a urinary tract infection (e.g., bladder infection) affects the chemical balance of the urine. Bacteria in the urinary tract release chemicals that neutralize urinary acid, 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 than men. 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.
Diagnosis
Diagnosis of renal stone disease involves a medical history, physical examination, laboratory evaluation, and imaging tests. The physician determines if the patient has a history of kidney stones, documents past medical conditions, and evaluates present symptoms. Physical examination may be difficult if the patient is experiencing severe pain and is unable to remain still. Lightly tapping on the kidney region often worsens the pain. Fever may indicate a urinary tract infection that requires antibiotics.
Laboratory tests
Include a urinalysis to detect the presence of blood (hematuria) and bacteria (bacteriuria) in the urine. Other tests include blood tests for creatinine (to evaluate kidney function), BUN and electrolytes (to detect dehydration), calcium (to detect hyperparathyroidism), and a complete blood count (CBC; to detect infection).
Imaging tests
Imaging tests used to diagnose kidney stones include ultrasound, KUB (plain abdominal x-ray), intravenous pyelogram (IVP), retrograde pyelogram, and computed tomography (CT) scan.
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 usually cannot detect small stones, especially those located outside the kidney. It is the preferred imaging method for kidney stone patients who are pregnant.
Intravenous Pyelogram (IVP)
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 can be precisely located using this procedure. There is a slight risk for an allergic reaction to the contrast agent during this study and overall kidney function must be normal. IVP can take a very long time if the blockage to the kidney is severe and it requires a radiologist be present.
Retrograde Pyelogram
A cystoscopy (i.e., a procedure in which a telescopic instrument is inserted into the bladder) is performed to locate the opening from the ureter to the bladder. The contrast agent is injected directly into this opening and an x-ray is taken to locate the kidney stone.
This procedure eliminates the risk of an allergic reaction to the contrast agent because the dye does not reach the bloodstream, but it may require anesthesia. While a retrograde pyelogram is the most reliable method for visualizing the urinary system and detecting stones, it is generally used only when other imaging methods are inadequate or unsuccessful.
Computerized tomography (CT Scan)
This test uses a scanner and a computer to create images of the urinary system. It is performed quickly but may have difficulty detecting small stones located near the bladder. CT scans can also help identify other medical conditions (e.g., ruptured appendix, bowel obstruction, abdominal aortic aneurysm) that cause symptoms similar to kidney stones.
Newer scanners do not require a contrast agent. The non-contrast CT scan is the most common imaging test used to evaluate a possible kidney stone attack. 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.
Prevention
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 will be asked to make lifestyle modifications such as increased fluid intake and changes in diet.
Orange juice and lemonade with real lemon juice are good sources 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 will substantially reduce the risk for future stone formation.
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