What are uric acid stones?
Uric acid stones are one of four major types of kidney stones, which include calcium stones (calcium oxalate and calcium phosphate), struvite stones, and cystine stones. A kidney stone is a hard mass of crystallized minerals that form in the kidneys or urinary tract.
How common are uric acid stones?
It is estimated that one in 10 people in the U.S. will have a kidney stone of one kind or another at some time in their lives. In the late 1970s, about 3.8% of the population had kidney stones, but this figure has now increased to about 8.8% of the population. Among men, the lifetime risk is about 19%; in women, it is 9%. Usually, the first incidence of kidney stones occurs after age 30. However, there are many cases that occur sooner, some in children as young as five years of age.
Symptoms and Causes
What causes uric acid stones?
Uric acid stones form when the levels of uric acid in the urine is too high, and/or the urine is too acidic (pH level below 5.5) on a regular basis. High acidity in urine is linked to the following causes:
- Inherited problems in how the body processes uric acid or protein in the diet can increase the acid in urine. This can be seen in conditions such as gout, which is known for its high levels of uric acid in the blood and painful deposits of crystals in the joints.
- Uric acid can result from a diet high in purines, which are found especially in animal proteins such as beef, poultry, pork, eggs, and fish. The highest levels of purines are found in organ meats, such as liver and fish. Eating large amounts of animal proteins can cause uric acid to build up in the urine. The uric acid can settle and form a stone by itself or in combination with calcium. It is important to note that a person’s diet alone is not the cause of uric acid stones. Other people might eat the same diet and not have any problems because they are not prone to developing uric acid stones.
- There is an increased risk of uric acid stones in those who are obese or diabetic.
- Patients on chemotherapy are prone to developing uric acid stones.
What are the symptoms of uric acid stones?
All types of kidney stones produce similar symptoms, including one or more of the following:
- Pain in the lower back, sides, abdomen or groin; the pain is the result of irritation or blockage inside the kidneys or urinary system
- Blood in the urine
- Nausea or vomiting
- Fever and chills
- Urine that smells bad or is cloudy during a urinary tract infection
Diagnosis and Tests
How are uric acid stones diagnosed?
- A healthcare provider will take down your medical history.
- A doctor will do a physical exam.
- An X-ray or other imaging tests can be used to allow for a picture of the size, number, and location of stones.
- If a stone is passed in the urine and can be retrieved, sending it to a laboratory for analysis can confirm what type of stone it is. Knowing the type of stone is helpful in the treatment and prevention. This is because each of the four major types of stones is treated differently.
- Stones may be removed from the kidney or urinary tract by the following minimally invasive surgical techniques:
- Shockwave lithotripsy involves breaking up the stone through external shock waves without entering the body.
- A tiny scope is used to enter the ureter (the tube leading from each kidney to the bladder) and/or kidney through the urethra and breaking up the stone with a laser. The stone fragments can then be removed with a tiny basket.
- Larger stones in the kidney can be removed through percutaneous nephrolithotomy (PCNL), which involves making a 1cm incision in the back and removing stones through a hollow, direct tract into the kidney.
- Blood and urine can be tested for abnormal levels of specific chemicals.
- Urine collection over a 24-hour period can determine the abnormal components in the urine and how much urine is being passed per day. If the urine volume is too low, the patient will be encouraged to drink more liquids to reach a target urine volume of 2.5L/day.
Management and Treatment
How are uric acid stones treated?
Stones smaller than seven millimeters (7 mm) in diameter may pass on their own with time. This can take up to three weeks. Even when stones do pass, however, it is important to seek treatment in order to prevent the formation of more stones. The use of medications known as alpha blockers may encourage the passage of stones located in the lower ureter.
Drinking more fluids is the most important step in treatment. Higher levels of fluids reduce the concentration of minerals in urine, and encourage urination which can flush away materials that might form stones. It is recommended that a person drink enough fluid each day to produce about 2.5 liters of urine. This usually requires intake of about three liters (3.1 quarts) of fluids per day, since some may be lost through sweating, particularly in hot weather, or as a result of work or exercise.
All types of fluids count toward the goal of increased intake, but the best fluid to drink is water. Hard water with high calcium content and soft water with high sodium content are best to avoid because they add extra minerals to urine.
In cases of stones that are large, block the flow of urine, cause infection, or have not passed after four to six weeks, surgery may be needed to remove them.
What are possible complications of uric acid stones?
Any kind of kidney stone increases the risk of developing long-term kidney disease. Once a stone is found, the risk of having another is about 50% in the next five to seven years.
How can uric acid stones be prevented?
You can help to prevent uric acid stones if you do the following:
- Drink at least three quarts (12 cups) of fluids daily; water is best.
- Decrease intake of red meats and shellfish, replacing them with more fruits and vegetables, low-fat dairy products, and whole grains.
- Limit the amount of alcohol you drink.
- Achieve or maintain a healthy weight.
- Avoid crash dieting, which increases uric acid levels in the blood.
- Follow the Dietary Approaches to Stop Hypertension (DASH) diet, which has been shown to reduce not only high blood pressure but also the risk of kidney stones.
Many patients may also need to take prescribed medications to prevent uric acid stones and keep them from coming back.