Kidney stones are hard collections of minerals and salts that form inside the kidneys. Stones made from high levels of uric acid are known as uric acid stones. Uric acid is a waste product from the body’s breakdown of substances called purines, found in many foods. While less common than calcium-based stones, these formations can cause problems as they move through the urinary system.
How Uric Acid Stones Form
Uric acid stones develop under two primary conditions: a high concentration of uric acid in the urine (hyperuricosuria) and persistently acidic urine with a low pH. When urine becomes too acidic, uric acid is less soluble and does not dissolve as easily. This environment allows uric acid crystals to form and aggregate, eventually growing into stones.
Diet plays a significant role in the formation of these stones. Consuming foods high in purines, such as red meat, organ meats like liver, and certain types of seafood, leads to higher production of uric acid. This increases the amount of uric acid the kidneys must filter, raising its concentration in the urine.
Dehydration is another major contributor. When fluid intake is insufficient, the urine becomes more concentrated. This higher concentration means there is less liquid available to dissolve waste products like uric acid, making it more likely that crystals will form. Even mild, chronic dehydration can significantly increase the risk of developing these painful stones.
Certain medical conditions are associated with an increased risk of uric acid stones. Gout, a form of arthritis, is characterized by high levels of uric acid in the blood, which can lead to higher levels in the urine. Diabetes and metabolic syndrome are also linked to stone formation, often because they are associated with more acidic urine. Chronic diarrhea can be a factor, as it leads to fluid loss and changes in urine chemistry.
A genetic component can also contribute to the risk of developing these stones. Some individuals have an inherited predisposition that affects how their body processes purines or regulates urine acidity. These genetic factors can lead to naturally higher levels of uric acid or a consistently lower urinary pH, creating an ideal setting for stones to form.
Symptoms and Diagnosis
Symptoms of uric acid kidney stones often do not appear until a stone moves from the kidney into the ureter, the tube connecting the kidney to the bladder. The most common symptom is severe pain in the side and back, just below the ribs. This pain can radiate to the lower abdomen and groin area and often comes in waves of varying intensity.
As the stone travels, other symptoms may develop. It is common to see blood in the urine (hematuria), which can make it appear pink, red, or brown. Individuals may also experience painful urination, a persistent urge to urinate, and cloudy or foul-smelling urine. Nausea and vomiting can accompany the intense pain as the body responds to the stress of the passing stone.
Diagnosing a kidney stone begins with a review of medical history and a physical examination. To confirm the presence of a stone and determine its type, a doctor will order a series of tests. Urine tests check for high levels of uric acid, crystals, and infection, while blood tests can reveal elevated uric acid in the bloodstream. A 24-hour urine collection may be requested for a more detailed analysis of the urine’s chemical composition.
Imaging studies are used for locating the stone and assessing its size. A non-contrast computed tomography (CT) scan is often the preferred method as it can reliably identify uric acid stones and determine their exact location and size. An ultrasound may also be used as a screening tool. Pure uric acid stones are often radiolucent, meaning they are not visible on a standard X-ray.
Treatment for Uric Acid Stones
The approach to treating an existing uric acid kidney stone depends on its size and the severity of symptoms. For smaller stones, a conservative approach is often successful. This involves increasing fluid intake by drinking more water to help flush the stone out naturally. Pain medication may be prescribed to manage discomfort as the stone passes.
When a stone is too large to pass on its own, causes unbearable pain, or obstructs urine flow, medical interventions are necessary. One procedure is shock wave lithotripsy (SWL), which uses high-energy sound waves to break the stone into smaller, passable fragments. Another option is ureteroscopy, where a small, flexible scope is passed through the urethra to either remove the stone or break it apart with a laser. For very large stones, percutaneous nephrolithotomy (PCNL) involves surgically removing the stone through a small incision in the back.
A unique aspect of treating uric acid stones is their potential to be dissolved with medication. Taking medications like potassium citrate can raise the urine’s pH, making it more alkaline. This change in acidity can help dissolve existing uric acid stones over time, providing a non-invasive treatment option. This approach targets the chemical composition of the stones directly.
Preventing Future Stones
Preventing the recurrence of uric acid stones involves lifestyle adjustments and sometimes medication. A primary focus is on dietary changes to lower the amount of uric acid in the body. This includes reducing the consumption of high-purine foods like red meat, organ meats, shellfish, and some fish. Limiting alcohol, especially beer, and sugary drinks with high fructose corn syrup also helps manage uric acid levels.
Increasing the intake of fruits and vegetables is another dietary strategy. Many fruits and vegetables can help make the urine less acidic, which discourages the formation of uric acid crystals. Foods that are good sources of potassium, like bananas, can be beneficial. Maintaining a healthy weight through a balanced diet and regular exercise is also recommended, as obesity is a risk factor.
Consistent hydration is key to prevention. Drinking plenty of water throughout the day, aiming for at least 2 to 3 liters, helps to dilute the urine. Diluted urine makes it more difficult for uric acid to form new stones. This simple habit is one of the most effective measures for preventing all types of kidney stones.
For individuals who continue to form stones despite these changes, preventative medication may be prescribed. A common medication is allopurinol, which works by reducing the body’s production of uric acid. This lowers the amount of uric acid that needs to be excreted by the kidneys. This medication is considered a long-term therapy for those with recurrent issues.