Stones are hard masses that can form at several sites in the body. The kidneys are one of the commonly affected organs with regards to stone formation. These stones can affect children and adults. Overall kidney stones will affect about 12% of American men and 7% of American women in their lifetime. Most of these stones are calcium stones with uric acid stones being the second most common type of kidney stones.
What are uric acid kidney stones?
Uric acid kidney stones are stones composed of uric acid that forms in the kidney. The stones typically form from the urine within the renal pelvis, the portion of the kidney where urine collects before being passed down the ureter to the bladder. These types of stones can also form from urine in the ureter and bladder. Sometimes small deposits of uric acid can form within the kidney tubules and lead to kidney disease which is known as uric acid nephropathy. It may be acute or chronic.
Causes of Uric Acid Kidney Stones
Uric acid is formed in the body as a result of the breakdown of purines, a substance found in high quantities within meat and meat products. The uric acid is filtered from the blood by the kidney and then excreted through urine. However, if the concentration of uric acid in the urine exceeds a certain point then it can longer remain dissolved. It precipitates and changes into an insoluble substance which can then form stones. Similarly if the fluid volume in urine is lower than normal, as may occur with dehydration, then there is a greater chance of uric acid precipitating.
Overproduction of uric acid increases the likelihood of precipitation and therefore kidney stone formation. This excess uric acid may be a result of consuming high quantities of purine-rich foods. It may also occur with enzyme defects that affect purine metabolism.
2. Abnormal Secretion and Reabsorption
Uric acid in the kidney tubules may be reabsorbed into the kidney tissue and secreted from the kidney tissue into the tubules. If there is excessive secretion or reduced reabsorption then uric acid concentration in the urine may increase. This may occur with some kidney diseases.
3. Reduced Excretion
Uric acid excretion may be reduced for several reasons which can cause an elevation in blood uric acid levels. Eventually this excess uric acid is processed by the kidney and there is a greater risk of uric acid stone formation. Ketogenic diets, certain drugs and some kidney diseases may be responsible.
If there is insufficient water intake or excessive fluid loss like with diarrhea or vomiting then the concentration of urine solutes increase. Diuretic like alcohol or drugs with a diuretic action may also cause fluid loss. Dehydration may therefore increase the risk of uric acid stone formation.
5. Cell Death
Purine is also found within human cells and is released when cells die. Although cells are constantly dying, sometimes there is higher than normal destruction of cells like in hemolytic anemia. This increases the level of uric acid in the body which may increase the risk of urice acid stone formation.
Read more on high uric acid levels.
Signs and Symptoms
The signs and symptoms of uric acid kidney stones are not significantly different from other types of kidney stones. It includes:
- Flank pain and/or back pain.
- Pain when urinating.
- Difficulty passing urine.
- Persistent urge to urinate.
- Abnormal urine volume evident as frequent urination or low volume urine output.
- Blood in the urine (hematuria).
- Fever, nausea and/or vomiting may also be present.
There is also an increased risk of urinary tract infections (UTIs). Most of the signs and symptoms overlap with kidney stone symptoms. In addition, people with high levels of blood urate levels (hyperuricemia) may also have symptoms of gout.
Read more on signs of kidney stones.
Treatment of Uric Acid Stones
The main treatment option is to make the urine more alkaline. These alkalinizing agents include sodium citrate or bicarbonate. When there is elevated levels of uric acid in the blood then substances like allopurinol may be used to decrease uric acid production. Ultimately this lowers the blood uric acid levels. Increased fluid intake is also important especially when dehydration is a factor.
Surgical intervention may be necessary to breakdown the stones and remove it. These surgical procedures may include ureteroscopic stone extraction, percutaneous nephrolithotomy and extracorporal shock wave lithotripsy. Sometimes open surgery may be performed to remove the stone but this is uncommon. The choice of procedure depends on several factor such as the size and location of the stone as well as the response to medication.
How to Prevent Uric Acid Stones?
Uric acid stones may be preventable. These preventative measures should be considered in people with elevated blood uric acid levels, even if uric acid kidney stones do not form. Always consult with a medical doctor about the treatment, management and prevention of uric acid stones.
- Increasing water intake is necessary and one of the most effective ways to prevent uric acid preciptation and therefore kidney stone formation. Fluid intake should be higher in summer, with strenuous physical activity and if conditions like gastroenteritis arise where fluid loss increases due to vomiting and/or diarrhea.
- Avoid substances that increase water loss through urine (diuretics). Alcohol and caffeine that are widely consumed may increase urine output. Without proper rehydration, there may be net fluid loss which ultimately results in dehydration.
- Cut back on purine-rich foods. Purine is abundant in all meats, poultry and fish. However, offals (organ meat) and certain oily fish such as herring and mackerel tend to have higher levels. While it is not necessary to stop meat consumption entirely, care should be taken to eat small quantities of meat.
- Consume certain vegetables in limited quantities as these foods may also be high in purine. Asparagus, beans, peas and spinach are some of the plant-based foods that can also be a problem along with meat. Moderate intake should not pose a problem but meals containing large quantities of these foods should be avoided.