Uric Acid Stone: Causes, Symptoms and Treatment

What is a Uric Acid Stone?

A uric acid stone is a type of kidney stone (nephrolithiasis) composed primarily of uric acid. Uric acid is a natural waste product formed during the breakdown of purines, which are found in foods such as organ meats, red meat, and shellfish. Uric acid stones account for approximately 5 to 10 percent of all kidney stones, making them the second most common type after calcium oxalate stones.

Causes

Uric acid stones develop when the urine is persistently acidic (low pH) and contains elevated levels of uric acid. Key risk factors include:

• Purine-rich diet: Regular consumption of organ meats, red meat, processed meats, and shellfish increases uric acid levels in both the blood and urine.
• Gout (hyperuricemia): Chronically elevated uric acid in the blood promotes stone formation.
• Metabolic syndrome and type 2 diabetes: These conditions are often associated with persistently acidic urine.
• Inadequate fluid intake: Dehydration raises the concentration of uric acid in the urine.
• Chronic diarrheal conditions: Fluid loss through the intestines leads to concentrated and acidic urine.
• Certain medications: Diuretics and uricosuric agents can increase urinary uric acid excretion.
• Genetic predisposition: A family history of uric acid stones or gout can increase the risk.

Symptoms

Small uric acid stones often cause no symptoms. Larger stones, however, may lead to:

• Renal colic: Severe, cramping pain in the flank that may radiate to the lower abdomen or groin.
• Blood in the urine (hematuria): The urine may appear reddish or brownish.
• Frequent urination and burning sensation during urination
• Nausea and vomiting
• Fever and chills (if a urinary tract infection is also present)

Diagnosis

Several diagnostic tests are used to identify uric acid stones:

• Urinalysis: Measurement of urine pH (typically below 5.5 in uric acid stone patients) and detection of uric acid crystals.
• Blood tests: Assessment of uric acid levels in the blood to identify hyperuricemia.
• Imaging: Unlike calcium stones, uric acid stones are radiolucent and are not visible on standard X-rays. They are best detected by computed tomography (CT) or ultrasound.
• Stone analysis: If a stone is passed, laboratory analysis can confirm its exact composition.

Treatment

Conservative Therapy

Many uric acid stones can be treated without surgery:

• Urine alkalization: Oral administration of alkaline citrate (e.g., potassium citrate) or sodium bicarbonate raises the urine pH to between 6.5 and 7.0. At this pH, uric acid stones can dissolve -- a process called chemolitholysis. This is a major therapeutic advantage over other stone types.
• Increased fluid intake: Drinking at least 2 to 2.5 liters of water daily dilutes the urine and lowers uric acid concentration.
• Dietary modifications: Reducing purine-rich foods, limiting alcohol (especially beer), and avoiding fructose-rich beverages.
• Uric acid-lowering medication: In patients with hyperuricemia, allopurinol may be prescribed to inhibit uric acid production.

Interventional and Surgical Therapy

Larger or treatment-resistant stones may require the following procedures:

• Extracorporeal shock wave lithotripsy (ESWL): Non-invasive fragmentation of the stone using external shock waves.
• Ureteroscopy (URS): Endoscopic removal of the stone through the urethra and ureter.
• Percutaneous nephrolithotomy (PCNL): A minimally invasive surgical procedure for very large stones, performed through a small incision in the back.

Prevention

The following measures can help prevent uric acid stone formation:

• Drink sufficient fluids (at least 2 liters per day)
• Reduce or avoid purine-rich foods
• Monitor blood uric acid levels regularly
• Consistent medical treatment of gout if present
• Maintain a healthy body weight

References

1. Turk C. et al. - EAU Guidelines on Urolithiasis, European Association of Urology (2023)
2. Moe O.W. - Kidney stones: pathophysiology and medical management, The Lancet (2006), 367(9507): 333-344
3. Deutsche Gesellschaft fur Urologie (DGU) - Guideline on Diagnosis, Therapy and Metaphylaxis of Urolithiasis (2022)