Uric acid is the product of the metabolic breakdown of purine nucleotides. It is composed of carbon, nitrogen, oxygen, and hydrogen. In the United States and Europe, 5-10% of urinary tract stones are uric acid stones [1]. This percentage increases in areas with hot, arid climates due to the tendency of low urine volume and acid urine pH which promotes precipitation of uric acid. Uric acid is essentially insoluble in acid urine and it becomes more soluble in alkaline urine. This Daily Doodle illustrates the risk factors of uric acid stones. One of the significant factors predisposing to uric acid stones is a history of gout. About 20% of patients with gout have uric acid stones.

Other Risk factors of uric acid stones can be divided into three different categories:

1. Over consumption (top left corner):

• Purine-rich diet: pork, turkey, anchovies, shrimp, alcohol, mushrooms…
• Medications: cytotoxic drugs

2. Over production (bottom right corner):

• Inborn errors of purine metabolism
• Psoriasis
• Myeloproliferative disorders
• Lymphoproliferative disorders
• Malignancies
• Hemolytic disorders
• Tissue hypoxia
• Down Syndrome
• Glycogen storage diseases

3. Acidic Urine (bottom left corner):

• Uncontrolled diabetes
• Dehyrdration
• Chronic diarrhea

Treatment of Uric Acid Stones:

“Because hydration and medical therapy can lead to dissolution of uric acid stones, more invasive procedures (such as extracorporeal shock wave lithotripsy) are usually not required” unless the stones are large, resistant to dissolution therapy or acutely symptomatic [1]. Dissolution therapy includes hydration, urine alkalinization, dietary modification and consideration of xanthine oxidase inhibitors, such as allopurinol.

Dissolution Therapy [2]:

• Patients are encouraged to maintain a high fluid intake so that urine output is approximately 2-3L per day.
• Potassium citrate can be used to alkalinize urine to pH of 6.5-7.
• For urinary acid excretion >1200mg/day, add allopurinol (inhibits conversion of hypoxanthine and xanthine to uric acid)

References:

1. G. Curhan and M. Becker. “Uric acid nephrolithiasis”. Uptodate. November 2014. http://www.uptodate.com/contents/uric-acid-nephrolithiasis
2. J. Reynard, S. Brewster, S. Biers. “Oxford Handbook of Urology”. Oxford University Press. 2013

“Uric Acid Stones” Daily Doodle by Michiko Maruyama