© Springer International Publishing Switzerland 2015
Manoj Monga, Kristina L. Penniston and David S. Goldfarb (eds.)Pocket Guide to Kidney Stone Prevention10.1007/978-3-319-11098-1_1010. Medical Management of Uric Acid Stones
(1)
Department of Medicine/Nephrology, Weill Cornell School of Medicine, 435 East 57 Street, New York, NY 10022, USA
Keywords
Uric acid stonesKidney stonesPotassium citrateSodium bicarbonateGeneral Principles
Uric acid is a weak organic acid whose solubility at pH 6.5 is approximately 11 times greater than at pH 5.0. Hence, uric acid stones will usually form only when the urinary pH is low most or all of the time.
Normally the pH of the urine rises after a meal. Parietal cells in the stomach form hydrochloric acid and transiently leave base in the blood which spills into the urine and dissolves any uric acid crystals which may have formed [1].
Uric acid stone formers lose this post-prandial alkaline tide which protects against uric acid crystal formation. Alkaline salt therapy will often dissolve a pure uric acid calculus as long as there is free flow of urine past the stone.
Situations Which Lead to a Persistently Acid Urine
1.
A defect in ammonia production requiring the kidney to use the phosphate buffer pair to excrete most of the acid load by keeping the urine pH low. This defect is frequently age related and is one reason uric acid stones are more common in patients over the age of 50.
2.
Obesity, diabetes, and the metabolic syndrome. Patients with insulin resistance have a defect in ammoniagenesis [2].
3.
A diet high in animal protein, the chief source of non-volatile acid the kidney must excrete.
4.
A ketogenic diet which is deliberately restricted in carbohydrate content.
5.
Lower gastrointestinal losses of base. Chronic diarrhea, whether from bowel disease, drugs such as mestinon (myasthenia gravis), or sertraline (depression).
Types of Bowel Disease
The prototype of intestinal dysfunction causing uric acid stones is the patient with an ileostomy. Lack of colonic function results in loss of base and results in an increased acid load that the kidney must excrete. Any situation which leads to an increase in stool volume can have the same effect. On the other hand, the patient with a small bowel resection for Crohn’s disease usually makes calcium oxalate stones. Malabsorption of fat leads to precipitation of divalent cations in the proximal bowel allowing oxalate to escape to the distal part of the GI tract where this anion is hyper-absorbed. To be sure, both types of GI problems can co-exist but the distinction is frequently useful.
Other Factors
1.
Dehydration is a risk factor for all types of stones. Some patients make pure uric acid stones only when traveling to hot climates. Hikers or skiers at high altitudes such as the Rocky Mountains will lose large amounts of water without realizing it because perspiration does not accumulate. Travelers may avoid fluids because of the nuisance of finding a bathroom.
2.
A high protein diet not only increases the amount of non-volatile acid the kidney must excrete. It also increases the purine load. Glandular meats, sausages and gravies are the worst.
3.
Rapid weight loss increases the uric acid to be excreted.
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