What causes kidney stones (and what to do)
Stone disease has plagued humanity since ancient times. Kidney stones have been identified in Egyptian mummies. The Hippocratic oath describes their treatment: “I will not use the knife, not even verily, on sufferers from stone, but I will give place to such as are craftsmen therein.”
Who gets kidney stones and why?
The lifetime risk of kidney stones among adults in the US is approximately 9%, and it appears that global warming may be increasing that risk. (As the climate warms, human beings are more likely to get dehydrated, which increases the risk of stone formation.) There are four major types of kidney stones: calcium oxalate/calcium phosphate, uric acid, struvite (magnesium ammonium phosphate), and cystine.
A risk factor for all stones, regardless of type, is dehydration. Anyone who is prone to kidney stones should pay attention to good hydration. A randomized trial has shown that drinking 2 liters of fluid a day reduces the likelihood of stone recurrence by about half. The American Urological Association guideline for medical management of kidney stones recommends that patients who form kidney stones should aim to drink more than 2.5 liters of fluid per day.
Anyone with symptoms of kidney stones should be referred to a urologist. The initial evaluation will often include blood, urine, and imaging studies. Decisions about testing, and ultimately treatment, should be made jointly by the physician and the patient. Let’s look at specific risk factors and treatment for each of the major stone types.
Calcium oxalate and calcium phosphate stones
Calcium stones are the most common type of kidney stones, and can be either calcium oxalate or calcium phosphate. As mentioned, good hydration is important to prevent calcium stones. It may be surprising, but results of a randomized clinical trial show that people with calcium kidney stones should not cut back on dietary calcium. In fact, they should consume the recommended daily allowance of calcium (1,000 mg/day for women younger than 50 years old and men younger than 70, and 1,200 mg/day for women over 50 and men over 70). Why? Calcium binds to oxalate in the intestine and prevents its absorption through the gut, so there is less in the urine to form stones. Ideally, calcium should come from food. Talk with your doctor before taking calcium supplements, and increasing fluid intake might be beneficial depending on how much calcium you take.
Foods high in oxalates (nuts, spinach, potatoes, tea, and chocolate) can increase the amount of oxalate in the urine. Consume these in moderation.
Calcium phosphate stones are less common than calcium oxalate stones. Causes include hyperparathyroidism (when the body produces too much parathyroid hormone), renal tubular acidosis (a kidney condition that causes a buildup of acid in the body), and urinary tract infections. It is important to understand if one of these conditions is behind the formation of calcium phosphate stones.
Good hydration can help prevent recurrence of calcium stones. In addition, thiazide diuretics such as hydrochlorothiazide can help the kidney absorb more calcium, leaving less of it in the urine where it can form stones. Potassium citrate is another medication that can bind to calcium and help keep calcium oxalate and calcium phosphate in the urine from forming into stones.
Uric acid stones
Most patients with uric acid stones don’t have too much uric acid. Instead their urine is too acidic. When that happens, normal levels of uric acid dissolve into the urine where it can crystalize into stones. Adjusting the pH of the urine, most commonly with the medication potassium citrate, reduces the risk of uric acid stone formation and can also help dissolve existing stones. Sodium bicarbonate can also be used to alkalinize the urine. Some people with uric acid stones do produce high amounts of uric acid. For these patients, eating less animal protein can help, as can a drug called allopurinol.
Struvite stones
Struvite stones are composed of magnesium ammonium phosphate, and form in alkaline urine. The most common cause of struvite stones is a bacterial infection that raises the urine pH to neutral or alkaline. Acetohydroxamic acid (AHA) can reduce urine pH and ammonia levels and help dissolve stones.
Cystine stones (the least common type)
Cystinuria is a genetic condition. It results in high levels of cystine (an amino acid) in the urine, which then forms into kidney stones. Most cystine stones can be managed by increasing hydration and medications that change the pH of the urine. If that isn’t enough to control stones, then another medication can be added.
The bottom line
All kidney stone sufferers should remember the phrase, “Dilution is the solution to the pollution.” Good hydration is a safe and useful therapy for all stone formers. That said, it is important to have a thorough discussion with a urologist about the right approach to evaluation, treatment, and strategies to keep new stones from forming.
References
Prevalence of Kidney Stones in the United States. European Urology, July 2012.
Climate-related increase in the prevalence of urolithiasis in the United States. Proceedings of the National Academy of Sciences, July 2008.
Urinary volume, water and recurrences in idiopathic calcium nephrolithiasis: a 5-year randomized prospective study. The Journal of Urology, March 1996.
Medical Management of Kidney Stones: AUA Guideline. American Urological Association, August 2014.
Meta-analysis of randomized trials for medical prevention of calcium oxalate nephrolithiasis. Journal of Endourology, November 1999.
Management of cystinuria. The Urologic Clinics of North America, August 2007.
Medical Stone Disease Management, AUA University, last updated September 2017.
