Greater precipitation, and not just higher temperatures, in a region may place its residents at increased risk for urinary stone disease, a new study suggests.
In a study of 63,994 patients who underwent stone procedures in California from 2010 to 2012, Kai B. Dallas, MD, of Stanford University School of Medicine in Stanford, California, and colleagues found that each 1-inch increase in rainfall was associated with an average of 0.019 surgeries per 1000 persons, according to a report published online ahead of print in the Journal of Endourology. Each 1-degree Fahrenheit increase in mean temperature was associated with an average of 0.029 surgeries per 1000 persons.
For the study, Dr Dallas’ group extracted data from the California Office of Statewide Health Planning and Development databases. They obtained climate information from the National Oceanic and Atmospheric Administration. They used patients’ zip codes to map their county of residence.
The investigators explained that a landmark study characterizing stone prevalence in the United States established the concept of a “stone belt.” The study, published in Kidney International (1994;46:893-899), demonstrated an increasing prevalence of urinary stone disease moving from north to south and west to east. “Many have interpreted the stone belt maps to support the conclusion that higher temperatures are associated with increased risk of stone disease,” the authors wrote. “This is postulated to occur secondary to insensible water loss from perspiration, which leads to dehydration, urine concentration, and urine supersaturation.”
They pointed out, however, that higher temperatures do not fully explain the stone belt phenomenon. “If temperature alone was the primary environmental driving factor, the American Southwest should have a similar stone prevalence to the Southeast.”
The researchers postulate that the increased urinary stone burden found in warmer places with higher precipitation could be related to increase inefficiency of human body thermoregulation in wet versus dry heat, “leading to greater insensible fluid losses.”
In a discussion of study strengths, Dr Dallas and colleagues noted that California is a large state with multiple microclimates, “making it an ideal region to explore the impact of climate variation on urinary stone disease.” In addition, they controlled for county-level patient intrinsic urinary stone factors, such as median age, sex, and prevalence of obesity and diabetes mellitus.
With regard to study limitations, the investigators noted that they could not fully account for climate variation within a county. In addition, patients’ reported zip code may not reflect where they actually live or spend most of their time.
Dallas KB, Conti S, Liao JC, et al. Redefining the stone belt: Precipitation is associated with increased risk of urinary stone disease. J Endourol 2017; published online ahead of print.