Redefining the Stone Belt: Precipitation is Associated with Increased Risk of Urinary Stone Disease: Beyond the Abstract

As urinary stone disease is a common and debilitating disease (lifetime prevalence estimated to be between 10-15% in the United States) there has been extensive research efforts defining it's risk factors. Geographic variation is one of the most studied extrinsic factors affecting urinary stone disease risk.

A landmark study characterizing stone prevalence in the United States using data from the Cancer Prevention Study II, established the concept of a "stone belt". It identified an increasing prevalence of urinary stone disease in the United States 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. This explanation alone is incomplete however as similarly warm areas (the southwest) do not have the same high incidence. 

We sought in our population based study of 63,994 patients, to discover what other environmental factors, in addition to temperature, are associated with urinary stone disease. We used the population adjusted county operative stone burden as a metric for overall stone disease prevalence. We obtained this data from the California Office of Statewide Health Planning and Development (OSHPD), which records all inpatient and outpatient kidney stone procedures, including ureteroscopy, extracorporeal shockwave, and percutaneous procedures. We accessed climate data from the National Oceanic and Atmospheric Administration (NOAA) over the study period and explored for associations with operative stone burden. We controlled for county level potential cofounders with data from the CDC including diabetes rates, obesity rates and gender breakdown.

Multivariate modeling revealed higher precipitation (0.019 average increase in surgeries per 1000 persons per inch, p<0.01) and higher mean temperature (0.029 average increase in surgeries per 1000 persons per degree, p<0.01) were both independently associated with an increased operative stone disease burden (Figure 1). 

(Figure 1: Increased temperature and precipitation are both associated with increased stone disease burden)

Our findings (increased precipitation is associated with stone disease) helps explain the discrepancy in the national trends of urinary stone disease (prevalence higher in Southeast than the Southwest despite having similar warm temperatures), as a major difference in climates between these two regions is increased precipitation in the southeast. We postulate that the increased rate of stone burden in hot climates with higher precipitation could be related to the increased inefficiency of human body thermoregulation in wet heat versus dry heat, leading to greater insensible fluid losses.

Our findings can be used to better identify populations/regions who are at increased risk for urinary stone disease, which may be changing. Prior studies have predicted an increase in urinary stone disease burden due increased temperatures secondary to global warming. However, the Environmental Protection Agency (EPA) predicts that global warming will also increase global precipitation which, based on our findings, would lead to a greater than forecasted increase in incidence of urinary stone disease.

Written by: Kai Dallas, MD, Urology Resident, Stanford Hospital and Clinics 

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