Geographic variation and environmental risk factors for the incidence of initial kidney stones in patients with spinal cord injury

Purpose: Although geographic variability in cases of kidney stones, primarily calcium stones, is reported in the general population, little is known about geographic variability in subjects with spinal cord injury, in whom struvite stones predominate. We examined regional variation in the incidence rate of initial kidney stones and clarified the contributing factors in a spinal cord injury cohort. Materials and Methods: We used data from the national spinal cord injury database between 1986 and 1999 on 7,784 participants from 21 spinal cord injury centers with 24,492 person-years of followup accumulated and 286 incident stone cases. A multilevel Poisson model was constructed to evaluate the ecological effects of latitude, air temperature, water hardness and sunlight index on stone formation while controlling for individual factors, including participant age, race, gender, severity of injury and bladder management. Results: The incidence rate was significantly greater in the southeast and tended to increase with decreasing latitude, similar to the geographic association with kidney stones in the general population. This finding was not explained by differences in individual risk factors. Decreasing water hardness had the strongest effect on stones during year 1 after injury (relative risk 0.6, p <0.001), whereas average annual temperature had the strongest association with stones after year 1 (relative risk 1.1 per 1C increase, p = 0.03). The sunlight index had no association. Conclusions: Our study implies that the increased stone risk in spinal cord injured subjects is potentially preventable by modifying environmental exposure. Etiological factors may be similar to those for kidney stones in the general population.