Decreased Temperature Associated with Increased ED Visits By Patients with Sickle Cell Disease

Introduction: Sickle cell disease (SCD) is a genetic disorder diagnosed at birth, characterized by repeated acute complications leading to significant morbidity and mortality. Patients subjectively state extreme temperatures trigger sickle cell complications such as vaso-occlusive crisis (VOC) and acute chest syndrome (ACS), leading previous studies to examine the relationship between environmental factors and rates of acute SCD complications with mixed results. Dessap et al. found a correlation between SCD admissions and daily temperature change but not mean daily temperature, whereas Nolan et al. found only windspeed to have a significant correlation. We hypothesized an association exists between temperature, windspeed, and humidity and ED visits for VOC and ACS.

Methods: We designed a retrospective observational study utilizing the Chicago Sickle Cell Disease Research Group clinical registry of patients with SCD (HbSC, HbSB⁰, HbSB⁺, or HbSS) seen in the emergency department (ED) from May 1, 2014 to May 31, 2016 at two pediatric hospitals in Chicago, Illinois. Encounters were included if a final diagnosis of VOC or ACS was recorded based on ICD-9 or ICD-10 codes. Weather data were obtained through the University of Chicago's Center for Data Intensive Science using data from the Environmental Data Commons, collected at Midway Airport. We used Spearman's rank correlation coefficient with n-2 degrees of freedom for correlations between frequency of ED encounters and daily weather conditions including mean temperature, relative humidity, and maximum windspeed. Encounter rates by weather marker were calculated using the number of encounters by unique individuals divided by the total number of days. Our analysis included encounters by unique individuals as multiple encounters from the same subject within a defined weather marker are likely to be correlated and can introduce bias.

Results: During the 2 year study period there were 139 unique individuals, ranging in age from 10 months to 23 years, with SCD (72.7% Hgb SS, 16.5% Hgb SC, and 10.8% Hgb Sβ^+/0). There were a total of 618 ED encounters with a qualifying final diagnosis. Mean daily temperature ranged from 0.4 - 84.8 degrees Fahrenheit, maximum daily windspeed ranged from 6.9 - 42.5 miles per hour, and daily relative humidity ranged from 17.1% - 90.2%. In our analysis, there was a correlation between lower mean daily temperature and increased number of ED encounters for VOC (ρ= -0.791, p<0.001) and for total ED encounters, which included both VOC and ACS (ρ= -0.757, p<0.001). A similar correlation was not seen in the analysis for relative humidity (ρ= 0.252, p=0.43) or for maximum windspeed (ρ= 0.318, p=0.23).

Conclusion: Our study demonstrates a correlation between lower mean daily temperature and increased number of ED visits for SCD complications when examining encounters by unique individuals. There was no significant correlation found for daily windspeed or daily humidity. Prior studies of temperature effects on SCD in France, Brazil and various sites in the United States have shown mixed results, with several showing no correlation. Our findings of low temperatures correlating with increased ED visits could be due to our location having a wider range of temperatures when compared with prior studies. This finding could significantly impact the advice given to patients in areas with periods of prolonged low temperatures as this could increase the rate of ED visits. In Chicago, our study conclusions could be utilized to make policy recommendations on school attendance and accommodations when the temperatures decrease during the frigid winter months, allowing for improved access to homebound instruction for patients with SCD.

Future directions include evaluating pertinent clinical markers such as hemoglobin change from baseline, hydroxyurea use at presentation, and ED disposition, which may show that weather change affects some children with SCD more severely than others. We believe continued investigation is warranted to attempt to more accurately characterize the effects of weather change on clinical outcomes of the pediatric sickle cell population.