Association of Wind Speed and the Occurrence of Sickle Cell Acute Painful Episodes: Results of a Case-Crossover Study.

The role of weather as a possible trigger of sickle cell acute painful episodes has been debated for over 30 years. Early studies based on anecdotal evidence, such as patients reporting pain during the colder parts of the day or when swimming in the cold ocean on a particularly hot day, argued for an association between weather and the occurrence of pain. Recently published studies have shown an association with cold and rainy seasons and with windy weather and low humidity. Other studies however, have found no associations. A limitation of these studies is that they are based on seasonal trend data, mean monthly temperatures, hospital-wide visit rates, but not data at the individual level. To more accurately describe the role of weather as a trigger of painful events, we conducted a case-crossover study of the association of local weather conditions with the occurrence of individual pain crises. From the Cooperative Study of Sickle Cell Disease, 813 patients with 3,580 acute painful episodes were identified. For each pain episode, the hazard period was defined as the 48 hours preceding the onset of pain, and control periods were two periods of 48 hours, two weeks before, and two weeks after the pain crisis. Local weather data including temperature, wind speed and relative humidity, were downloaded from weather-source.com for each of the 23 participating centers for the years 1979 through 1982. Weather data were merged with clinical data and the association between the occurrence of pain crises and local weather conditions were studied using conditional logistic regression. We found an association between wind speed and the onset of pain, specifically wind speed during the 24 hour period preceding the onset of pain. Continuous measures of wind speed, mean and median wind speed during the 24 first hours of the hazard/control windows, showed significant associations with the occurrence of pain (p = 0.03 and p = 0.009, respectively). Analyzing wind speed as a categorical trait, dichotomized at the median (10 mph) for the same 24 hour period, showed a 14% increase (95% CI: 4% – 12%) in odds of pain, when comparing the high wind speed to lower wind speed (p = 0.005). To determine the most likely induction time, average wind speeds were determined for 4 hour intervals and their association with the onset of pain analyzed. Assuming a non-specific induction time will bias the measure of association toward the null, the interval with the highest OR should contain the most relevant induction time. We found that the interval from 13 hours to 16 hours prior to onset of pain has the largest measure of association [OR =1.01 (1.00 – 1.02), p = 0.026]. These results are in agreement with another study that found an association between wind speed and hospital visits for pain in the United Kingdom (Jones et. al, BJH 2005). These findings lend support to recent physiological and clinical studies that have suggested that skin cooling is associated with sickle vasoocclusion (Mohan et al. Clin Sci, 1998), and perhaps pain (Resar et al., J Pediatr 1991). Though pain is a common complication, and likely to have many potential triggers, physicians may wish to advise patients to take precautions on windy days by limiting skin exposure.