Seasonal Variation in Incidence of Sarcoidosis: A Population-Based Study 1976-2013

Background/Purpose:   Sarcoidosis is a multi-systemic disorder of unknown etiology. Studies of seasonal and regional patterns of incidence of sarcoidosis may provide more understanding of potential environmental triggers of this disease. However, previous studies on the seasonality of sarcoidosis have yielded conflicting results.

Methods: A cohort of adult patients with incident sarcoidosis in 1976-2013 in a geographically well-defined population was identified based on comprehensive individual medical record review. Inclusion required physician diagnosis supported by histopathology and radiologic features of intrathoracic sarcoidosis, compatible clinical presentation and exclusion of other granulomatous diseases. The only exception to the requirement of histopathological confirmation was stage I pulmonary sarcoidosis that required only radiographic evidence of symmetric bilateral hilar adenopathy. Age and sex adjusted incidence rates were calculated using population estimates for adults based on decennial census counts as the denominators. Seasonal variation was compared using Poisson regression models.

Results: The cohort included 345 cases of incident sarcoidosis (mean age 35.4 years, 50% female, 90% Caucasian and 5% African-American). Patients in this cohort were less likely to have incident sarcoidosis in the autumn season with an age and sex adjusted rate of 2.0/100,000 (95% CI 1.5-2.5) compared with winter (3.2/100,000; 95% CI 2.6-3.8), spring (2.8/100,000; 95% CI 2.2-3.4) and summer (2.9/100,000; 95% CI 2.2-3.5; p=0.011; figure 1). Subgroup analysis per decade (1976-1985, 1986-1995, 1996-2005 and 2006-2013) consistently showed lower incidence of sarcoidosis in autumn (rate ratios for autumn compared to spring were 0.53, 0.70, 0.64 and 0.85, respectively), although statistical power was insufficient to demonstrate statistical significance at p <.05.

Conclusion: There is seasonal variation in the incidence of sarcoidosis, with lower rates consistently seen in autumn in the past more than three decades. The results may have implications for the understanding of the etiology of sarcoidosis.