Seasonal variation in sudden infant death syndrome and bronchiolitis--a common mechanism?

The purpose of this study was to compare epidemiologic features of sudden infant death syndrome (SIDS) and bronchiolitis and to apply statistical examination in order to examine the hypothesis that similar mechanisms could be at work in both conditions. The setting was Scotland from 1982 through 1990. We compared 1,211 deaths from SIDS with 10,058 hospital admissions for bronchiolitis in infancy. The comparisons included age, sex ratio, and seasonality. The sex ratios were similar (SIDS [M:F]:1.61:1; bronchiolitis: 1.63:1), but age distribution was different (chi 2 = 104.6, p < 0.001). When monthly rates throughout the year were compared using correlation of residuals from average season variation and by autocorrelation of residual series, no significant relationships were found between the two conditions (r2 = 0.0004). Once the seasonal pattern common to both conditions was accounted for, SIDS was not autocorrelated between months whereas bronchiolitis exhibited a high level of autocorrelation indicating an epidemic pattern for the latter but not for the former. While a common seasonal variation may indicate some shared etiologic factors associated with winter season, the two conditions do not appear to be closely related. The hypothesis that a common host susceptibility is at work is not supported. Further investigations of seasonal influences are warranted.