Sudden infant death syndrome: Insulation from bedding and clothing and its effect modifiers

Background. Thermal stress related to excessive insulation from bedding and clothing has been postulated to be associated with sudden infant death syndrome (SIDS). Methods. The parents of 393 (81%) of the infants who died of SIDS in the post neonatal period were interviewed at home. Interviews were also completed with the parents of 1592 (88.4% of total) controls, a representative sample of all hospital births. The study was conducted in regions of both the North and South Island of New Zealand in which 78% of all New Zealand births occurred in 1987-1990. Temperatures for the infant's bedroom estimated from the outside temperature and a model were used to predict the appropriate insulation for the lower critical temperature (temperature below which the metabolic rate is likely to increase). Results. Sudden infant death syndrome was associated with extra thermal insulation of > 2 tog above the lower critical value, the odds ratio (OR) was 1.70 (95% confidence interval [CI] : 1.31-2.20) after adjusting for season, After adjusting for a number of confounding factors the OR was reduced to 1.35 (95% CI :0.97-1.87). Also associated with SIDS was too little thermal insulation OR = 1.67 (95% CI : 1.13-2.48), and 2.63 (95% CI : 1.61-4.30) when adjustments were made for the confounding factors. The interaction effect between infants sleeping prone and > 2 tog extra thermal insulation was significant (OR = 6.07, 95% CI: 3.83-9.60). Infants with too little thermal insulation were at increased risk ii they were not tightly wrapped (OR = 3.81, 95% CI : 2.04-7.09). There were also small additive interaction effects if the mother smoked and the infants had > 2 tog extra thermal insulation, or if they were ill and had > 4 tog extra insulation. Interaction effects between thermal insulation and other factors were not significant. Conclusion. More thermal insulation than was necessary to maintain the lower critical temperature increased the risk of SIDS primarily among infants in the prone sleep position, and to a lesser degree in infants whose mothers smoked and in infants who were unwell.