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The investigation of the effect of maternal tobacco smoking on arousal in healthy infants1 concluded that maternal tobacco smoking increases arousal thresholds (i.e. impairing arousal) in infants of 2–3 months of age, during quiet sleep in the supine position. It is suggested that this may provide an explanation for the association between smoking and sudden infant death syndrome (SIDS).
This conclusion is not supported by the data because the study also found that maternal tobacco smoking reduces arousal thresholds in 2–3 month old infants, in active sleep in the prone position—the very position in which victims of SIDS are still most commonly found.2,3 Could passive smoking then be protective of death from SIDS?
Smoking is undoubtedly associated with SIDS. However, these contradictory findings do not support the hypothesis that an alteration of infants’ arousal thresholds by passive smoking is explanatory.
Is it not time that SIDS research concentrated less on smoking and more on alternative mechanisms?
In response to the letter from Tom Blyth and Sheila McKenzie,1 I wish to clarify the following points. Our study is the first to examine the effects of both sleeping position and maternal smoking, both factors that are associated with an increased risk for SIDS, on arousal from sleep. We had hypothesised that the effects of these two risk factors might be additive. Our findings, however, showed that sleeping position had no effect on arousal threshold in the smoking group, but arousal was impaired in the non-smoking group when they slept prone. The arousal responses to both stimulus induced and spontaneous arousal were, however, impaired in the smoking group in the supine position. The significant findings that Blyth and McKenzie highlight as being supportive of the idea that passive smoking is protective of SIDS may be explained by this finding that prone sleeping elevated arousal thresholds only in the non-smoking group.
We strongly disagree with the suggestion that passive smoking might be protective of SIDS when infants sleep prone. Our finding of depressed arousal responses in infants of smoking mothers is also supported by those of other workers.2,3 As yet the mechanism(s) that causes some infants to die suddenly and unexpectedly is unknown, it is thus of great importance that research should focus on how the known risk factors for SIDS might act. At present, a failure to arouse from sleep in the face of a life-threatening event is a leading hypothesis for SIDS. In support of this, prone sleeping, maternal smoking, recent infection, head covering, overheating, and prematurity—all risk factors for SIDS—have all been demonstrated to decrease arousability in otherwise healthy infants. Conversely, the use of pacifiers, which decrease the risk of SIDS,4 has been shown to increase arousability.3