Dear Editor

In response to the letter from Tom Blyth and Sheila McKenzie[1] we wish to clarify the following points. Our study is the first to examine the effects of both sleeping position and maternal smoking, both factors which are associated with an increased risk for Sudden Infant Death Syndrome (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 Drs 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 (Lewis and Bosque[2] 1995 and Franco et al.[3]). As yet the mechanism(s) which cause some infants to die suddenly and unexpectedly are 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]

References

(1) Blyth T, McKenzie S. SIDS, smoking and arousal thresholds: conclusions not supported by data [electronic response to Horne RSC et al., Effects of maternal tobacco smoking, sleeping position, and sleep state on arousal in healthy term infants] archdischild.com 2002.http://adc.bmjjournals.com/cgi/eletters/fetalneonatal;87/2/F100#143

(2) Lewis KL, Bosque EM. Deficient hypoxia awakening response in infants of smoking mothers: possible relationship to sudden infant death syndrome. J Pediatr 1995;127:691–9.

(3) Franco P, Pardou A, Hassid S, et al. Auditory arousal thresholds are higher when infants sleep in the prone position. J Pediatr 1998;132:240–3.

(4) L'Hoir MP. Proceedings 7th SIDS International Conference. Florence, 2002.

Dear Editor

The investigation of the effect of maternal tobacco smoking on arousal in healthy infants[1] concluded that maternal tobacco smoking increases arousal thresholds (i.e. impairing arousal) in infants of 2-3 months of age, in 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 since 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?

References

(1) Horne RSC, Ferens D, Watts A-M, Vitkovic J, Lacey B, Andrew A, Cranage SM, Chau B, Greaves R, Adamson TM. Effects of maternal tobacco smoking, sleeping posistion and sleep state on arousal in healthy term infants. Arch Dis Child Fetal Neonatal Ed 2002;87: F100-F105

(2) Fleming PJ, Blair PS, Bacon C et al. Environment of infants during sleep and the risk of sudden infant death syndrome: results of 1993-1995 case-control study for confidential inquiry into stillbirths and deaths in infancy. BMJ 1996;313:191-5.

(3) Skadberg BT, Marild I, Markestad T. Abandoning prone sleeping: Effect on the risk of sudden infant death syndrome. J Pediatr 1998;132(2):340-3.