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Editorial

SIDS: facts and controversies

We need to promote the established risk-reducing behaviours, which are based on strong scientific evidence

MJA 2000; 173: 173-174

  Over the period 1982-1986, Australian Bureau of Statistics figures show that there were an average of 457 deaths per year from sudden infant death syndrome (SIDS) in Australia (1.89 deaths/1000 live births).1 Ten years later, over the period 1992-1996, SIDS mortality had plummeted to 210 deaths per year (0.81 deaths/1000 live births).1 In spite of this dramatic decrease, SIDS still causes more deaths than traffic injuries, congenital anomalies and cancer combined in the 1-4 years age group.1 Here, I briefly discuss the established risk factors for SIDS and current areas of controversy.

Sleeping position: Studies dating back to the 1960s, but mostly in the 1980s, had suggested that prone sleeping position was associated with SIDS, but it was not until SIDS prevention campaigns had been successfully run in the Netherlands and New Zealand that the potential for reducing SIDS mortality by modifying this risk factor was recognised.2 In 1991, Australia launched its "Reducing the Risk" campaign, driven by SIDS organisations and supported by Red Nose Day funds. In New Zealand, we observed a close temporal relationship between Red Nose Day education campaigns and reduction in the prevalence of placing infants in the prone sleeping position.3 This illustrates the powerful synergy that can be created when professional and voluntary/parent groups work together. The fall in SIDS mortality can be attributed almost entirely to a change in the prevalence of placing infants in the prone sleeping position,4 supporting the contention that prone sleeping is part of the causal pathway and is a cause of SIDS. Recent evidence suggests that sleeping on the side doubles the risk of SIDS compared with sleeping in a supine position, probably because of infants turning to the prone position ("secondary prone").5 Infants who usually sleep supine but are placed prone (ie, are unaccustomed to the prone position) are at very high risk of SIDS.6,7

Smoking: Maternal smoking is the other major non-controversial risk factor for SIDS.8 Since the reduction in the prevalence of prone sleeping position, there have been eight studies examining maternal smoking and SIDS. The pooled unadjusted (not adjusted for confounders) relative risk (RR) determined from these studies is 4.7, which suggests that infants of mothers who smoke are at an almost fivefold greater risk of SIDS than infants of mothers who do not smoke.

Evidence for the effect of environmental tobacco smoke exposure can be obtained by examining the risk of SIDS from paternal smoking where the mother is a non-smoker. There have been six such studies. The pooled unadjusted RR for these studies was 1.4. The increased risk of SIDS with tobacco smoke is probably predominantly due to an in-utero effect of tobacco smoke rather than postnatal environmental tobacco smoke.8

Bedding and clothing: Excess bedding and clothing have been shown to increase the risk of SIDS in infants sleeping prone, but not for infants sleeping on their side or back. As few infants in Australia sleep prone,4 advice on the amount of bedding and clothing could be dropped.

Some 15%-20% of infants who die of SIDS are found with their head covered by bedding.9 Covering of the head might cause death by forcing an infant to rebreathe expired gases or by creating thermal stress. There have been several suggestions as to how to avoid covering of the head, including tucking bedding in firmly, removing bedding, placing infants at the foot of the cot, using the Dutch sleeping sack, and avoiding the use of duvets. The evidence to support these recommendations is limited.

Bed sharing: It is well established that infants who share a bed with mothers who smoked during the pregnancy are at increased risk of SIDS.5 Whether or not there is an increased risk for infants sharing a bed with mothers who were non-smokers has not been firmly established. If there is an increased risk it is likely to be quite small (pooled unadjusted RR, 1.4). Complicating the picture is the fact that in some cultures bed sharing is an established practice. Furthermore, others have advocated bed sharing to improve breastfeeding rates.10

Breastfeeding: Most studies have shown that the incidence of SIDS is lower in breastfed infants. However, breastfeeding in most developed countries is associated with socioeconomic advantage, and, when adjustment is made for socioeconomic factors, the protective effect of breastfeeding is less apparent.11,12 Some have concluded there is no decreased risk from breastfeeding,12 whereas others have argued that breastfeeding has a protective effect.11

Use of pacifier: An unexpected finding of several studies has been that pacifiers are associated with a reduced risk of SIDS.13 However, this benefit needs to be balanced against possible detrimental effects of pacifiers, such as a reduction in breastfeeding and increased incidence of otitis media.14

Vaccinations: In the past there was concern that vaccinations might cause SIDS, as the peak age for SIDS is 2-4 months, which coincides with the age for vaccinations. However, studies have shown that vaccinations are not associated with an increased risk of SIDS -- indeed, some studies have shown a reduced risk of SIDS at the time of vaccinations.15 Despite this, the media from time to time revive this old chestnut.

"Toxic gas": The "toxic gas" theory has received considerable media attention in the United Kingdom and New Zealand, but has not been substantiated.16 According to this theory, toxic gases are produced by the fungus Scopulariopsis brevicaulis as it metabolises chemicals containing arsenic, antimony and phosphorus in cot mattresses. Proponents of the theory recommend wrapping cot mattresses in polythene, but this is potentially dangerous advice in view of the evidence that plastic sheeting in a baby's sleeping environment can cause death through suffocation.17

Despite the success of the "Reducing the Risk" campaign, SIDS mortality remains unacceptably high among Indigenous Australians (mortality rates for the period 1992-1996, aggregated for South Australia, Western Australia and the Northern Territory, were 30 deaths per year among Indigenous Australians [5.29 deaths/1000 population] compared with 61 deaths/year among non-Indigenous Australians [0.81 deaths/1000 population]).

The cause or causes of SIDS remain largely unknown, although the most likely mechanisms include airway obstruction, rebreathing of expired gases, thermal stress and an "arousal defect" (reduced ability to respond to hypoxia or hypercapnoea by arousing or waking up). There is now little support for the (central) apnoea hypothesis, which was the major mechanism postulated in the 1970s and 1980s. Physiologists need to show how the established risk factors might operate, and researchers need to explore the reasons for the high rate of SIDS in disadvantaged and Indigenous communities.

We must also continue to promote the established risk-reducing behaviours, which are based on strong scientific evidence, and ensure that all new mothers receive this information. We need to devise and evaluate innovative methods for delivering these messages and changing behaviour among disadvantaged and Indigenous groups. New theories should be examined, and discredited ideas buried. The media have an important responsibility, as they are in a position either to create controversy and confusion about SIDS or to serve as a powerful force for producing change.

Ed A Mitchell
Associate Professor in Paediatrics
Department of Paediatrics University of Auckland, New Zealand.
e.mitchellATauckland.ac.nz

Acknowledgement: I am grateful to the Australian Bureau of Statistics for supplying mortality data.

  1. Australian Bureau of Statistics website <http://www.abs.gov.au>
  2. Engelberts AC, de Jonge GA. Choice of sleeping position for infants: possible association with cot death. Arch Dis Child 1990; 65: 462-467.
  3. Mitchell EA, Tonkin S. Publicity and infants' sleeping position. BMJ 1993; 306: 858.
  4. Dwyer T, Ponsonby AL, Blizzard CL, et al. The contribution of changes in the prevalence of prone sleeping position to the decline in SIDS in Tasmania. JAMA 1995; 273: 783-789.
  5. Scragg RKR, Mitchell EA. Side sleeping position and bed sharing in the sudden infant death syndrome. Ann Med 1998; 30: 345-349.
  6. L'Hoir MP, Engelberts AC, van Well GT, et al. Risk and preventive factors for cot death in The Netherlands, a low-incidence country. Eur J Pediatr 1998; 157: 681-688.
  7. Mitchell EA, Thach BT, Thompson JMD, Williams S. Changing infants' sleep position increases risk of sudden infant death syndrome. Arch Pediatr Adolesc Med 1999; 153: 1136-1141.
  8. Mitchell EA, Milerad J. Smoking and sudden infant death syndrome. In: International consultation on environmental tobacco smoke (ETS) and child health. Geneva: World Health Organization, 1999: 105-129.
  9. Beal SM, Byard RW. Accidental death or sudden infant death syndrome? J Paediatr Child Health 1994; 30: 144-150.
  10. McKenna JJ, Mosko SS, Richard CA. Bedsharing promotes breastfeeding. Pediatrics 1997; 100: 214-219.
  11. Ford RP, Taylor BJ, Mitchell EA, et al. Breastfeeding and the risk of sudden infant death syndrome. Int J Epidemiol 1993; 22: 885-890.
  12. Fleming PJ, Blair PS, Bacon C, et al. Environment of infants during sleep and risk of the sudden infant death syndrome: results of 1993-5 case-control study for confidential inquiry into stillbirths and deaths in infancy. Confidential Enquiry into Stillbirths and Deaths Regional Coordinators and Researchers. BMJ 1996; 313: 191-195.
  13. Fleming PJ, Blair PS, Pollard K, et al. Pacifier use and sudden infant death syndrome: results from the CEDI/SUDI case control study. Arch Dis Child 1999; 81: 112-116.
  14. Hunt L, Fleming P, Golding J. Does the supine sleeping position have any adverse effects on the child? I. Health in the first six months. The ALSPAC Study Team. Pediatrics 1997; 100: E11.
  15. Hoffman HJ, Hunter JC, Damus K, et al. Diphtheria-tetanus-pertussis immunization and sudden infant death: results of the National Institute of Child Health and Human Development Cooperative Epidemiological Study of sudden infant death risk factors. Pediatrics 1987; 79: 598-611.
  16. Expert Group to Investigate Cot Death Theories: toxic gas hypothesis. Chairman, Lady Limerick. Final report. London: Department of Health. May 1998.
  17. Kraus JF. Effectiveness of measures to prevent unintentional deaths of infants and children from suffocation and strangulation. Public Health Rep 1985; 100: 231-240.

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