Four modifiable and other major risk factors for cot death: the New Zealand study

Sudden infant death syndrome — a ‘diagnosis’ in search of a disease

Although Sudden Infant Death Syndrome (SIDS) is the major cause of death in infants aged between 1 week and 1 year in Western countries, it is one of the most enigmatic conditions encountered in paediatric forensic practice. SIDS has been recognized since Biblical times, and yet the definition continues to be debated and the aetiology remains obscure. In addition, there are no accepted pathognomonic features at post-mortem and the diagnosis is still one of exclusion. Emery once asked whether the term 'SIDS' is in reality more of a 'diagnostic dustbin' into which are placed a variety of unrelated entities.(1) To a certain extent this is true, as it is now recognized that a range of disorders can result in the sudden and unexpected death of an infant in a cot. It is also likely that the aetiology of SIDS is heterogeneous and that the term SIDS is not so much a diagnosis but a term covering a variety of mechanisms which result in a common lethal outcome. There are a number of controversies in the SIDS field which complicate the use of the term and which confound the assessment of causes and mechanisms of sudden infant death.

Physiological studies of gastro-oesophageal reflux and airway protective responses in the young animal and human infant

1. The mechanisms that underlie the Sudden Infant Death Syndrome (SIDS) must explain its two unique features; age at death and death during apparent sleep. 2. The occurrence of gastro-oesophageal reflux (GOR) during active sleep in infants presenting with apparent life threatening episodes (ALTE) and their similar age distribution to SIDS infants, suggested that reflux could be a cause of asphyxia. 3. Sleep related GOR was found to be a physiological and not a pathological event in normal, healthy term infants. 4. In healthy term infants, those infants that were formula-fed (who have a higher incidence of SIDS) had significantly longer oesophageal clearance times for acid reflux and significantly more active sleep compared with breast fed infants. 5. In very preterm infants (who are at increased risk for SIDS), both the frequency and duration of reflux during active sleep was significantly less at term equivalent age compared with healthy term infants, suggesting additional factors must operate to promote an ALTE. 6. One mechanism which may explain the pathogenesis of GOR could be that the reflux reaches the level of the pharynx and this, in turn, stimulates laryngeal receptors to produce apnoea. 7. Simulated reflux to the level of the pharynx in the sleeping piglet evoked airway protective responses, namely swallow, arousal and occasionally expectoration, but neither apnoea nor oxygen desaturation. 8. In the same piglets treated with pentobarbitone sodium, swallowing was impaired and arousal depressed. Simulated reflux to the pharynx produced significant apnoea and oxygen desaturation and death in two of five piglets.(ABSTRACT TRUNCATED AT 250 WORDS)

Are nappy sterilisers associated with SIDS? New Zealand Cot Death Study Group

Chemicals used to clean nappies have been suggested as a cause of sudden infant death syndrome (SIDS). Parents of 393 cases and 1592 controls were questioned about nappy cleaning procedures. Soaking in sterilisers followed by rinsing in water had a relative risk of 0.91 compared with other cleaning procedures. Nappy cleaning methods are not related to SIDS.

Sleeping position and sudden infant death syndrome in Norway 1967-91

OBJECTIVE

To investigate, in a population based national study, the association between sleeping position of infants and the occurrence of sudden infant death syndrome (SIDS).

DESIGN

A retrospective survey and registry based ecological study. A questionnaire based surveillance of sleeping position was obtained in a random sample (n = 34,799) and surveillance of SIDS was based on all infants born in Norway 1967-91, surviving the perinatal period. Variables studied from the questionnaire were usual sleeping position (placed), breast feeding at 3 months, and maternal smoking in pregnancy, and from the Medical Birth Registry maternal age, birth order, and birth weight.

RESULTS

Proportion of infants sleeping prone increased from 1970 (7.4%) to 1989 (49.1%) and dropped in 1990 (26.8%) and 1991 (28.3%). Occurrence of SIDS increased from 1970 (1.1/1000) to 1989 (2.0) before dropping in 1990 and 1991 (1.1). IMPLICATION AND RELEVANCE OF RESULTS: A cause effect relationship between prone sleeping and SIDS as suggested in previous studies is supported by the present; and so far only, national study of infants' sleeping position.

Sudden infant death syndrome in New Zealand: are risk scores useful? New Zealand National Cot Death Study Group

STUDY OBJECTIVE

To evaluate the Christchurch, Invercargill, Dunedin (CID) and Oxford record linkage study (ORLS) risk scores in five regions of New Zealand and examine the effect of risk factors for sudden infant death syndrome (SIDS), such as prone sleeping position, maternal smoking, breast feeding, measures of illness, the use of antenatal classes, community health care, and medical services on a high and low risk group delineated by the CID score.

DESIGN

This was a case-control study of infants dying of SIDS.

SETTING

Both the cases and controls were born in one of five health districts in New Zealand and their parents were interviewed between 1 November 1987 and 31 October 1990.

PARTICIPANTS

The cases were 485 infants who died of SIDS. The controls were a random sample drawn from the same five regions in which the cases were born, chosen so that their age on the day on which they were interviewed was similar to the age at death of the cases. Risk scores were calculated for 387 case and 1579 controls.

MEASUREMENTS AND MAIN RESULTS

Using the recommended cut off points the sensitivity and specificity of the CID and ORLS were found to be similar to those described for other samples. The differences among the regions were significant. There was, however, no evidence that the association between SIDS and the risk factors considered was different in the high and low risk groups delineated by the CID score. The relative attributable risk for smoking was 32.3% in the high risk group. The excess risk that could be attributed to a different prevalence of any of the other risk factors in the high risk group was small when compared with the low risk group.

CONCLUSIONS

Health care resources should be spent on promoting and evaluating good child care practices for all, rather than identifying and promoting special interventions for those in the high risk category.

Sudden infant death syndrome: effect of breast and formula feeding on frontal cortex and brainstem lipid composition

METHODOLOGY

Docosahexaenoic acid levels were measured by gas chromatography in samples of frontal lobe and brainstem taken from 28 and 26 infants, respectively, who had died of sudden infant death syndrome (SIDS).

RESULTS

Significantly higher levels of docosahexaenoic acid were present in the frontal lobe tissues derived from the 13 breast fed infants (age range = 3.3-36.3 weeks; mean 15.9 +/- 11.3 weeks) compared to the 15 formula fed infants (age range = 6.9-47.7 weeks; mean 19.3 +/- 10.6 weeks); mean (+/- s.d.) levels were 8.5 +/- 1.1% and 7.6 +/- 0.8% of total fatty acids (P = 0.019). There was, however, no significant difference in brainstem docosahexaenoic acid levels between breast and formula fed infants.

CONCLUSIONS

Given these variable findings, further investigation of the relationship between dietary fatty acid intake and cerebral lipid levels may help to clarify whether different modes of feeding have a role in the pathogenesis of SIDS.

Factors adversely associated with breast feeding in New Zealand

Control data from 1529 infants studied in a multicentre case-control study of sudden infant death in New Zealand were analysed to identify factors that might hinder the establishment and duration of breast feeding. Although 1300 infants (85%) were exclusively breast-fed at discharge from the obstetric hospital, this fell to 940 (61%) by 4 weeks. Logistic regression was used to identify factors that might adversely influence breast feeding 'at discharge', 'at 4 weeks' and the overall 'duration' of breast feeding. When adjusted for confounding factors, not exclusive breast feeding 'at discharge' was significantly associated with: twin pregnancy, being a Pacific Islander, mother not bedsharing, subsequent dummy use, birthweight less than 2500 g, heavy maternal smoking, not attending antenatal classes and mother less than 20 years old at first pregnancy. Mothers smoking more than 20 cigarettes a day were nearly twice as likely to not exclusively breast feed on discharge compared to those who did not smoke. A 'dose response' was apparent with the heaviest smokers having the least likelihood of establishing exclusive breast feeding. Being exclusively breast-fed at discharge but not 'at 4 weeks' was associated with: twin pregnancy, admission to a neonatal intensive care unit, subsequent dummy use and not being married. A shorter overall 'duration' of breast feeding was associated with maternal smoking, subsequent dummy use, mother not bedsharing, twin pregnancy, mother less than 20 years old at first pregnancy, low occupational status and not attending antenatal classes. These effects persisted when social and demographic factors, including birthweight, were taken into account.(ABSTRACT TRUNCATED AT 250 WORDS)

Clothing and bedding and its relevance to sudden infant death syndrome: further results from the New Zealand Cot Death Study

As part of a large nationwide case-control study covering a region with 78% of all births in New Zealand during 1987-90, the clothing and bedding of infants dying of sudden infant death syndrome (SIDS) and that of an appropriate control group were recorded. Cases consisted of 81% (n = 393) of all cases of SIDS in the study area and 88.4% (n = 1592) of 1800 control infants randomly selected from the hospital births and who completed a home interview. Use of a wool 'waterproof' underblanket was associated with a significantly reduced risk of SIDS (adjusted OR 0.44; 95% CI: 0.26-0.73) while sheepskin use was not. Firm tucking was identified as contributing to a reduced risk of SIDS even after adjusting for potentially confounding variables (adjusted OR 0.63, 95% CI: 0.46-0.86). Sixty case infants (15.6% of cases) were found dead with the head covered but there were no equivalent data for controls. Having been found previously completely covered by bedding was equally common in cases and controls (28.8% cases and 30.6% of control infants). Other differences of bedding and clothing between cases and controls were small; mattress characteristics were not studied. The exact methods in which babies are cared for are important and this study suggests that infants are at lower risk of SIDS when firmly tucked in and when sleeping on a 'waterproof' wool underblanket.

Observations on ethnic differences in SIDS mortality in New Zealand

INTRODUCTION

Within New Zealand the SIDS mortality rate is higher in Maori than in non-Maori, predominantly European.

AIMS

This paper addresses two questions (1) How should ethnicity be defined, by biological or cultural criteria? (2) Why is the SIDS rate higher in Maori, because of different risk factors or because of a higher prevalence of common risk factors?

METHODS

A nationwide case-control study.

RESULTS

The majority of mothers with some Maori blood (as reported on the infants birth registration form) report they are Maori (as recorded in the obstetric records or interview). Risk factors for SIDS are similar in the various ethnic groups in New Zealand.

CONCLUSIONS

Using a biological definition underestimates the number of Maori infants compared to the cultural definition. Differences in SIDS mortality appear to be explained by differences in prevalence of known risk factors, the most important of which, prone sleeping position, maternal smoking, lack of breast feeding and bed sharing, are culturally determined rather than biologically.

Travel and changes in routine do not increase the risk of sudden infant death syndrome

We investigated the relationship between travel and changes in routine and the sudden infant death syndrome (SIDS) among 485 SIDS cases compared with 1800 randomly selected control infants. There was no increased risk of SIDS with travel. Special events, such as christenings, were not associated with an increased risk of SIDS. However, visits to and by friends or relatives were associated with a significantly reduced risk of SIDS after controlling for potential confounders (odds ratios = 0.70; 95% confidence interval = 0.52, 0.96). These findings may indicate less social support in SIDS cases.

Sudden infant death syndrome in aboriginal and non-aboriginal infants

This study, based on routinely recorded data, was designed to compare the epidemiology of sudden infant death syndrome (SIDS) in Aboriginal and non-Aboriginal infants in Western Australia (WA). All cases of SIDS occurring in infants born in WA from 1980 to 1988 were included in the study. There were 66 Aboriginal (6.1 per 1000 live births) and 337 non-Aboriginal (1.7 per 1000 live births) infants who died from SIDS. It was found that there was a significant linear increase in the Aboriginal SIDS rate over the study period while the non-Aboriginal rate remained relatively constant. For non-Aboriginal infants, there was an elevated risk of SIDS for young maternal age, single marital status and male gender but this was not found for Aboriginal infants. There was a significant difference in the age at death distribution for the two populations. Low birthweight and preterm birth were risk factors for both Aboriginal and non-Aboriginal infants. There may be differences in the aetiology and/or classification of SIDS between the two populations.

Well health care and the sudden infant death syndrome

The aim of this study was to examine whether poor attendance at routine antenatal and postnatal 'well child' health services was associated with a higher risk of sudden infant death syndrome (SIDS, or cot death). A nationwide case-control study of SIDS in New Zealand enrolled 485 postneonatal deaths due to SIDS and 1800 control infants who were selected randomly. The risk for SIDS was found to be higher for infants whose mothers attended their first antenatal check later than 3 months into the pregnancy, made fewer antenatal visits, and did not go to antenatal education classes. However, this increased risk was largely explained by high parity, maternal smoking, the mother not being married, mother being < 20 years old at the birth of her first child, and delivery during the winter months. Infants not attending a 6 week postnatal check had an almost three-fold increased risk of SIDS compared with those who did attend (odds ratio [OR] 2.86; 95% confidence interval [CI] 1.93, 4.24). Similarly, infants not attending well child clinics were at increased risk of SIDS (OR 2.75; 95% CI 2.09, 3.62). These differences persisted when adjusted for likely confounders. This study demonstrates that infants who miss child health nurse clinics are those most at risk for SIDS and are those who warrant increased surveillance.

Reduction in mortality from sudden infant death syndrome in New Zealand: 1986-92

Mortality from sudden infant death syndrome (SIDS, or cot death) in New Zealand has been high by international standards (4/1000 live births). Within New Zealand the rate is higher in Maori than in non-Maori (predominantly European infants) and higher in South Island than in North Island. The National Cot Death Prevention Programme aims to reduce the prevalence of four modifiable risk factors for SIDS, namely infants sleeping prone, maternal smoking, lack of breast feeding, and infants sharing a bed with another person. The aim of this study is to describe the total postneonatal and total SIDS mortality in New Zealand from 1986 to 1992. Official publications from 1986 to 1990 and preliminary death notifications for 1991 and 1992 were examined. Deaths from all causes in the postneonatal age group (28 days to 1 year) and the total number of deaths from SIDS irrespective of age decreased markedly in 1990 and has continued to decrease. This decrease occurred particularly in non-Maori groups, in South Island, and in the winter months. The proportion of infants sleeping in a prone position has decreased from 43% to less than 5%. This suggests that the prone position is causally related to SIDS. The mechanism appears to be related directly or indirectly to environmental temperature.

Nicotine and cotinine levels in pericardial fluid in victims of SIDS

We have analyzed the levels of nicotine and cotinine in pericardial fluid in 24 consecutively autopsied cases of sudden unexpected death in infants aged one to six months. Our aim was to determine to what extent victims of sudden infant death may have been exposed to passive smoking near the time of death. Sixteen of the decreased infants were classified as SIDS at autopsy. Other contributing causes of death, predominantly infections, were found in eight cases. Eight infants (30%) had cotinine levels less than 2 ng, indicating that no significant exposure to nicotine had occurred near the time of death. Of the remaining 70%, five had been moderately exposed, seven markedly exposed and four heavily exposed (cotinine levels 2-10 ng, 10-50 ng and more than 50 ng, respectively). Since only 18% of Swedish women smoke after childbearing we conclude that nicotine exposure in infants who died suddenly was much higher than one would otherwise expect. It is hypothesized that high concentrations of nicotine and nicotine metabolites around the heart may affect cardiac function and thus play a role in the mechanisms causing SIDS or other categories of sudden unexpected death.

Bed sharing, smoking, and alcohol in the sudden infant death syndrome. New Zealand Cot Death Study Group

OBJECTIVES

To investigate why sharing the bed with an infant is a not consistent risk factor for the sudden infant death syndrome in ethnic subgroups in New Zealand and to see if the risk of sudden infant death associated with this practice is related to other factors, particularly maternal smoking and alcohol consumption.

DESIGN

Nationwide case-control study.

SETTING

Region of New Zealand with 78% of all births during 1987-90.

SUBJECTS

Home interviews were completed with parents of 393 (81.0% of total) infants who died from the sudden infant death syndrome in the postneonatal age group, and 1592 (88.4% of total) controls who were a representative sample of all hospital births in the study region.

RESULTS

Maternal smoking interacted with infant bed sharing on the risk of sudden infant death. Compared with infants not exposed to either risk factor, the relative risk for infants of mothers who smoked was 3.94 (95% confidence interval 2.47 to 6.27) for bed sharing in the last two weeks and 4.55 (2.63 to 7.88) for bed sharing in the last sleep, after other confounders were controlled for. The results for infants of non-smoking mothers were inconsistent with the relative risk being significantly increased for usual bed sharing in the last two weeks (1.73; 1.11 to 2.70) but not for bed sharing in the last sleep (0.98; 0.44 to 2.18). Neither maternal alcohol consumption nor the thermal resistance of the infant's clothing and bedding interacted with bed sharing to increase the risk of sudden infant death, and alcohol was not a risk factor by itself.

CONCLUSION

Infant bed sharing is associated with a significantly raised risk of the sudden infant death syndrome, particularly among infants of mothers who smoke. The interaction between maternal smoking and bed sharing suggests that a mechanism involving passive smoking, rather than the previously proposed mechanisms of overlaying and hyperthermia, increases the risk of sudden infant death from bed sharing.

Dummies and the sudden infant death syndrome

The association between dummy use and sudden infant death syndrome (SIDS) was investigated in 485 deaths due to SIDS in the postneonatal age group and compared with 1800 control infants. Parental interviews were completed in 87% of subjects. The prevalence of dummy use in New Zealand is low and varies within New Zealand. Dummy use in the two week period before death was less in cases of SIDS than in the last two weeks for controls (odds ratio (OR) 0.76, 95% confidence interval (CI) 0.57 to 1.02). Use of a dummy in the last sleep for cases of SIDS or in the nominated sleep for controls was significantly less in cases than controls (OR 0.44, 95% CI 0.26 to 0.73). The OR changed very little after controlling for a wide range of potential confounders. It is concluded that dummy use may protect against SIDS, but this observation needs to be repeated before dummies can be recommended for this purpose. If dummy sucking is protective then it is one of several factors that may explain the higher mortality from SIDS in New Zealand than in other countries, and may also explain in part the regional variation within New Zealand.

Ethnic differences in mortality from sudden infant death syndrome in New Zealand

OBJECTIVES

To examine the factors which might explain the higher mortality from sudden infant death syndrome in Maori infants (7.4/1000 live births in 1986 compared with 3.6 in non-Maori children).

DESIGN

A large nationwide case control study.

SETTING

New Zealand. 485 infants who died of sudden infant death syndrome were compared with 1800 control infants. There were 229 Maori and 240 non-Maori cases of sudden infant death syndrome (16 cases unassigned) and 353 Maori and 1410 non-Maori controls (37 unassigned).

RESULTS

Maori infants had 3.81 times the risk (95% confidence interval 3.06 to 4.76) of sudden infant death syndrome compared with non-Maori infants. The risk factors for sudden infant death syndrome within groups were remarkably similar. When Maori and non-Maori controls were compared the prevalence of many of the known risk factors was higher in Maori infants. In particular, mothers were socioeconomically disadvantaged, younger, and more likely to smoke and their infants were of lower birth weight and more likely to share a bed with another person. Multivariate analysis controlling for potential confounders found that simply being Maori increased the risk of sudden infant death syndrome by only 1.37 (95% CI = 0.95 to 2.01), not statistically significantly different from 1. Population attributable risk was calculated for prone sleeping position, maternal smoking, not breast feeding, and infants sharing a bed with another person. In total these four risk factors accounted for 89% of deaths from sudden infant death syndrome in Maori infants and 79% in non-Maori infants.

CONCLUSION

The high rate of sudden infant death syndrome among Maori infants is based largely on the high prevalence in the Maori population of the major risk factors. Other risk factors, not related to ethnicity, probably explain remaining differences between Maori and non-Maori children.

Postnatal depression and SIDS: a prospective study

This study was carried out in response to reports from nurses to a post-neonatal mortality review committee that a number of mothers of infants dying from sudden infant death syndrome (SIDS) appeared to be depressed before the child's death. The New Zealand Cot Death Study was a 3 year multicentre case-control study for SIDS. There were 485 SIDS cases in the post-neonatal age group in the study regions, and these were compared with 1800 control infants. Infants of mothers with either a self-reported use of medication for psychiatric disorders, a history of hospitalization for psychiatric illness or a family history of postnatal depression had a significantly increased risk of SIDS compared with infants of mothers who were either not using medication (odds ratio (OR) = 1.45; 95% confidence interval (CI) = 1.03, 2.04) or were without a history of hospitalization for psychiatric illness (OR = 1.80; 95% CI = 1.03, 3.11) or a family history of postnatal depression (OR = 1.61; 95% CI = 1.06, 2.43). All mothers of infants born in the study areas over a 1 year period were eligible to complete a questionnaire measuring maternal depression when the infant was 4 weeks of age. Thirty-three infants subsequently died from SIDS, and they were compared with 174 controls. Fifteen (45.5%) of the mothers of cases were depressed, compared with 28 (16.1%) of the mothers of controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Further evidence supporting a causal relationship between prone sleeping position and SIDS

A 3 year case-control study identifying the risk factors for SIDS was undertaken. Preliminary analysis of the data from the first year suggested that SIDS mortality could fall by 50% if the prevalence of the prone sleeping position changed from 40 to 0%. During the 3 year study the prevalence of the prone sleeping position among infants has fallen from 43% in the first year to 20% in the third year. SIDS mortality has fallen to 3.1/1000 live births, which is very close to that predicted. When considered with other available evidence this strongly supports a causal relationship between the prone sleeping position and SIDS.

Characteristics of the infant thermal environment in the control population of a case-control study of SIDS

This report examines the thermal environment during last sleep of a control population to investigate how the thermal environment of the infant's bedroom varies by season, external temperature and by certain maternal and infant characteristics. Two age-matched control infants were chosen for each case, one of which was also matched on birthweight. The home visits were not pre-arranged and were matched on climatic conditions, time of year and time period of day for the index case. The initial response rate for controls (n = 108) was 86%. Although there was a large amount of variation in the infant thermal environment, thermal insulation correlated with room temperature (r = -0.44, P = 0.0001) and external temperature (r = -0.30, P = 0.002). The thermal environment of the infant, as defined by excess thermal insulation for room temperature, did not vary by indoor or outdoor temperature, but higher average values were observed in teenage mothers (mean difference = 2.7 tog [95% Cl = 0.3, 5.2]), infants who slept in an adult bed (mean difference = 2.6 tog [-0.1, 5.4]) and infants with an illness (mean difference = 0.8 tog [-0.3, 1.9]). There was a tendency for the thermal environment of infants to be higher and more variable during winter, supporting previous hypotheses that paradoxical overheating may occur in some infants during winter. Further work is required to provide a set of recommendations on the optimal thermal conditions for post-neonatal infants.