Sudden infant death syndrome epidemiology: a review and update

Invited commentary: residential segregation and health--the complexity of modeling separate social contexts

When researching racial disparities in health, residential segregation cannot be ignored. Because of segregation, contextual differences by race are so pronounced that ignoring them may lead to mis-estimating the effect of individual-level factors. However, given the stark racial separation of social contexts, researching how residential segregation and neighborhood inequality contribute to racial health disparities remains methodologically challenging. Estimating the contribution of neighborhood effects to health disparities would require overlap in the racial distributions of neighborhood environment, for example, in the distributions of neighborhood poverty. Because of segregation, though, the extent of such overlap is extremely restricted. Previous analyses of the 2000 US Census found, on average, only a 24% overlap between the distribution of neighborhood poverty for black children and that for white children in metropolitan areas. Propensity score methods may be 1 useful tool for addressing limited overlap or exchangeability. However, as shown by their application to the segregation and health relation, their use should be informed by a sound conceptualization of the scale of the social exposure of interest, the hypothesized pathways between the exposure and the health outcome, and possible unmeasured confounders.

Pulse transit time and blood pressure changes following auditory-evoked subcortical arousal and waking of infants

STUDY OBJECTIVES

To establish a normal range of data in 3-month-old infants in relation to changes in cardiovascular measurements, with particular reference to pulse transit time (PTT), following subcortical arousals and awakenings from sleep.

DESIGN

Prospective study.

SETTING

Sleep laboratory, Dunedin Hospital

PARTICIPANTS

Twenty healthy infants aged 9-12 weeks.

METHODS

Nap studies were performed using a standard polysomnographic setup with the addition of a Portapres blood pressure (BP) cuff (wrist application) and a piezoelectric sensor on the foot. PTT was measured from the ECG-R waveform to the arrival of the pulse peripherally. Infants were exposed to white noise from 50 to 100 dB at 10 dB intervals within REM and NREM sleep.

RESULTS

Awakening thresholds were higher (P = 0.01) in NREM (>90 dB) than REM sleep (mean +/- SD; 74.3 +/- 9.4dB). Subcortical thresholds were always 10 dB below waking thresholds. Following awakening, there was an immediate increase in HR, SBP, and DBP of 21%, 14%, and 17%, respectively, and a 13% decrease in PTT returning to baseline within 25-30 seconds. PTT at baseline measured 140 +/- 11 and 139 +/- 9 msec in NREM and REM sleep, respectively, and decreased approximately 20 msec with waking. PTT changes were negatively correlated with heart rate (HR) but not BP, although a trend was evident.

CONCLUSIONS

At 3 months of age, infants provoked to arouse from sleep showed PTT changes that inversely mimicked BP trends, suggesting that PTT could be useful in infant studies as a marker for autonomic perturbations that occur during sleep in both clinical and research settings.

Seasonal variation of sudden infant death syndrome in Hawaii

OBJECTIVE

To test whether the sudden infant death syndrome (SIDS) rate displays the universal winter maximum and summer minimum in Hawaii where there is no appreciable seasonal variation of temperature.

DESIGN

The null hypothesis is tested that there is no seasonal variation of necropsied SIDS in Hawaii. The numbers of live births and SIDS cases by month for the years 1979 to 2002 were collected and the monthly SIDS distribution is predicted based on the age at death distribution.

SETTING

The state of Hawaii, located in the midst of the Pacific Ocean, has a semi-tropical climate with temperatures fluctuating diurnally as 25 +/- 5 degrees C throughout the year. Therefore homes are unheated and infants are not excessively swaddled. The Hawaii State Department of Health maintains vital statistics of all infant births and deaths.

MAIN RESULTS

The results reject the null hypothesis of no seasonal variation of SIDS (p = 0.026). An explanation for the seasonal effect of the winter maximum and summer minimum for Hawaiian SIDS is that it arises from the cycle of the school session and summer vacation periods that represent variable intensity of a possible viral infection vector. SIDS rates in both Hawaii and the United States increase with parity, also indicating a possible role of school age siblings as carriers.

CONCLUSIONS

The winter peak of the SIDS in Hawaii is support for the hypothesis that a low grade viral infection, insufficient by itself to be a visible cause of death at necropsy, may be implicated as contributing to SIDS in vulnerable infants.

Sudden infant death syndrome in Japan 1995-1998

This study examined the effects of birth characteristics, which reported on birth certificates, on sudden infant death syndrome (SIDS) based on vital statistics between 1995 and 1998 in Japan. One thousand eight hundred and fifty-nine cases of SIDS and 4,787,537 live births were analyzed. The mortality rate from SIDS was 0.388 SIDS deaths per 1000 live births. Based on multivariate analysis using Poisson regression model, risk factors associated with significantly increased SIDS rates included low birth weight, being a male infant, young maternal age, late birth in multiparity, maternal stillbirth experience, residential region of Kyusyu, Tokai or Hokuriku, and employment status as "unemployed or unknown". However, there was no significant difference between single and multiple birth groups after adjusting other characteristics. A decreasing postnatal age of death was observed as birth weight increased. However, there was no difference in postconceptional age of death between birth weight groups. Preterm infants died of SIDS at a later postnatal age than term infants, but there appeared to be little difference in postconceptional age of death amongst preterm infants. This association between birth characteristics and SIDS were consistent with other studies from western countries, suggesting that recommendations for SIDS in western countries are also applicable for Japan.

Perinatal and infant mortality and low birth weight among residents near cokeworks in Great Britain

With growing evidence of the adverse health effects of air pollution--especially fine particulates--investigators must concentrate on the fetus, neonate, and infant as potentially vulnerable groups. Cokeworks are a major source of smoke and sulfur dioxide. In the current study, the authors investigated whether populations residing near cokeworks had a higher risk of adverse perinatal and infant outcomes. Zones of 7.5-km radius around 22 cokeworks in Great Britain were studied, within which the authors assumed that exposure declined from highest levels within 2 km to background levels. Routinely recorded birth and death data for Great Britain during the period 1981-1992 were analyzed. Each individual record had a postcode that referred to a small geographical area of typically 15-17 addresses. The authors calculated expected numbers on the basis of regional rates, stratified by year, sex, and a small-area socioeconomic deprivation score. For all cokeworks combined, the observed/expected ratio (95% confidence intervals within parentheses) within 2 km of cokeworks was 1.00 (0.95, 1.06) for low-birth-weight (i.e., < 2,500 g) infants; 0.94 (0.78, 1.12) for still births; 0.95 (0.83, 1.09) for infant mortality; 0.86 (0.72, 1.03) for neonatal mortality; 1.10 (0.90, 1.33) for postneonatal mortality; 0.79 (0.30, 1.46) for respiratory postneonatal mortality; and 1.07 (0.77, 1.43) for postneonatal Sudden Infant Death Syndrome. Respiratory postneonatal mortality was low throughout the entire 0-7.5-km study area (observed/expected = 0.74 [0.61, 0.88]). There was no statistically significant decline in risk with distance from cokeworks for any of the outcomes studied. The authors concluded that there was no evidence of an increased risk of low birth weight, stillbirths, and/or neonatal mortality near cokeworks, and there was no strong evidence for any association between residence near cokeworks and postneonatal mortality. One must remember, however, the limited statistical power of the study to detect small risks.

Combined effects of sleeping position and prenatal risk factors in sudden infant death syndrome: the Nordic Epidemiological SIDS Study

OBJECTIVE

Prone sleeping is a strong risk factor for sudden infant death syndrome (SIDS). We investigated whether the combined effect of prone sleeping position and prenatal risk factors further increased the SIDS risk.

METHODS

In the Nordic Epidemiological SIDS Study, parents of SIDS victims in Denmark, Norway, and Sweden completed a questionnaire on potential risk factors for SIDS. Forensic pathologists verified the SIDS diagnosis. Four controls of the same gender, age, and place of birth were selected. This matched case-control study, which included 244 SIDS cases and 869 controls from 1992 to 1995, was analyzed by conditional logistic regression.

RESULTS

Odds ratios (ORs) for prone and side sleeping compared with supine sleeping for the last sleep were 13.9 (95% confidence interval 8.2-24) and 3.5 (2.1-5.7). Infants 13 to 24 weeks old had particularly high risk in prone and side sleeping, at 28.5 (7.9-107) and 5.9 (1.6-22). OR for prone sleeping was higher in girls, at 30.4 (11-88), than in boys, 10.3 (5.5-19). We found strong combined effects of sleeping position and prenatal risk factors (more than multiplicative). The OR for prone and side sleeping was increased for infants with birth weight <2500 g, at 83 (25-276) and 36.6 (13-107); for preterm infants, at 48.8 (19-128) and 40.5 (14-115); and for intrauterine growth retarded, at 38.8 (14-108) and 9.6 (4.3-22), compared with supine position in infants without these prenatal factors. The combined effect of nonsupine positions and intrauterine growth retarded was highest among 13- to 24-week-old infants. Effects of combined presence of nonsupine sleeping positions and each of the factors of smoking in pregnancy, young maternal age, higher parity, low level of maternal education, and single motherhood were more than additive. Attributable fractions in the population for prone and side sleeping were 18.5% and 26.0%.

CONCLUSIONS

Both prone and side sleeping increased the risk of SIDS. The risk was increased further in low birth weight infants, preterm infants, and infants at the age of 13 to 24 weeks, suggesting that SIDS may be triggered by nonsupine sleeping in infants with prenatal risk factors during a vulnerable period of postnatal development.

Geophysical variables and behavior: LXXXII. A strong association between sudden infant death syndrome and increments of global geomagnetic activity--possible support for the melatonin hypothesis

A significant subpopulation of young infants who die suddenly (SIDS) often exhibit anomalies consistent with disruptions within the pineal-limbic system. We have hypothesized that sudden decreases in nocturnal melatonin by a specific range of geomagnetic activity would precipitate sudden infant death. A correlation of .90 was found between the numbers of cases of Sudden Infant Death Syndrome per month during the years 1960 and 1961 in Ontario and an increase (primarily) in numbers of days per month with average geomagnetic activity between 11 and 20 nT and 31 through 40 nT but a decrease in the numbers of days with values between 21 and 30 nT. This nonlinear sensitivity, presumably associated with specific ripple frequencies within the geomagnetic field, may explain the failure by other researchers to detect linear associations between the numbers of these types of deaths and gross, scalar indicators of geomagnetic activity.

An analysis of sudden infant death syndrome in aboriginal infants

The purpose of this case-control study was to identify antenatal and perinatal risk factors for sudden infant death syndrome (SIDS) in Aboriginal infants in Western Australia (WA). Cases were all Aboriginal infants born in WA from 1980 to 1990 inclusive and classified as dying from SIDS in WA. Controls consisted of a matched group and a random group both selected from liveborn Aboriginal infants born from 1980 to 1990. Multivariate modelling showed that SIDS in Aboriginal infants was strongly related to young maternal age (< 20 years, odds ratio (OR) = 2.89), high parity (parity > 3, OR = 4.40) and being small-for-gestational age (OR = 3.36) but was not associated with single marital status (OR = 0.95) or male sex (OR = 0.97). Although the study was based on routinely collected data, results do highlight some important groups for SIDS prevention. To gain further knowledge in terms of SIDS in Aboriginal infants, there is an urgent need to collect information concerning infant care practices in the Aboriginal community.

Overheating alone can trigger malignant hyperthermia in piglets

Seven out of eight piglets which were susceptible to malignant hyperthermia (MHS) died when subjected to a heat challenge which was well tolerated by controls. The piglets which succumbed developed the classical clinical and biochemical changes of malignant hyperthermia before they died. These results show that overheating alone can trigger malignant hyperthermia in susceptible animals. Because the biochemical basis of malignant hyperthermia is similar in both humans and pigs, these observations suggest that overheating can also trigger malignant hyperthermia in humans. The susceptibility to overheating in malignant hyperthermia susceptible humans and animals probably explains why the myopathy which predisposes to this condition has also been reported to predispose to heat-stroke and the sudden infant death syndrome. In view of this, particular care to prevent overheating should be taken in infants of parents who are susceptible to malignant hyperthermia.

Carbon monoxide--does fetal exposure cause sudden infant death syndrome?

The epidemiological features of sudden infant death syndrome (cot death) include a peak incidence between 8 and 13 weeks of age, a time of death or conception occurring during the winter months and an excess of deaths in infants born to young multiparous women of low socioeconomic status who smoke. We suggest that, through hypoxia, carbon monoxide in tobacco smoke and in the home exerts a noxious effect on the developing central respiratory control mechanism of the fetal brain which then remains particularly susceptible to further insults in the early postnatal period from infection and hyperthermia, resulting in death from central respiratory dysfunction.

Incidence of sudden infant death syndrome in Olmsted County, Minnesota: 1945 through 1992

OBJECTIVE

To apply uniform diagnostic criteria for sudden infant death syndrome (SIDS) for an extended period for comparison of incidence rates from 1945 to 1992 in Olmsted County, Minnesota, to investigate the influence of a person-time or live birth denominator on the estimate of incidence, to calculate the proportionate mortality rate for SIDS over time, and to evaluate the accuracy of death certificates for case ascertainment and the role of interobserver variation in case classification.

DESIGN

We retrospectively reviewed the autopsy results and complete medical records for all infant death from 1945 through 1992 for residents of Olmsted County, Minnesota.

MATERIAL AND METHODS

Cases were identified from a computerized list of all Olmsted County deaths of infants occurring at ages 48 hours to 365 days. All resident cases were categorized as non-SIDS, possible SIDS, SIDS, or incomplete, on the basis of findings from autopsy and clinical history. Incidence rates were calculated for two different SIDS groups and with use of denominators of person-time and live births.

RESULTS

For the study period, 82 cases of SIDS were identified (97% white and 3% Asian). The mean age at death was 12.5 weeks; male infants constituted 59% of cases. No significant trend in seasonal distribution was noted; 73% of deaths occurred between midnight and noon. The incidence rate, defined as SIDS definite and possible deaths per 1,000 resident live births, increased from 0.55 in 1950 through 1953 to 1.28 in 1990 through 1992. The secular trend was best described by a linear model with constant positive slope. Similar trends were observed with other definitions of incidence. During the study period, SIDS as a percentage of total infant deaths dramatically increased, ranging from 2.5 in 1950 through 1953 to 17.9 in 1990 through 1992. The death certificate diagnosis correctly predicted 72% of SIDS cases before 1970 and 100% of cases after 1970.

CONCLUSION

Since 1945, the incidence of SIDS apparently has increased, although diagnostic transfer from other causes of death probably contributes to the observed trend. The comparison of live births versus person-time as denominators showed no significant difference in incidence rates. Interobserver reliability is modest for SIDS cases diagnosed before 1970 and may contribute to the variability in reported SIDS incidence rates.

The timing of SIDS deaths in premature infants in an urban population

Previous reports have demonstrated that premature infants are at greatly increased risk for sudden infant death syndrome (SIDS). Although only 9% of infants are born at less than 36 weeks' gestation, 20% of SIDS victims are former premature infants. The objective of this study was to characterize the time course of SIDS in premature infants and to determine why SIDS occurs at such a high rate in this patient population. A database of all cases of SIDS in Philadelphia from 1987 through 1991 was used to establish the time course for SIDS deaths in term and preterm infants. Gestational age was established by Dubowitz exam. To evaluate distinctly different age groups, infants from 32-36 weeks were excluded from analysis. Age at death and postconceptional age of death were compared for both groups. Data are described in weeks (mean +/- SEM), and analyzed using unpaired t-test and log-rank test to compare survival rate between term and preterm infants. A significant difference (P < 0.01) was noted in age at death of term versus preterm infants. No difference was found in postconceptional age of death. The survival rates were also different (P < 0.001). Preterm infants showed a much wider distribution in age of death from SIDS. The term infants followed the classic SIDS curve. By 32 weeks' postnatal age, 95% of all SIDS had taken place in the term group, but only 75% in the preterm group. The age at death for SIDS differs in the preterm infant.(ABSTRACT TRUNCATED AT 250 WORDS)

Adolescent parity and infant mortality, Minnesota, 1980 through 1988

The association of parity and infant mortality was studied using linked birth-death files for 46,985 infants born to 11- to 19-year-old Minnesota residents between 1980 and 1988. Compared with infants of primiparas, infants of multiparas were at twice the risk for infant and postneonatal death but at no increased risk for neonatal death. They were also at two to three times the risk for deaths due to accidents, infections, and sudden infant death syndrome. The higher sociodemographic disadvantage and poorer prenatal care of multiparas did not explain the excess infant mortality risk that was concentrated in causes of death that are potentially preventable through primary care and parent education.

The Tasmanian SIDS Case-Control Study: univariable and multivariable risk factor analysis

A population-based retrospective case-control study has been conducted in Tasmania since October 1988. Study measurements pertained to the scene of death of last sleep, as well as a verbal questionnaire on relevant exposures. From 1 October 1988 to 1 October 1991, 62 cases of sudden infant death syndrome (SIDS) occurred. Case response rate for retrospective interviews was 94% (58/62). The initial control response rate was 84% (101/121). After stratification for maternal age and birthweight, there was no increase in risk associated with the usual side position (odds ratio [OR] 1.05 [0.27, 5.02]), compared with the supine position (OR 1.00, reference). The prone position was associated with increased risk [OR 5.70 (1.67, 25.58)], relative to the supine position. In the final multivariable model, predictors of SIDS in this study were usual prone position (P < 0.001), maternal smoking (P = 0.008), a family history of asthma (P = 0.045) and bedroom heating during last sleep (P = 0.039). Protective factors were maternal age over 25 years (P = 0.013) and more than one child health clinic attendance (P = 0.003). The results provide further support for current health education activities which aim to inform parents of modifiable risk factors for SIDS, including the prone sleeping position, thermal stress and infant exposure to tobacco smoke.

Is the higher postneonatal mortality in lower social status groups due to SIDS?

In order to study the influence of sociodemographic factors, postneonatal mortality of all live births surviving the neonatal period registered in the Norwegian Medical Birth Registry in 1978-1982 were examined (n = 209,030). Postneonatal deaths (n = 634) were divided into two categories; deaths due to the sudden infant death syndrome (SIDS) (n = 359) and deaths due to other causes (non-SIDS) (n = 275). SIDS and non-SIDS deaths showed different relationships to sociodemographic factors, and the associations appeared to be different for first-born and later born children. SIDS mortality was highest for first-born offspring when the mother was young (adjusted relative risks (RR) 2.3) and had a low educational level (adjusted RR 4.9). For later-born offspring no association between maternal educational level and SIDS was found, while young maternal age (adjusted RR 4.4) and unmarried status (adjusted RR 2.3) were closely associated with SIDS. In the multivariate model, however, there were no statistically significant associations between non-SIDS and sociodemographic factors for first-born or later-born children. Thus it appears that the increased postneonatal mortality in lower social groups can be explained by an association with SIDS.

The art and science of home infant apnea monitoring in the 1990s

The use of home apnea monitoring (HAM) continues as an accepted or recommended intervention for infants with certain signs and symptoms or medical diagnoses. Results of HAM in terms of case outcomes versus cost-effectiveness and efficacy remain matters of controversy in relation to the limited number of studies that show conflicting results. There are no studies that document the effectiveness of apnea monitoring. When an apnea monitor is prescribed, nursing can provide quality care through education and emotional support of families using HAM.

Beta-endorphin immunoreactivity in spinal fluid and hypoxanthine in vitreous humour related to brain stem gliosis in sudden infant death victims

Beta-endorphin may induce respiratory depression and bradycardia. Elevated levels of hypoxanthine (HX) in vitreous humour (VH) may possibly indicate hypoxia before death. Furthermore, gliosis in the brain stem may reflect a previous hypoxic/ischaemic injury in the brain. In the present study we relate beta-endorphin immunoreactivity (BENDI) in the CSF to the presence or absence of reactive astrocytosis in the nucleus olivae inferior (NOI). The relationship between the HX concentration in VH and the number of reactive astrocytes in sudden infant death (SID) cases (n = 17) and controls (n = 23) was also studied. The number of reactive astrocytes was examined in the NOI by immunohistochemical demonstration of glial fibrillary acidic protein (GFAP). The BENDI in CSF and the number of reactive astrocytes in the NOI divided the SID victims into two subpopulations (P < 0.01). One had a median of < 4 fmol/ml BENDI in CSF (range < 4) and 2 reactive astrocytes (range 0-15), and was similar to the controls that died from infections. The other subpopulation had a median of 260 fmol/ml BENDI in CSF (range 160-400) and 13 reactive astrocytes (range 7-33), similar to the control infants with previous hypoxia. In this latter SID subpopulation the number of reactive astrocytes correlated positively with BENDI in CSF (r = 0.7, P < 0.05). All the SID victims had elevated levels of HX in VH. In the SID subpopulation with high level of BENDI in CSF and increased number of activated astrocytes, the correlation factor between HX in VH and activated astrocytes was r = 0.7 (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Urban-rural and regional differences in infant mortality in Taiwan

We analyzed data from death certificates for all infant deaths in Taiwan from 1981 to 1988. During this 8-year period, the crude infant mortality rate decreased from 8.9 to 5.3 per 1000 live births. Deaths due to infectious diseases, which constituted a major fraction of this mortality, also declined from 3.4 to 1.2 per 1000 live births. While infant deaths due to nearly all causes declined, deaths due to injury and accidents rose from 0.62 to 0.71 per 1000 live births, and the sudden infant death rate rose from 0.13 to 0.46. Notable geographic differences included a high death rate in the small islands off the coast and in the eastern mountainous counties (9.1-11.2/1000 live births); this rate was twice that in Taipei (4.5/1000 live births). In addition, the level of urbanisation was also an important determinant of death rate; urban areas had much lower rates than rural areas. The highest rate (15.4) was persistently observed in the rural areas where the aboriginal tribes reside. This differential rate between urban and rural areas was most prominent for the vaccine-preventable diseases; the aboriginal areas had rates that were 12.9 times those in the cities. Moreover, since neonatal deaths are severely under-reported in Taiwan, especially in less urbanized areas, our data presumably underestimate the urban-rural health differences. Our findings identify high risk areas for various causes of infant death, and indicate that more targeted intervention such as improving education and health care as well as environmental hygiene in some specific areas may be warranted.

Secular trends of sudden infant death syndrome in Norway 1967-1988: application of a method of case identification to Norwegian registry data

In Norway, towards the end of the 1980s, sudden infant death syndrome (SIDS) was the most frequent cause of infant death. Both SIDS and the total post-perinatal mortality rates had increased. This paper presents a procedure for identifying SIDS from death certificates. Supplemented with additional information, a database was established to evaluate secular trends of SIDS and for further analytical research. The Medical Birth Registry of Norway comprises 1.3 million births from 1967 to 1988. Of these, 5447 infants died in the post-perinatal period. The cause of death was reviewed by an expert panel and 1984 cases of SIDS were retrieved. Low maternal age, higher birth order, male gender, and lower birth-weight were confirmed as risk factors for SIDS. In 1988, the rate for SIDS and for total post-perinatal deaths reached 2.69 and 5.02 per 1000 infants at risk. The incidence of SIDS increased 2.2 times from the period 1967-1971 to the period 1987-1988. Adjusted for maternal age, birth order, and birthweight, the odds ratio was 3.1. The increase is due to factors not yet accounted for. Adjusted mortality rates for the other post-perinatal deaths were not different from the crude rates.

Risk factors for SIDS in Japan: a record-linkage study based on vital statistics

This study examines the effect of items as reported on birth certificates on sudden infant death. We linked infant death certificates with birth certificates for the infants born in 1989 to residents of the Tohoku, Tokai and Kyushu regions in Japan (n = 409,679), that is, about one-third of the infants born in Japan that year. The mortality rate from sudden infant death, including 88 deaths from sudden infant death syndrome (SIDS) and 17 deaths from instantaneous death, was 25.6 per 100,000 live births. Elevated risk of sudden infant death was associated with low birthweight, late birth order, illegitimacy, male gender and young maternal age. These results correspond to previous studies from Western countries, suggesting a similar pattern for SIDS in Japan.

A temporal-spatial cluster of sudden infant death syndrome in Navarre, Spain

An apparent temporal-spatial cluster of Sudden Infant Death Syndrome (SIDS) was noted in the Autonomous Community of Navarre, Spain, when four unrelated children aged between 1 and 6 months died unexpectedly within a 7-day interval in January, 1990. The population under one year of age in Navarre was approximately 4,800. The scan test of temporal clustering showed that the sudden infant deaths occurred closer to one another in time significantly more often than would be expected by chance. All four infants lived in a neighbourhood of the capital of Navarre, which accounts for approximately half the region's population. The clustered cases coincided with an outbreak of influenza type A detected by the epidemiological surveillance system and seen by the increase in 1990 over the same period in the previous year in the number of paediatric emergency-ward admissions during the 'epidemic' days. The results confirm the presence of a temporal-spatial cluster of SIDS and favour an environmental etiology where exposure to influenza A viruses is implicated.

Factors potentiating the risk of sudden infant death syndrome associated with the prone position

BACKGROUND

In several studies the sudden infant death syndrome (SIDS) has been significantly associated with sleeping in the prone position. It is not known how the prone position increases the risk of SIDS.

METHODS

We analyzed data from a case-control study (58 infants with SIDS and 120 control infants) and a prospective cohort study (22 infants with SIDS and 213 control infants) in Tasmania. Interactions were examined in matched analyses with a multiplicative model of interaction.

RESULTS

In the case-control study, SIDS was significantly associated with sleeping in the prone position, as compared with other positions (unadjusted odds ratio, 4.5; 95 percent confidence interval, 2.1 to 9.6). The strength of this association was increased among infants who slept on natural-fiber mattresses (P = 0.05), infants who were swaddled (P = 0.09), infants who slept in heated rooms (P = 0.006), and infants who had had a recent illness (P = 0.02). These variables had no significant effect on infants who did not sleep in the prone position. A history of recent illness was significantly associated with SIDS among infants who slept prone (odds ratio, 5.7; 95 percent confidence interval, 1.8 to 19) but not among infants who slept in other positions (odds ratio, 0.83). In the cohort study, the risk of SIDS was greater among infants who slept prone on natural-fiber mattresses (odds ratio, 6.6; 95 percent confidence interval, 1.3 to 33) than among infants who slept prone on other types of mattresses (odds ratio, 1.8).

CONCLUSIONS

When infants sleep prone, the elevated risk of SIDS is increased by each of four factors: the use of natural-fiber mattresses, swaddling, recent illness, and the use of heating in bedrooms.