Climatic temperature and variation in the incidence of sudden infant death syndrome between the Australian states

Geographic Variation in Sudden Unexpected Infant Death in the United States

OBJECTIVES

To assess the geographic variation of sudden unexpected infant death (SUID) and test if variation in geographic factors, such as state, latitude, and longitude, play a role in SUID risk across the US.

STUDY DESIGN

We analyzed the Centers for Disease Control and Prevention's Cohort Linked Birth/Infant Death dataset (2005-2010; 22 882 SUID cases, 25 305 837 live births, rate 0.90/1000). SUID was defined as infant deaths (ages 7-364 days) that included sudden infant death syndrome, ill-defined and unknown cause of mortality, and accidental suffocation and strangulation in bed. SUID geographic variation was analyzed using 2 statistical models, logistic regression and generalized additive model (GAM).

RESULTS

Both models produced similar results. Without adjustment, there was marked geographic variation in SUID rates, but the variation decreased after adjusting for covariates including known risk factors for SUID. After adjustment, nine states demonstrated significantly higher or lower SUID mortality than the national average. Geographic contribution to SUID risk in terms of latitude and longitude were also attenuated after adjustment for covariates.

CONCLUSION

Understanding why some states have lower SUID rates may enhance SUID prevention strategies.

Ambient Heat and Sudden Infant Death: A Case-Crossover Study Spanning 30 Years in Montreal, Canada

BACKGROUND

Climate change may lead to more severe and extreme heat waves in the future, but its potential impact on sudden infant death-a leading cause of infant mortality-is unclear.

OBJECTIVES

We sought to determine whether risk of sudden infant death syndrome (SIDS) is elevated during hot weather.

METHODS

We undertook a case-crossover analysis of all sudden infant deaths during warm periods in metropolitan Montreal, Quebec, Canada, from 1981 through 2010. Our analysis included a total of 196 certified cases of SIDS, including 89 deaths at 1-2 months of age, and 94 at 3-12 months. We estimated associations between maximum outdoor temperatures and SIDS by comparing outdoor temperatures on the day of or day before a SIDS event with temperatures on control days during the same month, using cubic splines to model temperature and adjusting for relative humidity.

RESULTS

Maximum daily temperatures of ≥ 29°C on the same day were associated with 2.78 times greater odds of sudden infant death relative to 20°C (95% CI: 1.64, 4.70). The likelihood of sudden death increased steadily with higher temperature. Associations were stronger for infants 3-12 months of age than for infants 1-2 months of age, with odds ratios of 3.90 (95% CI: 1.87, 8.13) and 1.73 (95% CI: 0.80, 3.73), respectively, for 29°C compared with 20°C on the day of the event.

CONCLUSIONS

High ambient temperature may be a novel risk factor for SIDS, especially at ≥ 3 months of age. Climate change and the higher temperatures that result may account for a potentially greater proportion of sudden infant deaths in the future.

Temperature differences during sleep between fullterm and preterm neonates at matched post-conceptional ages

OBJECTIVE

Altered physiologic behaviors during sleep have been described for healthy preterm neonates at post-conceptional fullterm ages. These differences may reflect brain dysmaturity as a result of conditions of prematurity. The present study examines if differences in state-specific temperature changes exist in a healthy preterm cohort as another expression of brain dysmaturity.

METHODS

Rectal and skin temperatures during sleep state transitions are reported in 59 asymptomatic post-conceptional age term infants, comparing 25 full term and 34 preterm infants. Three-hour 24-channel electroencephalogram (EEG)-sleep studies were recorded for each child. One of 4 sleep states were assigned for each of 7339 min, based on both cerebral and non-cerebral measures. For each study, average rectal and skin temperatures for each sleep state were calculated. Repeated measures MANOVA were performed using 4 explanatory variables, average skin and rectal temperatures and variance of rectal and skin temperatures, comparing preterm/fullterm status and 4 sleep states.

RESULTS

Rectal temperature differences between neonatal cohorts during specific sleep states were noted: F=8.58, P<0.0001. Significant differences were noted for both average and variance of rectal temperatures during all 4 sleep states with higher temperatures in the preterm group. For all neonates, both skin and rectal temperature differences were also noted among sleep states (F=4.22, P<0.0004). Differences were specifically noted between two specific EEG segments, mixed frequency active sleep and tracé alternant quiet sleep (P<0.0004).

CONCLUSIONS

In summary, significant differences in temperatures were noted across sleep state transitions for two neonatal cohorts, with higher average rectal temperatures in the preterm cohort. These findings highlight an altered measure of brain function during sleep in preterm infants affecting temperature regulation. This altered physiologic behavior reflects adaptation of the infant's brain function to conditions of prematurity which may contribute to vulnerabilities at older ages.

Thermal stress in sudden infant death: Is there an ambiguity with the rebreathing hypothesis?

OBJECTIVE

To assess the role of thermal stress in the cause of sudden infant death syndrome (SIDS), and to compare risk factors with those of rebreathing.

METHODOLOGY

Analysis of publications concerning the epidemiology and physiology of thermal stress in SIDS.

RESULTS

A strong association between thermal regulation and ventilatory control was found, specifically for prolonged apnea. Infections, excessive room heat and insulation, and prone sleeping produce significantly increased odds ratios for SIDS. Although some of the risk factors for rebreathing could be explained by the effects of thermal stress, several factors for thermal stress could not reasonably be explained by the rebreathing hypothesis.

CONCLUSIONS

Although the risk of thermal stress is widely accepted abroad, it has received relatively little attention in the United States. The incidence of SIDS in the United States can likely be further reduced by educating the public against the dangers of overheating, as an integral part of the back-to-sleep campaign.

Living at high altitude and risk of sudden infant death syndrome

OBJECTIVE

To investigate the association between altitude of residence and risk of sudden infant death syndrome (SIDS).

METHODS

A retrospective, case control study in the Tyrol, Austria enrolled 99 infants with SIDS occurring between 1984 and 1994, and 136 randomly selected control cases. Data on pregnancy, delivery, child care practice, and socio-demographic characteristics including altitude of residence were collected with a standardised questionnaire.

RESULTS

The risk of SIDS increased gradually with increasing altitude of residence. This relation remained independently significant when the analysis was adjusted for gestational age, birth weight, prenatal care, mother's age at delivery, educational level of parents, and cigarette smoking during pregnancy. The prone sleeping position emerged as an obligatory cofactor in this association. In the whole of Austria, a similar trend of association emerged between the average altitudes in the 99 political counties and the rates of SIDS.

CONCLUSIONS

This study identified altitude of residence as a significant risk predictor of SIDS, primarily in combination with the prone sleeping position. Respiratory disturbances, reduced oxygen saturation, and lower temperatures at high altitude might explain this association.

Is baby too warm? The use of infant clothing, bedding and home heating in Victoria, Australia

Overheating of infants has been associated with a raised risk for sudden infant death syndrome (SIDS). Data on the use of heating, bedding and clothing and other measures affecting the thermal environment of 4 week old infants were collected at a home interview for infants of women born in Australia (Anglo-Celtic background), in Southern Europe, in Asia and of women who had a planned out-of-hospital birth. These groups have different risks of SIDS not explained by the classic social and perinatal risk factors nor associated with the currently promoted 'new' risk factors. Thermal insulation of the infant's bedding and clothing and excess thermal insulation (for any observed room temperature) were calculated. Bed sharing differed significantly between the groups as did the use of a sheepskin, tucking in firmly, the closing of doors and windows and the use of heating in the infant's room. After stratifying by bed sharing practice and season of interview, it was found that bed sharing infants had more thermal insulation than those sleeping alone irrespective of season of interview. Infants sleeping alone in the Asian-born and Southern European-born groups were kept warmer than infants in the other two groups. Cultural factors appear to affect the thermal environment in which infants are raised. Some bed sharing infants in all four groups were inappropriately warm, particularly in colder weather, but this was more likely in the Asian-born (low risk) group than in the home birth (high risk) group. These results do not explain the differences in SIDS incidence between the groups.

Sudden infant death syndrome: a hypothesis

A study of the strikingly low incidence of sudden infant death syndrome in Eastern countries revealed significant differences in infant handling thought to have an etiological bearing; therefore this writer suggested that adoption of certain Eastern methods of nursing may reduce the incidence of sudden infant death syndrome. A dramatic fall in incidence has resulted from implementing one of the suggestions made by the writer in 1983, namely the abandonment of the prone position, after initial opposition. The present hypothesis sets out to give a scientific explanation for this fall, and is a unified hypothesis explaining certain puzzling and disparate features of sudden infant death syndrome such as the remarkable winter incidence, age incidence, and the occurrence of sudden infant death syndrome during sleep, and is based on a postulated disturbance in thermoregulatory function (a unique hypothermia). Recommendations are made for evolving a test for sudden infant death syndrome-proneness and a possible method of treatment of a fatality within a short time frame.

Infant dreaming and fetal memory: a possible explanation of sudden infant death syndrome

During rapid-eye-movement sleep, when we dream, the brain is thought to be processing stored memory. The memory of a newborn infant is dominated by its fetal experience, and the infant is likely to dream about its life in the womb. Research with lucid (or conscious) dreaming has shown that dream images are supported by the corresponding body actions, using those muscles which remain active during rapid-eye-movement sleep. We suggest that sudden infant death syndrome or cot death may be a result of an infant dreaming about its life (or memory) as a fetus. In the course of that dream, since a fetus does not breathe (in the usual sense) the infant may cease to breathe and may die. This simple hypothesis is consistent with all of the known facts about sudden infant death syndrome (pathological and epidemiological), such as the age at death curve (the observed exponential decay and possibly the peak at 2-3 months), the higher risk with the prone sleeping position (but not excluding the supine position), and the observed climatic variation (seasonal and regional) in the incidence of sudden infant death syndrome. Many of these well-established facts have no other known explanation and other theories can generally only account for a few of the known facts about sudden infant death syndrome. Our hypothesis is also supported by recent findings that, as a group, sudden infant death syndrome infants have a higher proportion of rapid-eye-movement sleep, and also that they have an average higher heart rate (corresponding to possible fetal dreams) but only during rapid-eye-movement sleep.(ABSTRACT TRUNCATED AT 250 WORDS)

Ethnic differences in the incidence of the sudden infant death syndrome (SIDS) in Victoria, Australia 1985-1989

In order to describe ethnic differences in the incidence of the sudden infant death syndrome (SIDS) records of all livebirths in the State of Victoria, Australia, 1985-1989, excluding those who died in the first month of life, were linked to death certificates. Cases were defined as infants dying with a diagnosis of SIDS between 1 month and 1 year of age (n = 601) from the cohort of 308,052 neonatal survivors. Ethnicity was defined by the mother's country of birth. The SIDS incidence was 2.04/1000 in infants of Australian-born mothers. The relative risk of SIDS was 0.28 (95% confidence interval (CI) 0.15, 0.55) in infants whose mothers had been born in Southern Europe and 0.48 (95% CI 0.29, 0.78) in infants whose mothers had been born in Asia. SIDS in infants of Australian-born mothers was associated with low maternal age, high parity, marital status other than married, male sex, multiple birth, low birthweight and preterm birth. After adjustment for those factors in a case-control analysis using a logistic regression model the adjusted odds ratio for SIDS was 0.34 (95% CI 0.17, 0.69) comparing infants whose mothers were born in Southern Europe with infants of Australian-born mothers, and 0.60 (95% CI 0.35, 1.04) for infants whose mothers were born in Asia, compared with infants of the Australian-born. Thus there are substantial ethnic differences in SIDS which are not explained by the classic social and perinatal risk factors.

An epidemiological survey of SIDS in the Sydney metropolitan area

Records of 1049 cases of Sudden Infant Death Syndrome (SIDS) (100%) in the Sydney metropolitan area (SMA) from 1980 to 1989 were analysed in relation to ambient temperature, geographical distribution and socio-economic scale. The SIDS rate varied between eastern and western Sydney and between statistical subdivisions; it peaked sharply in July, coinciding with the monthly mean minimum daily temperature (MMMDT; 6.5 and 5.4 degrees C, respectively). The inverse linear relationship between the SIDS rate and the MMMDT for the regions and the subdivisions was highly significant (P < 0.001). The SIDS rate showed a significant inverse linear relationship to two socio-economic scales (allotment and house value) for 82 of the 87 suburbs (94%) of the SMA. Temperature, indicative of cold weather, was the major factor determining SIDS rates, and this, together with socio-economic factors, largely explains the observed geographical distribution of SIDS rates in the SMA.

SIDS--a developmental neurological perspective

Most SIDS deaths occur between 2 and 4 months of age. Neural transformations may be important in SIDS. Evidence of delayed myelination has been found in SIDS victims. However a simple neuro-developmental model is insufficient to explain SIDS. The age of death profile is reportedly similar for pre-term and full-term SIDS victims. Another developmental change is that IgG levels are lowest when SIDS risk peaks, and there is data linking SIDS and infections. Prone position may also increase risk. While a developmental model emphasizing neural transformation is useful, other factors affect risk also.

Effect of ambient temperature on SIDS rate

OBJECTIVE

To examine the relationship between ambient temperature and rate of sudden infant death syndrome (SIDS).

SUBJECTS AND SETTING

All infants with SIDS occurring during 1980-1989 in the Sydney metropolitan area.

RESULTS

The SIDS rate varied more than threefold over the months of the year and peaked at the minimum daily temperature in July. The rate was negatively linearly related to both the monthly mean minimum and maximum daily temperatures. Both relationships were highly significant (P < 0.001).

CONCLUSION

The significant relationship between daily temperature and SIDS rate means either that daily temperature is an excellent indicator of the cold weather conditions that have a profound effect on the rate of SIDS or that temperature itself plays a direct role in the cause of SIDS.

Sudden infant death syndrome: factors contributing to the difference in incidence between Victoria and Tasmania

OBJECTIVE

To examine how much of the difference in incidence of sudden infant death syndrome (SIDS) between Tasmania and Victoria could be accounted for by the effect of differing climatic temperature and the effect of the differing prevalence of maternal and infant characteristics in the two State populations.

DESIGN

A two population ecological comparison. Two previously published predictive models were applied to quantify the contribution of several factors to the higher incidence of SIDS in Tasmania compared with Victoria.

SETTING

A population based study involving the two Australian States of Tasmania and Victoria.

PATIENTS

The characteristics of the 1985 to 1987 live birth cohorts of Tasmania and Victoria were examined. Cases were defined as all infants dying in 1985 to 1987 whose cause of death was stated as SIDS.

RESULTS

The rate of SIDS for Tasmania and Victoria 1985 to 1987 was 3.76 per 1000 live births and 2.18 per 1000 live births respectively. Adjustment of the Tasmanian rate for the effect of the interstate difference in climatic temperature resulted in a lower Tasmanian rate of 2.92 per 1000 live births. Adjustment for the effect of interstate differences in maternal age, birthweight, infant sex, month of birth and intention to breast-feed at hospital discharge decreased the Tasmanian rate to 2.47 per 1000 live births.

CONCLUSION

Approximately 82% of the interstate difference in SIDS incidence between Tasmania and Victoria from 1985 to 1987 can be accounted for by differences in climatic temperature, maternal age, birth-weight, infant sex, month of birth and feeding intention at hospital discharge.

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.