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.

The black infant's susceptibility to sudden infant death syndrome and respiratory infection in late infancy.

Between 2 and 11 months of age, the risk of sudden infant death syndrome (SIDS) declines more slowly in black infants than in infants of other races. This phenomenon might also be a feature of certain non-SIDS causes of death. Identifying these causes may through analogy provide support for the theory that SIDS is a disease of the central nervous system, an unusual consequence of respiratory infection, or a form of suffocation. We used logistic regression analysis on details of infant deaths in the United States, 1985-1991, to examine the difference between the rates of decline with increasing age in the mortality rates of black infants and infants of other races. We defined slower rate of decline in black infants as a positive difference. The magnitude and direction (positive) of the difference for deaths due to respiratory infection were similar to those for SIDS. It is unlikely that this difference in the rates of decline for respiratory infection can be explained by diagnostic cross-misclassification between respiratory infection and SIDS. SIDS appears to be a disease of the respiratory system caused by infection that affects that system's control centers.

The influence of race and gestational age on the age of maximum risk of SIDS in infancy.

Risk of sudden infant death syndrome (SIDS) reaches a maximum in the third month. Thereafter, it decreases by half every 40 days or so. It is proposed that the relative sparing of the very young infant is a consequence of an innate (but temporary) characteristic possessed by the newborn infant. Interpretation of available data suggests that this innate characteristic is negatively associated with the infant's level of maturity. This is the basis for the hypothesis that the age at which the risk of SIDS begins to decline at a uniform rate decreases as the infant's gestational age increases. Because of a greater level of maturity at birth, the age at which this occurs in the black infant should be earlier than average. An analysis of data on 32 573 instances of SIDS within the United States between 1985 and 1991 provides support for the hypothesis.

The effect of the weekend on the risk of sudden infant death syndrome.

OBJECTIVE: The risk of sudden infant death syndrome (SIDS) is associated strongly with socioeconomic status. However, many infants who live in one socioeconomic environment, with its attendant level of risk of SIDS over the weekend, often are exposed to a different level of risk during the work week (because of day care for the infant). If the association between SIDS and socioeconomic status acts through the quality of supervision of the infant, then there could be an immediate change in the level of risk as the infant moves from home to outside care to home again. In this scenario, infants of economically disadvantaged parents would have a higher risk of SIDS over the weekend than they do during the week. On the other hand, infants of economically advantaged parents would be at lower risk over the weekend. Therefore, the relative risk of SIDS associated with the weekend (risk over the weekend vs risk during the work week) should be found to decrease as the number of years of maternal education (a surrogate for socioeconomic status) increases. Testing this prediction is the objective of the study. METHODOLOGY: Instances of SIDS in the postneonatal period (28-364 days) among the cohort of all infants born in the United States between January 1989 and December 1991 were analyzed. The number 798.0, taken from the International Classification of Diseases, was used to identify 14 996 cases of SIDS. Deaths among hospital patients were distinguished from all other deaths. The latter were divided into four categories: 1) death occurred in the emergency department; 2) the infant was dead on arrival at the emergency department; 3) death occurred at a residence; and 4) death occurred at some other place. Maternal education was divided into four categories: <12, 12, 13 to 15, and >/=16 years. The weekend ratio was defined as the ratio of SIDS cases on Saturday and Sunday (times 5) and Monday through Friday (times 2). The predicted trend in this ratio by maternal education was tested by applying a chi(2) test-for-trend. RESULTS: The overall weekend ratio was 1.00, indicating that the risk of SIDS was no higher over the weekend than it was Monday through Friday. However, for infants of mothers with <12 years of education, the ratio was 1. 13. For infants of mothers with >/=16 years of education, it was 0. 55. The trend in the ratio as maternal education increased (1.13, 0. 99, 0.86, and 0.55) was highly significant (chi(2) = 74.2; 1 degree of freedom). Each of the four ratios, with the exception of 0.99, was significantly different from 1.00 (z = 3.74, 2.45, and 6.09, respectively). The ratios for infants of mothers with 13 to 15 and >/=16 years of education also were significantly different from each other (z = 4.57). For all causes of death combined (including the relatively small number of SIDS cases) among hospital inpatients, there was no significant trend in the weekend ratio as the level of maternal education increased. However, among deaths not attributable to SIDS or accidents occurring outside the hospital, there was a slight but significant declining trend (chi(2) = 8.4; 1 degree of freedom) The risk of an accidental death was highest over the weekend for all four maternal education categories. On an average working day, the risk of SIDS among offspring of mothers with <12 years of education was found to be 3.9 times greater than that among offspring of mothers with >/=16 years of education. At the weekend, the relative risk increased to 7.9. A plot of the weekend ratio against single years of maternal education revealed a unimodal distribution with a peak at 11 years. CONCLUSIONS: First, the results of the study are consistent with the level of risk of SIDS, changing promptly toward the risk level obtained in the baby's new environment. Variability in the observation of unusual respiratory events seems the most likely explanation. It is unlikely that confounding factors played a role in the results for tertiary-educated mothers

The seasonal distribution of infant deaths by age: a comparison of sudden infant death syndrome and other causes of death.

OBJECTIVE: To examine the possibility that among deaths in infancy the increase in the winter/summer ratio with increasing age is not peculiar to sudden infant death syndrome (SIDS). METHODOLOGY: Details of the winter (December-February)/summer (June-August) ratio among deaths in neonates (< 28 days) and post neonates dying in the United States of America between 1979 and 1990 were abstracted from published statistics. The primary causes of death were classified according to the ninth Revision of the International Classification of Diseases. RESULTS: For every non-traumatic cause of death including SIDS, the winter/summer ratio was higher among postneonates than neonates. This was not seen for deaths due to trauma. Cases of SIDS and deaths due to infection had the highest ratios in both age categories. Causes of death occurring predominantly in the neonatal period (e.g. anencephaly) had the lowest overall ratios. CONCLUSIONS: Neither the greater number of SIDS cases in the winter, nor the increasing winter/summer ratio with increasing age is unique to SIDS.

Are bedding and rebreathing suffocation a cause of SIDS?

Suffocation by bedclothes became a popular diagnosis in the 1940s but gradually became replaced with the diagnostic label of Sudden Infant Death Syndrome (SIDS). In 1991 a paper purported that, instead of SIDS, pillows filled with polystyrene beads had caused death by rebreathing suffocation; this conclusion was reached on the basis of experiments with anesthetized rabbits breathing through a doll's head that was placed face down on the pillow. Because of the anesthesia, rabbits could not change their face down position. The doll's nares could not collapse, which would have resulted in rapid death due to conventional suffocation. The rabbits required up to 3 hours or more to die of hypercarbia and hypoxia. Studies in normal infants revealed that they turned from the face-down position after only 2 minutes. (The only infant who retained CO2 soon died of a fatal neurologic disorder, with central hypoventilation). Using the rabbit/doll's head and mechanical models, a wide range of bedding was indicted, including cushions, sheepskins, pillows, comforters, foam mattresses, and even simple blankets and sheets as potentially causing fatal rebreathing. Except for the use of pillows in general, as well as mattresses filled with kapok and bark, there has been no epidemiologic support for these indictments. Although normal infants are unlikely to succumb to rebreathing suffocation, infants with blunted ventilatory responsiveness and delayed arousal due to prior hypoxia were hypothesized to be at increased risk. Support for this concept was found in the pathology of the brain stem in victims of SIDS that was attributed to prior hypoxic injury. In infants who survived prolonged apnea, less than 20% have demonstrated a diminished ventilatory responsiveness to hypercarbia, but, more significantly, none had an absent response. Arousal to hypercarbia, an abnormality which is crucial to the hypothesis of rebreathing suffocation, is regularly present in normal subjects, but the threshold is higher in near-SIDS infants; however, no instances of failure to arouse have been reported in near-SIDS. If the infant is placed on his or her back or side, the issue of bedding could become moot; unfortunately, a sizable percentage of infants are still being placed prone for sleep. Instead of confusing parents with an ever-expanding list of "dangerous bedding," the message "Back to Sleep" should be emphasized.

Negative effect of a short interpregnancy interval on birth weight following loss of an infant to sudden infant death syndrome.

It was hypothesized that a short interpregnancy interval immediately following the birth of an infant that had succumbed to sudden infant death syndrome (SIDS) (and no other cause of death) would be associated with a reduced mean birth weight in the next infant. Mothers who had given birth to two children in the state of Oregon between 1975 and 1984 and whose first child had died in infancy from either SIDS (n = 84) or some other cause (n = 305) were identified from vital records. A multiple regression analysis in which adjustment was made for possible confounding variables (including the birth weight of the deceased child) was conducted. When the firstborn child had succumbed to SIDS, the mean birth weight of the next baby was 314 g (2,978 g vs. 3,292 g, p = 0.04) lower when the interpregnancy interval was less than 6 months versus greater than 6 months. In contrast, a less-than-6-month interval had a slightly positive effect (60 g) on the mean birth weight of the next baby when the firstborn child had died due to a cause other than SIDS. These results suggest that parents who have lost a child to SIDS may wish to delay a new pregnancy for at least 6 months.

Recommendations to avoid the prone sleeping position and recent statistics for sudden infant death syndrome in the United States.

OBJECTIVE: To determine whether two recent, nonsynchronized recommendations to avoid the prone position for sleeping infants were each followed by a decline in the incidence or expected number of cases of sudden infant death syndrome (SIDS). DATA SOURCES: Data were collected from SIDS counseling programs, state vital statistics, and medical examiner records of 44 states and the city of Los Angeles, Calif. Data for a state were excluded because of either incompleteness or nonresponsiveness to request. DATA SYNTHESIS: In the 8 months following an article in the Seattle Times advising against the use of the prone position for sleeping infants, the incidence of SIDS fell by 52.0% in King County (where 32 households in every 100 receive the Seattle Times) and by 19.9% in Snohomish County (16 in 100 households). In the remaining 37 counties of Washington State (on average, < 1 in 100 households are subscribers), the incidence rose 3.4%. Examination of medical examiner records for King County and Snohomish County revealed no compensatory increase in other causes of death and no cases attributed to aspiration. In the 12 months following the initial 8 months, the number of SIDS cases in King County remained at approximately half the previous annual average (25 vs 49 cases). At the national level, the American Academy of Pediatrics' recommendation on April 15, 1992, was followed in the next 6 months by a decrease of 12.0% in the number of SIDS cases compared with the previous year. CONCLUSIONS: The results are consistent with those of intervention programs in other countries. A national campaign to inform parents of the risk of the prone position in early infancy should be given serious consideration.

Redefinition of the sudden infant death syndrome: the disadvantages.

For many years the definition of SIDS has been the sudden death of an infant that was unexpected by history and in which a thorough postmortem examination failed to demonstrate an adequate cause of death. In 1991 a report was published in this journal from a panel convened by the NICHD which recommended that the diagnosis of SIDS not be made unless a death scene investigation has been conducted. The panel recommended further exclusions from the diagnosis of SIDS of certain "unresolved" cases. We believe the changes recommended by the NICHD panel are impractical and may have a serious negative impact on SIDS research and on the surviving family members of the SIDS victims.

Birth order and risk of sudden infant death syndrome: is the true relationship negative?

The risk of SIDS has been reported consistently as being positively related to parity or birth order. However, the reports in question have failed to take into account the possible confounding effects of sibship size. In the present study it was reasoned that if this were done the relationship would be negative, not positive. In an analysis of births stratified by sibship size positive. In an analysis of births stratified by sibship size occurring in the years 1975-84 in the state of Oregon, it was found that the risk of SIDS in the age range 7-364 days did indeed tend to decrease with increasing birth order when sibship size was held constant. An expected tendency for SIDS rates to increase with increasing sibship size when birth order was held constant was also confirmed. This tendency is probably explained by a negative correlation between family size and socio-economic status. Non-SIDS rates behaved in a similar manner to SIDS rates. The tendency for the risk of SIDS to decrease with increasing birth order was more evident when births following a pregnancy interval of less than 12 months were excluded.

The effect of parity on the relation between maternal history of spontaneous pregnancy loss and the risk of sudden infant death syndrome in offspring.

We conducted a population-based case-control study, using the 1984-1989 linked Washington State birth and death certificate data, to determine whether maternal prior spontaneous pregnancy loss was associated with the risk of sudden infant death syndrome (SIDS). We identified 872 autopsied SIDS cases and 3,704 controls matched to cases on year of birth. To reduce the confounding from unmeasured socioeconomic and behavioral factors, we analyzed the data separately for infants with and without father's race provided on the birth certificate. Among infants whose father's race was known, those of multiparous women with a prior spontaneous pregnancy loss before 20 weeks gestational age were at reduced risk of SIDS, after adjustment for maternal age, prenatal smoking,a nd gravidity. An increased number of prior pregnancy losses was associated with a further reduction in the risk of SIDS: one prior pregnancy loss was associated with a 25% decrease [odds ratio (OR) = 0.75; 95% confidence interval (CI) = 0.52-1.08], and two or more prior pregnancy losses was associated with a 47% decrease (OR = 0.53; 95% CI = 0.30-0.94). There was no such association, however, among infants of women with no prior livebirth. Similar associations were observed between prior spontaneous pregnancy loss during the entire gestational period and the risk of SIDS in offspring. To explain the different associations between maternal prior pregnancy loss and the risk of SIDS observed among infants of women with and without a prior livebirth, we hypothesize an upward bias resulting from the presence of a larger proportion of infants of human leukocyte antigen-compatible couples among first liveborn infants.

Sleeping prone and the risk of sudden infant death syndrome.

OBJECTIVE: To critically analyze reports that show a relationship between sudden infant death syndrome (SIDS) and the prone sleeping position in infants. DATA SOURCES: Peer-reviewed articles, published letters, book chapters, and local and national health statistics were used, without time or language restrictions. These studies represented three races, four continents, and seven countries; none was published in North America. STUDY SELECTION: No studies were ignored, but only those with case controls were reviewed in detail; we regarded a recent cohort analytic (prospective) study as particularly strong, in addition to six before-and-after (intervention) trials. DATA EXTRACTION: Hill's criteria for decision making were used to assess the quality and validity of the data. DATA SYNTHESIS: Without exception, all studies demonstrated an increased risk for SIDS associated with the prone sleeping position. The published likelihood ratios (relative risk or odds ratio) for SIDS in the prone position compared with SIDS in any other position ranged from 3.5 to 9.3 in seven studies. Publicity against the use of the prone position has been associated with reduction of SIDS by 20% to 67%, paralleling the reduction in use of the prone position, with no increase in deaths from aspiration or in other diagnostic categories. CONCLUSIONS: We recommended avoidance of the prone sleeping position for infants in the first 6 months of life unless there is a specific medical indication for it. Reports from the Netherlands, Great Britain, Australia, and New Zealand indicate that avoiding the prone position for infants in the first 6 months of life could reduce the number of SIDS deaths by as much as 50%. Unfortunately, these findings have received little attention in North America. We offer here an analysis of these reports so that physicians can assess the evidence and advise parents accordingly.

A seasonal association between SIDS deaths and kindergarten absences.

Data from linked birth and death certificates from the State of Oregon were used to determine the monthly distribution of deaths from sudden infant death syndrome (SIDS) for the years 1976 through 1984. The monthly number of SIDS deaths increased in winter and decreased in summer, when schools usually are not in session. Absences from kindergarten were determined from school records for the period 1979-84. School absences, previously shown to reflect incidence of predominantly infectious diseases, were found to be positively correlated with occurrences of SIDS. The role of ordinarily nonlethal infections in occurrences of SIDS has been established by history, histology, and viral isolation. The authors concluded that the seasonality of SIDS is related to the seasonality of respiratory infections and that the seasonality is influenced by respiratory infections being spread from school children to infants during periods when schools are in session.

Relative risk of early and late death in infancy among offspring of non-native-born parents residing in Washington State.

It has been suggested that geographic variation in the early neonatal (0-6 days) and postperinatal (7-364 days) mortality rates for the United States derives from positive and negative associations, respectively, with the probability of an average couple sharing one or more HLA antigens. If this theory is correct, then the ratio of 0-6-day deaths to 7-364-day deaths among the offspring of migrant parents should be similar to that seen in the area from which the mother and the father originally migrated. Information on deaths among the offspring of migrant couples by the mother's region of birth was obtained from computerized vital statistics for Washington state for the period 1968-1977. Information on infant deaths occurring in the mother's region of birth was obtained from published vital statistics for the United States for 1980-1984. Ratios of 0-6-day deaths to 7-364-day deaths in 9 geographic regions for the period 1980-1984 were placed in 4 intervals. These intervals became the standard against which the 0-6/7-364-day ratios in the offspring of migrant parents were compared by means of a test for trend in proportions. As predicted, a significant positive trend was observed when the mother and the father were born in the same region but not when they were born in different regions.

Inverse relationship between risk of SIDS and early neonatal mortality: study of state mortality rates.

It was predicted that at the state level early neonatal (0-6 days) mortality rates and rates for sudden infant death syndrome (SIDS) in the age range 7-364 days would be negatively correlated. Using published data for each of the 50 states and the District of Columbia over the 5-year period 1980-1984, the actual correlation was -0.68 (p less than 0.001). In a breakdown of early neonatal mortality by age at death, rates for infants aged less than 1 hour proved to be the exception; their rates were not found to be negatively correlated with the rates for SIDS (r = +0.20). This finding probably reflects uniformity in the quality of obstetric care. The geographic variation in mortality rates for infants aged 1 hour to 6 days might be explained by variation in the probability of a mother and fetus sharing one or more HLA antigens.