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-reprinted with permission from the Information Exchange the newsletter of the National SIDS Resource Center

The best hope for eliminating SIDS lies in discovering its cause. For this reason, research is crucial. Part I summarizes the various components of the overall research effort, explains prevailing theories, and explores the issue of risk factors. Part 2 of the article will describe specific new avenues of inquiry, focusing on the research of the National Institute on Child Health and Human Development (NICHD). Part 3 completes the series with highlights of the SIDS-related projects funded as Special Projects of Regional and National Significance (SPRANS) by the Maternal and Child Health Bureau (MCHB).


According to data from the National Center for Health Statistics, there were 5,476 SIDS infant deaths in the United States in 1988, the latest year for which final data is available. This figure refers to deaths classified as SIDS (code 798.0) by the International Classification of Diseases, Ninth Revision. Finding the cause for SIDS is difficult because statistically speaking the number of cases of SIDS is low. Also, it takes time to analyze and apply results from other research methods to SIDS studies. To prevent any fatal phenomenon, scientists look for a "marker"-a health condition that victims have and nonvictims do not-so that those at risk can be identified and measures can be developed to prevent their death. To predict and ultimately prevent SIDS, scientists will need multiple markers with strong predictive value. In addition to identifying babies at risk, another important function of SIDS research is to differentiate among causes of death. An unambiguous morphologic pattern that distinguishes SIDS cases will help to protect bereaved parents from accusations of child abuse and neglect and, possibly, rule out inherited metabolic disorders, which will provide useful knowledge for the parents' future family planning. (The possible role of inherited metabolic disorders in SIDS is one of the targets of investigation.)


Research data about SIDS are collected from animal models and studies in other areas such as sleep physiology, respiratory and neurological disorders, and epidemiology. Much of what is known about SIDS begins with the findings from the death investigation conducted by the medical examiner or coroner, pathologist, and law enforcement personnel. All state laws require investigation of cases of sudden infant death. An accurate determination of SIDS requires details of the death scene and circumstances of death; medical histories of the infant, family, and pregnancy; the demographics of the family (such as ethnicity, income, and employment and educational status); and an autopsy performed to look for any unusual characteristics in the infant.

The information produced by these investigations forms the basic foundation of all research. Retrospective epidemiological studies draw upon these data, usually comparing the information on victims with similar information on "control" infants-normal babies who are matched with the SIDS babies for the characteristics the researcher wants to control(keep the same), such as ethnicity or birth weight, in the hope of identifying other characteristics as markers.

Certain limitations exist in the method of collecting data on SIDS: death investigations are not conducted by precisely the same standards across the country, making data comparison difficult, and reports of infants' behavior and health are based largely on the observations of parents and caregivers rather than on scientific measurement. To help improve uniformity of information, the Division of Maternal and Child Health of the Department of Health and Human Services held a meeting in 1975 which resulted in investigative and autopsy protocols for examining the SIDS infant. These protocols were included in an article entitled "The Examination of the Sudden Infant Death Syndrome Infant: Investigative and Autopsy Protocols." This article was published in Journal of Forensic Sciences, Vol. 21, No. 4, pp. 833-841, 1976.

The largest SIDS research effort ever conducted was the NICHD SIDS Cooperative Epidemiological Study, which looked at prenatal, neonatal, and post neonatal characteristics of 757 SIDS infants and twice as many control infants. The findings of this study will be summarized in Part 2 of this article. Another large study is the U.S. Collaborative Perinatal Project, supported by the National Institute of Neurologic Disorders and Stroke.


Fetal Origin

A central hypothesis in both of the major studies mentioned above, as well as other research, is that babies who die of SIDS are born with one or more vulnerabilities that probably result from adverse conditions or abnormalities in the early stages of fetal development. These vulnerabilities may prevent the infants from responding normally to internal and external influences that place special demands on their bodies. A prenatal insult (trauma or irritation), such as hypoxia (a deficiency in the amount of oxygen reaching bodily tissues), might impair a baby's nervous system so that the sleeping infant does not produce the normal arousal response.

A nervous system abnormality alone would probably not account for sudden death; an event, such as sleep deprivation or infection, would be necessary to trigger the abnormal response. Research on the normal development of the central and peripheral nervous systems and normal recovery mechanisms will help to elucidate this line of thought. The brain stem controls breathing and heart rates, and it is possible that defects in brain stem circuits result in abnormal reactions to stresses. To pursue this possibility, new imaging techniques are making it possible to create three-dimensional representations of the anatomical and neurochemical composition of the brain stem on a computer screen. The maturation of the brain may be delayed in SIDS infants: myelin, a fatty substance that facilitates nerve signal transmission, appears to mature more slowly in victims than in controls.

Learning and Behavior

Other theories under investigation involve the learning process. Babies exhibit strong natural reflexes in the early stages of infancy, which are gradually replaced by learned voluntary behaviors. It may be that babies learn, possibly between the ages of 2 and 4 months, how to struggle for breath when needed. If so, this could help to explain why almost all SIDS deaths occur among this age group.

In the NICHD study, SIDS babies were frequently described as passive, "listless", and "droopy" just prior to death. These behavioral clues are being pursued to determine what underlying abnormalities they may indicate. Many healthy infants exhibit such behavior, however, and it is not necessarily a cause for fear of SIDS.


A small number of cases appear to be related to inherited metabolic disorders. Research is under way to identify which disorders are involved and how they interact with environmental and internal triggers to result in sudden death.


Research on SIDS is based on observed similarities among victims and differences between victims and normal babies. Characteristics that occur more frequently in victims than in controls are called risk factors. These can include features of the mother's health and behavior during pregnancy, the autopsy findings, and the baby's health and behavior. It is hypothesized that any such factor may be a clue to finding the cause, but risk factors themselves are not causes and are not reliable in predicting SIDS.

Many families that exhibit known risk factors are not afflicted by SIDS, and there are SIDS cases that exhibit no known risk factors. For example, because about 95 percent of SIDS deaths occur between 2 and 4 months of age, being in this age group is a risk factor, although obviously not a cause or predictive factor.

Risk factors include maternal smoking during pregnancy, low birth weight, young maternal age (under age 20), and high parity (number of children born to the mother), and intrauterine growth retardation (low growth rate for gestational age, whether premature or full-term). Neonatal apnea (prolonged pauses in breathing) was not found to be a marker for SIDS in the NICHD study and is now discounted by some researchers as causal influence. However, rapid heartbeat and respiratory distress among newborns were found to be significantly more common in SIDS infants.

Knowledge of risk factors can be put to practical use by prospective parents and health professionals. Smoking during pregnancy, which is also a risk for a number of other health problems, should be avoided. Proper nutrition as well as early and frequent prenatal care can help avoid low birth weight, which is also a multiple-disease risk factor.

The NICHD study found associations "of moderate strength" between SIDS and other maternal factors of pregnancy: sexually transmitted disease, weight gain less than 20 pounds from conception to delivery, anemia, and urinary tract infection. Although the relationship, if any, between these factors and sudden infant death is not known, these health problems are obviously best avoided for a healthy pregnancy outcome.

The cause of SIDS remains unknown, but promising new leads are being followed that may illuminate the means of prevention. In the meantime, existing knowledge points to healthy prenatal behavior as a worthwhile precaution and continues to reinforce the need for compassionate support of families bereaved by this unpredictable, and still unavoidable, tragedy.

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