What is sudden infant death syndrome (SIDS)?

Causes and Symptoms

The distribution of sudden infant death syndrome (SIDS) is worldwide. According to the Centers for Disease Control and Prevention (CDC), 1,500 infants died of SIDS in the United States in 2013.

Cultural practices may make the incidence rate vary. In England, a Birmingham study found that 22 percent of Asian babies were put to sleep on their backs, compared with 3 percent of European babies. Sleeping prone is significantly more common in infants dying of SIDS than in controls. In the same study, 98 percent of Asian babies slept in the same room as their parents for the first year, 34 percent in the same bed. Only 65 percent of European infants slept in the same room as their parents. Perhaps the risk of sudden infant death increases in proportion to the amount of time an infant spends asleep out of parental earshot. In Zimbabwe, SIDS practically does not exist. According to English pediatrician Duncan Keeley, who served in that country for two years, Zimbabwean infants almost invariably sleep with their mothers, at least until they are six months old and often until they are a year old.

The cause of sudden infant death syndrome is unknown, but a variety of genetic, environmental, and social factors have been associated with an increased risk of SIDS. Besides sleeping in the prone position, other associations include cold weather, overheating, the hours of the day from midnight to 9:00 a.m., and poor socioeconomic conditions, including overcrowding. The young, unmarried mother, especially if she has had no prenatal care, is more likely to have an infant with SIDS; so is the mother who smokes (either before or after the birth), is anemic, or ingests narcotics. Prematurity, especially with a history of apnea or damage to the immature lungs from elevated levels of inspired oxygen while on a respirator, also increases the risk.

Males are at a higher risk for SIDS than are females; so are the brothers and sisters of infants with SIDS. Likewise, a previously aborted episode of SIDS (that is, a “near miss”) increases risk. On average, Apgar scores (a measure of infant health immediately after birth) are lower in infants with SIDS than they are in surviving peers. In a family that has lost an infant to SIDS, the risk for the next or subsequent child is about five times the usual risk. Most risk factors, however, are associated with only a twofold or threefold elevation of incidence. Therefore, predicting which infants will die unexpectedly is extremely difficult. Recent immunization is not a risk factor. Breast-feeding is not associated with a decreased risk, as was originally thought. Although the peak incidence of SIDS is around three months of age and coincides with normally low levels of circulating immunoglobulins, the syndrome is not associated with any known pathogen.

Pathologists report a wide variety of findings in their postmortem reports—especially changes in the brain and other parts of the body that suggest chronic or intermittent hypoxemia. Yet pathologists also fail to find an increase in the number of cells in tissue of the carotid bodies, a chemoreceptor that responds to decreases in blood oxygen tension; such a finding weighs against the presence of chronic hypoxia.

Like many other aspects of this disease, the mechanism or mechanisms of death in SIDS are unknown. Does the infant stop breathing, or does some cardiac irregularity occur? An immature cardiorespiratory control mechanism involving the nervous system is the most common hypothesis.

D. P. Davies and Madeleine Gantley of the University of Wales College of Medicine have stated that an important mechanism underlying SIDS is failure of respiratory control at a vulnerable stage of development—more a physiological syndrome than a disease in the accepted sense. These doctors hypothesized that the disturbance to this delicate equilibrium might upset the regulation of breathing, sometimes leading to death. Epidemiological risk factors, such as an upper respiratory infection (which is not uncommon), are somehow linked with destabilizing influences to breathing. By avoiding or modulating these factors, the risk of death can be reduced.

Although the pathogenesis of SIDS remains unclear, Anne-Louise Ponsonby and her colleagues at the University of Tasmania in Australia proposed that SIDS be considered as a biphasic event, with the first set of factors operating to predispose the infant and the second set of factors acting as loading factors that operate at a critical stage of the infant’s development. According to the Australian doctors, a warm environment could lead to sudden infant death through direct hyperthermia; a thermolabile, sudden fall in blood pressure leading to a diminished oxygen supply to the brain; impaired respiratory control; altered sleep state; or depressed arousal. An asphyxial mode of death would also be more likely, particularly in heavily dressed infants found prone (face down).

Additionally, there is increasing evidence to suggest that those infants who succumb to SIDS are born with brain abnormalities that make them vulnerable to the syndrome. Studies of SIDS children have found these abnormalities in a network of nerve cells that use serotonin to transmit signals from one nerve cell to another. This network is located in the arcuate nucleus part of the brain, which controls major bodily functions—breathing, heart rate, temperature, and waking up from sleep. An infant with these abnormalities is thus less able to cope with environmental challenges, including overheating, inhalation of cigarette smoke, and sleeping in a prone position. However, medical experts do not believe that brain abnormalities are the only cause of SIDS, as the discussion of risk factors above suggests.

Concern for the confusion of SIDS with child abuse should not be ignored, nor should the efforts of the National Sudden Infant Death Syndrome Foundation to provide information about psychosocial support groups and counseling for families of SIDS victims.

Treatment and Therapy

Since the causes and mechanisms of death from SIDS may continue to be unknown, strategies that might reduce the incidence of this syndrome seem imperative. Cold weather and the hours of midnight to 9:00 a.m. bring increased risks for SIDS. A closer look explains that other risk factors are involved. Overheating as a response to cold weather and leaving the infant alone at night (particularly in Western countries) may be more important. Babies sleeping alone might lose external sensory stimulation that may help stabilize breathing patterns. Davies and Gantley, citing experimental work with mothers and infants co-sleeping in sleep laboratories, have shown how patterns of breathing may interact. They say that the alertness of the babies’ caregivers to early symptoms of illness might also be important.

French doctors studied the seasonal variation of death from SIDS in their country for a two-year period in the early 1980s. They concluded that for babies born in the spring, the third month of age was not necessarily associated with the highest SIDS risk. Babies born during other seasons, however, exhibited a normal pattern of increasing risk between the first and third months. Age was an especially critical factor among babies who reached three months of age during the winter months. If they reach this age in July or August, they are less susceptible to SIDS.

This finding, then, leads to a consideration of the risk of overheating. Explanations for the association between cold weather and SIDS include hypothermia, increased viral illness, and indirect hyperthermia. New Zealand doctors looked at the role of thermal balance in SIDS by investigating the death scene. They found that infants who died of SIDS were significantly more likely to be overdressed for the room temperature at the death scene and in the prone position, when compared to control infants. They also suggest that parents may have responded to infections in their babies by increasing the amount of clothing and bedding or by otherwise warming the infant.

The government of New Zealand initiated a program of education for parents recommending that the prone sleeping position be avoided, that mothers not smoke, and that breast-feeding be encouraged. (Most experts believe that breast-feeding itself does not reduce risks for SIDS. Rather, closer and more frequent contact with mothers is the operative factor.)

A similar education program for parents in Avon, England, was initiated, but it omitted advice on breast-feeding and included suggestions to avoid overheating after a retrospective case-control study that suggested a nearly ninefold relative risk for SIDS from infants sleeping prone. New Zealand and Avon both reported fewer deaths from SIDS after their parental education programs were introduced. The Department of Health extended Avon’s campaign nationally.

In an editorial note in 1986, the US National Center for Health Statistics acknowledged that “the rapid decline of infant mortality rates in the 1970s has been attributed largely to the advent of medical technology in the area of premature and other clinically ill newborns.” Yet, “in the 1980s, this decline has slowed considerably—partly because of a lack of progress in primary prevention of conditions which lead to infant death.” Undoubtedly, the United States would benefit from a massive, national program of education for parents. For example, cigarette packages carry a warning of the harmful effects of smoking on the fetus; perhaps they should also include a warning about the dangers to infants of maternal smoking. Another possibility for intervention exists in the area of infections: pertussis (whooping cough) could be prevented by the immunization of infants under six months of age. In the long term, all nations should work toward improving the socioeconomic status and health care of the poor.

Finally, improved medical technology will be less important over the long haul than will efforts to educate parents in infant care practices. The ability of parents and other members of the household to monitor infants and respond appropriately to both true and false alarms is crucial, as is appropriate training in infant CPR (cardiopulmonary resuscitation) and the proper use of monitory equipment. Even if all SIDS is eliminated in at-risk children, there will continue to be cases among children not known to have been at risk.

Perspective and Prospects

The term “sudden infant death syndrome” was popularized by Abraham Berman’s book on SIDS in 1969, which grew out of a conference on that subject. Since then, recognition of the syndrome has led to the creation of organizations dealing with it. The Sudden Infant Death Foundation merged, on January 1, 1991, with the National Center for the Prevention of SIDS to form one organization, the SIDS Alliance. In 2002, as the organization's goals and efforts continued to expand, it changed its name to First Candle.

In dealing with SIDS, one factor looms most important: education of parents makes all the difference. In 1991, for example, England’s Scarborough district reported a 50 percent fall in the SIDS death rate after parents were advised not to overwarm their small infants. That same year, four other districts in England reported a similar reduction after parents were advised not to let their infants sleep in a prone position. The Foundation for the Study of Infant Deaths and the US Department of Health recommend both procedures: a supine sleeping position and prevention of overwarming.

These successes raise two issues: the overall decline in rates of SIDS worldwide in industrial countries and parental guilt. For a number of years, the incidence of SIDS was generally falling. This decline slowed considerably in the 1980s. How much, then, did the parental education programs actually lower the incidence rate in these English districts? No one can say with certainty, but one thing is clear: if doctors make recommendations regarding sleeping positions and warming, they run the risk of inducing guilt in parents who have not followed their recommendations—or, alternatively, who have followed the recommendations but have still lost an infant to SIDS. Parents who have lost a child to SIDS are grief-stricken. They are not prepared for such a tragedy, and their grief is compounded by guilt, because no definitive cause for SIDS has been identified and, as a result, parental behavior seems to be implicated. Investigations conducted by police, social workers, or others who become involved only add to this guilt. Parents may be confronted by questions of whether they positioned their infant correctly or overdressed the child. Regardless of these behaviors, however, the factors causing the death may not have been under the parents’ control.

SIDS will continue to occur until the exact etiologies of the syndrome, its mechanisms, and its correct treatment—based on fact, not simply risks alone—are identified. Until that time, it is expected that incidence rates will continue to go down, based on what is now known of the risk factors and recommendations against prone sleeping positions and overwarming.

In 2015, a study published in the journal Pediatrics continued to emphasize that while sleeping position does play a significant role in causing SIDS, other factors also influence its occurrence. Specifically, the researchers leading the study reported that there are three major factors that impact the risk of SIDS: an infant's intrinsic predisposition, an infant's period of development, and the sleeping position. Decreased numbers of women smoking or drinking while pregnant, an increase in mothers choosing to breastfeed, and increased access to prenatal care have also contributed to the declining instances of SIDS in conjunction with the change in sleeping position, the researchers suggested. Therefore, public health education needs to focus on maintaining these behaviors as well.

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