131 Sudden Infant Death Syndrome

The sudden infant death syndrome (SIDS) is a diffi­cult condition to define because medical scientists do not yet fully understand its nature. In the typical SIDS case, an apparently healthy infant, who may recently have suffered some minor respiratory ail­ment, is put to bed in the evening and is found dead in the crib next morning.

The Second International Conference on Causes of Sudden Death in Infants, held in Seattle in 1969, described SIDS as “the sudden death of any infant or young child which is unexpected by history, and in which a thorough postmortem examination fails to demonstrate an adequate cause for death” (Berg­man, Beckwith, and Ray, eds. 1970). That definition still applies. Physicians make the diagnosis of SIDS by excluding other causes of death in infants be­tween one month and one year.

In attempting to understand SIDS, medical profes­sionals have identified epidemiological patterns that characterize its victims, their parents, and the set­tings in which SIDS death occur. These characteris­tics will be discussed in subsequent sections.

Distribution and Incidence

The vast majority of reported and published SIDS cases come from countries and continents in the Earth’s temperate zones (e.g., the United States, Can­ada, Europe, Australia, New Zealand, Japan, Hong Kong, and Israel) (Golding et al. 1985; Culbertson, Krous, and Bendell, eds. 1988; Guntheroth 1989; Irgens, Skjaerven, and Lie 1989; Lee et al. 1989). But SIDS occurs worldwide, in countries in tropical and frigid zones, in the mountains and at sea level (Gol­ding et al. 1985; Culbertson et al. 1988; Guntheroth 1989). SIDS tends to receive less attention in coun­tries with high infant death rates from problems such as infectious diseases and malnutrition.

The occurrence of SIDS has probably not changed much over the centuries around the world (Bergman 1986; Culbertson et al. 1988). Rates range generally from 1.5 to 3.5 cases per 1,000 live births per year, though the incidence varies from country to country. It is difficult to compare these rates because of differ­ences in the way each study team obtains and ana­lyzes its data (Culbertson et al. 1988). In the United States, SIDS rates have ranged from 1.2 to 3.4 deaths per 1,000 live births. The range is higher in Britain (2.1 to 4.0) and the Antipodes (New Zealand and Australia) (1.6 to 4.1), and lower on the Euro­pean continent (0.5 to 2.7), in Israel (0.3 to 0.7), Japan (0.5 to 1.2), and Hong Kong (0.036 to 0.3) (Golding et al. 1985; Culbertson et al. 1988; Gun- theroth 1989; Irgens et al. 1989; Lee et al. 1989).

SIDS accounts for approximately 6,000 to 7,000 infant deaths per year in the United States (Berg­man 1986). It is the greatest killer of infants be­tween 1 month and 1 year of life (Culbertson et al. 1988).

Epidemiology

SIDS’ outstanding epidemiological characteristic is the age at which it strikes children. Most deaths occur at between 1 month and 6 months of age, with a peak between ages 2 and 3 months. Very vew cases occur before 1 month; the incidence drops signifi­cantly after 6 months old. SIDS deaths thus occur at an age when babies are undergoing their most rapid systemic development, and when their needs for effi­cient bodily processes and outside sources of energy to fuel them are greatest. Infants at this time are adjusting, for example, their sleep patterns to chang­ing internal needs and to the outside environment, their gastrointestinal systems to changing foods, their immune systems to new antigens and patho­gens, and their nervous systems to a variety of new motor and sensory stimuli.

SIDS strikes children of both sexes, of all social, economic, ethnic, and racial groups, and at all times of the year. The distribution of SIDS within these groups and seasons is not equal, however. About 60 percent of SIDS deaths occur among boys. SIDS occurs more commonly, but by no means exclu­sively, during the colder months of the year (au­tumn and winter), in both the northern and south­ern hemispheres. Members of lower socioeconomic groups generally suffer a higher incidence of SIDS than do others. The distribution of SIDS also seems to follow racial lines in the United States: Afro- Americans (blacks) show the highest incidence, followed by Euro-Americans (whites), followed by Asian-Americans. That racial distribution may be deceptive, as it probably reflects more the generally lower socioeconomic status (SES) of blacks in the United States compared to other groups. Low SES does not always translate into high risk for SIDS, however. Studies in both Chicago (Guntheroth 1989) and California (Grether and Schulman 1989) show, interestingly and, at present, inexplicably, that His­panics of low SES have a SIDS rate comparable to or lower than that for whites.

Certain other characteristics of babies, mothers, and families appear to be risk factors associated with a higher incidence of SIDS in infants. None of these factors is predictive of SIDS and none will be found in all SIDS cases, but all increase the risk in a child vulnerable to SIDS. All, it should be noted, can be related to low SES. Prematurity and low birth weight are both important risk factors in SIDS. SIDS occurs more frequently in children of the fol­lowing: multiple births (increased risk due to small birth size or prematurity), younger mothers, moth­ers who smoke, mothers of greater parity, higher birth rank in the family, single mothers, mothers who are drug abusers, mothers with poor prenatal care, and families in which a SIDS death has previ­ously occurred (slightly increased risk).

Etiology and Pathology

At present, the etiology of SIDS remains a mystery. It is not even clear whether SIDS has a single cause, has several causes, or is the result of a combination of factors working together.

Before the medical profession took an interest in the sudden, unexplained deaths of infants in the eighteenth century, people attributed the demise of these children to accidental suffocation in bed­clothes, or to accidental smothering and overlaying by sleeping parents. Less charitable people accused parents or nursemaids of infanticide. These theories persisted throughout the nineteenth and early and mid-twentieth centuries concomitantly with medical theories that ascribed sudden infant deaths to an enlarged thymus or a thymic condition (see the His­tory section for more on this). Since the 1940s, when researchers took a renewed interest in the etiology of sudden unexplained infant deaths, medicine has proposed numerous theories to explain why these children die.

When medical examiners in the 1940s and 1950s tested the blood of infants who had died suddenly and inexplicably, they often found fulminant infec­tions that could easily have caused death. For the next several years, bacterial and viral infections were considered a major cause of sudden infant deaths. But when those deaths from infection were weeded out, there still remained a large number for which pathologists could find no infectious agents. Researchers then found other possible causes of death, including the following: powerful allergic re­actions to cow’s milk, to house dust mites, or to some unidentified allergen; botulism, beginning in 1976 when a number of infants infected with Clostridium botulinum were discovered in California; a severe, undetected respiratory viral infection; a response to vaccination against childhood diseases; overheating; hypothermia; high sodium in the blood; deficiency of a trace element like magnesium, zinc, copper, cal­cium, selenium, or manganese; a vitamin deficiency; and high or low levels of thyroid hormones.

Most current research relates the “final pathway” of SIDS to a malfunctioning or immaturity of the respiratory or cardiovascular system. Etiologic theo­ries under consideration include preexisting hy­poxia, heart conduction problems (arrhythmias), and apnea (Culbertson et al. 1988; Schwartz, South­all, and Valdes-Dapena 1988; Guntheroth 1989). Evi­dence indicates that children who die of SIDS pos­sess physical risk factors such as small size, slower growth rate, fatty changes in the liver, and thymic changes Campatible with previous infection. These risk factors are not specific to SIDS but, like the social factors listed in the Epidemiology section, re­flect increased risk to all infant deaths. When a young patient possesses what Abraham Bergman (1986) calls a “critical mass” of these physical and social factors, all that is needed is a trigger to cause SIDS to occur. “Something must happen during sleep to tip the balance,” he speculates, because vir­tually all SIDS deaths occur during sleep. The na­ture of that trigger is the mystery of SIDS.

SIDS leaves few pathological footprints in its young victims’ bodies. Postmortem examination re­veals little for the physician to use in understand­ing the pathology of the condition. The very defini­tion of SIDS incorporates this fact, stating that negative postmortem findings help to classify an infant’s cause of death as SIDS. Pathologists study­ing large numbers of SIDS cases have, over the years, noted only a few consistent postmortem find­ings that might at some time help explain the na­ture of SIDS. These include, according to one SIDS researcher (Guntheroth 1989), “intrathoracic pete- chiae, patchy pulmonary edema and emphysema,” indicative of respiratory problems; “histopathology suggesting pre-existing hypoxia, such as changes in pulmonary arteries and right ventricle, smaller thy­mus, extra-medullary erythropoiesis, increased peri­adrenal brown fat cells, [and] enlarged adrenal chromaffin cells”; and neuropathological changes such as “astroglial proliferation in brain stem, Ieu- komalacia, and delayed loss of dendritic spines in reticular substance,” consistent with underdevelop­ment or a subtle chronic disorder.

History

The medical profession and society did not recognize SIDS until the late twentieth century. And yet people from Biblical times onward described sudden unex­plained infant deaths that matched the typical his­tory of a SIDS death of today. Because the deaths almost always occurred at home or in private situa­tions, and to seemingly healthy children, most peo­ple, including parents and caregivers, generally as­cribed the cause of death to accidential or intentional smothering or suffocation. When discovering their infants, with whom they regularly slept, dead next to them after a night’s sleep, with no signs of any dis­ease or disturbance, and no cries during the night, parents believed that they had unknowingly overlaid and smothered their children. Or, if they had not slept with their infant, but found it lifeless where they had put it down for the night or for a nap, parents as­sumed the child had suffocated in its bed clothes. In either circumstance, parents blamed themselves for the tragedy. Worse, community members suspected not just parental negligence, but overt infanticide. Because SIDS leaves no telltale marks on its little victims, no one could determine if the infant’s demise was truly accidental or if it was intentional. As a result, society assumed parental negligence and pun­ished the parents or whoever was responsible for the child’s care. Medical people were not consulted in these situations except perhaps to confirm the death. It was purely a societal matter dealt with by reli­gious, and later by secular, authorities.

Perhaps the first recorded Western case of SIDS is found in the famous Bible story in 1 Kings 3:19 of the two women who went before King Solomon with claims to motherhood of an infant boy. One of the women had awakened, found her son dead, thought she had overlaid him, and secretly switched the child with another. Solomon’s proposed solution was to cut the living boy in half so each mother could have part of the child. Medieval church rules enunci­ated specific punishments for those who overlaid their children, and forbade parents from taking in­fants to bed with them. As early as the sixteenth century, Florentine craftsmen designed a wooden arch that fit over, and kept blankets away from, the child, thus preventing potential suffocation with bed clothes.

The power of ecclesiastical courts began to wane in the Renaissance. As secular authorities gained power during the subsequent centuries, civil courts investigated cases of overlaying and smothering to determine causes of death. At this same time, medi­cine was learning more about human anatomy and physiology. In 1761 an Italian physician, Giovanni Morgagni, published his book On the Seats and Causes of Disease, which correlated specific autopsy findings with disease signs and symptoms during a patient’s illness. The resultant development of patho­logical anatomy in the early nineteenth century helped medicalize the previously nonmedical condi­tions of sudden unexplained infant death. As autop­sies of these children revealed large thymuses (actu­ally a normal finding), physicians explained death on the basis that the thymus gland cut off the tra­cheal airway or overly reduced the size of the tho­racic cavity in which the heart and lungs had to func­tion. Such explanations relieved parents of blame for their children’s deaths. Despite evidence presented by other physicians during the nineteenth and early twentieth centuries that neither an enlarged thy­mus nor a similar but more complex condition called status thymico-lymphaticus could cause sudden in­fant death (Cone 1979), many people, including judges in courts, used thymic death to absolve par­ents of guilt. By the end of the nineteenth century, medical people were divided over sudden unex­plained infant deaths. For example, a police surgeon in Dundee, Scotland, in 1892 openly accused parents of neglect, ignorance, carelessness, and drunkenness in overlaying their children (Templeman 1892), whereas William Osler (1904) still wrote of thymic enlargement as a cause of sudden infant death in the 1904 edition of his influential and widely used text­book of medicine.

Recognition of the condition now known as SIDS began to occur in the 1940s and 1950s with the publication of studies (e.g., Weme and Garrow 1953) demonstrating the extreme difficulty of overlaying a child or smothering a child in bed clothes, and the importance of performing full autopsies on these chil­dren. As medical scientists and epidemiologists gath­ered information during the 1960s and 1970s, they better characterized SIDS (Bergman 1986; Gun- theroth 1989). Public awareness and political cam­paigns since the 1970s have succeeded in removing much of the parental stigma associated with sudden infant deaths (Bergman 1986).

Todd L. Savitt

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