Post #3 Back-to-Sleep series
What do we know about SIDS?
In her first post, Ms. Fama introduced the concept of the “Back to Sleep” campaign, including the changing recommendations for sleep position and the correlation (emphasis on correlation) between the declining rate of sudden infant deaths and back sleeping. In her second post she explored the possible downsides of supine sleep for infants.
A key part of the critical appraisal of back sleeping involves understanding sudden infant death syndrome. Today, Ms. Fama offers us a thorough explainer. JMM
What is Sudden Infant Death Syndrome?
The sudden death of an apparently healthy child is a devastating tragedy, and if it’s preventable, the costs of supine sleep may be worthwhile.
What do we know about SIDS and how effective back sleep is at reducing it?
In popular (lay) understanding, SIDS represents a common danger that a well-cared for and apparently healthy baby will suddenly and terrifyingly be found dead in the morning. Its relative rarity and the many risk factors beyond prone sleep and soft bedding are not widely discussed.
SIDS is one of three subcategories of “Sudden Unexpected Infant Death (SUID),” along with “Unknown Cause” and “Accidental Suffocation and Strangulation in Bed” (ASSB). (Figure 2.)
SIDS is defined as “The sudden death of an infant under one year of age which remains unexplained after a thorough case investigation, including performance of a complete autopsy, examination of the death scene, and review of the clinical history” (per the NICHD expert panel convened on June 20, 1989). The death has an “unknown cause” rather than SIDS if the investigation or autopsy is missing, or an alternative diagnosis cannot be ruled out (see Duncan and Byard, 2018, p. 6).
The failure-to-rouse model of SIDS hypothesizes that when CO2 levels rise in the baby’s breathing environment, she is unable to move her head to find clean air due to a genetic flaw. Researchers comb through the victims’ genomes for mutations in cardiorespiratory function and autonomic arousal systems.
Scientists have found dozens of mutations in babies with SIDS deaths, but no clear answer. Putative causes related to SIDS include ion channelopathies, cardiomyopathies, metabolic disease, genes that mediate inflammatory responses, serotonin uptake and regulation, genes responsible for sodium channels, genes involved in immunity, and genes involved in the development of the autonomic nervous system.
Dr. Paul N. Goldwater, a researcher and professor at the University of Adelaide and Adelaide Medical School, told me in a call that for decades we have known that autopsies of SIDS babies have common characteristics: 80% have intrathoracic petechiae (broken blood vessels visible on particular organs, likely indicative of upper airway obstruction), liquid (unclotted) blood in the chambers of the heart, an empty bladder, and a heavy liver, thymus, and brain—all potentially consistent with a sepsis (infection) event.
Most SIDS babies have congested lungs, and 75% have a history of a recent cold, or an existing cold. Up to 81% have Clostridium perfringens enterotoxin in their intestinal tracts, compared with 20% of healthy controls. In his work, Dr. Goldwater describes a kind of toxic-shock (cytokine) reaction in the body that results from the combination of a cold virus plus being infected with a common bacterium—each pathogen relatively harmless on its own but deadly together. This sort of sepsis would produce many of the common autopsy findings. The regional, seasonal, and socioeconomic correlates of SIDS also suggest a link to illness rather than a pure genetic defect interacting with sleeping environment.
I wonder whether an old diagnosis from the late 1960s (see Bergman et al 1970, quoted by Froggatt on p. 132) of laryngospasms may have been discarded too quickly—the intrathoracic petechiae, the empty bladder, and the fact that infants are sometimes soaked in sweat could all indicate a struggle.
In sum, despite decades of study, SIDS remains a diagnosis of exclusion. There is no accepted causal mechanism or biological marker to define SIDS.
How Many Infants Die of SIDS?
The CDC reports that in the US in 2020 there were 93 Sudden Unexpected Infant Deaths per 100,000 live births (Fig. 2). Of these, 38 were attributed to SIDS (41%), 29 due to unknown causes (32%), and 25 due to “Accidental Suffocation or Strangulation” ASSB (27%).
SIDS declined substantially from 1990, a year in which there were 155 unexplained deaths per 100,000 live births, of which 130 were SIDS, 21 Unknown Cause, and 3 ASSB. In total, there were about 3.6 million babies born in 2020, and the CDC reports there were about 1,389 SIDS deaths.
The overall level of sudden unexpected death (the purple line) declined from 1990 to 1997, but then plateaued thereafter, with a troubling rise in other causes coinciding with the decline in SIDS. Figure 2 leads me to wonder how much of the apparent drop in SIDS is due to a change in classification—former SIDS deaths now attributed to Unknown Cause or ASSB.
The definition of SIDS has in fact changed over time, mostly becoming stricter. The definition of SIDS adopted at the Second International Conference of Sudden Death in Infants in 1969 did not require a death scene examination. The CDC admits that “in recent years, SUIDs are being classified less often as SIDS and more often as ASSB or unknown cause.”
Indeed, cause of death is coded differently over time and across counties, states, and countries. A 2021 AAP article by Parks and colleagues says,
“Lack of uniformity in defining and classifying SUID is a long-standing limitation of surveillance. Rates of SUID and SUID subtypes are frequently calculated by using death certificate data. Cause-of-death determination is subject to death certifier preferences, training and experience, and state and local guidance. Inconsistent reporting practices have resulted in a diagnostic shift among SUID subtypes and adversely affects reliable monitoring of SUID and SUID subtypes which impacts understanding of the magnitude of and causes underlying SUID.”
(See also this article entitled “Continuing Major Inconsistencies in the Classification of Unexpected Infant Deaths.”)
A 2016 study by Lambert et al in The Journal of Pediatrics shows how variable death investigations can be. They looked at 770 sudden unexpected infant deaths (SUID) across seven states between 2010 and 2012 to see how comprehensive the autopsies and death-scene investigations were. Every death had an autopsy performed, but the quality of the investigations varied widely. On average, only 73% of cases across all states had data on whether the infant’s airway was obstructed. Only 45% of investigations across all states asked caregivers to re-create the scene, while 37% had scene re-creation using a doll—an investigation technique that is prized by some researchers as a kind of gold standard to help tease out sudden unexpected infant death (SUID) from SIDS.
Hesitation to request scene re-creation is understandable: in many small jurisdictions, the coroner is an elected official who knows almost every family by name, and asking parents to re-live the moment they discovered their unresponsive child may feel like an unnecessary cruelty. Yet even autopsy labs varied, with only 41% of cases including blood chemistry results. One state performed blood chemistry labs 100% of the time and another 0% of the time. Only 23% of all cases across all seven states did genetic testing. Regarding sleep position, 85% recorded position placed, 86% reported position found, with the “low” state reporting this data only 76% of the time.
Without uniform and methodical investigations of each death, it seems inevitable that what we call SIDS could be many things, and that some percentage of the deaths are natural (that is, due to disease or natural process), but are too subtle for less-than-meticulous autopsies to uncover.
The Roberts Program on Unexpected Death in Pediatrics at Boston Children’s Hospital does approach SIDS in this meticulous way in order to bring medical closure to families (and to prevent future tragedy in the case of, for example, inherited epilepsy).
In their first study, Roberts Program scientists attempted to retroactively “solve” 14 cases of sleep-related unexplained child death, 8 of which are under age 12 months and thus called SIDS. By exhaustively looking at autopsies, blood labs, and tissue samples, and by implementing neuropathology protocols and genetic testing of the entire family, they were able to medically explain 5 of the 8 SIDS deaths as natural, with causes such as sepsis and early-stage meningitis, cardiac arrhythmia due to long QT syndrome, lymphatic meningoencephalitis, and a temporal lobe epilepsy event without genetic variants detected.
This case series argues that a ruling of SIDS can be a catch-all category for very difficult cases.
The Cause(s) of Declining SIDS Rates
Although I struggled to find data before 1990, it seems that SIDS rates were already decreasing before the Back to Sleep campaign started in 1994, and even before the AAP’s 1992 recommendation of “back or side.”
See Figure 1 (in Post #1), which goes back to 1988, and glance ahead at Figure 3, which graphs SIDS data from about 1973, and seems to show that SIDS peaked in about 1980 but was in decline for a dozen years before the 1992 push toward non-prone sleep.
A vintage, digitized CDC article summarizing SIDS says,
“From 1980 through 1988, 47,932 infants born to U.S. residents died from SIDS. During that time, overall SIDS rates declined 3.5% for white infants and 19.2% for black infants…”
Another vintage CDC article summarizes SIDS in the U.S. between 1983-1994:
“During 1983-1990, the rate of SIDS decreased an average of 1.6% per year; during 1990-1994, the rate decreased an average of 5.6% per year.”
SIDS captures parents’ attention, but it is not the dominant cause of infant mortality. According to CDC data, 544 infants died per 100,000 live births in 2021. Of these deaths, 20% were due to congenital abnormalities; 15% were due to short gestation and low birthweight; and 7% (39 deaths), were classified as SIDS—about the same percentage as deaths due to unintentional injuries (6.5%). Most infant deaths (64% in 2021) occur due to knowable causes in the first 28 days of life, a period when SIDS is rare.
Other Health Gains Coincided with Back to Sleep
Infant deaths of major causes have declined 28% since 1995. About a third of SIDS infants are born prematurely, and supine sleep was promoted during the same period that antenatal steroids and lung surfactant were developed.
Improvements in overall infant illness and mortality for both preterm and term infants also occurred due to more advanced and more widely provided maternal healthcare.
The world changed in other ways: lower pollution, increased breastfeeding initiated at birth (in the U.S., from 57% in 1993 to 82.5% in 2013); fewer teen births; and new childhood vaccinations. The Haemophilus Influenzae type b (Hib) vaccines were introduced for children under two in the early 1990s, eliminating a disease that affected 20,000 children every year. A safer version of the pertussis vaccine was introduced in 1996. The pneumococcal vaccine arrived in 2000, solving 700 cases of meningitis, 17,000 cases of bloodstream infections, 200 deaths and five million ear infections per year in children. A rotavirus vaccine debuted in 2006. Since roughly 75% of SIDS victims have had a respiratory or gastric infection in the two weeks before death, one theory of SIDS suggests the after-effects of infections are responsible for some cases (see also here). Fewer infections could therefore lower the rate of SIDS. Many of these childhood illnesses can also cause death in a manner that would be similar on autopsy to SIDS, and mitigating them would again lower SIDS rates directly.
In Figure 3, only Unintentional Injuries and Short Gestation/Low Birthweight increased between 1990 and 2015, although more recent CDC data shows the latter category has declined each year since 2015.
Goldstein et al show that the rate of SIDS has declined almost exactly apace of the decline in overall deaths, except for a brief window between 1994 and 1996 where SIDS declined faster than other causes of death, which they attribute to Back to Sleep. The effect disappears entirely after those couple of years.
In next week’s post, Ms. Fama explores the evidence linking sleep position and SIDS. There is (some) evidence, but there are no randomized trials. JMM
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The vaccine-SIDS argument is strong. I thought about commenting in detail on it, but then stopped once I realized my references/discussion exceeds three pages. No one reads the literature. It's plain as day. Plenty of cases of twins dropping dead right after vax. And tons of evidence of vaccines inducing cardiorespiratory changes. And DTP and possible some other vaccines having negative nonspecific effects (i.e. increasing other infections), which the author has touched on perhaps not realizing the possible relation to vaccines.
Three kids here. All co slept on stomach.