What is SIDS?
An overview for the science-minded parent
© 2009 -2014, Gwen Dewar, Ph.D., all rights reserved
What is SIDS? After years of research, scientists can offer only partial answers. What we do know is this:
or sudden infant death syndrome, seems to involve a failure to arouse
from sleep during a life-threatening event--like a severe episode of
sleep apnea or esophageal reflux (Franco et al 2004).
Why do babies fail to arouse?
not entirely clear. As I note below, some babies seem to suffer from
abnormalities of the brain stem and the serotonin system. These babies
show alterations in the brain cells that are sensitive to serotonin, a
neurotransmitter that regulates both sleep and our ability to gasp in response to asphyxiation (Audero et al 2008; Garcia et al 2013).
But even if we don’t understand everything about the causes of SIDS, we know quite a bit about lowering the risk of SIDS.
studies suggest that SIDS deaths occur when vulnerable babies are
exposed to one or more environmental risk factors, like second-hand
smoke or prone sleeping. To the degree that we can control these SIDS
risk factors, we may be able to reduce the number of babies that die
For example, SIDS rates in the United States have
declined since medical workers began promoting the supine position for
sleep (i.e., putting infants on their backs). In 1992, the United States
SIDS rate was 1.2 deaths per 1000 live births. Ten years later, the
rate had fallen to 0.57 deaths per 1000 live births (American Academy of
Pediatrics Task Force on SIDS 2005).
So here is an overview of
the science of SIDS--what researchers mean by “SIDS," what are the
likely causes of SIDS, and what environmental risk factors may
contribute to SIDS.
For a checklist of SIDS prevention tips, see this article on
specific ways that you can reduce the risk of SIDS.
What SIDS is not
The term “SIDS" does not apply to all cases of sudden infant
death. The terminology “sudden infant death syndrome" was coined in
1960s to describe sudden, unexpected mortalities that couldn’t be
ascribed to any known causes (Bergman et al 1969).
definition, “SIDS" excludes deaths caused by known diseases, child
abuse, or accidents (including accidental suffocation or strangulation).
In this sense, “SIDS" began as a label for any deaths that couldn’t be explained.
But—and here’s the big “but"—the reason why SIDS was recognized as a syndrome was that researchers noticed certain recurrent features associated with sudden infant death (Krous et al 2004):
• SIDS appears to strike when babies are asleep.
SIDS is associated with a specific age range: Babies older than 21 days
and younger than 9 months. SIDS risk peaks around 2 to 3 months of age.
pattern led researchers to speculate that SIDS wasn’t merely a
collection of different diseases and accidents. SIDS was suspected of
being a new, if poorly-understood, medical condition.
The problem was: What exactly was it?
What is SIDS?
An impaired ability to arouse from sleep
later, SIDS is still not fully understood. But researchers are piecing
together the puzzle. The general idea is that SIDS victims fail to
awaken when something potentially life-threatening happens during
sleep—like a severe episode of sleep apnea, or esophageal reflux, or a
pillow pressed up against the face (Franco et al 2004; Garcia et al 2013).
normal circumstances, these events don’t lead to death. That’s because
the infant’s brain stem detects a drop in oxygen levels and wakes the
baby up. She wiggles, she cries. Her heart rate and blood pressure
increases, her breathing normalizes, and all is well.
But in the SIDS victim, something goes wrong. Why?
Biological cause of SIDS
Brain stem abnormalities
Postmortem analysis suggests that SIDS victims may suffer from
abnormalities of the brain stem, the part of the brain that regulates
breathing and sleep (Mitchell 2009; Randall et al 2013). For instance, studies have reported that victims of SIDS had
more abnormalities in the serotonin pathways of the medulla, the lower
portion of the brain stem (Paterson et al 2006; Randall et al 2013). Serotonin is a
neurotransmitter that directs all sorts of basic functions, including
heart rate, breathing, gasping, appetite, body temperature, and vomiting.
Researchers speculate that an abnormal serotonin system prevents some
babies from awakening when their breathing is obstructed. It may also interfere with the effectiveness of the gasp reflex (Garcia et al 2013).
A second hypothesis--not incompatible with the brain stem hypothesis--is that a fatal SIDS event can be triggered by “re-breathing asphyxia,"
which is what happens when a baby keeps re-breathing the carbon dioxide
that he exhales (Paluszynska et al 2004). Re-breathing asphyxia would
be a risk whenever babies sleep on their stomachs—and particularly when
babies are sleeping on soft bedding that can trap gases. Re-breathing
asphyxia might also occur if the baby’s face is covered or pressed up
against a pillow, duvet, or heavy blanket.
Other biological risk factors:
• Prenatal exposure to smoke and/or alcohol. Babies whose
mothers smoked or drank alcohol during pregnancy are more likely to
suffer from abnormalities in the serotonin system. They also have higher
SIDS rates (Fifer et al 2009).
• Infection. SIDS rates are higher among babies recovering from a respiratory or intestinal infection (Horne et al 2002; Platt et al 2000).
• Sleep deprivation.
Experiments have shown that babies exposed to short-term sleep
deprivation experience more sleep apnea and are harder to arouse from
sleep (Franco et al 2004). This suggests that sleep deprivation might
trigger SIDS, an idea that is consistent with studies showing that SIDS
victims sleep significantly less in the 24 hours preceding death than do
controls (Blair et al 2000).
• Preterm birth. Premies
and low-birth weight babies are at higher risk of SIDS than are
full-term babies. This may reflect the fact that premies have more
trouble arousing from quiet sleep, the infant equivalent of adult “deep"
sleep (Horne 2006).
• Sex. Boys are more likely to
die of SIDS than are girls (American Academy of Pediatrics Task Force
on Sudden Infant Death Syndrome 2005). Possibly, this reflects sex
differences in the frequency of brain stem abnormalities (Paterson et al
2006) or the effects of testosterone on the immune system (Moscovis et al 2014).
• Genes. Perhaps some genotypes make
babies more vulnerable to SIDS (Nonnis Marzano et al 2008). This seems
plausible, but the research has only just begun, and the genetic
contributions to SIDS risk are not yet understood. For example, the
Serotonin System FEV Gene affects the serotonin pathways of the brain.
One study showed that victims of SIDS were more likely to carry a
mutation in this gene than were controls (Rand et al 2007). However,
these correlations held for African Americans only. Among Caucasians,
there was no link between SIDS and the FEV gene.
Understanding environmental factors that affect arousal—and the risk of SIDS
Studies have identified several environmental risks factors for SIDS. Here’s what we know about the most important of them.
Smoking and alcohol
As noted above, parental smoking and maternal alcohol consumption are important
environmental risk factors for SIDS.
For example, prenatal exposure to cigarettes and alcohol increases the risk of SIDS, perhaps because such exposure alters the
development of the serotonin system (Kinney et al 2003; Duncan et al
2008) and causes defects in the lungs and nervous system (Lavezzi et al 2014).
Postnatal smoke exposure is also associated with higher
SIDS rates. Babies exposed to second hand smoke are harder to arouse
from sleep, and they experience fewer spontaneous arousals (Horne et al
2004; Richardson et al 2009).
The smoking-related risk of SIDS
appears to be magnified for babies born preterm (Schneider et al 2008)
and for babies who share beds with smokers (Horsley et al 2007).
SIDS rates are higher among babies who sleep on their stomachs. Why
might the prone sleep position contribute to SIDS? Experimental research
suggests that prone sleep can trigger several problems, including an
impaired ability to arouse from sleep (Horne et al 2001), rebreathing
asphyxia (Paluszynska et al 2004), and a fall in blood pressure
(Yiallourou et al 2008).
SIDS case studies have revealed that sleeping prone is riskier in
colder latitudes, higher altitudes, and during the winter. The prone
position is also riskier when an infant is over-bundled and/or sweating
(Mitchell 2009). Such findings suggest that overheating plays a role in
Why would high temperatures matter? Perhaps the brain has
more trouble awakening when it’s overly warm. A study of young mice
revealed that overheating, when combined with reduced oxygen levels,
could interfere with an individual’s ability to resuscitate himself
(Kahraman and Thach 2004).
In addition, an experiment on human
babies found that room temperatures of 82 degrees Farenheit / 28 degrees
Celsius were associated with fewer arousals than were temperatures of
75F/24C (Franco et al 2001).
Covered heads or faces
In a recent analysis of published SIDS studies, Peter Blair and
colleagues found that head covering is a major SIDS risk factor (Blair
et al 2008). Indeed, these studies suggest that if we can prevent head
covering, we might reduce the risk of SIDS by over 25%. Why?
may make babies hot. They may also trap exhaled gases, putting the baby
at risk for rebreathing asphyxia. In addition, covers seem to affect
the way baby’s sleep. When something covers a baby’s face—even something
as sheer as a bed sheet—the baby is less likely to arouse from sleep in
response to noise (Franco et al 2002).
Dangerous sleep surfaces
SIDS case studies suggest that sofas, armschairs, and waterbeds are
particularly dangerous places for babies to sleep (e.g., Tappin et al
Soft mattresses are also considered hazardous. If a baby
sleeps with his face turned against a soft mattress, his exhalations
could create a dangerous accumulation of CO2, triggering rebreathing
asphixia and possibly SIDS (Colditz et al 2002).
Soft bedding and toys
SIDS research suggests that soft bedding—including pillows, bed
sheets, towels, blankets, and duvets—can cause asphyxiation and/or
overheating (e.g., Hauck et al 2003; Kanetake et al 2003). As noted
above, they might also interfere with a baby’s ability to arouse from
Bed sharing with young infants
Some studies of SIDS deaths in Western countries have reported a
correlation between SIDS and bed sharing in babies under 20 weeks old
(Tappin et al 2005; Carpenter et al 2004; McGarvey et al 2006). In
response, several agencies have issued general recommendations against
bed sharing for young infants (e.g., Task Force on Sudden Infant Death Syndrome 2011).
However, it’s not clear why bed sharing per se--as
opposed to specific bed sharing practices--would increase the risk of
SIDS. Because babies experience more spontaneous arousals when
they sleep with their mothers, there is reason to suspect that bed
sharing--practiced with safeguards by sober, nonsmoking parents--could help
protect babies from SIDS (Mosko et al 1997; Mao et al 2004). For babies in the Western studies, it seems this protective effect was counteracted by other risk factors.
What is very clear is that babies who bed-share are at a much greater risk of SIDS when they are exposed to second-hand smoke.
In addition, common sense suggests that bed sharing is dangerous when the sleeping environment includes known hazards, like soft mattresses or loose bedding.
Research also indicates that SIDS risk is increased when babies share a bed with
• someone who smokes, drinks, or who is impaired by illness or drugs (Horsley et al 2007)
• more than one other person, and/or with a non-parent (Hauck et al 2003)
• someone who is very tired (as when the infant's longest sleep bout in the last 24 hours was 4 hours or less-—Blair et al 1999)
can pose other hazards, too--as when an exhausted caregiver falls
asleep on a sofa or armchair while holding an infant.
shared sleep surfaces are very dangerous indeed. In one study, the risk
of mortality increased 60-fold for babies cosleeping on sofas (Tappin et
For more information, see this article
bed sharing and safety.
Research has identified a number of things to avoid. Are there any
positive steps we can take to reduce the risk? Perhaps. Studies suggest
that several practices--including breastfeeding, proximal sleeping,
pacifiers, and even electric fan use—might have a protective effect. For
more information, see this
evidence-based article on lowering SIDS risk.
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Content of "What is SIDS?" last modified 2/14