Don’t worry, I haven’t suddenly turned into a conspiracy theorist 😉 . I’m talking about a new systematic review which appeared in the May issue of Archives of Disease in Childhood. It identifies head-covering during sleep as a major risk factor for SIDS.
While the article notes that bedsharing babies who die of SIDS are less likely to be found dead with their head covered, another review of some of the literature published by some of the same research team members last month in Pediatrics points out (emphasis mine):
In the New Zealand study but not GeSID, bed sharing was associated with a decreased risk for being found with the head covered. Head covering during co-sleeping is common. Baddock et al reported that there were 102 head-covering episodes for the 22 infants studied. The majority of head covering during co-sleeping was by inadvertent movements by the adults as they changed position during sleep. Sixty-eight percent of head uncovering was facilitated by the mother, and half of these events were prompted by the infant. It is possible that for SIDS cases, when mothers wake up, they uncover the infant’s head. They then discover that the infant has died, thus reporting the low incidence of head covering while bed sharing.
The three Baddock et al studies from New Zealand, are a very interesting read. Each examines a certain aspect of an experiment comparing 40 bedsharing mother-infant pairs vs. 40 cot-sleeping (cot being non-American English for crib) ones over two nights. While I wish I could tell you the conclusions from these studies coalesce into one big “A-ha!”, they don’t. The information gleaned from them is, nevertheless, quite valuable in providing some more puzzle pieces, so to speak, and will probably serve as an important basis and experimental model for future research.
The first study, Bed-sharing and the infant’s thermal environment in the home setting, describes the experiment’s setup: The 80 babies, all born at term and aged 0-6 months, were attached to various monitors recording heartbeat, blood oxygen saturation, body (rectal and shin) temperature, respiratory pattern and air quality around the infant’s nose, and electrocardiogram. The babies and their mothers/parents were also recorded by video all night long. 40 of those babies shared a bed with their parents, all of whom expressed ideological reasons for doing so rather than ‘reactive bedsharing’. The other 40 were kept in cots in the parents’ bedroom.
The main conclusion of this particular study was that bedsharing infants tend to sleep in a warmer environment than cot sleepers, but they tolerate this well and do not overheat. However, I found table 5 in the study to be most informative (click on the thumbnail to see a screenshot of the table):
Bedsharing infants had their faces covered for X5.62 the time period that the cot-sleeping babies did. They also woke up more often, fed more frequently during the night and were awake for a longer period of time (though this last prameter was not statistically significant).
The second study, Differences in Infant and Parent Behaviors During Routine Bed Sharing Compared With Cot Sleeping in the Home Setting, also elaborates upon this in the abstract:
Head covering above the eyes occurred in 22 bed-sharing infants and 1 cot-sleeping infant. Five of these bed-sharing infants were head covered at final waking time, but the cot-sleeping infant was not.
The study further divulges that most of the covering happened as a result of the parents shifting in bed. Uncovering was usually performed by the mother; most often, the impetus for this was the baby’s movements (see Table 4).
There is also a discussion regarding the fact (also seen in Table 5 of the first study) that most of the bedsharing infants spent the large part of the night in the side-lying position, rather than the recommended supine position, as do most of the cot-sleepers. Baddock et al tend to think that side-sleeping might not be as dangerous for bedsharers as compared to non-bedsharing babies, as the typical mother-infant bedsharing position tends to prevent the baby rolling from side to prone sleeping. However, one infant out of 23 side-sleepers in this study did, in fact, roll into the prone position during the night.
The third study, Sleep Arrangements and Behavior of Bed-Sharing Families in the Home Setting, focuses on the bedsharing arm of the experiment, consisting of 40 bedsharing infants and mothers. We learn that the mothers are predominantly of Anglo-Saxon origin, most are highly educated, and all breastfed. 3 out of the 40 mothers (7.5%) smoked, despite the admonition not to bedshare if one smokes. None of the mothers drank alcohol to excess.
Some more interesting tidbits:
1) In most cases of bedsharing, fathers were absent. (Do fathers count in the “family” part of the “family bed”? Doesn’t seem like it.)
2) In 2 of the 4 instances the bed was shared by siblings, a sibling spent time sleeping next to the baby – also a no-no according to safe cosleepng rules.
I was intrigued that despite the fact that various measurements of the infants’ physiological states (pulse oximetry, ECG, carbin dioxide levels around the babies’ noses) were ,obviously, painstakingly gathered, none were provided except for the thermal measurements and an offhand reference to “no oxygen desaturation events <90% with head covered (data not given) …was observed” in the second study. So I contacted Dr. Baddock via email asking her about this, and she was kind enough to respond with the following (emphasis mine):
I have previously presented to the International SIDS Conference an
analysis of episodes during the night when the inspired CO2 was elevated
to at least 3%. Analysis of 80 of these events showed they were
associated with an increase in the frequency of breathing and that
oxygen saturation was maintained. In the majority of events the increase
in CO2 was associated with covering of the face. The 80 events were from
22 bedshare infants and one cot sleep infant. Our suggestion is that
normal healthy infants maintain oxygen saturation when the head is
covered through increasing ventilation but we hypothesize that infants
with impaired cardiorespiratory control may not do this successfully.
While desaturation below 90% was recorded on many occasions during the
night, the level of CO2 for the preceding 30 seconds was seldom above
1%. I have not linked these events specifically to the time scale on the
video, but the low CO2 values would suggest these desaturations were not
associated with head covering. They were preceded by central apnea and
were twice as common in bedshare infants.
The episodes of central apnea more common iin bedsharing infants look like McKenna’s findings from 1998; it would be interesting to know if the apneas precipitated other physiological events such as bradycardia, and how long the episodes were.
Those “infants with impaired cardiorespiratory control” who may have trouble responding to a respiratory challenge such as head covering seems to jive with the recently-popularized SIDS/serotonin theory (emphasis mine):
Researchers examined brain tissue from SIDS victims and found that their serotonin receptor binding was lower than normal in the brain stem, an area that helps control vital functions like breathing.
More recently scientists found that SIDS cases were more likely to have a certain variation of a gene that produces the serotonin transporter. This cell component can pump the serotonin back into the brain cell to mute the messaging process. The researchers suspect that people with the variation harbor more effective transporters than other people. Preliminary examinations of brain stem tissue also suggest that some SIDS victims have an excess of these more effective serotonin transporters.
Together these results could mean that the serotonin communication system in some infants does not work properly, perhaps sending out fewer messages than normal. Possibly the faulty system prevents children from responding to life-threatening events during sleep, such as increased levels of carbon dioxide, a harmful waste product eliminated by the lungs during breathing. Babies can experience excessive levels of the gas when they rebreathe air trapped in bedding, for instance. Normally, the serotonin system may help sense the problem and trigger mechanisms that increase breathing to expel the carbon dioxide.
Animal research supports this idea. For example, scientists discovered that normally an increase in carbon dioxide strongly stimulates cells that contain serotonin in the brain stem. Also early evidence indicates that a drug used for depression, which inhibits the transporter’s activities and increases messaging in the serotonin system, enhances the response of rats to carbon dioxide. In ongoing research, scientists also find that they can decrease an animal’s response to carbon dioxide by killing cells that contain serotonin in the brain stem. In addition, mice bred to lack most of their serotonin cells have abnormal breathing and some die during infancy.
So there you go…no definite conclusions, but these seem to me to be some very few pieces of the great puzzle that SIDS/SUDI currently is that can be put together : A respiratory challenge such as head covering or prone sleeping (though by no means exclusive to those two mechanisms!) to an infant who is, due to a biochemical brain imbalance, unable to respond sufficiently to clear his airway. This would help to explain why small babies who bedshare are at risk for SIDS/SUDI, even if precautions are taken. And also that even “ideological” bedsharing parents are not completely immune to dangerous cosleeping…
Filed under: Infant sleep |