Sudden infant death syndrome and brainstem abnormalities

Many aspects of our lives are more or less governed by fashions, and parenting is no exception. From the rather dry mode of parenting of our grandparents (Babies fed and changed? Then let them cry themselves to sleep) to some of the extremes of the “attachment parenting” of today, parenting styles have varied together with recommendations of the medical community, traditions, innovations and social trends. In some cases, different school of thoughts may coexist: pacifier or no pacifier, baby in recliner next to parent or in a wrap carrier on parent, etc. In others, change should be readily embraced as a crucial step to improve babies’ health and safety. A baby’s sleep position belongs to the latter category.

I have often heard my mother say that recommendations from the medical community to put babies to bed in a prone position (lying on their belly) was one of the stupidest things told to young parents in the 80’s. She could not have been more right. A prone sleeping position was recognized as a risk factor for sudden infant death syndrome (SIDS) in the 90’s, and parents were advised to put babies to bed in a supine position (lying on their back).

  • Sudden infant death syndrome

SIDS is defined as the sudden and unexpected death of a 1 month- to 1 year-old baby, with death apparently occurring during sleep and remaining unexplained even after thorough investigation (complete autopsy and review of the circumstances of death). The reported SIDS rate in the US is about 6 in 10,000 and SIDS accounts for about 20% of all post-neonatal deaths (1,2). Often, asphyxia is identified as the potential cause of death, however clear situations of asphyxia such as strangulation, wedging, or getting trapped in bedding are witnessed in only a small fraction of cases, and the reason why babies stopped breathing in the rest of the cases remains unknown.

There is now ample epidemiological data showing that, as babies are increasingly put to bed in a supine position (on their back), rates of SIDS have decreased worldwide. Other sleep conditions are now also known to increase the risk of sudden infant death: these include head covering, sleeping on an adult mattress, soft or excessive bedding, bed-sharing (co-sleeping), and mild upper respiratory infection. What these conditions have in common is that they can lead to asphyxia, hypoxia (low oxygenation), or homeostatic stress (for example overheating), and potentially cause the baby to stop breathing.

Sudden, unexpected, and unexplained infant death may also occur in apparently safe sleeping conditions (about 10% of SIDS cases). What happened then? Why did the baby stop breathing? It is of course possible that the investigation looking into the circumstances of death may have missed something, but recent research suggests that biological abnormalities in the brainstem might play a role.

  • Sudden unexpected infant death and serotonin

Normally, when babies have trouble breathing while asleep (for example because of a cold), they will wake up, gulping for air. The serotonin-related network in the brainstem is an important component of these kind of protective respiratory and autonomic responses to sleep-related asphyxial or homeostatic stress. Deficiencies in one or more of the brain chemicals involved in these responses may prevent babies from automatically waking up when they’re not getting enough oxygen.

In a study (3) published in Pediatrics (the official journal of the American Academy of Pediatrics) in November, researchers led by Hannah Kinney from Boston Children’s Hospital and Harvard Medical School compared a number of neurochemical parameters in the brainstem of babies who had died suddenly in their sleep from unknown reasons with those of babies who had died suddenly from known causes (accident or natural cause such as congenital heart disease or pneumonia). They found that the babies who had died suddenly, unexpectedly, and without any explanation, had neurochemical abnormalities in the brainstem serotonin network compared with babies who had died from known causes. These results confirmed the findings from previous work published in 2006 and 2010.

In the current study, the researchers also tried to go one step further in their analysis and asked if there could be a gradation in brainstem abnormalities that would relate to how susceptible babies were to asphyxial stressors. They therefore classified the cases of sudden, unexpected, and unexplained deaths in different groups based on the presence or absence of potential asphyxia-related conditions, and then compared the levels of certain neurochemical parameters (belonging to the brainstem serotonin network) between these two groups of babies. They found no difference: both groups had abnormal levels of the parameters studied compared to control infants who had died of known causes, but babies who had been found dead in safe sleeping environments did not show worse abnormalities than babies who had been found dead in the presence of potential asphyxial stressors.

So what could have triggered the death of those babies even in safe sleeping conditions? Since the classification of sleeping environments was based on death scene investigation records, the researchers point out that some details may have been overlooked or may be missing from the reports, leading to an inaccurate classification of death cases in the study. However, it is also very likely that other unknown factors contributed to the infants’ sudden deaths, be it abnormalities in other neurochemical parameters not analyzed in the study, or other environmental conditions not recognized as risk factors and/or not recorded.

We certainly know very little about sudden infant death, but studies like this begin to shed some light. Hopefully, research will improve our understanding of what precipitates death, and allow the detection of infants who are particularly at risk as well as the development of prevention strategies.

In the meantime, since we do not what babies are at risk of SIDS, and which ones are more susceptible than others, simple measures ensuring safe sleeping conditions should be taken. The American Academy of Pediatrics published in 2011 a list of recommendations to help prevent sleep-related sudden infant deaths. These include a supine sleeping position, a firm sleep surface, breastfeeding, room-sharing without bed-sharing, routine immunizations, and avoidance of soft bedding, overheating, and exposure to tobacco smoke. The full list of recommendations, with details and references, can be found here (4).

References

1. International trends in sudden infant death syndrome: stabilization of rates requires further action. Hauck FR, Tanabe KO. Pediatrics. 2008 Sep;122(3):660-6. doi: 10.1542/peds.2007-0135
PMID: 18762537
(Free access)

2. The sudden infant death syndrome. Kinney HC, Thach BT. N Engl J Med. 2009 Aug 20;361(8):795-805. doi: 10.1056/NEJMra0803836
PMID: 19692691

3. Potential Asphyxia and Brainstem Abnormalities in Sudden and Unexpected Death in Infants. Randall BB, Paterson DS, Haas EA, Broadbelt KG, Duncan JR, Mena OJ, Krous HF, Trachtenberg FL, Kinney HC. Pediatrics Vol. 132 No. 6 December 1, 2013 pp. e1616 -e1625. doi: 10.1542/peds.2013-0700
PMID: 24218471

4. SIDS and other sleep-related infant deaths: expansion of recommendations for a safe infant sleeping environment. Task Force on Sudden Infant Death Syndrome, Moon RY. Pediatrics. 2011 Nov;128(5):1030-9. doi: 10.1542/peds.2011-2284
PMID: 22007004
(Free access)

ResearchBlogging.orgRandall BB, Paterson DS, Haas EA, Broadbelt KG, Duncan JR, Mena OJ, Krous HF, Trachtenberg FL, & Kinney HC (2013). Potential asphyxia and brainstem abnormalities in sudden and unexpected death in infants. Pediatrics, 132 (6) PMID: 24218471

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