Interaction between bedding and sleeping position in the sudden infant death syndrome: a population based case-control study

Title: Interaction between bedding and sleeping position in the sudden infant death syndrome: a population based case-control study

Authors:
Peter J Fleming (corresponding author), FRCP
Ruth Gilbert, MRCP
Yehu Azaz, MRCP
P Jeremy Berry, MRCPATH
Peter T Rudd, MD
Alison Stewart, SRN
Elizabeth Hall, FRCPATH

Summary: This study analyzed the supine (back) sleep position and the quantity of bedding put on an infant prior to sleep. The objective was to determine in relation between the quantity of bedding and sleep position and the risk of Sudden Infant Death Syndrome (SIDS). The study analyzed 72 infants who had died suddenly and unexpectedly (of which 67 eventually were deemed to have SIDS) and 144 control infants.

Key Findings:
* 10.45% of the infants that died of SIDS were born premature.
* 2.99% of the control infants were born premature.
* 4.48% of the infants that died of SIDS were born low birth weight.
* 3.73% of the control infants were born low birth weight.
* The sleep position data for the study infants (n=216) was collected in Avon and Somerset between November 1987 and April 1989.
* The sleep position data for the ALSPAC infants (n=2,395) was collected in Avon between June 1991 and October 1991.
* The control infants in the study were selected by Health Visitors who selected 2 infants from their caseload in the neighborhood that were closest in age to the infant that died of SIDS.
* 17.2% of the control infants in the study slept supine (Nov. 1987 to Apr. 1989).
* 1.5% of the infants that died of SIDS slept supine (Nov. 1987 to Apr. 1989).
* 2.8% of the infants in the ALSPAC sample slept supine (Jun. 1991 to Oct. 1991).
* The supine sleep rate is greater than 500% lower than the supine sleep rate of the control infants in the study. Considering the late 1980's and early 1990's was a time when the supine sleeping recommendations were first being reported in the media it is perplexing that a 500% decrease would occur in the area of Avon during this time. But, it's also possible that there was an inherent bias in the way the Health Visitors selected the controls.
* The authors conclude that overheating and the prone sleep position are independently associated with an increased risk of an infant dying of sudden infant death syndrome and that this is particularly true for infants more than days old.
* The day an infant died the parents were contacted by their General Practitioner and a health visitor. The Health Visitor was then asked to select two infants in the same neighborhood that were closest in age to the infant that died of SIDS. This selection process has a built in bias in that it is possible that the Health Visitor was biased towards selecting controls from parents that were easier to interact with socially.
* The mean age of the infants that died of SIDS was 94.4 days and the mean age of the control infants was 97.0 days. For the control infants the mean age was within 10 days for 65 of the infants that died of SIDS. For 35 of the infants that died of SIDS the mean age was within 3 days.
* Results:
DATE: November 1987 to April 1989
LOCATION: Avon and Somerset
Infants that died of SIDS (n=67)
Prone = 62 (92.5%)
Side = 4 (6.0%)
Supine = 1 (1.5%)
Unknown = 0 (0.0%)
Control Infants (n=134)
Prone = 76 (56.7%)
Side = 32 (23.9%)
Supine = 23 (17.2%)
Unknown = 3 (2.2%)
ALSPAC Sample
DATE: June 1991 to October 1991 (Prior to the SIDS Back to Sleep Campaign began in the UK but after this study was published)
LOCATION: Avon (ALSPAC data)
n= 2,395
Prone= 9.4%
Side= 81.6%
Supine= 2.8%
Key Finding: The Control infants in the 1990 study (collected between Nov. 1987 to Apr. 1989 data) are completely different from the ALSPAC data collected between June 1991 and October 1991. For the control infants in the article 17.2% of infants yet in the ALSPAC data 2.8% of the infants slept supine. This is curious since if anything it would seem there would be no difference or that the supine sleep position results would be higher than 17.2%. This suggests that there was a 500% decrease in the supine sleeping position between 1987 and 19991 at a time when the medical community was beginning to report that the supine sleep position was theoretically supposed to prevent SIDS. This seems highly unlikely. What seems much more likely is that the selection of the Controls in the 1990 study was somehow biased to include more supine sleeping infants than simply by chance.
* Some authors in subsequent letters to the editor suggested recall bias could have influenced the results. That is that it is possible that some parents may have told the investigator that their baby slept on it's stomach since much of the medical advice up until this study advised parents to put their infants to sleep on their stomachs. The parents of the control infants would have much less of a need to tell investigators this since their infants had not died. My suspicion is that the Health Visitor tended to pick as control the two infants from her case load that had the most easy to interact with parents. It is possible that these parents were more attuned to current medical advice and news and that a disproportionate number of them were putting their infants to sleep on their backs. This is just an hypothesis.
Key Findings (From Subsequent Letter to the Editor):
Number (percentage) of infants who were born either pre-term or low birth weight
The following data is from a letter in BMJ (Volume 301, September 8, 1990) and is in response to a letter to the editor by a Dr. Englebert and a Dr. de Jonge. The letters authors are PJ Fleming, PJ Berry, and Ruth Gilbert.

I took the results out to 2 decimal points and here is how I report them.
Infants that died of SIDS (n=67)
<35 weeks: 3(4.48%)
35-36 weeks: 4 (5.97%)
<2500 g: 3 (4.48%)

Control Infants (134)
<35 weeks: 1 (0.75%)
35-36 weeks: 3 (2.24%)
<2500 g: 5 (3.73%)

Infants who were born pre-term consisted of 10.45% of the infants who died of SIDS. For the control sample, infants that were born pre-term consisted of 2.77% of the sample. This is a difference of 7.46%.

Infants that were born low birth weight consisted of 4.48% of the infants who died of SIDS. For the control sample, infants that were born with low birth weight consisted of 3.47% of this sample. This is a difference of 0.75%.