Used Infant Mattresses and Sudden Infant Death Syndrome (SIDS) in

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					Used Infant Mattresses and Sudden Infant Death Syndrome (SIDS) in

Scotland. A Case-control study.

David Tappin*, Hazel Brooke+, Russell Ecob~, Angus Gibson#.

Clinical Senior Lecturer*, Executive Director+ & Chairman#, Research Statistician~.

Scottish Cot Death Trust, Royal Hospital for Sick Children, Glasgow, G3 8SJ.

Correspondence to and reprints from Dr D. M. Tappin, Paediatric Epidemiology and

Community Health (PEACH) Unit, Department of Child Health, Royal Hospital for

Sick Children, Glasgow, G3 8SJ

email:, tel: 0141 201 0176, fax: 0141 201 6943

Running heading: Used Infant Mattresses and SIDS

Text 1996 words plus references.

Objective This new study examined the proposition that a used infant mattress is
associated with an increased risk of Sudden Infant Death Syndrome (SIDS) in Scotland.

Design Case-Control study.

Setting Scotland, UK, population 5.1 million, with approximately 53,000 births per

Participants 131 infants who died of SIDS between 1/1/96 and 31/5/00 and 278 age,
season and obstetric unit matched control infants.

Main outcome measures Routine and last sleep use of an infant mattress previously
used by another child.

Results Routine use of an infant mattress previously used by another child was
significantly associated with an increased risk of SIDS (multivariate OR 3.07, 95% CI

1.51, 6.22). Last sleep use of a used infant mattress was also associated (multivariate

OR 6.10, 95% CI 2.31, 16.12).

Conclusion A valid significant association exists between use of a used infant mattress
and an increased risk of SIDS particularly if the mattress is from another home.

Insufficient evidence is available to judge whether this relationship is cause and effect.

Keywords: child care, sudden infant death/ep, beds, case-control studies


In 1997 we reported a case-control study of Sudden Infant Death Syndrome (SIDS) in

Scotland 1992-5, focusing on infant care practices.1 This study suggested that an infant

mattress previously used by another child (hereafter referred to as used infant mattress)

was associated with an increased risk of SIDS. This paper reports new case-control data

from Scotland from 1/1/96 to 31/5/00 using a revised questionnaire. The aim is to

examine the null hypothesis that a used infant mattress is not associated with an

increased risk of SIDS.


Ethics and funding
Ethics approval was given by ethics committees for all 15 Health Board areas in

Scotland. The Scottish Cot Death Trust provided funding.

The Registrar General for Scotland reported to us all infant deaths occurring after the

seventh day of life to the end of the first year. The computerised maternity record was

provided. For sudden unexpected deaths our office was notified directly by the

pathologist. Consistent classification was sought by using a standard necropsy protocol

with agreed diagnostic criteria.2 All infants deaths were scrutinised for possible

misclassification. Overall, 195 out of 751 postperinatal infant deaths were categorised as

SIDS between January 1996 and May 2000.

We identified two controls for each SIDS case - the births immediately before and after

the index case in the same maternity unit. Controls were therefore matched for age,

season, and maternity unit. Home visits to complete a questionnaire were made within

28 days of the index case's death to minimise differences in age related circumstances

between cases and controls. Questionnaires were completed on 131 of 195 cases and

278 controls. Failure to acquire data on 64 cases was due to delay in notification by the

pathologist making a visit within 28 days of the death impossible. The characteristics of

the cases with and without an interview were compared and were similar in terms of

maternal age and deprivation category.3

Data collection
The questionnaire provided core medical and social data, as well as details of infant care

practices. The difference from the previous study1 was that data from control families
about child care practices on the day/night before interview were available to compare

with SIDS cases on the day/night of death (hereafter referred to as last sleep).
Socio-economic status was assessed by two sets of indicators. The first was assessment

of deprivation (DEPCAT) based on postcode of residence in seven categories in

ascending order of deprivation.3 The second was an individual assessment based on

mother’s marital status, mother living alone, mother and father currently employed, age

mother and father left full-time education, pre-birth mother not in paid employment.

Exposure to smoking was assessed using cigarettes smoked by mother, father and other

household members.

Co-sleeping information for last sleep included, where, for how long and with whom.

Exposure to a used infant mattress was assessed by asking parents about routine night

and day sleeping place for both cases and controls and ascertaining the status of the

mattress; if new for this baby, if not how many other infants had used it and whether or

not it came from another home. For cases the sleep place was also ascertained for the

last sleep whether day or night, and for controls the day and night immediately prior to


Data analysis
The full dataset included 131 cases and 278 controls (table 1 website version). Those

babies who routinely bed shared at night i.e. their place of sleep was the parental bed (14

cases and 13 controls) were excluded as they did not sleep on an infant mattress. The

main analysis was performed on this slightly reduced dataset (Dataset A, table 1) with

117 cases and 265 controls and used the random effects logistic regression procedure

(xtlogit) in the software package STATA,4 which allows the inclusion of all the data,

including those cases with no controls. All variables which were significantly (p<0.05)

related to SIDS on univariate analysis, were treated as potential confounding variables.

They were entered sequentially in groups (socio-economic, not easily modifiable,
possibly modifiable) into the logistic regression of case versus control and removed

singly in a stepwise manner starting with the least significant until all remaining
variables showed statistical significance with a probability of < 0.10. The next group of

variables was then added and examined similarly. Quadratic functions were included for

continuous and multicategorical variables and were retained if non-linear effects were

present. The model which best described the difference between cases and controls was

established without the used infant mattress variables (table 1) and included: DEPCAT,

maternal age and its quadratic function, infant age and its quadratic function, parity
linear, admission to neonatal intensive care (yes/no), sharing a bed couch or chair

during last sleep (yes/no), dummy use during last sleep (no/a little), mother only smokes,

both parents smoke and laid prone to sleep. To this model the routine used infant

mattress variable (yes/no) was added (table 1). We then forced into the model the
strongest other socioeconomic variable pre-birth mother not in paid employment.

Further analyses were made of the excess risk of 2 or more mattress users versus one

user, and used mattress other home versus used mattress same home. Sensitivity

analyses were performed by a) retaining only cases occurring on a Sunday to Thursday

(n=91) and their matched controls (n=205), as no designated last sleep for controls was

on a Friday or Saturday (all interviews being carried out Monday to Friday) and b)

restricting cases to those found on infant mattresses who had not shared at any stage

during their last sleep (n=44) and controls who had slept on infant mattresses on their

designated night and had not shared (n=205).

For Dataset B, using only cases (n=111) with one or more matched controls (n=202), a

multivariate model was built in the same way as the main analysis, but conditional fixed

effects logistic regression (clogit) was used to establish that the random effects method

had not allowed bias. The model included DEPCAT, maternal age, parity, intensive

care, sharing, dummy, mother smokes, both parents smoke, laid prone and laid on side.
Routine use of a used mattress was then added to the model (table 1 website version).


Table 1 shows the frequencies of all variables collected in the questionnaire. The main

analysis performed on dataset A showed a significant association between SIDS and

routine use of a used infant mattress OR 3.07 95%CI 1.93,5.06. When the variable pre-

birth mother not in paid employment was forced back into the multivariate model, the
association remained unchanged OR 3.04 95%CI 1.49,6.16. The association remained

when Friday/Saturday cases and controls were removed OR 3.31 95%CI 1.50,7.34 and

when the dataset was restricted to those on an infant mattress throughout last sleep OR

6.10 95%CI 2.31, 16.10. A stronger association was seen for two or more previous users

compared with one, but this difference did not reach statistical significance. With used

infant mattress from the same home as the comparison group, an infant mattress from

another home was significantly associated with SIDS OR 2.91 95%CI 1.02, 8.33. A
parallel multivariate conditional logistic regression with only matched cases and

controls confirmed a significant association OR 9.59 95%CI 2.10, 43.80.


While undertaking our previous case-control study1 we uncovered a new possible risk

factor for SIDS, a used infant mattress. This paper aims to test the null hypothesis that a

used infant mattress is not associated with an increased risk of SIDS, based on new data
collected in Scotland from January 1996 to May 2000.

The first step is to assess if a valid statistical association exists, that chance, bias and

confounding are unlikely alternative explanations. As the focus of this study was on two

issues, sharing a bed couch or chair during last sleep and use of a previously used

infant mattress, a spurious finding of statistical significance because of multiple
statistical tests could not arise. Used infant mattress rejects the null hypothesis of no

association with a p value of less than 1 in 1000. A chance finding is therefore unlikely.

Bias is possible where the choice of cases and controls is affected by a systematic error.

Only 131 of 195 cases were interviewed due to slowness of notification by pathologists

resulting in no interview within 28 days of death, an inclusion criterion. However, cases

interviewed and not interviewed were not significantly different in terms of socio-

economic status measured by DEPCAT or in terms of maternal age. Controls were the

births just before and after the case in the same hospital. A systematic difference in

infant age for questions about last sleep (night before interview for controls, night of

death for cases) was present (mean 18.5 days). The multivariate random effects model

included a continuous linear variable for infant age and a quadratic function to cope

with non-linearity, so it is unlikely that this source of bias explains the association.

When cases where death occurred on a Friday and Saturday were excluded because no

designated last sleep for controls was on a Friday or Saturday, the association remained.

Finally a parallel conditional logistic regression using only cases with matched controls,
and so controlling for any possible bias due to systematic differences in the cases for

which controls did or did not exist, showed the same association. A number of possible

sources of bias to explain the observed association have been ruled out.

Confounding from socio-economic status is the most obvious possibility with SIDS and

use of a used infant mattress. DEPCAT a non-questionnaire information item was the

most resilient socio-economic variable when constructing the multivariate models. As

this was the only socio-economic variable which remained prior to introduction of used

infant mattress, we attempted to force in a further strong variable pre-birth mother not

in paid employment. This did not change the significance of used infant mattress which
argues against the presence of residual socio-economic confounding. Other obvious

confounding items such as parity and bedsharing on the night of death were in the

multivariate model. Restriction of the dataset to look at used infant mattress during last

sleep in those not sharing during last sleep removed any bed sharing effect, but the
association remained.

Chance, bias and confounding have been examined and the findings are relatively robust

over a range of different statistical approaches. We conclude that there is a valid

statistical association between use of a used infant mattress and SIDS.

The next stage is to examine cause and effect. The association is relatively strong OR 3,

the time sequence of use of mattress and death is compatible, toxigenic bacteria,

implicated in SIDS5,6, do reside in used infant mattresses7. However, a dose response

was not demonstrated and consistency with other studies is not established, as the only

other study with similar information did not show a link between ‘old’ infant mattresses

and SIDS8.

In conclusion, a valid statistical association exists between SIDS and use of an infant

mattress previously used by another child particularly if from another home. We are
therefore able to reject the null hypothesis. There remains insufficient evidence to

establish a cause and effect relationship.

1. Brooke H, Gibson A, Tappin D, Brown H. Case-control study of Sudden Infant Death

Syndrome in Scotland 1992-5. BMJ 1997:314:1516-20.

2. Gibson AAM. The use of a standard SIDS post mortem protocol in Scotland: two

                Third SIDS international conference, Stavanger. Norway, 1994. Oslo:
years' audit. In:

Norwegian SIDS Society, 1994.

3. Carstairs V, Morris R. Deprivation and health in Scotland. Aberdeen: Aberdeen

University Press, 1991.

4. STATA (2001) Stata7 User’s Guide, Release 7, Stata Press, college Station, Texas.

5. Blackwell CC, Saadi AT, Raza MW, Stewart J, Weir DM. Susceptibility to infection

in relation to SIDS. J Clin Pathol 1992; 44(II Suppl): 20-4.

6. Blackwell CC, MacKenzie DA, James VS, Elton RA, Zorgani AA, Weir DM,

Busuttil A. Toxigenic bacteria and sudden infant death syndrome (SIDS):

nasopharyngeal flora during the first year of life. FEMS Immunol Med Microbiol 1999;

25: 199-206.

7. Jenkins R. Cot mattresses: a reservoir of toxigenic bacteria. Proceedings of the

Scottish Cot Death Trust grant holders meeting. March 2002.

8. Sudden Unexpected Death in Infancy. CESDI SUDI studies 1993-1996. Eds. Fleming

P, Bacon C, Blair P, Berry PJ. The Stationary Office London 2000. pp 50-1.

This week in the BMJ

Previously used infant mattress is associated with increased risk of cot death.

Sleeping routinely on an infant mattress previously used by other children may increase

the risk of Cot Death. A new study carried out by researchers from the University of

Glasgow and The Scottish Cot Death Trust confirms previously published findings that

there is an association of some kind, regardless of other factors, between use of a

previously used infant mattress and Cot Death. A used infant mattress from another

home showed a significantly stronger association than a mattress used by siblings in the

same home. The researchers found that the associated risk was particularly high if the

infant was sleeping on the used mattress at time of death.

What is already known about this topic

Placing infants to sleep on their front is established as a risk factor which causes Sudden

Infant Death Syndrome (SIDS). Most countries now advise parents to place their infants

to sleep on their the back – the Back to Sleep Campaign . Since the early 1990s, the

SIDS rate has halved worldwide associated with a reduction in prone sleeping.

Parental smoking, particularly maternal smoking during and after pregnancy, has also

been established as an important risk factor for SIDS. It is estimated that the present

SIDS rate worldwide would be halved again if maternal smoking during pregnancy was

eliminated. However effective interventions to help women stop smoking during

pregnancy have yet to be established.

The risk of prone sleeping has implicated the infant mattress as part of the explanation

for SIDS. A previous theory that toxic nerve gases were given off by infant mattresses

under certain conditions was not substantiated after careful repeated studies. However

bacteria such as staphylococcus aureus, which produce toxins, have been carefully

studied as a possible cause of SIDS. The foam matrix of previously used infant

mattresses may be a harbour for such bacteria. A previous case control study published

in this journal implicated a previously used infant mattress as a possible risk factor for


What this study adds

This study, based on new case control data, confirms that the association between a

previously used infant mattress and SIDS is valid. This association is significantly
stronger if the mattress is from another home, and remains when night of death is

compared with a designated control night and when cosleepers are removed. Insufficient

evidence is available to judge whether this relationship is cause and effect.

Contributorship Statement

The study was the idea of HB and AG who applied for ethics approval, secured funding,

constructed the questionnaire, coded the data and helped write the paper. DT helped

construct the questionnaire, code the data, worked closely with RE during the analysis,

and wrote the paper. RE analysed the data and helped write the paper. DT is guarantor.


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