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Sudden infant death syndrome in Canada:

Trends in rates and risk factors, 1985-1998

Vol. 25 No. 1, 2004

ID Rusen, Shiliang Liu, Reg Sauve, KS Joseph and Michael S Kramer

Abstract

In Canada, sudden infant death syndrome (SIDS) remains the leading cause of postneonatal death. However, SIDS rates have been declining in many countries, including Canada. This decline has been largely attributed to recommendations to avoid placing infants to sleep in the prone position. We examined the postneonatal rate of mortality due to SIDS and to other causes in relation to the initial risk reduction campaign. The postneonatal mortality rate due to SIDS decreased from 0.97 to 0.54 per 1,000 neonatal survivors between 1985-1989 and 1994-1998 (relative risk [RR] = 0.56, 95% confidence interval [CI] 0.51-0.62). The rate of postneonatal mortality due to other causes also decreased during the same period, though to a smaller extent, from 1.19 to 0.86 (RR=0.72, 95% CI 0.66-0.78). With the exception of seasonality, established risk factors for SIDS remained essentially unchanged between the two time periods. The observed reduction in postneonatal SIDS is consistent with a positive impact of the initial recommendations regarding risk reduction. However, the lack of reliable risk factor data limits the extent to which the decline can be attributed directly to the campaign.

Key words: postneonatal mortality; sleep position; sudden infant death syndrome

Introduction

Sudden infant death syndrome (SIDS) is the leading cause of postneonatal mortality in Canada.1 In 1999, 144 or 26% of all postneonatal deaths were caused by SIDS.1 However, the incidence of this syndrome has declined markedly in Canada and many other parts of the world.2 This decline has been largely attributed to recommendations made in the early 1990s against placing infants to sleep in the prone position.2 In 1993, Health Canada, the Canadian Paediatric Society, the Canadian Foundation for the Study of Infant Deaths and the Canadian Institute for Child Health released their first joint statement on reducing the risk of SIDS.3 The rate of decline in the Canadian SIDS rate has not been reported in relation to the recommendations and in comparison with rates of infant death due to other causes. Similarly, the consistency of this decline across Canadian provinces and territories has not been examined. Finally, various demographic, perinatal and other factors have been previously associated with an increased risk of SIDS in Canada.4 The relevance of these risk factors may have changed following adoption of the sleep position recommendations.

The purpose of this study was to examine trends in Canadian, provincial and territorial SIDS rates, as well as changes in the importance of various factors known to be associated with an increased risk of SIDS.

Methods

We used data on all live births from Statistics Canada's live birth database for the years 1985 to 1998 and data from the mortality database for 1985 to 1999. Information in these databases was obtained from birth and death registrations supplied by Canadian provincial and territorial registries of vital statistics.5 A probabilistic linkage was carried out using previously validated methods to link infant deaths to their respective birth registrations.6 Uncertain linkages were resolved after manual examination of the relevant birth and death registration documents. Infant deaths for which a birth record could not be found were noted as "unlinked deaths" and retained in the analysis.

Births to mothers residing in Ontario were excluded because of documented problems with data quality7 and large numbers of unlinked infant deaths. Births to mothers residing in Newfoundland were also excluded, because data from that province were not available at the national level prior to 1991.

The computerized files of Canadian infant death registrations list one underlying cause of infant death. In this study, postneonatal deaths were categorized as being due to SIDS (International Classification of Diseases, Ninth Revision [ICD-9] code 798.0) versus other causes of postneonatal death combined. Limiting the analysis to the postneonatal period (28-364 days of life) captured over 90% of SIDS deaths in Canada while eliminating the uncertain diagnosis of SIDS in the neonatal period.

To accurately assess the impact of sleep position recommendations, the appropriate comparison or control group would be postneonatal deaths due to causes with no known intervention during the study period. Previous analysis has demonstrated a marked decrease in infant deaths due to congenital anomalies during the period of interest, likely due to increasing use of prenatal diagnosis and termination of affected pregnancies.8 Congenital anomalies (ICD-9 codes 740-759) were therefore excluded from the "other causes of death" group.

Traditionally, postneonatal mortality rates are calculated as ratios by dividing the number of postneonatal deaths in a calendar year by the number of live births in the same (index) year. The postneonatal rates of mortality due to SIDS and due to other causes described in this study are based on birth cohorts and represent the proportion of infants who died in the postneonatal period among neonatal survivors in the year of interest. This cohort approach enabled the linkage of postneonatal deaths to risk factors of interest, such as maternal age and gestational age. However, this change in the method of computing postneonatal mortality means that the temporal patterns presented in this study may differ slightly from those reported elsewhere.

We examined the change in the postneonatal mortality rates due to SIDS and due to other causes between two time periods (1985-89 and 1994-98), selected to capture the periods before and after the release of the first Canadian joint statement on reducing the risk of SIDS. In addition to changes in national rates, we calculated changes in the provincial and territorial postneonatal SIDS rates. The significance of temporal changes was estimated using relative risks (RRs) and 95% confidence intervals (CIs).

We examined the population distribution of established demographic and perinatal risk factors for SIDS in the two time periods. The risk factors examined were limited to those available in the Canadian Birth Database. The distribution of these demographic and perinatal characteristics in each time period as well as the percentage change (and 95% CI) between 1985-1989 and 1994-1998 were calculated. We also examined potential changes in the significance of these risk factors for SIDS over the two periods. Both crude and adjusted (using full, unconditional logistic regression model) odds ratios were estimated for postneonatal deaths due to SIDS for each period. (Odds ratios obtained from logistic regression models were interpreted as relative risks as the rare disease assumption was satisfied.) Subsequently, again using logistic regression models, we examined the time period effect on postneonatal SIDS rates sequentially adjusted for key demographic and perinatal risk factors.

Finally, using the Canadian Mortality Database, we examined the proportion of all postneonatal SIDS deaths that occurred in each season (January-March, April-June, July-September, October-December) in the period before and after release of the first joint statement. Chi-square tests were used to determine the statistical significance of seasonal predominance in each of the two periods.

All analyses were carried out using SAS PC version 8 statistical software (SAS Institute, Cary, North Carolina).

Results

During the period 1985-1998, the Canadian postneonatal mortality rate due to SIDS and due to other causes decreased (Figure 1). Between 1985-1989 and 1994-1998, the postneonatal mortality rate due to SIDS decreased markedly from 0.97 to 0.54 per 1,000 neonatal survivors: RR = 0.56 (95% CI 0.51-0.62). The postneonatal mortality rate due to other causes decreased by a smaller magnitude during the same period, from 1.19 to 0.86: RR 0.72 (95% CI 0.66-0.78).

FIGURE 1
Postneonatal mortality rate (PNMR) per 1,000 neonatal survivors due to SIDS and other causes (excluding SIDS and congenital anomalies) in Canada (excluding Newfoundland and Ontario), 1985-1998

Note: Different scales have been used for PNMR due to SIDS and due to other causes to depict the divergent trends over time.


Examination of provincial and territorial postneonatal mortality due to SIDS revealed wide variation of rates in both periods (Table 1). In 1985-1989, Quebec had the lowest postneonatal SIDS rate at 0.47 (95% CI 0.41-0.55), and the Northwest Territories had the highest rate at 2.16 (95% CI 1.24-3.51). In 1994-1998, Quebec and British Columbia had the lowest postneonatal SIDS rates at 0.38 (95% CI 0.32-0.44) and 0.41 (95% CI 0.33-0.50) respectively. Yukon and the Northwest Territories had the highest rates in 1994-1998 at 2.26 (95% CI 0.73-5.26) and 2.18 (95% CI 1.24-3.53) respectively. Most provinces and territories experienced a decline in the postneonatal SIDS rate between 1985- 1989 and 1994-1998. However, the extent of the observed decline varied considerably (Table 1); the greatest decline was observed in British Columbia: RR 0.27 (95% CI 0.22-0.35). No province or territory experienced a statistically significant increase in postneonatal mortality due to SIDS between 1985-1989 and 1994-1998

TABLE 1
Trends in postneonatal SIDS deaths per 1,000 neonatal survivors, provinces and territories, Canada (excluding Newfoundland and Ontario), between 1985-1989 and 1994-1998

 

1985-1989

1994-1998

Relative risk of postneonatal death due to SIDS between 1985-1989 and 1994-1998
(95% CI*)


Postneonatal SIDS deaths

Postneonatal SIDS rate
(95% CI*)


Postneonatal SIDS deaths

Postneonatal SIDS rate
(95% CI*)

P.E.I.

8

0.82
(0.35-1.62)

4

0.49
(0.13-1.25)

0.59
(0.18-1.97)

Nova Scotia

72

1.18
(0.92-1.48)

25

0.48
(0.31-0.71)

0.41
(0.26-0.65)

New Brunswick

45

0.93
(0.68-1.24)

26

0.63
(0.41-0.92)

0.68
(0.42-1.10)

Quebec

200

0.47
(0.41-0.55)

157

0.38
(0.32-0.44)

0.80
(0.65-0.98)

Manitoba

63

0.74
(0.57-0.95)

50

0.65
(0.48-0.86)

0.86
(0.60-1.25)

Saskatchewan

94

1.10
(0.89-1.34)

61

0.92
(0.71-1.19)

0.84
(0.61-1.16)

Alberta

291

1.36
(1.21-1.53)

153

0.80
(0.68-0.94)

0.59
(0.49-0.72)

BC

318

1.50
(1.34-1.67)

93

0.41
(0.33-0.50)

0.27
(0.22-0.35)

Yukon

3

1.25
(0.26-3.63)

5

2.26
(0.73-5.26)

1.81
(0.43-7.57)

NWT

16

2.16
(1.24-3.51)

16

2.18
(1.24-3.53)

1.01
(0.50-2.01)

Canada

1110

0.97
(0.91-1.03)

590

0.54
(0.50-0.59)

0.56
(0.51-0.62)

* CI - confidence interval


Examination of the population distribution of key demographic and perinatal risk factors for SIDS revealed some changes over the two periods (Table 2). The proportion of postneonatal survivors with very young maternal and paternal ages as well as advanced maternal and paternal ages increased. There were increases in the proportions of very preterm and preterm births, as well as increases in both the proportion of very low birth weight babies and babies weighing greater than 4,000 grams. Examination of the significance of these risk factors for SIDS demonstrated no change in the associations with specific risk factors after the release of the first Canadian joint statement. In the period 1985-1989, young maternal age, young paternal age, male sex, preterm birth, low birth weight and increasing parity were all associated with an increased risk of SIDS in the postneonatal period. In the period immediately after the release of the joint statement, these factors remained associated with an increased risk of SIDS (Table 3). As demonstrated in Table 4, adjusting for several key demographic and perinatal risk factors had minimal impact on the time period effect on postneonatal SIDS rates.

TABLE 2
Population distribution of selected demographic and perinatal risk factors for SIDS, Canada (excluding Newfoundland and Ontario), 1985-1989 and 1994-1998

   

Number (%) of postneonatal survivors 1985-1989

Number (%) of postneonatal survivors 1994-1998

Percentage change 1985-1989
to 1994-1998
(95% CI*)

1.

Maternal age (years)

< 20

73,090 (6.4)

71,080 (6.6)

+3 (+2, +4)

20-24

288,890 (25.2)

221,595 (20.4)

-19 (-19, -18)

25-29

452,434 (39.5)

354,493 (32.7)

-17 (-17, -17)

30-34

255,594 (22.3)

306,635 (28.3)

+27 (+26, +27)

>= 35

76,612 (6.7)

130,414 (12.0)

+80 (+78, +82)

2.

Paternal age (years)

< 20

11,166 (1.1)

17,164 (1.7)

+62 (+58, +65)

20-24

133,777 (12.9)

111,668 (11.3)

-12 (-13, -12)

25-29

385,435 (37.1)

274,143 (27.8)

-25 (-26, -25)

30-34

327,221 (31.5)

336,968 (34.1)

+8 (+8, +9)

>= 35

180,059 (17.4)

247,163 (25.0)

+44 (+44, +45)

3.

Sex of infant

Male

587,599 (51.2)

556,314 (51.3)

0 (0, 0)

Female

559,212 (48.8)

527,952 (48.7)

0 (0, 0)

4.

 

Gestational age (weeks)

20-27

1,894 (0.2)

2,255 (0.2)

+26 (+18, +34)

28-31

6,016 (0.5)

6,308 (0.6)

+10 (+7, +14)

32-33

8,414 (0.7)

8,684 (0.8)

+9 (+6, +12)

34-36

52,770 (4.6)

55,750 (5.1)

+12 (+10, +13)

37-41

1,019,374 (88.9)

987,048 (91.0)

+2 (+2, +2)

>= 42

58,343 (5.1)

24,393 (2.3)

-56 (-56, -55)

5.

Birth weight (grams)

500-1499

6,800 (0.6)

7,436 (0.7)

+15 (+11, +19)

1500-2499

54,204 (4.8)

51,187 (4.7)

-1 (-2, +1)

2500-3999

949,926 (83.5)

891,292 (82.5)

-1 (-1, -1)

4000-5999

126,836 (11.2)

129,982 (12.0)

+8 (+7, +9)

6.

Parity

1

479,020 (42.7)

458,59l (42.6)

0 (-1, 0)

2

390,722 (34.9)

374,711 (34.8)

0 (-1, 0)

>= 3

251,496 (22.4)

243,568 (22.6)

+1 (0, +1)

*CI - confidence interval

TABLE 3
Selected demographic and perinatal risk factors for SIDS, Canada (excluding Newfoundland and Ontario), 1985-1989 and 1994-1998

 

1985-1989
*Crude relative risks*
(95% CI**)

1994-1998
*Crude relative risks*
(95% CI**)

1.

Maternal age (years)

< 20

4.13 (3.46-4.93)

4.20 (3.28-5.38)

20-24

1.91 (1.64-2.22)

2.53 (2.04-3.13)

25-29

1

1

30-34

0.88 (0.72-1.06)

0.64 (0.48-0.84)

>= 35

0.91 (0.67-1.24)

0.65 (0.45-0.95)

2.

Paternal age (years)

< 20

3.18 (2.28-4.45)

2.51 (1.74-3.62)

20-24

1.51 (1.29-1.78)

1.29 (1.03-1.61)

25-29

1

1

30-34

0.64 (0.55-0.75)

0.45 (0.36-0.56)

>= 35

0.61 (0.50-0.75)

0.29 (0.22-0.39)

3.

Sex of infant

Male

1.67 (1.48-1.89)

1.62 (1.37-1.91)

Female

1

1

4.

 

 

Gestational age (weeks)

20-27

5.56 (2.88-10.75)

3.85 (1.44-10.30)

28-31

6.43 (4.54-9.11)

6.54 (4.13-10.35)

32-33

3.89 (2.67-5.67)

5.50 (3.58-8.43)

34-36

2.32 (1.90-2.85)

2.60 (2.01-3.37)

37-41

1

1

>= 42

1.00 (0.75-1.33)

1.15 (0.67-2.00)

5.

Birth weight (grams)

500-1499

6.05 (4.35-8.41)

5.44 (3.43-8.61)

1500-2499

2.47 (2.04-2.99)

3.61 (2.87-4.55)

2500-3999

1

1

4000-5999

0.65 (0.51-0.82)

0.87 (0.65-1.15)

6.

Parity

1

1

1

2

1.48 (1.28-1.71)

1.59 (1.29-1.96)

>= 3

2.19 (1.89-2.53)

2.82 (2.30-3.46)

* Adjustment using logistic regression for all listed factors did not change the relative risks.

** Confidence interval.

TABLE 4
Crude and sequentially adjusted relative risks (95% confidence interval [CI]) for time period effect on postneonatal SIDS rates, Canada

Risk factor

Relative risk

(95% CI)

Period 1994-1998 vs. 1985-1989 (crude)

0.56 (0.51-0.62)

Period 1994-1998 vs. 1985-1989 Adjusted for

 

 

Maternal age

0.59 (0.53-0.65)

 

Plus parity

0.59 (0.53-0.65)

 

Plus infant sex

0.59 (0.53-0.65)

 

Plus gestational age

0.58 (0.53-0.65)


Examination of the proportion of postneonatal SIDS deaths that occurred in each season revealed a significant seasonal pattern present only in the period before the release of the 1993 joint statement. Between 1985 and 1989, 29.4% of all postneonatal SIDS deaths occurred during January-March, as compared with 25.2%, 20.4% and 25.1% in April-June, July-September and October- December respectively (p = 0.0493). This winter predominance was no longer significant in 1994-1998, 27.2% of postneonatal SIDS deaths occurring in January-March, as compared with 26.6%, 21.9% and 24.4% in April-June, July-September and October-December respectively (p = 0.41).

Discussion

The Canadian postneonatal mortality rate due to SIDS declined markedly between 1985 and 1998. Our analysis demonstrated a similar pattern of decline in the late 1980s and early 1990s for postneonatal mortality due to all other causes of death (excluding SIDS and congenital anomalies). Furthermore, the initial decline in the postneonatal SIDS rate preceded the release of the first joint statement on reducing the risk of SIDS. However, our analysis also demonstrates that the reduction in the postneonatal mortality rate due to SIDS before and after the release of the first joint statement was significantly greater than the reduction in the postneonatal mortality rate due to other causes. This finding is consistent with a positive impact of the initial Canadian efforts to reduce the risk of SIDS and with findings in many other countries, including New Zealand, Australia, Norway, Denmark, Sweden and England and Wales.2,9,10 Furthermore, more recent provincial/territorial data obtained by the Canadian Foundation for the Study of Infant Deaths suggest that the SIDS rate has declined further in 1999 and 2000.11

The positive impact of risk reduction efforts would be further supported by data demonstrating changes in the prevalence of risk factors in the population. In Australia, almost 70% of the reduction in the SIDS incidence was attributed to the reduction in the prone sleep position.12 In The Netherlands, a relation between changes in prone sleep position and SIDS deaths has also been demonstrated over time.13 In Canada, no data are available at the national level to make similar assessments in relation to the initial risk reduction campaign. In Australia, changes in other risk factors, such as smoking, were responsible for less than 10% of the observed decline in the SIDS rate.12 Canadian data on prenatal smoking and infant exposure to environmental tobacco smoke are also very limited for the period of this study. National and regional prenatal smoking rates are available in the period after the release of the first joint statement.14 However, the absence of comparable data on tobacco use for the period before the initial joint statement further limits the extent to which the observed decline in postneonatal mortality due to SIDS can be attributed to the joint statement on risk reduction.

Our study shows that provincial and territorial SIDS rates vary considerably, as do the observed changes in SIDS rates in relation to the initial risk reduction campaign. This observed variation may be due to regional differences in the prevalence of various risk factors for SIDS in the population. Once again, reliable and comparable interprovincial/territorial data on factors such as sleep position and smoking are not available for either before or after the release of the initial risk reduction statement.

Other possible explanations for the observed variation in provincial/territorial rates include differences in the composition of the regional populations. In several countries, including Canada, Aboriginal populations have been identified as being at particularly high risk of SIDS.15,16 Moreover, a less marked decline in SIDS rates in certain ethnic and lower socioeconomic subgroups has been attributed to a lower awareness of SIDS prevention opportunities in these groups.2 National data do not permit comparison of SIDS rates in relation to risk factor information for specific subgroups in the Canadian population, e.g., those with lower socioeconomic status, Aboriginal groups and recent immigrants to Canada. Finally, variation in the postneonatal SIDS rates as well as the observed decline may also be due to provincial/territorial-specific public health efforts.

As expected, we found some changes in the population distribution of established demographic and perinatal risk factors for SIDS over the two time periods. In particular, increases in advanced maternal age and increasing rates of preterm birth in the Canadian population have been well documented.14 With regard to the significance of these established risk factors, we found essentially no change between the two periods. This finding has been reported elsewhere, including The Netherlands and England.17,18 Of greatest importance was that our consideration of an adjusted time period effect on the decline of postneonatal SIDS rates demonstrated that any changes observed in these demographic and perinatal risk factors over the two periods were not responsible for the marked decline in Canadian SIDS rates. This finding further supports a positive impact of the risk reduction campaign.

Our study also demonstrates that a significant winter predominance in postneonatal SIDS deaths was present before the initial risk reduction efforts but disappeared in the period following the release of the joint statement. This elimination of a seasonal pattern for SIDS deaths has been reported previously. A UK study reported a decrease in the proportion of deaths in the cold months over time, from 34% in 1990-1991 to 27% in 1995-1996.18 In contrast, the persistence of a seasonal effect, though diminished in magnitude, has been reported for other countries, including Australia.19 The etiologic nature of a seasonal effect, as well as the reasons for the discrepancies in trends in seasonality, are not fully understood.

Our study has several potential limitations. First, data on causes of death were extracted from death certificates that recorded only a single underlying cause of death. This may result in misclassifying the cause for some infant deaths. Furthermore, if a diagnosis of SIDS is not available at the time of completion of the death certificate, an updated diagnosis must be forwarded to Statistics Canada in time for the publication of mortality statistics. Failure to meet this deadline would result in an underestimate in the number of SIDS deaths.

Second, some transcription and other errors are inevitable in large databases.

Third, as SIDS is a diagnosis of exclusion, its diagnosis may vary depending on the expertise of the coroner/medical examiner to detect alternative diagnoses. For example, a study in Quebec reported that a diagnosis other than SIDS was more likely if the autopsy was performed in a centre with expertise in pediatric pathology.20

Fourth, the exact timing of the "intervention" for this study is not well defined. The initial Canadian joint statement was released in 1993 and was followed by promotional campaigns in Canada in 1994 and 1995. Additionally, the American Academy of Pediatrics released a statement on sleep position in the United States in 1992,21 and recommendations may have been adopted by some Canadians at that time. Finally, the absence of comparable risk factor data at the national level before and after the initial joint statement is an additional limitation of the study.

In conclusion, Canadian postneonatal mortality due to SIDS declined significantly between 1985 and 1998. The reduction in the SIDS rate before and after the initial joint statement on reducing the risk of SIDS was greater than the decline observed in the postneonatal mortality due to other causes. Furthermore, adjusting this time period effect for key demographic and perinatal risk factors for SIDS did not alter the observed decline. These findings are consistent with a positive impact of initial risk reduction efforts. However, the absence of detailed risk factor data at the national and provincial/territorial levels limits the extent to which the SIDS rate reduction can be directly attributed to the initial joint statement. Future risk reduction campaigns should consider the available data sources and gaps to ensure rigorous evaluation.

Acknowledgements: Dr. Joseph is supported by a Clinical Research Scholarship from the Dalhousie University Faculty of Medicine and the Peter Lougheed/CIHR New Investigator award from the Canadian Institutes of Health Research. Dr. Kramer is a Senior Investigator of the Canadian Institutes of Health Research. We thank the vital statistics registrars of the provinces and territories who gave us access to the data. We acknowledge the SAS programming efforts of Sudha Busavaraj and Ling Huang.

References

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  16. Alessandri LM, Read AW, Stanley FJ, et al. Sudden infant death syndrome and infant mortality in aboriginal and non-aboriginal infants. J Paediatr Child Health 1994;30:234-41.
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Author References

ID Rusen, Shiliang Liu, Health Surveillance and Epidemiology Division, Centre for Healthy Human Development, Health Canada, Ottawa, Ontario, Canada

Reg Sauve, Departments of Paediatrics and Community Health Sciences, University of Calgary, Calgary, Alberta, Canada

KS Joseph, Perinatal Epidemiology Research Unit, Departments of Obstetrics and Gynecology, and Pediatrics, Dalhousie University, Halifax, Nova Scotia, Canada

Michael S Kramer, Departments of Pediatrics, and Epidemiology and Biostatistics, McGill University, Montreal, Quebec

Correspondence: Dr. ID Rusen, MD, MSc, Health Surveillance and Epidemiology Division, Health Canada, AL: 1910C, Tunney's Pasture, Ottawa, Ontario, Canada, K1A 0L2; Fax: (613) 941-9927; E-mail: CPSS@hc-sc.gc.ca


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