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Is sudden infant death syndrome still more common in very low birthweight infants in the 1990s?

Beverley Sowter, Lex W Doyle, Colin J Morley, Anne Altmann and Jane Halliday
Med J Aust 1999; 171 (8): 411-413.
Published online: 18 October 1999
Research

Is sudden infant death syndrome still more common in very low birthweight infants in the 1990s?

Beverley Sowter, Lex W Doyle, Colin J Morley, Anne Altmann and Jane Halliday

MJA 1999; 171: 411-413

Abstract - Introduction - Methods - Results - Discussion - References - Authors' details
- - More articles on Paediatrics


Abstract Objective: To determine the rate of sudden infant death syndrome (SIDS) in very low birthweight children (VLBW) relative to children with low (LBW) and normal birthweights.
Design, setting and subjects: Cohort study of consecutive live births in Victoria, 1993-1997 inclusive.
Main outcome measures: All sudden unexpected deaths in early childhood over this five-year period; all deaths from SIDS (defined as a sudden unexpected death without a definite pathological explanation); and the proportion of SIDS in live births in three birthweight subgroups (VLBW, 500-1499 g; LBW, 1500-2499 g; and normal birthweight, > 2499 g).
Results: There were 316 028 live births (with known birthweight) in Victoria over the five-year period; 224 (0.71 per 1000 live births) died unexpectedly. In 10 of these deaths there was a definite pathological explanation, giving a rate of SIDS of 0.68 per 1000 live births. The rate of SIDS in VLBW children was 2.52 per 1000 live births, lower than the rate reported before the 1990s. The rate of SIDS in VLBW children was not significantly different from the rate in LBW children of 1.98 per 1000 live births (difference per 1000 live births, 0.53; 95% CI, 21.45 to 2.52), but was significantly higher than the rate in normal birthweight children of 0.59 per 1000 live births (difference per 1000 live births, 1.93; 95% CI, 0.06-3.79).
Conclusions: The rate of SIDS in VLBW children has fallen in the 1990s, along with the overall fall in the rate of SIDS, but remains higher than that in normal birthweight children.


Introduction Most parents learn to live with the fear that their baby may die of sudden infant death syndrome (SIDS). However, the fear of SIDS can be even greater for parents of babies with low birthweight, or those with babies who have been in intensive care or who have had apnoea. Before discharge, many parents attend education sessions on reducing the risks of SIDS and on infant resuscitation. At one of these sessions the parents may ask the difficult question: "Is my baby more likely to die of SIDS because he (or she) was so tiny when born?".

Before the 1990s, children with very low birthweight (VLBW, 500-1499 g) were known to have a higher rate of SIDS than those with a normal birthweight (> 2499 g).1-3 With the advent of preventive measures, the overall rate of SIDS in Australia has fallen dramatically in the 1990s (from 1.87 per 1000 live births in 1990 to 0.78 per 1000 live births in 1995).4 However, it is unclear whether the rate of SIDS has also fallen in VLBW children. We aimed to determine the rate of SIDS in the 1990s for VLBW children relative to children of other birthweights (low birthweight [LBW], 1500-2499 g; and normal birthweight, > 2499 g).


Methods We studied a cohort of all consecutive live births in Victoria during the five-year period from 1993 to 1997, inclusive, and recorded all sudden deaths in early childhood (divided into the first 28 days [the neonatal period], postneonatal infancy [29-365 days], and early childhood [more than 1 year]). SIDS was defined as a sudden unexpected death without definite pathological features to explain the death. In the SIDS group, some children were considered by the pathologist to have pathological features, but these were insufficient to explain the death. Some had only a minor condition, and in the remainder no pathological features were found. All births with unknown birthweight were excluded.  

Data sources Data on deaths were obtained from the annual reports of the Consultative Council on Obstetric and Paediatric Mortality and Morbidity,5-9 a government-legislated surveillance body. The Council collects data on all perinatal deaths from 20 weeks' gestation, and all infant and child deaths up to 14 years of age. Death registrations are forwarded directly from the Registry of Births, Deaths and Marriages, and information on all sudden unexpected deaths is supplemented by the Victorian State Coroner. These cases are then all reviewed and classified by an expert pathologist working in the field.

Data on births in Victoria were supplied by the Perinatal Data Collection Unit of the Public Health and Development Division of the Department of Human Services. The Unit collects data on all births in Victoria from 20 weeks' gestation under a legislated notification system.  

Statistical analysis
The proportions of deaths from SIDS (and 95% confidence intervals) in each of the birthweight subgroups (VLBW, 500-1499 g; LBW, 1500-2499 g; and normal birthweight, > 2499 g) were calculated,10 and comparisons made between the groups.10


Results Over the five-year period 1993-1997, there were 316 028 live births in Victoria for which birthweight was known. (Birthweight was not known for a total of 90 live births.) Over the same period, there were 224 sudden unexpected deaths with known birthweight, of which 10 (4.5%) had a definite pathological explanation, leaving 214 deaths from SIDS (0.68 per 1000 live births). Of these 214 children with SIDS, 23 (10.7%) died in the neonatal period (including one who died during the primary hospitalisation), 171 (79.9%) died in postneonatal infancy, and 20 (9.3%) died after the age of 1 year.

VLBW children made up less than 1% of all live births over this period; the rate of SIDS in VLBW children was 2.52 per 1000 live births (Table). This was not significantly higher than the rate in LBW children of 1.98 per 1000 live births (difference per 1000 live births, 0.53; 95% CI, 21.45 to 2.52), but was significantly higher than the rate in normal birthweight children of 0.59 per 1000 live births (difference per 1000 live births, 1.93; 95% CI, 0.06-3.79). LBW children had a significantly higher rate of SIDS than normal birthweight children (difference per 1000 live births, 1.39; 95% CI, 0.69-2.09).

Of the seven deaths in VLBW infants, definite pathological features were found in five, but these were insufficient to explain the death; three of these infants had respiratory disease (pneumonia, bronchiolitis, or tracheobronchitis).


Discussion Our study showed no significant difference in the rate of SIDS between VLBW and LBW infants; however, the rate in each of these subgroups was significantly higher than in normal birthweight children.

Before recommendations for reducing the risk of SIDS were introduced in 1991, the SIDS rate was higher in VLBW infants compared with those with normal birthweight, for both hospital and regional cohorts. In a VLBW hospital cohort, 1977-1978, the rate of SIDS before 2 years of age was 30.3 per 1000 live births (7/231);1 and in a regional cohort in New Zealand (children born in 1986), the incidence of SIDS in the VLBW group was 13 per 1000 live births, more than three times the rate of 4.0 per 1000 live births for all NZ children born in 1986.2 In a Californian study of 2962 children dying of SIDS between 28 days and 1 year of age in 1978-1982, the overall incidence was found to be 1.5 per 1000 live births. The highest incidence was in the VLBW group (7.5 per 1000 live births), decreasing to 1.3 per 1000 live births for the normal birthweight group.3

Studies have not shown evidence of a fall in the rate of SIDS in VLBW infants just before the recommendations were introduced. For example, in 1985-1991, the rate of SIDS in VLBW infants in the first year of life for singleton births in the United States remained relatively constant (average, 3.66 per 1000 live births) compared with rates for normal birthweight singleton infants (average, 1.07 per 1000 livebirths).11

Studies comparing the period before and after the recommendations have shown a change in rate of SIDS in VLBW children around the beginning of the 1990s. One study cited by l'Hoir et al12 estimated the rate of SIDS in VLBW children in the Netherlands to have decreased from 10 per 1000 in 1983 to 1 per 1000 (presumably live births) in 1995-96, a change in the rate of SIDS over time similar to that comparing our results with rates in the late 1980s. A more recent US report described a smaller reduction in the rate of SIDS in VLBW children before and after the recommendations about sleeping position, and the reduction was similar across birthweight subgroups (between 1991 and 1995 reductions of 37%, 36% and 30% were found for birthweight subgroups 500-1499 g, 1500-2499 g, and > 2499 g, respectively).13

Avoidance of risk factors probably explains the fall in the rate of SIDS in VLBW infants in the 1990s, the same reason that it has fallen for infants overall. The reason SIDS remains more prevalent in VLBW infants compared with normal birthweight infants may relate to the underlying pathological features, which were more common in VLBW infants with SIDS.

So, what should we be telling parents when they ask if their tiny baby is more likely to die of SIDS?

  • Firstly, VLBW and LBW children are at increased risk of SIDS. However, 399 out of 400 VLBW children, and 499 out of 500 LBW children, do not die of SIDS.

  • Secondly, parents can help to decrease the risk of SIDS by following the recommendations: putting their baby to sleep supine, not smoking, and not allowing the baby to become overheated or covered over by bedding.4

  • Thirdly, as most VLBW children who died of SIDS had a definite pathological condition at autopsy, parents should seek medical advice early if the baby appears unwell in any way.

    Parents often recognise that their baby is unwell, but it is not always easy for them to decide what is a minor illness and when they need to seek medical advice. A scoring system such as "Baby Check" 14 can be used both by parents and general practitioners to help them to determine whether or not a baby is seriously ill. In a recent review of 37 sudden unexpected infant deaths, 3 (8%) scored very highly for serious illness on a retrospective score with Baby Check, suggesting that such a scoring system could have identified serious illness before death and led to appropriate treatment.15 The three VLBW children with significant respiratory disease in our study probably would have shown signs of illness before they died.

    In conclusion, the rate of SIDS in VLBW children has fallen from over 10 per 1000 live births before the 1990s to 2.5 per 1000 live births at the end of the 1990s, but remains higher than the rate in normal birthweight children. LBW children are also at greater risk of SIDS at the end of the 1990s.


  • References
    1. Kitchen WH, Yu VYH, Lissenden JV, Bajuk B. Collaborative study of very-low-birthweight infants: techniques of perinatal care and mortality. Lancet 1982; 1: 1454-1457.
    2. Darlow BA, Horwood LJ, Mogridge N, Clemett RS. Prospective study of New Zealand very low birthweight infants: outcome at 7-8 years. J Paediatr Child Health 1997; 33: 47-51.
    3. Grether JK, Schulman J. Sudden infant death syndrome and birth weight. J Pediatr 1989; 114: 561-567.
    4. Henderson-Smart DJ, Ponsonby AL, Murphy E. Reducing the risk of sudden infant death syndrome: a review of the scientific literature. J Paediatr Child Health 1998; 34: 213-219.
    5. The Consultative Council on Obstetric and Paediatric Mortality and Morbidity. Annual Report for the Year 1993, incorporating the 32nd Survey of Perinatal Deaths in Victoria. Melbourne, 1994.
    6. The Consultative Council on Obstetric and Paediatric Mortality and Morbidity. Annual Report for the Year 1994, incorporating the 33rd Survey of Perinatal Deaths in Victoria. Melbourne, 1995.
    7. The Consultative Council on Obstetric and Paediatric Mortality and Morbidity. Annual Report for the Year 1995, incorporating the 34th Survey of Perinatal Deaths in Victoria. Melbourne, 1996.
    8. The Consultative Council on Obstetric and Paediatric Mortality and Morbidity. Annual Report for the Year 1996, incorporating the 35th Survey of Perinatal Deaths in Victoria. Melbourne, 1997.
    9. The Consultative Council on Obstetric and Paediatric Mortality and Morbidity. Annual Report for the Year 1997, incorporating the 36th Survey of Perinatal Deaths in Victoria. Melbourne, 1998.
    10. Gardner MJ, Altman DG. Statistics with confidence - confidence intervals and statistical guidelines. London: BMJ, 1989.
    11. Bigger HR, Silvestri JM, Shott S, Weese-Mayer DE. Influence of increased survival in very low birth weight, low birth weight, and normal birth weight infants on the incidence of sudden infant death syndrome in the United States: 1985-1991. J Pediatr 1998; 133: 73-78.
    12. l'Hoir MP, Engelberts AC, van Well GT, et al. Case-control study of current validity of previously described risk factors for SIDS in the Netherlands. Arch Dis Child 1998; 79: 386-393.
    13. Malloy MH. Birth weight and gestational age specific sudden infant death syndrome (SIDS) mortality: 1991 vs 1995. Pediatr Res 1999; 45: 249A.
    14. Morley CJ, Thornton AJ, Cole TJ, et al. Baby Check: a scoring system to grade the severity of acute systemic illness in babies under 6 months old. Arch Dis Child 1991; 66: 100-105.
    15. Cole TJ, Gilbert RE, Fleming PJ, et al. Baby Check and the Avon infant mortality study. Arch Dis Child 1991; 66: 1077-1078.
    (Received 21 Jun, accepted 6 Sep, 1999)


    Authors' details Division of Paediatrics, Royal Women's Hospital, Melbourne, VIC.
    Beverley Sowter, RN, Case Manager.
    Lex W Doyle, MD, FRACP, Paediatrician; and Associate Professor, Department of Obstetrics and Gynaecology, and Department of Paediatrics, University of Melbourne.
    Colin J Morley, MD, FRACP, Paediatrician.

    Consultative Council on Obstetric and Paediatric Mortality and Morbidity, Melbourne, VIC.
    Anne Altmann, MB BS(Hons), MPH, FAFPHM, Epidemiologist.

    Victorian Perinatal Data Collection Unit, Melbourne, VIC.
    Jane Halliday, PhD, Epidemiologist.

    Reprints will not be available from the authors.
    Correspondence: Associate Professor L W Doyle, Division of Paediatrics, The Royal Women's Hospital, 132 Grattan Street, Carlton, VIC 3053.
    l.doyleATobgyn-rwh.unimelb.edu.au






    Sudden unexpected deaths in early childhood in Victoria, 1993-1997

    Birthweight subgroup

    500-1499 g1500-2499 g> 2499 gTotal

    Live births278115 630297 617316 028
    Sudden unexpected deaths (≤ 28 days)07*1623
    Sudden unexpected deaths (> 28 days)724170201
    Total sudden unexpected deaths731186224
    Sudden unexpected deaths explained at autopsy001010
    Total SIDS731176214
    Pathological features
    Definite586174
    Minor21887107
    None052833
    Rate of SIDS (per 1000 livebirths) 2.521.98 0.590.68
    (95% CI)(1.04-5.19)(1.29-2.68)(0.50-0.68)(0.59-0.77)

    Data are numbers of infants, unless indicated otherwise. * One infant died while still in hospital after birth. SIDS = Sudden infant death syndrome (sudden unexpected deaths, excluding those explained at autopsy).
    Back to text

    Received 21 September 2018, accepted 21 September 2018

    • Beverley Sowter
    • Lex W Doyle
    • Colin J Morley
    • Anne Altmann
    • Jane Halliday


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