Sudden Infant Death Syndrome

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PatientPlus articles are written by UK doctors and are based on research evidence, UK and European Guidelines. They are designed for health professionals to use, so you may find the language more technical than the condition leaflets.

Synonyms: cot death (especially amongst the general public), SIDS

Sudden infant death syndrome (SIDS) describes the sudden and unexplained death of a child under the age of 1 year.

It is a diagnosis of exclusion and can be made only after detailed history with examination of the site of death and postmortem examination. All other possible causes of death must be excluded for this diagnosis to be tenable.[1] It usually occurs in apparently healthy children during sleep, with no warning. The terms 'sudden and unexpected infant death' (SUID) or 'sudden unexpected death in infancy' (SUDI) are used to describe all deaths, regardless of cause. Cases of SUID that remain unexplained after postmortem examination and review of the history and circumstances surrounding the death are classified as SIDS. 80% of SUID cases are due to SIDS. 20% have a clear cause such as severe infection, inherited disorders of fatty acid oxidation or genetic cardiac channelopathies.[1]

In the affluent West, SIDS is the most common cause of death of children between the ages of 1 month and 1 year,[2] with the majority of the deaths occurring between the ages of 4 and 6 months.[2] The true incidence may be masked due to discrepancies in the diagnosis used on death certificates - for example, accidental suffocation being misdiagnosed as suffocation and vice versa.

  • More than 300 babies per year die from SIDS in the UK.[3]
  • Among industrialised nations, Japan has the lowest reported SIDS rate (0.09 cases per 1,000) whilst New Zealand has the highest rate (0.8 cases per 1,000).[1]
  • The UK rate for SIDS is 0.41 per 1,000 live births.[2]
  • Cot death is more common in male infants (about 60%) of cases.[2]
  • The number and rate of SIDS cases has been falling since 1989. This fall was most marked between 1991 and 1992, when the 'Reduce the Risk' campaign was launched.[3] The USA implemented a similar campaign - 'Back to Sleep'.
  • In 2009, 78% of all SIDS cases were in infants aged less than 3 months.[3] 90% of SIDS deaths occur in the first six months of life.[2]
  • In 2009, babies with a low birth weight (less than 2500 g) were five times more likely to die than those of normal birth weight.[3]
  • The most common season is the winter and the most common time is early morning. This may be related to changing sleep patterns as the infant matures, increased risk of respiratory infections in the colder months, and 'biological clocks'.

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Although sudden infant death syndrome cannot be prevented, there are several factors which are known to be associated with increased risk. A 'Triple Risk Model' was proposed in 1994, emphasising the role and interaction of a number of factors in the pathogenesis of SIDS.[1] In this model, SIDS occurs when three factors are present simultaneously. These factors are: an underlying vulnerability in the infant (eg, low birth weight or prematurity), a critical developmental period (usually 1-3 months of age) and an 'exogenous stressor' (eg, sleeping prone).[1]

Maternal factors

  • Smoking during and/or after pregnancy:
    • In an American study,[4] of all SIDS cases, 21% were attributable to maternal smoking.
    • Maternal smoking during pregnancy is associated with a significantly increased risk of SIDS.[2]
    • A relationship between maternal smoking and changes in the brainstem of the infant has also been shown.[5]
  • Late or no antenatal care.
  • Low pregnancy weight gain.
  • Age less than 20 at first pregnancy:
    • In 2009, the cot death was highest for babies born to mothers aged 20 years or under at the time of birth (the risk was 3.6 times higher).[3]
  • Placental abnormalities:
    • Such abnormalities may account for low birth weight, which is a risk factor for SIDS.
  • Alcohol and substance abuse.
  • History of sexually transmitted disease.
  • Socio-economic group. The cot death rate in 2009 was three times higher in babies born to fathers with a manual job compared to fathers with a managerial or professional profession.[3]

Infant factors

1. Sleeping factors

  • Overheating during sleep:
    • There is an association with high tog factor bedding and clothes.[6]
    • Baby sleeping bags prevent overheating, stop the head from becoming covered with bedclothes and make it more difficult for a young baby to roll on to his or her front or side.
    • Sleeping in the prone position (on the front). The prone sleep postion more than triples the rate of SIDS:[1]
    • Infant blood pressure (BP) is modified by the sleep state and sleeping position.[7] A tendency for BP to fall in the prone position appears to be prevented by elevating the heart rate at 2-4 weeks and 5-6 months, but not at 2-3 months, coincident with the age of greatest risk for SIDS. An uncompensated fall in BP in the prone position at this age could increase the possibility of circulatory failure and SIDS in vulnerable infants.
  • Prone sleeping can increase the risk of asphyxia, especially on surfaces other than those intended for infant sleep (for example, an adult's bed):
    • In an American study of SIDS cases, only 25% of infants were found sleeping in a crib. When found, 70% of babies were on a surface not intended for infant sleep (eg, an adult bed).[8] 
    • Both high tog bedding and sleeping in a position other than on the back may contribute to the high risk associated with bed sharing.[6] Sleeping on the side is also a risk for SIDS.
  • Other factors have been thought to influence the development of SIDS, including type of bedding and mattress, bottle feeding and the use of a 'dummy'. The exact mechanism of action is largely unknown.
    • Parents are advised to buy a new mattress for each new baby. This is because a small association between SIDS and the use of a secondhand mattress was shown by a Scottish study. Breast-feeding and the use of a dummy are both associated with some reduction of the risk of SIDS, but they are not completely protective.
    • Dummies are advised from 1 month of age (use too early can negatively impact on the establishment of breast-feeding)[9] until 6 months of age. Use beyond 2 years is not advised due to the risk of dental malocclusion. However, dummies are also associated with an increased risk of otitis media and gastrointestinal infections, and of oral candida colonisation.
    • It is advised that babies under 6 months of age sleep in the same room as their parent(s).[2]

2. Environmental factors

  • Exposure to cigarette smoke in utero or after birth.

3. External factors

  • Family history of SIDS (see below, under 'Recurrent infant deaths').
  • Sometimes SIDS will follow an immunisation. This is inevitable, as infant vaccination schedules occur at the same time as the peak age for SIDS - ie any association is temporal rather than causal. There is no increased risk after vaccination and the figures may even suggest a slight but insignificant protection.[9] Children regarded as being at high risk for SIDS should have their immunisation schedule as usual.
  • Some feel that the large yield of postmortem bacteriological cultures positive for Staphylococcus aureus and Escherichia coli, in otherwise unexplained cases of SIDS, suggests that these bacteria could be associated with this condition.[10]

It is important to note that approximately 10% of SIDS occur in babies lying supine and who are not co-sleeping with an adult; nor is their face covered with bedding. This reinforces the point that the factors above are not causative and the exact mechanisms are not known.

The sudden death of a child is likely to be very traumatic for all concerned, and that includes the attending doctor. Useful guidelines for GPs have been produced by the Foundation for the Study of Infant Deaths.[3] Parents are likely to be in a state of shock and any professional attending in such a situation will need to be very sensitive and considerate in their handling of the family.

  • When first looking at the infant, note the position in which the child is lying, clothes the child is wearing, any secretions, etc on the child's face, and make an accurate recording of these observations as soon as possible, so as to have contemporaneous notes.
  • Once it has been established that death has indeed taken place, the initial concern must be for the parents and other members of the family in attendance. After allowing a little time for them to accept the fact of the death, it must be gently explained to them that all cases of sudden death from any cause must be reported to the coroner or, in Scotland, the procurator fiscal, that police officers will call, and that this is a routine process and not because of any suspicious circumstances.
  • Ask if there is anyone that you can call to come and stay with them, or look after siblings, particularly in the case of single parents.
  • Many parents, on finding their baby lifeless will have attempted some form of resuscitation, or will have removed the child from the site of death to attempt to get help, and a detailed history of events will need to be taken using open questions such as "Can you tell me what happened?" and "Had the baby been ill recently?". Further detailed questioning will be undertaken by the police officers, who will also perform a detailed examination of the site of death, and again it is important to reassure parents that this, together with a postmortem examination, is a routine process in order to ascertain the cause of death, and not due to any suspicion of foul play.
  • The family of the child is likely to need support through the period of investigation and mourning, and the death should be reported to their usual doctor and health visitor at the earliest opportunity.
  • Sometimes, a lone mother, in particular, will be fearful that the father will blame her for the death of the baby. This will be a special problem if there is a history of domestic violence.
  • The family is likely to have a number of questions in the ensuing weeks and months including "Why did it happen?", "Is it likely to happen to any future children?" and "Is there anything we could have done to prevent the death?", and should be given the opportunity to ask these questions and be given information on local and national agencies who are able to provide information for grieving families.

In approximately 37% of cases, a postmortem will identify a cause of death:[11]

  • Disease
  • Genetic disorder
  • Accidental injury
  • Non-accidental injury

It is now widely believed that infants that are affected by SIDS are born with a predisposition which makes them more vulnerable to physiological stresses, and current research is focusing on identifying subtle abnormalities in the brain and brain stem, particularly in those areas responsible for the control of breathing and heart rate.[1]

Although this has been known for several years, more recent research[12] has suggested that the role of serotonin and its deficiency in the possible pathogenesis of SIDS is more important than had previously been realised. This may also help to explain the vulnerability of boys. It may also explain some susceptibility of families.

Apparent life-threatening event syndrome used to be called near miss cot death:

  • It affects predominantly children younger than 1 year.
  • There are frightening symptoms with some combination of apnoea, change in colour, change in muscle tone, coughing or gagging. Approximately 50% of these children are diagnosed with an underlying condition that explains the event.
  • The problems are digestive (up to 50%), neurological (30%), respiratory (20%), cardiac (5%), and endocrine or metabolic (<5%).
  • The cause remains unknown in around half.[13]
  • These children do appear to be at greater risk of developing SIDS, although most cases are benign.[14]

The value of apnoea monitors is controversial, as they have not been proven to prevent SIDS. However, parents often feel reassured that they are 'doing everything they can', whilst using one.

There are a few cases, documented by covert video surveillance, in which parents have induced illness in their children. This can result in serious neurological damage and even death. The implications are discussed in the separate article Fabricated or Induced Illness by Carers (FII).

There are occasions when more than one infant death occurs within a family and explanations are sought. There are three possibilities that immediately arise.

  • If SIDS is truly a random event, then having more than one in a family is extremely bad luck. This does not mean that it cannot happen. The chance of winning the National Lottery is 1 in 14 million but at least one person does it most weeks. If the risk of SIDS is, say, 1 in 2,000, the risk of losing both of two children is 1 in 4 million (2,0002) and the chance of losing all of three is 1 in 8 billion (2,0003).
  • This has made people wonder if perhaps the children had been intentionally killed.
  • The third possibility is that there is a predisposition for SIDS than may run in some families and so it is not a random event.

The Care of the Next Infant (CONI) programme is available throughout most of the UK. All second infant deaths until 1990, were studied for one paper which reported:[15]

  • 57 infant deaths, giving a rate of 8.9 per 1,000 that is rather higher than would have been expected with no past history.
  • Of the deaths, 9 were inevitable, and 48 were unexpected.
  • Of the 48 unexpected deaths there were 2 in each of 2 families and a single one in the other 44.
  • Of the 46 first CONI deaths, 40 were natural but the other 6 were probable homicides.
  • There were 5 committed by one or both parents and 2 resulted in criminal conviction.

The conclusion is that repeat deaths occur and are usually natural but murder by parents does occur.

SIDS cannot be prevented completely but experience shows that it can be reduced. This requires attention to the various risk factors outlined above. In particular, placing the baby to sleep on the back and avoidance of smoking are important.

Supine sleeping position has, however, increased the incidence of flattening of the occiput (deformational plagiocephaly). To try to prevent this, infants should have supervised 'tummy time' when awake - spending as much time as possible in the prone position.[9]

Further reading & references

  1. Kinney HC, Thach BT; The sudden infant death syndrome. N Engl J Med. 2009 Aug 20;361(8):795-805.
  2. Moon RY, Horne RS, Hauck FR; Sudden infant death syndrome. Lancet. 2007 Nov 3;370(9598):1578-87.
  3. Advice for professionals; The Lullaby Trust
  4. Shah T, Sullivan K, Carter J; Sudden infant death syndrome and reported maternal smoking during pregnancy. Am J Public Health. 2006 Oct;96(10):1757-9.
  5. Matturri L, Ottaviani G, Lavezzi AM; Maternal smoking and sudden infant death syndrome: epidemiological study related to pathology. Virchows Arch. 2006 Nov 8;.
  6. McGarvey C, McDonnell M, Hamilton K, et al; An 8 year study of risk factors for SIDS: bed-sharing versus non-bed-sharing. Arch Dis Child. 2006 Apr;91(4):318-23. Epub 2005 Oct 21.
  7. Yiallourou SR, Walker AM, Horne RS; Effects of sleeping position on development of infant cardiovascular control. Arch Dis Child. 2008 Oct;93(10):868-72. Epub 2008 May 2.
  8. Schnitzer PG, Covington TM, Dykstra HK; Sudden unexpected infant deaths: sleep environment and circumstances. Am J Public Health. 2012 Jun;102(6):1204-12. Epub 2012 Apr 19.
  9. Adams SM, Good MW, Defranco GM; Sudden infant death syndrome. Am Fam Physician. 2009 May 15;79(10):870-4.
  10. Weber MA, Klein NJ, Hartley JC, et al; Infection and sudden unexpected death in infancy: a systematic retrospective case review. Lancet. 2008 May 31;371(9627):1848-53.
  11. Weber MA, Ashworth MT, Risdon RA, et al; The role of post-mortem investigations in determining the cause of sudden unexpected death in infancy. Arch Dis Child. 2008 Dec;93(12):1048-53. Epub 2008 Jun 30.
  12. Paterson DS, Trachtenberg FL, Thompson EG, et al; Multiple serotonergic brainstem abnormalities in sudden infant death syndrome. JAMA. 2006 Nov 1;296(17):2124-32.
  13. Hall KL, Zalman B; Evaluation and management of apparent life-threatening events in children. Am Fam Physician. 2005 Jun 15;71(12):2301-8.
  14. Dewolfe CC; Apparent life-threatening event: a review. Pediatr Clin North Am. 2005 Aug;52(4):1127-46, ix.
  15. Carpenter RG, Waite A, Coombs RC, et al; Repeat sudden unexpected and unexplained infant deaths: natural or unnatural? Lancet. 2005 Jan 1-7;365(9453):29-35.

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. EMIS has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.

Original Author:
Dr Hayley Willacy
Current Version:
Peer Reviewer:
Prof Cathy Jackson
Document ID:
2819 (v22)
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