Sudden Infant Death Syndrome, SIDS, is the leading cause of deaths in infants aged 1-12 months. “Syndrome” is the word used when symptoms and physical findings are often found together. In other words, SIDS is not a disease or a medical condition, it is the term used when infants die suddenly and unexpectedly. The most frequent age that infants are affected by SIDS is between 2-4 months. In Canada, 90% SIDS cases occur when the infant is under 6 months old. Approximately 1 out of 2,000 infants die of SIDS/year, every year.

SIDS can happen at any time of day but is most likely to occur at night. As implied by the word “sudden”, SIDS can happen without warning, but the babies do not seem to suffer.
What makes SIDS particularly frightening for parents and loved ones is that the precise cause of death might not be clear, even after a thorough investigation. This investigation includes a death scene investigation, a post-mortem examination, and a review of the infant’s medical history.
The sudden death of babies is not new; it has been happening since ancient times. The term SIDS was first used in 1968, and in the 20 years afterwards support groups for parents developed. Also a great deal of information was gathered about SIDS. This could be described as “passive research”, collecting information that was already available in official records. Much of this information was confusing, or was about things that could not be changed. As an example slightly more boys and girls die of SIDS (about 60% of the deaths are boys).
But in the late 1980s it was realized that there were some things about the baby’s sleep that seemed to be important in SIDS and that parents would be able to change.
These so-called “modifiable risk factors” were:

  • Sleeping on the tummy or the side
  • Exposure to tobacco smoke
  • Sharing a sleep surface with an adult
  • Not breast-feeding

For the next several years, carefully carried out “real life” studies confirmed that some sleep environments were much safer than others. In the studies, the families of babies who died of SIDS were visited as soon as possible after the death, sometimes the same day and usually within a week, by researchers. The researchers matched the baby who died with 2, 3 or 4 babies who survived and were as alike to the SIDS babies in as many different ways as possible. Parents of the babies who died and those who survived were asked the same questions about their babies and how they cared for them.
The conclusions were clear. Babies should:

  • Sleep on their back
  • Never be exposed to tobacco during pregnancy or after birth
  • Sleep in their own crib
  • Be breast-fed wherever possible.

It was also clear that all the recommendations mattered. Parents couldn’t safely choose one and ignore the others.
It was soon seen that this research could be the basis of campaigns to prevent SIDS. There were “Safe Sleep” campaigns in most countries from 1992 onwards, and the number of babies dying of SIDS fell dramatically. Thousands of lives have been saved worldwide by parents following these simple recommendations. In Alberta nearly a thousand babies are now alive who might have died if these facts had not been known. SIDS rates are given as numbers who died related to births in that year. In Alberta in 1977 there were 34,406 births and 77 SIDS deaths, a SIDS rate of 2.24/1000. By 2013 there were many fewer SIDS – 26, even although there were many more births – 56, 078. The SIDS rate had fallen to 0.46/1000.
But while following the safe sleep recommendations are important, they are not the whole story. Some researchers have tied things together with the “Triple Risk Model”. The idea here is that three things need to happen before a baby dies of SIDS. Firstly there needs to be a “predisposition” or liability to have the condition, secondly the baby needs to be at a specific stage of development, and thirdly the baby needs to be exposed to an unsafe environment. So we know a lot about two of the three things (age and environment) but not nearly enough about the first one. Some of the things we do know in terms of liability are that when babies are exposed to tobacco during pregnancy, or when a baby is born prematurely, the chance of SIDS goes up. All parents need to be very careful with the sleep environment, the parents of preterm infants, or mothers who smoke during pregnancy need to be even more careful. Current research focuses on how breathing and arousal are controlled, and the results of this research may give more clues as to what some babies are liable to an unsafe sleep environment and other babies are not.

Triple-Risk Model

  • A vulnerable infant. An underlying defect or brain abnormality can make the baby vulnerable. Factors such as defects in parts of the brain that control respiration, heart rate, or genetic mutations qualify as vulnerabilities.
  • The critical developmental period. During the infant’s first 6 months of life, rapid growth and changes in homeostatic controls occur. These changes may be evident (e.g., sleeping and waking patterns), or they may be subtle (e.g., variations in breathing, heart rate, blood pressure, and body temperature). Some of these changes may destabilize the infant’s internal systems temporarily or periodically.
  • Outside stressor(s). Most babies can survive environmental stressors, such as a stomach sleep position, overheating, secondhand tobacco smoke, or an upper respiratory tract infection. However, an already vulnerable infant may not be able to overcome them. Although these stressors are not believed to single-handedly cause infant death, they may tip the balance against a vulnerable infant’s chances of survival.

Critical Developmental Period
External Stressors
Vulnerable Infant

According to the Triple-Risk Model, all three elements must be present for a sudden infant death to occur:

  • The babies vulnerability is undetected.
  • The infant is in a critical developmental period that can temporarily destabilize his or her systems.
  • The infant is exposed to one or more outside stressors that he or she cannot overcome because of the first two factors.

If parents and guardians can remove a factor from this list, such as placing the baby on their back to sleep, eliminating an outside stressor then the risk of SIDS will be reduced.

Brain Abnormalities

As SIDS research develops, there is more and more evidence that suggests babies who are born with unseen brain abnormalities are more likely to succumb to SIDS. These abnormalities are usually found in the portion of the brainstem that that likely controls breathing, heart rate, blood pressure, temperature and waking from sleep.
Scientists, however, believe that these brain abnormalities alone might not be enough to cause SIDS. Many medical professionals say that other events must also occur in order for SIDS to strike, such as lack of oxygen, overheating etc.
Normally, infants will sense a problem in their sleep and be able to correct it. For example, if a baby is asleep on their stomach with the exhaled air being re-breathed, they will sense the inadequate air flow and their brains will trigger them to wake up or change breathing patterns. If an infant has an abnormality in their brain stem, these protective mechanisms could be less effective, leading to SIDS.

Genetic Polymorphisms

It’s unlikely that one defective gene is the cause of SIDS, but certain genes may combine with exterior risk factors that could result in SIDS. These genes could affect the metabolic and immune systems, as well as other conditions that affect the brain stem or cause neurochemical imbalances in the brain. Polymorphisms may predispose infants to death in critical situations.
Many SIDS infants have an activated immune system which may indicate that they were already vulnerable to infection. In one study, around 50% of infants who died of SIDS had a mild upper airway infection before death.

Genetic Mutations

Genetic mutations can lead to disorders that could be a cause of Sudden Infant Death Syndrome. Genetic screening before conception can help determine potential disorders, such as a metabolic disorder, that could be passed onto an infant. If a mutation is found the baby can be tested soon after birth. If the condition is not identified, the resulting death could be mistaken for SIDS.  
When SIDS occurs, medical professionals look for infants with genetic mutations because they would be excluded from a SIDS diagnosis. Some infants who have been diagnosed with SIDS have rare mutations that affect the function of the cardiac conduction system.