Sudden Infant Death Syndrome (SIDS) refers to the sudden and unexpected death of an apparently healthy infant less than one year of age, which remains unexplained even after a full investigation. Such events are so widespread that they are now recognized as a disease syndrome not caused by suffocation or by parental neglect. The incidents are almost invariably discovered when babies are supposedly asleep, hence the term cot death or crib death. This phenomenon concerns a very young child, usually from two to six months of age, who was either quite well or had only trivial symptoms. SIDS babies invariably die during the night, during their sleep, so some people have suggested there is a disorder in the parts of the brain controlling sleep and arousal from sleep. However, breathing and heartbeat irregularities are common among even healthy infants, and severe problems, while much more rare, do not seem to predict SIDS.
In the USA in 1996 the incidence was 1 in 1000 SIDS deaths. In Belgium SIDS kills around 180 babies a year (0.8 deaths per thousand), whereas in Canada the figure is about 400 deaths. In both countries SIDS is the leading cause of death in infants between one month and one year of age. The rate of SIDS in the Netherlands is 0.17 per thousand, one of the lowest in the world. African-American, and Canadian and American Aboriginal infants have a risk of SIDS that is three to four times higher than the risk to non-Aboriginal infants. Premature and low-birth weight babies of any ethnic type are at most risk.
Although the specific cause of SIDS remains unknown, there is some knowledge of certain risk factors. The age range within which cot death occurs is from two weeks to two years. It is thus not a condition of new-born children; death in the first couple of weeks of life almost always excludes this syndrome. The great majority of cases occur between two and six months, with a peak at four months. Few deaths occur after nine months, and those in the second year are very rare. There is a significant preponderance of boys over girls; most of the evidence suggests that such deaths affect boys more than girls in the ratio of about 3 : 2.
There is a striking seasonal variation in the deaths. Cot deaths occur in the cold, wet seasons when respiratory infections are at their height. The preponderance of cot deaths in the lower income groups also suggests that over-crowding within households, especially relating to the sleeping places ("bedsharing") and possibly to raised cross infection, are significant contributing factors. Researchers looked at 84 infant deaths in Cleveland between 1992 and 1996 that were classified as SIDS deaths. At the time of their deaths, more than 30 of the infants were sleeping one or more adults on either a bed or a sofa. Whilst the study does not prove that bedsharing was the cause of the deaths (most probably from suffocation after adults roll over or lay on the infant), it does indicate that bedsharing may be a risk factor for SIDS-like deaths (some say because adult beds are too soft for babies).
Studies in England, the Netherlands, Australia and New Zealand report that putting the baby to sleep on its back and not using the prone sleeping position (sleeping on the tummy) reduced SIDS deaths. In the UK a public awareness programme was called the "Back to Sleep" campaign. In the USA, the incidence fell 30% in the 2 years before 1995 and was attributed to a campaign to put babies to sleep on their backs.
Heat stress campaign heat stress is considered a preventable risk factor in crib deaths. Excessive sweating, high room temperature, infection with fever and excessive bedclothes are factors in heat stress. Re-breathing of exhaled air is another putative cause. Both are minimized by avoidance of prone sleeping, soft bedding and covering the head. It is possible that both thermal stress and re-breathing are interrelated co-factors of a lethal process that could lead to (death) through failure to arouse or respiratory failure. Defects in brain stem function from earlier injuries could increase the susceptibility to either mechanism. The risk of both uncontrollable overheating and poor air circulation is likely to be higher in winter, and thus this hypothesis is supported by the fact that more deaths occur in winter than summer.
One view in New Zealand is that cot death is caused by gaseous poisoning. The gases concerned are phosphines, arsines and stibines, which are all extremely toxic nerve gases. They are produced in a baby's cot (or any other bed where the baby sleeps) by the action of common household fungus on compounds of phosphorus, arsenic and antimony present in the mattress (and in certain other underbedding, e.g. sheepskins).
A number of studies have shown that the cot-death syndrome is very closely connected with smoking during pregnancy: an American study covering nearly 20,000 births showed that 70% of the mothers whose children had fallen victim to the syndrome had smoked during pregnancy. According to a 1992 British report, one cot death every day occurs because the baby's mother is a smoker. More than 1 in 4 crib deaths are attributed to either the mother having smoked while pregnant or during the first few months of the infant's life. The same report states an estimated 1 in 3 women continues to smoke throughout her pregnancy.
Exposure to second-hand tobacco smoke is another significant risk factor. An infant who has been exposed either before or after birth is placed at an increased risk for SIDS compared to those infants who have not been exposed. If the mother smokes, the infant has 6 times the risk of dying of SIDS. Bottle-fed babies have 3 times the risk of SIDS that breast-fed babies do.
Not all cot deaths are unexpected, nor are all unexpected deaths of infants cot deaths; they may be the result of unsuspected conditions such as a heart or kidney disorder.
Through the 1970s and 1980s in the USA, it was believed that cot death was associated with prolonged apnoea (non-breathing). This was later proven to be based on distorted evidence from the 1970s, notably from a family with five cases of cot death. The mother was convicted in the 1990s for multiple murder of her infants. This raised the question of how many single cases of cot death are actually death by deliberate smothering.