The more that is discovered about SIDS risk the more it is recognised that causation cannot be reduced to a single factor. The triple-risk model (or triple risk hypothesis) is the best current consensus explanation for SIDS encompassing three key factors:
- a vulnerable infant
- a critical developmental period
- an external stressor.
Age is significant in infants as this critical period of development appears between 2-4 months of life (the age at which SIDS peaks). Vulnerability includes all intrinsic risk factors, such as gender, prematurity, genetic polymorphisms and prenatal exposure to harmful chemicals. Environment refers to extrinsic risk factors, for example sleeping position, bedding and sleep location. Each factor is outlined with a few examples below.
The critical developmental period for most infants appears to be 2-4 months of age. This is when most SIDS deaths occur. Infants who are born prematurely are likely to have a delayed critical period compared to term babies.
e.g Asphyxia and brainstem abnormalities - Prematurity Smoke exposure during gestation
There are some intrinsic factors of infants (characteristics they are born with) that may make them more vulnerable than others to SIDS, such as brain-stem abnormalities. Studies are increasingly homing in on the relationship between breathing regulation and neurochemical abnormalities in the brainstem that can impair protective responses. Prematurity is likely to be associated with SIDS due to an inability of some of these infants to mount a normal arousal response in the face of a physiological challenge. Smoke exposure in utero has a similar explanation in blunting an infant's normal arousal response.
Sleeping position - Head covering - Overheating - Post-natal smoke exposure - Formula feeding - Sleeping in a room alone - Soft bedding - Bed-sharing - Soft toys etc.
Although studies demonstrated a strong association between prone (on front) sleep and SIDS, and encouraging parents to sleep their infants in a supine position was associated with a dramatic fall in the SIDS-rate wherever such recommendations were introduced, we still do not know what it is about prone sleep that increases a baby's chances of dying suddenly and unexpectedly. Various explanations such as toxic mattress gases and rebreathing exhaled carbon diolxide have been investigated and debunked. One potential finding relates to sleep patterns -- babies experience more deep sleep and fewer spontaneous arousals in the prone position: it may be that babies some babies' brains are not well enough developed to arouse themselves from particularly deep sleep when confronted with a physiological stressor, such as head covering.
Subsequent studies identified further key risks: parental smoking-, head-covering, overwrapping, and infant illness also were associated with increased risk (Fleming et al. 2003; Fleming et al. 1996) along with soft bedding, soft sleep surfaces, overheating (Flick et al. 2001; Moon, et al 2007), breastfeeding for less than two weeks, and 'co-sleeping' (Vennemann et al. 2009).
For reasons that remain unclear, the risk of SIDS is particularly high for infants who sleep with parents on a sofa (Blair et al. 1999).? Pacifier use during sleep is apparently protective (Moon et al. 2007; Vennemann et al. 2009) although there is some debate as to whether this might be a marker for something else such as change in family routine. Infants who sleep in a separate room from their parents are at increased risk compared to infants who sleep in the same bedroom (Blair et al. 1999).
Seasonal and climatic variation
Winter months are associated with a higher SIDS rate. Environmental risk factors are twofold: overheating and overbedding. These are both more likely in winter due to an increased use of central heating at night and the extra use of bedding and clothing, regardless of the indoor temperature. This is the reasoning behind the rise in unexplained infant deaths in February 2013 when monthly temperature was below average (ONS: 2015). In this year 28% of unexplained infant deaths occurred in the winter, as opposed to 22% in summer.
Overheating is a danger for infants, their thermal balance of significantly favouring heat conservation, particularly at three months. Several elements come together at this point: metabolic rate and mass to surface area ratio are both elevated, a thicker layer of subcutaneous fat develops and peripheral vasomotor response to cold is more effective.
When sleeping in bedding up to 85% of an infant’s heat loss is through the head (Fleming et al. 1992). If covered by bedding this heightens the risk of hyperthermia, as well as hypoxia due to rebreathing of air and lack of oxygen. Studies (Ponsonby et al., 1991; Williams et al., 1996) have found infants with SIDS were more heavily wrapped infants and households with heating on all night in comparison to control infants with matched variables.
Exemplifying the triple risk model, these intrinsic age-related biological vulnerabilities meet the more unfavourable winter environment, resulting in a higher SIDS rate.