Under the WHO International Classification of Diseases, infant deaths are categorised according to whether they were expected or unexpected, and if unexpected, whether or not they were explained. The categorisation SUDI (Sudden Unexpected Death in Infancy -- in the US known as SUID) covers all unexpected infant deaths, including both explained, and unexplained. Explained SUDI may include congenital issues, sudden onset illnesses,accidents and infanticides. If a suddent infant death is unexplained it is often classified as SIDS.
What is Sudden Infant Death Syndrome (SIDS)?
Sudden Infant Death Syndrome was defined in 1965 under code 795 of the International Classification of Diseases (ICD-8) for infant deaths as the sudden unexpected death of an infants with no apparent cause. SIDS is a category of exclusion for designating the death of an infant where a post-mortem examination (and often a death scene investigation) fails to determine a specific cause (Willinger et al. 1991).
Whilst deaths attributed to sudden infant deaths have been on a downward trajectory since the 1990s, this trend has recently begun to falter. In both 2008 and 2013 there were troubling increases in sudden and unexplained infant deaths. In 2013 SIDS was attributed to 162 of 249 unexplained infant deaths, a rate of 0.36 per 1,000 live births (ONS: 2016).
Research into SIDS characteristics was prioritized in New Zealand where rates were especially high, with a national SIDS case-control study that became particularly influential (Mitchell et al. 1992; Mitchell 2009). Three factors accounted for 79% of SIDS deaths: prone infant sleeping position, maternal smoking, and not breastfeeding, triggering the launch in 1991 of the New Zealand Cot Death Prevention Programme (Mitchell 2009).
The launch of 'Back to Sleep' campaigns
Parents were encouraged to place their infants for sleep in a non-prone position, keep them smoke-free, and to breastfeed. Following further data analyses, avoidance of parent-infant bed-sharing was added in 1992 (Mitchell et al 1992; Mitchell 2009). Other countries with substantial SIDS-rates such as the UK, Norway and Ireland (e.g. Fleming et al. 1996; Oyen et al. 1997; McGarvey et al. 2003) also conducted national case-control studies. All confirmed the association between prone infant sleep position and SIDS, leading to wide-scale Back to Sleep campaigns, and dramatic reductions in infant deaths (e.g. Golding et al. 1992; De Jonge et al. 1993; Irgens et al. 1995; Markestad et al. 1995).
Encouraging parents to sleep their infants in a supine position was associated with a dramatic fall in the SIDS-rate in many Western countries over the past two decades. Subsequent studies identified further key risks which are now incorporated into the 'Triple Risk Model': this is the best explanation we have for SIDS to date.
SIDS risk across cultures
In the US, African American, Alaskan Native, and Native American communities are disproportionately affected by SIDS (NICHD 2001), and recently a higher incidence of SIDS in child-care settings has been identified (Moon, et al. 2008). In New Zealand, SIDS-risk is substantially higher in Maori families (Mitchell et al. 1993), but not those of Pacific Island origin (Scragg et al. 1995; Schluter et al. 2007). UK immigrants from 'New Commonwealth' countries (India, Pakistan, Bangladesh, and Caribbean) have a greatly reduced risk of SIDS compared with the White British population (Balarajan et al. 1989; Davies and Gantley 1994; ONS 2000), while in Holland infants of Moroccan immigrants have a SIDS-risk three-times lower than the Dutch population but infants of Turkish immigrants exhibit a significantly increased SIDS-risk (van Sleuwen et al. 2003).
Other sleep-related infant deaths (explained SUDI)
SIDS refers to sudden unexplainable infant deaths, and is grouped with other sudden explainable infant mortality under the heading Sudden Unexpected Death in Infancy (SUDI). The clinical characteristics of SIDS and explained SUDI are similar: infants in both groups have generally poorer health, a higher frequency of symptoms, and a history of apparent life-threatening events (ALTE) (Ward-Platt et al. 2000), however, maternal smoking during pregnancy is particularly relevant to the etiology of SIDS. Sleep environments are also implicated in infant suffocation deaths, and various lethal sleep environments have been described in detail (Byard et al. 1994; 2001). The differentiation of SIDS and explained SUDI is problematic due to ambiguity in the pathological separation of SIDS and soft suffocation. In the case of bed-sharing deaths the evidence used is often circumstantial, and when infants die while bed-sharing coroners sometimes assign the death to the 'Unexplained/Unascertained' category rather than SIDS or SUDI due to the presence of a sleep-partner (O'Hara et al. 2000). This skews national figures and comparisons within populations, especially where bed-sharing is more prevalent in some sub-groups than others.