Historical perspective

Sudden death in infancy has been reported since the earliest times. In the Book of Kings in the Bible there is a report of an infant being overlain and found dead. From Medieval to Victorian times babies were taken into their parents’ beds in the winter to protect them from the cold and some were found dead in the morning. As socioeconomic conditions and the care of newborn infants improved, these types of death became less common. In the 1960s, many babies in newborn intensive care units were nursed prone because of increased stability of the chest wall and better oxygenation in this position, and this sleep position became increasingly used for well babies at home, especially when it was realized that infants went to sleep faster in this position and that it was also associated with less gastro-oesophageal reflux.

In the 1970s and 1980s high rates of sudden infant death were recognized throughout the world and were considered to be so high that a number of large epidemiological studies were set up to try to determine the reason for these deaths. It was usual at this stage for the term ‘sudden infant death syndrome’, or SIDS, to be applied to these deaths. Both Australia and New Zealand were found to have some of the highest rates of SIDS in the world at this time. The apparent emergence of a new condition was almost certainly mainly related to diagnostic transfer as health professionals and coroners increasingly used the term (recognized with an International Classification of Diseases (ICD) 9 code of 798 in 1977 and an ICD10 code of R95 in 1992). This is strongly suggested because total post-neonatal mortality (within which are almost all SIDS cases) did not increase as SIDS rates rose. Case–control studies identified a number of risk factors associated with SIDS, and the dramatic decrease in SIDS and post-neonatal mortality in countries that intervened around these risk factors (primarily the prone sleep position) strongly suggest a causative relationship. This early research was followed by a plethora of basic science research to try to establish the mechanism by which the risk factors contributed to any individual infant’s risk of sudden death.

Despite these efforts there remain a significant number of infants who die suddenly and unexpectedly each year. It has increasingly been recognized that these infants represent a particular group and that many have been found sleeping in an unsafe sleep position or situation. For this reason, terminology has changed over time and this group of deaths is now more commonly discussed under the label ‘sudden unexpected death in infancy’ (SUDI).

Definitions

SUDI (sudden unexpected death in infancy), sometimes described as SUD or SUID, is a term used for all unexpected infant deaths, whether the explanation is determined after a thorough investigation or remains unknown. Traditionally all unexplained SUDI deaths have been labelled as sudden infant death syndrome (SIDS), but since the mid-1990s a number of forensic pathologists are becoming increasingly reluctant to use this diagnosis as a cause of death when certain risk factors are present (co-sleeping, prone positioning, issues surrounding appropriate parental care, minor pathological changes, etc.), preferring to use the terms ‘unascertained’, ‘borderline SIDS’, ‘undetermined’, ‘unspecified’ or ‘unknown’ when they report their findings to the coroner. The formal definition of SIDS has undergone several revisions since the term was initially coined by Beckwith in 1969, with the broad definition from 1989, ‘the sudden death of an infant less than one year of age which remains unexplained after a thorough case assessment, including performance of a complete autopsy, investigation of the death scene, and review of the clinical history’, still being the most widely used. Subsequent suggestions have involved including the need for the death to have occurred during sleep, and also several subclassifications have been suggested, none of which has entered into general usage.

Incidence and risk factors

As can be deduced from the discussion above, the problems of definition make comparisons over time and between countries problematic! Figure 3.10.1 shows both SIDS mortality and total post-neonatal mortality in 1990 (before risk reduction campaigns in these countries) and in 2005, 15 years later.

Fig. 3.10.1 SIDS and post-neonatal mortality in selected countries in the years 1990 and 2005.

Many studies have described the characteristics of babies who die from SIDS, and risk factors have been elucidated by a series of case–control studies. There has been much speculation as to why very few babies die in this manner in the first month of life, with the peak age of death at 24 months of age. Other factors such as prematurity and growth restriction at birth are more understandable, as are many risk factors that are difficult to change such as socioeconomic deprivation. More recently there has been concern that these deaths appear to be occurring more frequently in the first month of life.

The focus of much research has been on identifying ‘modifiable’ risk factors – by comparing the circumstances around the baby who has died with those of babies (matched for age) who have not. The key modifiable risk factors are nicely summarized by the International Society for the Study and Prevention of Perinatal and Infant Death (ISPID) and are:

Two key protective factors have been described – breastfeeding and immunization – with some ongoing discussion over consistent data from many studies suggesting that the use of a pacifier or dummy at the time of going to sleep is also protective.

Aetiology of sudden death in infancy

Much research has been undertaken in the past few decades to try to find a reason for the death of infants where the clinical history, examination of the scene of death and autopsy has not found a fatal diagnosis. These infants have been labelled as dying from SIDS, and research has indicated that they do have some common characteristics. For 20–30% of infants who appear to have died suddenly and unexpectedly, a clear cause of death is found through either the clinical history, death scene examination or autopsy. These cases can then be considered explained SUDI deaths.

Explained SUDI

Infection

Sudden and overwhelming infection is a possible cause of sudden infant death. Pneumococcal or meningococcal septicaemia or meningitis are often of particularly sudden onset, especially in the young infant. The diagnosis should be clear at autopsy. As evidence of minor infection may be found in infants who have a final diagnosis of SIDS, the pathologist must be convinced that the evidence of infection found is of a degree sufficient to explain the death.

Homicide

Homicide must always be kept in mind, especially in the situation where there have been other siblings who have apparently died from SIDS. In general, it is thought that less than 5% of SIDS cases may in fact be homicide. As intentional suffocation is virtually impossible to distinguish from SIDS at autopsy, this diagnosis may not be confirmed unless there are other significant injuries or a confession.

Accidental asphyxia

For some infants it is clear at the death scene that death has been due to accidental asphyxia. Some infants have been found dead caught in the bars of their cot or found strangled on a stray piece of cord. It is likely that any infant would die if they got themselves into such a situation. For other infants, the scene may have been suspicious for accidental asphyxia, for example if the infant was found lying under a parent, but it cannot be proven that the infant’s airway was completely obstructed at the time of death. The question then arises as to whether the particular infant may have been more vulnerable in a potentially unsafe sleep situation than an otherwise normal infant would be expected to be.

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