Sudden infant death syndrome--insights from epidemiological research.

Sudden infant death syndrome (SIDS) was not recognised as an entity until the early 1960s, and an accepted definition did not emerge until 1969. At that time it was anticipated that the condition might contain subgroups. However, investigators have proceeded in the main as if it was a single condition. The original definition, "the sudden death of any infant or young child which is unexpected by history and in which a thorough postmortem examination fails to demonstrate an adequate cause for death",' was revised by the National Institutes of Health, USA, in 1989. The revised definition included only infants dying suddenly under one year of age whose deaths remain unexplained after a complete necropsy examination, including death scene investigation and case history review, had been conducted.2 Thus the current definition of SIDS remains one of exclusion. However, using standard necropsy protocols to define SIDS as a cause of death it is now possible to obtain good agreement between pathologists independently assessing a case. It might have been expected that following the identification of the syndrome the first epidemiological studies would have been of a descriptive nature, with analytical studies chasing the leads generated by that work and by concurrent laboratory studies. However, since 1969 both descriptive and analytical epidemiological investigations have proceeded in parallel and it is not at all clear that the hypotheses generated in the descriptive studies have been the basis for much of the analytical work. We shall nonetheless discuss some of the major findings from the two areas in turn, recognising that some of these distinctions between study types are somewhat arbitary.