The Sudden Infant Death Syndrome
Hannah C. Kinney, M.D. and Bradley T. Thach, M.D.
sudden infant death syndrome (sids), which is characterized by the sudden death of a seemingly healthy infant during a sleep period, has long been considered one of the most mysterious disorders in medicine.1,2However, in recent years, SIDS has been substantially demystified by major advances in our understanding of its relationship to sleep and homeostasis, environmental and genetic risk factors, and biochemical and molecular abnormalities. The most important advance has been the discovery that the prone sleep position more than triples the risk of SIDS,3 which in the early 1990s led to national and international campaigns advocating a supine sleep position for infants. Since then, it has been estimated that rates of SIDS have declined by more than 50%, and thousands of infant lives have been saved.3-6 However, SIDS still remains the leading cause of postneonatal infant death in the United States and is the third leading cause of infant mortality overall.7 This review highlights the major advances in our understanding of SIDS.
DEFINITION AND INCIDENCE OF SIDS
In 1969, a National Institutes of Health consensus conference led to the first standardized definition of sudden infant death as the “sudden death of an infant or young child, which is unexpected by history, and in which a thorough post mortem examination fails to demonstrate an adequate cause of death.”8 The definition mandated an autopsy for infants who died from a condition diagnosed as SIDS, which would demarcate a set of infants with similar characteristics for whom vital statistics, research, and family counseling were needed. Although SIDS was defined as a syndrome and thus potentially the result of more than one disease, many observers still viewed SIDS as a single entity because of its distinctive features, which included a peak incidence at 2 to 4 months of age, male predominance, and the presence of intrathoracic petechiae. Subsequent modifications of the definition restricted its application to infants under the age of 12 months,9added the requirement of a death-scene investigation,9 or linked the death to a sleep period (i.e., the time when the majority of deaths occurred10). Of note, it is unclear whether SIDS occurs during sleep itself or during the many transitions between sleep and arousal that occur during the night, since such deaths are typically not witnessed. No single definition of SIDS is universally accepted, and contradictions among SIDS studies are due in part to the use of various definitions of the syndrome around the world.11
Among the industrialized nations, Japan has the lowest reported SIDS rate (0.09 case per 1000 infants), New Zealand has the highest rate (0.80 per 1000), and the United States has an intermediate rate (0.57 per 1000).6A striking discrepancy exists among racial and ethnic groups that have been studied, with SIDS rates that are two to seven times the national averages among Native Americans12 and blacks13 in the United States; among persons of mixed ancestry in Cape Town, South Africa14; among Maoris in New Zealand4,15; and among aboriginal Australians.15 In a recent review of SIDS data from 13 predominantly industrialized countries,16 the majority of countries had a major decrease in SIDS rates from 1990 to 2005 (the most recent year for which data were available), with the largest decreases occurring before 2000.16 These decreases ranged from 40% in Argentina to 83% in Ireland. The true incidence of SIDS may be masked by a so-called diagnostic shift, or the use of diagnoses other than SIDS on death certificates (e.g., accidental suffocation, positional accidental asphyxia, and indeterminate cause). Diagnostic shift may explain, in part, why postneonatal death rates for both SIDS and non-SIDS have remained static since approximately 2000.