SIDS — A Syndrome in Search of a Cause
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《新英格兰医药杂志》
The sudden infant death syndrome (SIDS) was initially defined in 1969 with the goal of identifying infants who had similar characteristics before sudden death so that a common underlying mechanism of death might be discovered. The original definition was quite broad, and its development was driven in part by parents of infants whose deaths were unexplained. Since then, the definition has been refined, first by an expert group convened by the National Institute of Child Health and Human Development (NICHD) and more recently by an expert panel of forensic pathologists whose definition includes several categories of SIDS. This most recent approach defines SIDS generally as "the sudden unexpected death of an infant less than 1 year of age, with the onset of the fatal episode apparently occurring during sleep, that remains unexplained after a thorough investigation including performance of a complete autopsy and review of the circumstances of death and the clinical history."1
Despite attempts to define SIDS, the classification of deaths of infants and the tracking of trends in SIDS in the United States remain inexact. The classification of a death as caused by SIDS depends on a death certifier's compliance with the current definition of the syndrome and the version of the International Classification of Diseases (ICD) coding for the underlying cause of death that is currently in use by the National Center for Health Statistics. After SIDS was defined in 1969, the number of sudden, unexplained deaths of infants reported in the United States began to increase dramatically (see Figure). In 1979, the rates of death due to SIDS began to be published in the Vital Statistics of the United States (Vol. II, Mortality, Part A) as a separate category of death; SIDS accounted for the vast majority of sudden, unexplained infant deaths. In 2001, the deaths of 2234 infants in the United States were attributed to SIDS.2
Figure. Rates of Sudden, Unexplained Infant Death and Death Due to SIDS.
The specific diagnosis of SIDS was not used for reporting sudden, unexplained infant deaths in the United States until after 1978. The number of sudden, unexplained infant deaths — the majority of which were attributed to SIDS — peaked in 1979; it began to decrease sharply after 1992.
A number of case–control studies conducted between 1970 and 1990 in Europe, Australia, and New Zealand reported an association between the prone positioning of infants for sleep and the occurrence of SIDS. Although no specific mechanism had been identified, hypotheses to explain the risk associated with prone positioning included oropharyngeal obstruction, rebreathing with associated hypoxia and hypercarbia, and overheating. On the basis of these observations, several investigators began advocating positioning infants on their backs for sleep — recommendations that were highly publicized in the lay press. Subsequently, preliminary reports from New Zealand and the United Kingdom showed decreases in the incidence of SIDS.
In the United States, the Task Force on Infant Positioning and SIDS of the American Academy of Pediatrics (AAP) published a document in 1992 recommending that infants be placed on their backs for sleep in order to prevent SIDS.3 The NICHD then joined forces with the AAP and several SIDS Alliance groups to begin a national "Back to Sleep" campaign in 1994.4 Thanks to this effort, the proportion of infants in the United States who were placed in the prone position for sleep was reduced from approximately 70 percent in 1992 to 20 percent in 1998, and the number of deaths classified as caused by SIDS decreased by more than 50 percent between 1992 and 2001. Although much of this decrease is probably attributable to the avoidance of the prone position, it is possible that some of the reduction may be explained by alternative classifications of these deaths. In this regard, it is notable that the rate of SIDS continued to decrease from 1999 to 2001, while the overall postneonatal mortality rate stabilized.2
Given the difficulty of identifying the features of a sudden infant death that make it classifiable as SIDS, it is perhaps understandable that little progress has been made in identifying a particular cause or causes of this syndrome. Whereas a classic epidemiologic investigation involves definite clinical and pathological findings that can be used to identify a disease state, a diagnosis of SIDS essentially indicates the absence of a known explanation for the death. Demographic characteristics of the mother that are associated with SIDS thus reflect the characteristics of populations at risk for other causes of infant death — namely, youth, a low level of education, and disenfranchisement. Other risk factors for SIDS such as preterm birth and maternal smoking have not been shown to be causally related to SIDS by any specific mechanism and are known to be associated with other causes of infant death. Bed sharing has been associated with SIDS, primarily in association with mothers who smoke or abuse alcohol or drugs. Rates of recurrence within families remain uncertain, and when SIDS occurs in two or more siblings, concern may be aroused about possible infanticide.
The hypothesis that SIDS occurs in biologically vulnerable infants who are exposed to an environmental risk factor implies that these infants should be identifiable before the event. Recordings of neonatal cardiorespiratory activity in large populations, however, have not been useful for prospective identification of such infants.
In this issue of the Journal, Smith et al. (pages 978–986) report a positive association between second-trimester levels of maternal serum alpha-fetoprotein and the subsequent risk of SIDS. They conclude that the association may be mediated in part through impaired fetal growth. Alpha-fetoprotein, however, is a nonspecific marker of placental and fetal dysfunction that may be associated with a number of causes of fetal and infant death. The clinical importance, relative to other demographic and behavioral risk factors, of the observed doubling of the risk of SIDS that is associated with a second-trimester alpha-fetoprotein level in the fourth or fifth quintile as compared with the lowest quintile remains uncertain.
The cause of SIDS is not all that remains unknown about this syndrome. A clearer, more identifiable presentation and a more precise definition would also be helpful. In the meantime, one hopes that as death-scene investigations and autopsy procedures become more thorough and widespread, the number of infant deaths that are sudden and unexplained will continue to diminish.
Source Information
From the Department of Pediatrics, University of Texas Medical Branch, Galveston.
References
Krous HF, Beckwith JB, Byard RW, et al. Sudden infant death syndrome and unclassified sudden infant deaths: a definitional and diagnostic approach. Pediatrics 2004;114:234-238.
CDC Wonder. Compressed mortality file: underlying cause of death: mortality for 1979-1998 with ICD 9 codes: mortality for 1999-2001 with ICD 10 codes. (Accessed August 13, 2004, at http://wonder.cdc.gov/mortSQL.html.)
AAP Task Force on Infant Positioning and SIDS. Positioning and SIDS. Pediatrics 1992;89:1120-1126.
Willinger M, Hoffman HJ, Hartford RB. Infant sleep position and risk for sudden infant death syndrome: report of meeting held January 13 and 14, 1994, National Institutes of Health, Bethesda, MD. Pediatrics 1994;93:814-819.(Michael H. Malloy, M.D.)
Despite attempts to define SIDS, the classification of deaths of infants and the tracking of trends in SIDS in the United States remain inexact. The classification of a death as caused by SIDS depends on a death certifier's compliance with the current definition of the syndrome and the version of the International Classification of Diseases (ICD) coding for the underlying cause of death that is currently in use by the National Center for Health Statistics. After SIDS was defined in 1969, the number of sudden, unexplained deaths of infants reported in the United States began to increase dramatically (see Figure). In 1979, the rates of death due to SIDS began to be published in the Vital Statistics of the United States (Vol. II, Mortality, Part A) as a separate category of death; SIDS accounted for the vast majority of sudden, unexplained infant deaths. In 2001, the deaths of 2234 infants in the United States were attributed to SIDS.2
Figure. Rates of Sudden, Unexplained Infant Death and Death Due to SIDS.
The specific diagnosis of SIDS was not used for reporting sudden, unexplained infant deaths in the United States until after 1978. The number of sudden, unexplained infant deaths — the majority of which were attributed to SIDS — peaked in 1979; it began to decrease sharply after 1992.
A number of case–control studies conducted between 1970 and 1990 in Europe, Australia, and New Zealand reported an association between the prone positioning of infants for sleep and the occurrence of SIDS. Although no specific mechanism had been identified, hypotheses to explain the risk associated with prone positioning included oropharyngeal obstruction, rebreathing with associated hypoxia and hypercarbia, and overheating. On the basis of these observations, several investigators began advocating positioning infants on their backs for sleep — recommendations that were highly publicized in the lay press. Subsequently, preliminary reports from New Zealand and the United Kingdom showed decreases in the incidence of SIDS.
In the United States, the Task Force on Infant Positioning and SIDS of the American Academy of Pediatrics (AAP) published a document in 1992 recommending that infants be placed on their backs for sleep in order to prevent SIDS.3 The NICHD then joined forces with the AAP and several SIDS Alliance groups to begin a national "Back to Sleep" campaign in 1994.4 Thanks to this effort, the proportion of infants in the United States who were placed in the prone position for sleep was reduced from approximately 70 percent in 1992 to 20 percent in 1998, and the number of deaths classified as caused by SIDS decreased by more than 50 percent between 1992 and 2001. Although much of this decrease is probably attributable to the avoidance of the prone position, it is possible that some of the reduction may be explained by alternative classifications of these deaths. In this regard, it is notable that the rate of SIDS continued to decrease from 1999 to 2001, while the overall postneonatal mortality rate stabilized.2
Given the difficulty of identifying the features of a sudden infant death that make it classifiable as SIDS, it is perhaps understandable that little progress has been made in identifying a particular cause or causes of this syndrome. Whereas a classic epidemiologic investigation involves definite clinical and pathological findings that can be used to identify a disease state, a diagnosis of SIDS essentially indicates the absence of a known explanation for the death. Demographic characteristics of the mother that are associated with SIDS thus reflect the characteristics of populations at risk for other causes of infant death — namely, youth, a low level of education, and disenfranchisement. Other risk factors for SIDS such as preterm birth and maternal smoking have not been shown to be causally related to SIDS by any specific mechanism and are known to be associated with other causes of infant death. Bed sharing has been associated with SIDS, primarily in association with mothers who smoke or abuse alcohol or drugs. Rates of recurrence within families remain uncertain, and when SIDS occurs in two or more siblings, concern may be aroused about possible infanticide.
The hypothesis that SIDS occurs in biologically vulnerable infants who are exposed to an environmental risk factor implies that these infants should be identifiable before the event. Recordings of neonatal cardiorespiratory activity in large populations, however, have not been useful for prospective identification of such infants.
In this issue of the Journal, Smith et al. (pages 978–986) report a positive association between second-trimester levels of maternal serum alpha-fetoprotein and the subsequent risk of SIDS. They conclude that the association may be mediated in part through impaired fetal growth. Alpha-fetoprotein, however, is a nonspecific marker of placental and fetal dysfunction that may be associated with a number of causes of fetal and infant death. The clinical importance, relative to other demographic and behavioral risk factors, of the observed doubling of the risk of SIDS that is associated with a second-trimester alpha-fetoprotein level in the fourth or fifth quintile as compared with the lowest quintile remains uncertain.
The cause of SIDS is not all that remains unknown about this syndrome. A clearer, more identifiable presentation and a more precise definition would also be helpful. In the meantime, one hopes that as death-scene investigations and autopsy procedures become more thorough and widespread, the number of infant deaths that are sudden and unexplained will continue to diminish.
Source Information
From the Department of Pediatrics, University of Texas Medical Branch, Galveston.
References
Krous HF, Beckwith JB, Byard RW, et al. Sudden infant death syndrome and unclassified sudden infant deaths: a definitional and diagnostic approach. Pediatrics 2004;114:234-238.
CDC Wonder. Compressed mortality file: underlying cause of death: mortality for 1979-1998 with ICD 9 codes: mortality for 1999-2001 with ICD 10 codes. (Accessed August 13, 2004, at http://wonder.cdc.gov/mortSQL.html.)
AAP Task Force on Infant Positioning and SIDS. Positioning and SIDS. Pediatrics 1992;89:1120-1126.
Willinger M, Hoffman HJ, Hartford RB. Infant sleep position and risk for sudden infant death syndrome: report of meeting held January 13 and 14, 1994, National Institutes of Health, Bethesda, MD. Pediatrics 1994;93:814-819.(Michael H. Malloy, M.D.)