Sudden Infant Deaths: Sleep Environment and Circumstances
Sudden Infant Deaths: Sleep Environment and Circumstances
Our descriptive analysis of population-based multistate CDR program data showed that only one quarter of SUID victims were sleeping in a crib or on their back when found; 70% of infants were on a surface not intended for infant sleep, and 64% with documentation of their position when found were on their stomach or side. Importantly, 64% of SUID victims in our study were sharing a sleep surface, and 49% of these infants were sleeping with an adult. Infants whose deaths were classified as resulting from suffocation or undetermined causes were significantly more likely than were those whose deaths were classified as resulting from SIDS to be found in an adult bed, a couch or chair, or another surface not intended for infant sleep; they were also significantly more likely to be sharing that sleep surface with an adult.
Our findings are largely consistent with those of other studies that have described the sleep environment and other characteristics of SUIDs documented in CDR or medical examiner data from single (urban or state) jurisdictions. Although these studies are not all directly comparable to ours because they report details on only suffocation deaths, omit SIDS or undetermined cause deaths, or do not report findings by final classification of death, the key findings are consistent and indicate that a large proportion of SUIDs involve hazards in the sleep environment such as nonsupine sleep position, use of surfaces not intended for infant sleep, and the presence of people (bed sharing) or objects (bedding) in the sleep environment.
Our findings are also consistent with welldocumented SIDS risk factors, although it is important to note that most studies identifying sleep risk factors for SIDS were conducted before the diagnostic shift in classifying SUIDs as suffocation and undetermined cause rather than SIDS. In fact, it has been suggested that stricter adherence to the definition of SIDS might explain this diagnostic shift. To be classified as SIDS, the sudden death of an infant must remain unexplained even after an autopsy, a thorough death scene investigation, and a review of the infant's clinical and medical history.
In one example of strict adherence to the SIDS definition, the New York City Office of the Chief Medical Examiner classifies SUIDs according to a protocol that prohibits classifying a death as SIDS if any environmental events or sleep-related risk factors were present at the time of death. Deaths are classified as suffocation when sufficient evidence of suffocation is present during the death scene investigation. Deaths in which there is insufficient evidence of suffocation but hazards in the environment are identified, such as an infant sleeping with others in an adult bed but no report or witness of overlay, would be classified as resulting from undetermined causes.
Medical examiners in other jurisdictions may be using similar criteria for classifying SUIDs, but such strict protocols are not universal. For example, in a study conducted in Kentucky, Shields et al. described a 3-month-old infant who "succumbed to sudden infant death syndrome" while sleeping with hermother, a sleeprelated circumstance defined as "consistent with SIDS" according to Kentucky's 2003 classification scheme for SUIDs.
Our results are consistent with stricter adherence to the SIDS definition on the part of at least some of the death certifiers in the 9 states included in our analysis. That is, infants whose deaths were classified as suffocation were signi ficantly more likely to be sleeping on a surface not intended for infant sleep, to be sharing that surface with an adult, and to have documentation of an obstructed airway when found. In addition, the proportion of infant suffocation deaths for which there was documentation that the infant was found under, between, or tangled in or wedged, pressed, or rolled into people or objects such as soft bedding was 2-fold higher than the proportion of undetermined cause deaths and 4-fold higher than the proportion of SIDS deaths. These results indicate that although sleep-related risk factors were present for some of the deaths classified as SIDS, deaths with clearly documented hazards in the sleep environment were more likely to be classified as suffocation or, to a lesser extent, undetermined cause.
It is notable that the percentage of deaths in each SUID category differed from percentages reported previously in the literature. Shapiro-Mendoza et al. reported that, in 2004, 59% of SUIDs were classified as SIDS, 14% as suffocation, and 27% as unknown cause, whereas in our 2005 to 2008 data 31% were classified as SIDS, 29% as suffocation, and 39% as undetermined. This difference is likely caused, at least in part, by the different data sources used; Shapiro-Mendoza et al. reported cause-specific mortality data from death certificates, and we used data from CDR programs.
The National Center for Health Statistics (NCHS) assigns International Classification of Diseases (ICD) codes to death certificates based on the reported underlying cause of death. The SIDS code (R95) is assigned to death certificates with such designations as "infant death unknown cause" and "sudden unexpected infant death." As a result, deaths identified as cause unknown in the NCDR-CRS might be coded as SIDS on the death certificate. This would result in a higher proportion of SUIDs being classified as SIDS in national mortality statistics than in CDR data. Another explanation for the inconsistency might be a continuation of the diagnostic shift from classification of SIDS to classification of suffocation or undetermined cause, given that nationally available mortality data typically lag several years and that we used CDR data from 2005 to 2008.
Our study is not without limitations. For example, although many state CDR programs attempt to review all child deaths, not all reviews are completed, and not all data are entered by the end of a calendar year (in fact, data on some deaths are not entered for more than a year after the death occurs). In addition, not all counties in one of the states included in our analysis participate in the NCDR-CRS. As a result of this lag time and incomplete coverage, we could not confidently determine an appropriate denominator for the deaths included in our study, precluding calculation of mortality rates. Furthermore, without access to a nonaffected comparison group, risk cannot be determined. As a result, our analyses focused only on identifying the proportions of deaths in each of the 3 SUID categories across infant, caregiver, and sleep environment characteristics.
The NCDR-CRS is a relatively new system and has grown rapidly in a short time. In her description of the system, Covington explained the potential limitations of the data in detail. Of note, data quality can differ across states, particularly states new to the system. Although data from more than 3000 infant deaths were available for our analysis, inclusion of data from only 9 states may limit the generalizability of our results, especially given some of the documented differences in classification of SUIDs by jurisdiction.
In addition, the database includes more than 1700 data elements, and large proportions of missing data are more likely when novice users are responsible for entering information into the system. The observation that the proportion of missing data submitted by a state decreases with time was a factor in selecting states that had participated in the database from early in its existence. Even so, several of the variables reported had large proportions of missing data, and we did not include other sleep environment data elements because they involved even larger proportions of missing data. Our imputation of missing data in our regression analyses allowed inclusion of all SUIDs and likely produced less biased results. The reasons particular data elements involve large proportions of missing data are not known at this time; however, our findings can be used in future state training initiatives to improve data quality.
Finally, given the nature of the NCDR-CRS, only information on deceased infants was available for analysis. Although survey data are now available that describe usual infant sleep practices among living infants, the etiological component of infant sleep environment characteristics with respect to risk of SUIDs cannot be determined without an analytic study in which the sleep environment and other characteristics of infants who die suddenly and unexpectedly are compared with the same characteristics in living infants. Such an investigation is beyond the scope of the NCDR-CRS data.
Despite their limitations, the NCDR-CRS data have a number of inherent strengths. For instance, these data are population based and consist of standard elements that allow aggregation of data across states. The NCCDR provides training and support for NCDR-CRS users, including a comprehensive data dictionary to facilitate consistency in completion of data elements across jurisdictions. The number of states participating in the NCDR-CRS continues to grow, and states are continually gaining experience in using the system; thus, although the NCDR-CRS data are relatively new, they have the potential to inform our understanding of the circumstances and risk factors associated with all causes of child death, particularly injury deaths. The use of these collective data for prevention is a goal of the NCCDR.
To our knowledge, this is the first populationbased study in which CDR data from multiple states have been used to examine infant, caregiver, and sleep circumstances and to compare them across 3 SUID categories. Notably, we included sleep environment details for more than 3000 infant deaths, a sufficiently large number to allow calculation of stable proportions of specific infant, caregiver, and sleep environment characteristics stratified by SUID category and assessment of independent associations of key sleep environment risk factors.
This study makes an important contribution to the existing SIDS research and to the growing evidence from smaller SUID studies that identify hazards in the infant sleep environment as likely contributors to SUIDs. As such, our findings have important implications for preventing injuries and reducing SUID mortality. We identified modifiable sleep environment risk factors in a large proportion of SUIDs, regardless of the ultimate cause of death classification.
Discussion
Our descriptive analysis of population-based multistate CDR program data showed that only one quarter of SUID victims were sleeping in a crib or on their back when found; 70% of infants were on a surface not intended for infant sleep, and 64% with documentation of their position when found were on their stomach or side. Importantly, 64% of SUID victims in our study were sharing a sleep surface, and 49% of these infants were sleeping with an adult. Infants whose deaths were classified as resulting from suffocation or undetermined causes were significantly more likely than were those whose deaths were classified as resulting from SIDS to be found in an adult bed, a couch or chair, or another surface not intended for infant sleep; they were also significantly more likely to be sharing that sleep surface with an adult.
Our findings are largely consistent with those of other studies that have described the sleep environment and other characteristics of SUIDs documented in CDR or medical examiner data from single (urban or state) jurisdictions. Although these studies are not all directly comparable to ours because they report details on only suffocation deaths, omit SIDS or undetermined cause deaths, or do not report findings by final classification of death, the key findings are consistent and indicate that a large proportion of SUIDs involve hazards in the sleep environment such as nonsupine sleep position, use of surfaces not intended for infant sleep, and the presence of people (bed sharing) or objects (bedding) in the sleep environment.
Our findings are also consistent with welldocumented SIDS risk factors, although it is important to note that most studies identifying sleep risk factors for SIDS were conducted before the diagnostic shift in classifying SUIDs as suffocation and undetermined cause rather than SIDS. In fact, it has been suggested that stricter adherence to the definition of SIDS might explain this diagnostic shift. To be classified as SIDS, the sudden death of an infant must remain unexplained even after an autopsy, a thorough death scene investigation, and a review of the infant's clinical and medical history.
In one example of strict adherence to the SIDS definition, the New York City Office of the Chief Medical Examiner classifies SUIDs according to a protocol that prohibits classifying a death as SIDS if any environmental events or sleep-related risk factors were present at the time of death. Deaths are classified as suffocation when sufficient evidence of suffocation is present during the death scene investigation. Deaths in which there is insufficient evidence of suffocation but hazards in the environment are identified, such as an infant sleeping with others in an adult bed but no report or witness of overlay, would be classified as resulting from undetermined causes.
Medical examiners in other jurisdictions may be using similar criteria for classifying SUIDs, but such strict protocols are not universal. For example, in a study conducted in Kentucky, Shields et al. described a 3-month-old infant who "succumbed to sudden infant death syndrome" while sleeping with hermother, a sleeprelated circumstance defined as "consistent with SIDS" according to Kentucky's 2003 classification scheme for SUIDs.
Our results are consistent with stricter adherence to the SIDS definition on the part of at least some of the death certifiers in the 9 states included in our analysis. That is, infants whose deaths were classified as suffocation were signi ficantly more likely to be sleeping on a surface not intended for infant sleep, to be sharing that surface with an adult, and to have documentation of an obstructed airway when found. In addition, the proportion of infant suffocation deaths for which there was documentation that the infant was found under, between, or tangled in or wedged, pressed, or rolled into people or objects such as soft bedding was 2-fold higher than the proportion of undetermined cause deaths and 4-fold higher than the proportion of SIDS deaths. These results indicate that although sleep-related risk factors were present for some of the deaths classified as SIDS, deaths with clearly documented hazards in the sleep environment were more likely to be classified as suffocation or, to a lesser extent, undetermined cause.
It is notable that the percentage of deaths in each SUID category differed from percentages reported previously in the literature. Shapiro-Mendoza et al. reported that, in 2004, 59% of SUIDs were classified as SIDS, 14% as suffocation, and 27% as unknown cause, whereas in our 2005 to 2008 data 31% were classified as SIDS, 29% as suffocation, and 39% as undetermined. This difference is likely caused, at least in part, by the different data sources used; Shapiro-Mendoza et al. reported cause-specific mortality data from death certificates, and we used data from CDR programs.
The National Center for Health Statistics (NCHS) assigns International Classification of Diseases (ICD) codes to death certificates based on the reported underlying cause of death. The SIDS code (R95) is assigned to death certificates with such designations as "infant death unknown cause" and "sudden unexpected infant death." As a result, deaths identified as cause unknown in the NCDR-CRS might be coded as SIDS on the death certificate. This would result in a higher proportion of SUIDs being classified as SIDS in national mortality statistics than in CDR data. Another explanation for the inconsistency might be a continuation of the diagnostic shift from classification of SIDS to classification of suffocation or undetermined cause, given that nationally available mortality data typically lag several years and that we used CDR data from 2005 to 2008.
Limitations and Strengths
Our study is not without limitations. For example, although many state CDR programs attempt to review all child deaths, not all reviews are completed, and not all data are entered by the end of a calendar year (in fact, data on some deaths are not entered for more than a year after the death occurs). In addition, not all counties in one of the states included in our analysis participate in the NCDR-CRS. As a result of this lag time and incomplete coverage, we could not confidently determine an appropriate denominator for the deaths included in our study, precluding calculation of mortality rates. Furthermore, without access to a nonaffected comparison group, risk cannot be determined. As a result, our analyses focused only on identifying the proportions of deaths in each of the 3 SUID categories across infant, caregiver, and sleep environment characteristics.
The NCDR-CRS is a relatively new system and has grown rapidly in a short time. In her description of the system, Covington explained the potential limitations of the data in detail. Of note, data quality can differ across states, particularly states new to the system. Although data from more than 3000 infant deaths were available for our analysis, inclusion of data from only 9 states may limit the generalizability of our results, especially given some of the documented differences in classification of SUIDs by jurisdiction.
In addition, the database includes more than 1700 data elements, and large proportions of missing data are more likely when novice users are responsible for entering information into the system. The observation that the proportion of missing data submitted by a state decreases with time was a factor in selecting states that had participated in the database from early in its existence. Even so, several of the variables reported had large proportions of missing data, and we did not include other sleep environment data elements because they involved even larger proportions of missing data. Our imputation of missing data in our regression analyses allowed inclusion of all SUIDs and likely produced less biased results. The reasons particular data elements involve large proportions of missing data are not known at this time; however, our findings can be used in future state training initiatives to improve data quality.
Finally, given the nature of the NCDR-CRS, only information on deceased infants was available for analysis. Although survey data are now available that describe usual infant sleep practices among living infants, the etiological component of infant sleep environment characteristics with respect to risk of SUIDs cannot be determined without an analytic study in which the sleep environment and other characteristics of infants who die suddenly and unexpectedly are compared with the same characteristics in living infants. Such an investigation is beyond the scope of the NCDR-CRS data.
Despite their limitations, the NCDR-CRS data have a number of inherent strengths. For instance, these data are population based and consist of standard elements that allow aggregation of data across states. The NCCDR provides training and support for NCDR-CRS users, including a comprehensive data dictionary to facilitate consistency in completion of data elements across jurisdictions. The number of states participating in the NCDR-CRS continues to grow, and states are continually gaining experience in using the system; thus, although the NCDR-CRS data are relatively new, they have the potential to inform our understanding of the circumstances and risk factors associated with all causes of child death, particularly injury deaths. The use of these collective data for prevention is a goal of the NCCDR.
To our knowledge, this is the first populationbased study in which CDR data from multiple states have been used to examine infant, caregiver, and sleep circumstances and to compare them across 3 SUID categories. Notably, we included sleep environment details for more than 3000 infant deaths, a sufficiently large number to allow calculation of stable proportions of specific infant, caregiver, and sleep environment characteristics stratified by SUID category and assessment of independent associations of key sleep environment risk factors.
This study makes an important contribution to the existing SIDS research and to the growing evidence from smaller SUID studies that identify hazards in the infant sleep environment as likely contributors to SUIDs. As such, our findings have important implications for preventing injuries and reducing SUID mortality. We identified modifiable sleep environment risk factors in a large proportion of SUIDs, regardless of the ultimate cause of death classification.
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