. Author manuscript; available in PMC: 2022 Oct 1.

Published in final edited form as: Pediatrics. 2021 Sep 20;148(4):e2021053746. doi: 10.1542/peds.2021-053746

TABLE 1.

Selected Terminology and Definition or Criteria to Classify Unexplained Sudden Deaths in Infants and Accidental Asphyxiation

Terminology	Case Definition
Beckwith, 1969¹
Sudden Infant Death Syndrome (SIDS)	“The sudden death of any infant or young child, which is unexpected by history, and in which a thorough post-mortem examination fails to demonstrate an adequate cause for death.”
Willinger et al, 1989⁶
Sudden Infant Death Syndrome (SIDS)	“The sudden death of an infant under one year of age, which remains unexplained after a thorough case investigation, including performance of a complete autopsy, examination of the death scene, and review of the clinical history.”
Undetermined or unexplained cause	“Cases that are autopsied and carefully investigated, but which remain unresolved may be designated as ‘undetermined,’ ‘unexplained,’ or the like. ‘Unresolved’ cases are those for which the history, investigation, or autopsy reveals information that places death outside the SIDS category but does not explain the cause of death. Examples of the latter, are suspected cases of abuse, neglect, or accidental suffocation; cases with episodes of vomiting or diarrhea in 24 hours prior to death without pathologic evidence of infection; or cases in which the information regarding death is not reliable.”
Krous et al, 2004⁵
General definition: Sudden Infant Death Syndrome (SIDS)	“Sudden unexpected death of an infant less than 1 year of age, with 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.”
Category IA SIDS: classic features of SIDS present and completely documented	“Infant deaths that meet the requirements of the general definition and also all of the following requirements. Clinical More than 21 days and <9 months of age. Normal clinical history, including term pregnancy (gestational age of ≥37 weeks). Normal growth and development. No similar deaths among siblings, close genetic relatives (uncles, aunts, or first-degree cousins), or other infants in the custody of the same caregiver. Circumstances of Death Investigation of the various scenes where incidents leading to death might have occurred and determination that they do not provide an explanation for the death. Found in a safe sleeping environment, with no evidence of accidental death. Autopsy Absence of potentially fatal pathologic findings. Minor respiratory system inflammatory infiltrates are acceptable; intrathoracic petechial hemorrhage is a supportive but not obligatory or diagnostic finding. No evidence of unexplained trauma, abuse, neglect, or unintentional injury. No evidence of substantial thymic stress effect (thymic weight of <15 g and/or moderate/severe cortical lymphocyte depletion). Occasional ‘starry sky’ macrophages or minor cortical depletion is acceptable. Negative results of toxicological, microbiologic, radiologic, vitreous chemistry, and metabolic screening studies.”
Category IB SIDS: classic features of SIDS present but incompletely documented	“Infant deaths that meet the requirements of the general definition and also meet all of the criteria for category IA except that investigation of the various scenes where incidents leading to death might have occurred was not performed and/or 1 of the following analyses was not performed: toxicological, microbiologic, radiologic, vitreous chemistry, or metabolic screening studies.”
Category II SIDS	“Infant deaths that meet category I criteria except for ≥1 of the following: Clinical Age range outside that of category IA or IB (ie, 0–21 days or 270 days [9 months] through first birthday). Similar deaths among siblings, close relatives, or other infants in the custody of the same caregiver that are not considered suspect for infanticide or recognized genetic disorders. Neonatal or perinatal conditions (for example, those resulting from preterm birth) that have resolved by the time of death. Circumstances of Death Mechanical asphyxia or suffocation caused by overlaying not determined with certainty. Autopsy Abnormal growth and development not thought to have contributed to death. Marked inflammatory changes or abnormalities not sufficient to be unequivocal causes of death.”
Unclassified sudden infant death	“Infant deaths that do not meet the criteria for category I or II SIDS but for which alternative diagnoses of natural or unnatural conditions are equivocal, including cases for which autopsies were not performed.”
Goldstein et al, 2019¹⁵
Unexplained sudden death in infancy or Sudden Infant Death Syndrome	“The sudden unexpected death of an apparently healthy infant under one year of age that remains unexplained after a thorough case investigation, including performance of a complete autopsy with ancillary testing, examination of the death scene, and review of the clinical history.” Special note: “Infant deaths with adequate death scene investigation and autopsy, with a history of bed/sleep surface sharing, soft bedding, or non-supine sleep, and without physical evidence of asphyxia, may be more appropriately certified as unexplained sudden death in infancy or sudden infant death syndrome.”
Other ill-defined or unspecified causes of death (undetermined)	“The investigation, death scene examination, or autopsy was substantially limited, incomplete or insufficient (eg, legal/religious restrictions, delayed report of death that limits scene investigation, or decomposition).” Or “The investigation, death scene examination, or autopsy had inconsistent accounts or other findings raise competing conclusions about the cause of death.”
Unintentional threat to breathing (accidental asphyxia): certification of asphyxia	“Adequate evidence must be documented to substantiate asphyxiation, given the decedent’s age and stage of development. There cannot be a reasonable competing cause of death after a complete autopsy with ancillary testing, examination of the death scene (with a doll re-enactment when appropriate), and review of the clinical history. Bed/sleep surface sharing, soft bedding, or prone sleep, without adequate evidence for airway obstruction or chest wall compression, are insufficient to certify a death as due to asphyxia. These deaths may be more appropriately certified as unexplained sudden death or SIDS. The use of ‘possible’ or ‘probable’ asphyxia will result in the death being classified as asphyxia.”
National Association of Medical Examiners Panel on Sudden Unexpected Death in Pediatrics, 2020^10,14
Unexplained sudden death (no identified intrinsic or extrinsic factors)	“Infant less than one year of age in apparent good health that dies suddenly and unexpectedly. For Sleep-related Deaths: Placed alone, supine, in infant-specific sleep environment (eg, crib, bassinet, portable crib, play pen) with flat, firm sleep surface, uncluttered by objects, and without potential areas of entrapment. Found unresponsive or dead, in the same sleep environment, with no obstruction of the nose and/or mouth or compression of neck/chest to cause asphyxia given the developmental abilities of the infant, as described by finder and demonstrated by doll reenactment. The infant was not overly dressed or bundled for the environmental temperature. Competent caregiver not impaired by drugs or alcohol. Physical findings on body and at scene consistent with history provided by caregiver. Completion of scene investigation and doll reenactment unless caregiver declines. Review of child medical records and family health history. Complete autopsy with histology, comprehensive toxicology testing (including vitreous chemistries if possible), and skeletal survey. No anatomic, metabolic, toxicological, chemical, historical, or external cause of death identified. Genetic testing is recommended but not required for this certification. No extrinsic or intrinsic risk factors are identified.”
Unexplained sudden death (intrinsic factors identified)^a	“A cause of death cannot be determined and criteria for Unexplained Sudden Death (No Identified Intrinsic or Extrinsic Factors) are not met due to intrinsic/natural abnormalities that are either known risk factors for sudden death (including, but not limited to, low birth weight, preterm birth, small for gestational age, concurrent non-lethal illness, febrile seizures) or are of unknown significance (including, but not limited, to mutations of unknown significance). Trauma and other unnatural etiologies are sufficiently excluded.”
Unexplained sudden death (extrinsic factors identified)^b	“A cause of death cannot be determined and criteria for Unexplained Sudden Death (No Identified Intrinsic or Extrinsic Factors) are not met due to the presence of unintentional extrinsic factors that increase risk for unnatural death. This may include, but is not limited to, non-lethal injuries or injuries of unknown significance, nonlethal toxicological findings of unknown significance, or circumstances otherwise concerning for unnatural death.”
Unexplained sudden death (intrinsic and extrinsic factors identified)^a,b	“A cause of death cannot be determined and criteria for Unexplained Sudden Death (No Identified Intrinsic or Extrinsic Factors) are not met due to a combination of intrinsic and extrinsic factors as described above.”
Undetermined (not further specified)	“A cause of death cannot be determined due to circumstances or findings that make the above classifications inapplicable. Examples may include: Inconsistent histories and/or other evidence that raise uncertainty about manner of death, and competing causes of death. Cases which remain undetermined but were not sudden.”
Undetermined (insufficient data)	“A cause of death cannot be determined because investigation, death scene examination, or autopsy were substantially limited, incomplete, or insufficient. Examples may include legal/religious restrictions, delayed report of death that limits scene investigation, and/or decomposition.”
Asphyxia	“The case must have a complete/full autopsy. Toxicology, histology, vitreous electrolytes, cultures, and review of medical history are to be performed, as necessary as determined by investigation and autopsy. The infant must have obstruction of both nose and mouth or compression of the neck or chest, that is reliably witnessed or demonstrated by doll reenactment, or other reliable evidence of overlay or entrapment. Asphyxiation must be probable given infant’s age and stage of development. There cannot be a reasonable competing cause of death.”

“Intrinsic factors are: natural conditions or risk factors associated with abnormal physiology or anatomy that are concerning as contributors to death but are insufficient as a cause (eg, low birth weight, preterm birth, small for gestational age, concurrent non-lethal illness, history of febrile seizures), or natural conditions of unknown significance (eg, cardiac channelopathy or seizure gene variants of unknown significance).”

“Extrinsic factors are: conditions in the child’s immediate environment that are a potential threat to life but cannot be deemed the cause of death with reasonable certainty (eg, side or prone sleep if unable to roll to supine, over-bundling without documented hyperthermia, objects in immediate sleep environment, sleep environment not specifically designed for infant sleep, soft or excessive bedding, and sleep surface sharing), injuries or toxicologic findings that are either non-lethal or of unknown lethality, or circumstances/findings otherwise concerning for unnatural death.”