Sudden Unexpected Infant Death (SUID) and Sudden Infant Death Syndrome (SIDS)

SIDS is considered by most researchers in the field to be a heterogeneous disorder presenting as sudden, predominantly sleep-related death in a seemingly well infant. Its etiology involves an interplay between usually survivable, modest external threats to breathing and inapparent intrinsic vulnerabilities in an infant (1). For example, prone sleep position is not fatal in the vast majority of infants, but it is strongly associated with SIDS and its reduction is associated with reduced SIDS rates (2). Although these deaths raise child protection concerns and lead to mandated forensic investigations, one subsequent investigation found evidence of nonaccidental harm in < 4% of cases (3). Repeated heritability studies show a greater than 4-fold increased risk of SIDS to a subsequent sibling of an infant who died of SIDS (4, 5).

SIDS deaths that occurred while infants were on monitors have revealed a crucial subset of SIDS in which the terminal events, in infants who are bradycardic and hypoxemic, involved a failure of self-recovery (autoresuscitation) that entails gasping without a cardiac response, so-called cardiorespiratory uncoupling, and a failure to arouse (6). Research to understand this subset has found deficiencies in serotonin, its receptors, and its precursors in the brain stems of approximately 40% of studied infants (7), and scientists have replicated this mechanism of death in animal models (8). Other recent key insights include pathologic changes in the hippocampus that are also seen in epilepsy in 41% of infants (9) and causative genetic variants in 11%, divided between cardiac and neurologic mechanisms of susceptibility (10). In one large cohort study, approximately 4% of infants who died of SIDS were found to have a pathogenic or likely pathogenic cardiac gene mutation that would be potentially useful in evaluating and/or treating family members; this percentage is smaller than previously described (11, 12). The majority of these mutations was for inherited arrhythmia syndromes, including channelopathies.

• Autopsy, examination of death scene, and review of clinical history

SUID is a category of death and not a specific cause.

SIDS and undetermined causes of death are cause-of-death diagnoses of exclusion that are used when a postmortem assessment does not identify a specific etiology or reason for the death. (Ingestions, unrecognized head trauma, precipitous infection, or blood clotting disorders are examples of specific causes that are infrequently found.) Accidental suffocation and strangulation in bed (ASSB) is a specific cause and requires sufficient evidence beyond the mere presence of a risk factor for suffocation for the medical examiner to conclude that the mechanism of suffocation caused the death.

As stated above, conflation of risk factors and cause is a central issue in this area, and research shows that the diagnostic constructs are not independent. Many medical examiners object to the concept of a sudden infant death syndrome and use alternative diagnoses, even when diagnostic criteria for SIDS are met. Current diagnostic classifications do distinguish between SIDS, ASSB, and an undetermined cause of death (1).

Diagnosis is based on a thorough case investigation, including performance of a complete autopsy with ancillary testing, examination of the death scene, and a review of the clinical history.

The details of what constitutes a complete autopsy are not uniform. Variations range from cases where an autopsy is not performed beyond an external examination to the standard use of specialized autopsy tools that include routine neuropathologic assessment and genomic testing.

Examination of the death scene includes a standardized set of observations about the infant’s final sleep period, including found position. In tandem with the child protective services (CPS) investigation, other factors that may have impacted the child’s health and safety are also investigated. SIDS is used as the diagnosis only when all of the elements of a complete investigation have been satisfied and investigators determine that nothing discovered is the cause of death.

An infant dying suddenly, unexpectedly, and without a determined cause is a dire and exceptional challenge for any family. Beyond the immeasurable loss of an infant’s life, SUID or SIDS is a family crisis. Grief is profound. Parents report that the death continues to affect them on a daily basis more than a decade later (1). One year after a death, prolonged grief disorder occurs in 10% of people grieving the loss of an older life partner but occurs in 60% of mothers grieving the loss of an infant to SIDS (2). The shock of finding a dead infant, the brutality of resuscitation, and sometimes harsh treatment while the death is investigated leave many parents with intrusive and distressing thoughts related to the death.

During the postmortem investigation, clinicians working with the family can help them navigate the investigation and interpret any findings. Given the variation in postmortem evaluations, clinicians involved with the family can recommend programs for further, more thorough, assessment. If there are findings that warrant further evaluation of the parents or living siblings, clinicians can play an important role in helping bereaved parents address any implications for the family’s health. In every case, support services, whether a referral to a bereaved parent organization for peer support or to individualized counseling with a mental health professional, are important for parents who may be struggling to understand or advocate for their needs.

The following recommendations from the American Academy of Pediatrics (AAP) are based on consistent, quality, patient-oriented evidence) (1):

• Back to sleep for every sleep (as soon as developmentally possible and medically reasonable for hospitalized preterm infants); infants who can roll prone-to-supine and supine-to-prone can remain in the sleep position they assume.

• Use a firm, flat, noninclined sleep surface to reduce the risk of suffocation or wedging/entrapment. 

• Feed infants human milk.

• Have infants sleep in the parents’ room, close to the parents’ bed, but on a separate surface designed for infants, ideally for at least the first 6 months.

• Keep soft objects, such as pillows, pillow-like toys, quilts, comforters, mattress toppers, fur-like materials, and loose bedding (such as blankets and nonfitted sheets), away from the infant’s sleep area to reduce the risk of SIDS, suffocation, entrapment or wedging, and strangulation. 

• Offer a pacifier at naptime and bedtime. 

• Avoid smoke and nicotine exposure during pregnancy and after delivery. 

• Avoid alcohol, marijuana, opioids, vapes, and illicit drug use during pregnancy and after delivery. 

• Avoid overheating and head covering in infants. 

• Obtain regular prenatal care when pregnant. 

• Immunize infants in accordance with guidelines from the AAP and Centers for Disease Control and Prevention (CDC). (See also Childhood Vaccination Schedules.)

• Do not use home cardiorespiratory monitors as a strategy to reduce the risk of SIDS. 

• Supervise short periods of tummy time beginning soon after hospital discharge while the infant is awake to facilitate development and minimize the risk of positional plagiocephaly (goal of at least 15 to 30 minutes total daily by age 7 weeks).

The following English-language resources may be useful. Please note that The Manual is not responsible for the content of these resources.

1. American SIDS Institute

2. Safe to Sleep