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CHAPTER 118: Sudden Infant Death Syndrome 745 Sudden Infant Death Syndrome Erik Akopian Ilene Claudius Joel Tieder Sudden infant death syndrome (SIDS) is the unexpected death of an infant under 1 year of age for which no pathologic cause can be deter mined by an examination of the death scene, an autopsy, and a review of the infant’s medical history. SIDS is a type of sudden unexpected infant death, a term that encompasses all unexpected infant deaths, both deaths from SIDS and those for which a cause is ultimately determined. Of the 3500 U.S. yearly cases of sudden unexpected infant death, 27% to 43% are due to SIDS. 2,3 A number of other terms are used in pediatrics to refer to sudden and unexpected death. Sudden unexpected death of an infant is interchangeable with sudden unexpected infant death, and sudden death in the young (SDY) refers to such a death in any child ≤19 years old. Sudden unexplained early neonatal death is limited to infants succumbing within the first week of life and is often caused by congenital abnormalities. Sudden intrauterine unexpected death syn drome refers to stillbirths for which a postmortem exam cannot identify a cause, and sudden unexpected death in epilepsy is an unexpected death in an individual with epilepsy (excluding trauma or drowning) in which a postmortem exam does not reveal an anatomic or toxicologic cause. EPIDEMIOLOGY AND RISK FACTORS SIDS is the leading cause of death for infants between 1 month and 1 year of age. SIDS rates have declined considerably from 130.3 deaths per 100,000 live births in 1990 to 38.7 deaths per 100,000 live births in 2014. 2 This significant decrease can largely be attributed to the safer sleep practices promoted by the American Academy of Pediatrics’ “Back to Sleep” and “Safe to Sleep” campaigns and, to a lesser extent, classifying deaths due to asphyxiation or strangulation in sleep separately from SIDS. 4 The incidence of SIDS peaks between 2 and 4 months of age, and boys account for 60% of SIDS deaths. 5 There are a disproportionate number of SIDS deaths in lower socioeconomic groups, although this is true for deaths in infancy from all causes. Other risk factors include low birth weight and prematurity. 6 There is ethnic variation, with Asian Americans at lower risk and African Americans and Native Americans at higher risk. Mothers of SIDS victims are more commonly <20 years old, overweight, unwed, use drugs, and have made few prenatal and postpartum visits. Prenatal and postnatal maternal smoking increases the incidence of SIDS. SIDS is more likely to occur during the winter months and while the infant is asleep. Thirty percent to 50% of SIDS patients have some acute infection, usually an upper respiratory tract infection, at the time of the event. Sleep position has received a great deal of attention as a modifiable SIDS risk factor. The prone sleep position is associated with an odds ratio of 4.92 for SIDS. In clinical trials, prone infants were found to rebreathe expired air and experience hypercarbia. 7 In addition, infants normally dissipate heat through their head, and prone sleeping may inhibit heat loss, thereby exacerbating hyperthermia, another noted risk factor for SIDS. 8 A disproportionate number of infants succumb to SIDS while with a babysitter. 9 Many of these infants are found in the prone position.

ia. 7 In addition, infants normally dissipate heat through their head, and prone sleeping may inhibit heat loss, thereby exacerbating hyperthermia, another noted risk factor for SIDS. 8 A disproportionate number of infants succumb to SIDS while with a babysitter. 9 Many of these infants are found in the prone position. For some infants, this is the first time they have been placed in the prone position, and investigators have proposed that these infants have poor strength and tone in their neck muscles. Side-sleeping is also considered a risk factor for SIDS and discouraged. Because of these observations related to the prone position, the American Academy of Pediatrics has recommended supine sleeping for normal infants since 1992 and issued a statement in 2011 addressing specific issues related to sleep. A supine sleep position does not increase aspiration risk in infants with GI reflux and should apply to premature infants as well, even in neonatal intensive care units, as soon as they are medically stable. Infants should be placed on a firm mattress in a crib, bassinet, or portable crib (rather than adult bed) and protected from overheating. Bed sharing is discouraged, particularly in situations where the infant is less than 3 months of age; if the parent is a smoker, excessively tired, or on sedative substances (particularly alcohol); or on soft surfaces such as couches or beds with soft bedding. 4 Prone sleeping, bed sharing, or sleeping in a location other than a crib or bassinet was associated with 92.2% of SIDS deaths. 10 Sofas are a particularly hazardous sleep environment, accounting for 12.9% of sleep-related infant deaths.11 Swaddling, a common practice to minimize crying and colic, has been shown to increase the risk of SIDS, particularly in those who slept prone and those >6 months of age. 12 High ambient temperature might also contribute.13 Use of a pacifier, 14 breastfeeding, 15 and immunizations 16 are protective against SIDS. Home monitoring has not been shown to prevent SIDS. Although a recognized case of abuse would not meet the criteria for SIDS, 1% to 5% of cases of sudden unexpected infant death are attrib utable to infanticide. 18 Familial cases of “SIDS” raise the possibility of abuse. The presence of traumatic head injury, bruises, long-bone frac tures, rib fractures, internal hemorrhages, evidence of physical neglect, or blood around the nares suggests abuse. 19 It is so common for a SIDS victim to have a risky sleep position that a SIDS death without that factor might increase suspicion of abuse. Similarly, channelopathies are a common cause of sudden unexpected infant death that need to be excluded before a death is determined to be SIDS. A registry has recently been developed to capture accurate and com prehensive information on all sudden death in pediatric patients and to further understand the epidemiology and risk factors for both SIDS and sudden unexpected infant death. PATHOPHYSIOLOGY The exact cause of SIDS is not known, but it is felt to be multifacto rial. Certain autopsy features seem relatively consistent among infants succumbing to SIDS, including intrathoracic petechial hemorrhages, thyromegaly, encephalomegaly, microcardia, unclotted blood in the heart, kidney growth restriction, and an empty bladder and rectum. The hypothesized mechanism of SIDS includes an interplay between an infant’s underlying vulnerability, a critical period of development, and exogenous stressors—the triple-risk theory of SIDS . A genetic basis for infant vulnerability has been heavily studied recently. Although no single genetic locus for SIDS has been identified, the 10-fold increased risk among the siblings of SIDS victims suggests a genetic component.

critical period of development, and exogenous stressors—the triple-risk theory of SIDS . A genetic basis for infant vulnerability has been heavily studied recently. Although no single genetic locus for SIDS has been identified, the 10-fold increased risk among the siblings of SIDS victims suggests a genetic component. Recently, polymorphism in the interleukin-10 gene promoter has been associated with SIDS and sudden unexpected death associated with infection. Brain biomarkers ergothioneine, nicotinic acid, succinic acid, adenosine monophosphate, and azelaic acid have been associated with and may be predictive of SIDS. 22 Recent studies on the brains of infants who have died from SIDS demonstrate the presence of medullary serotonergic (5-hydroxytryptamine) pathology, including abnormal firing, synthesis, release, and clearance, 23,24 and morphologic differences in the brainstem and hippocampus. 25,26 This likely leads to an inadequate response to hypoxemia. Autonomic dysfunction has also been postulated to play a role. Infancy may represent a critical period for hypoxia, a likely contributor to SIDS. In general, infants are more likely to have apnea, sustained desaturations, and poor ventilatory control. Neonates less than 1 month of age have a better anaerobic capacity for survival and ability to raise their partial pressure of arterial oxygen with a gasp. The fact that SIDS is uncommon in this age group supports a ventilator theory. Studies that look at infants in various altitudes have found an increase in SIDS risk in children who live in higher altitudes, specifically greater than 8000 ft. An example of environmental stressors can be seen with pre- and perinatal nicotine exposure. It is hypothesized that nicotine blunts ventilation responses to hypercapnia and reduces central respiratory chemoreception by changing the expression of serotonin receptors. 29 The fact that supine sleeping has decreased the incidence of SIDS dramatically also has made investigators look closely at how supine sleep may affect autoregulation of respiratory and cardiac centers. It is possible that supine sleep increases sympathetic tone and decreases rapid eye move ment sleep, both of which are thought to decrease the risk of adverse CHAPTER Tintinalli_Sec12_p0669-0996.indd 745 8/2/19 7:50 PM