Understanding SIDS Risk Factors and Pathogenesis

Sudden infant death syndrome (SIDS) refers to the unexpected death of an infant younger than 12 months during sleep with no known cause. Although the National Institutes of Health first defined SIDS in 1969, no universally accepted definition currently exists. Campaigns to promote the supine sleep position have reduced SIDS rates by about 50%, yet SIDS remains the leading cause of postnatal infant death in the U.S. (JW Pediatr Adolesc Med Nov 4 2005 and JW Pediatr Adolesc Med Mar 24 2006).

The SIDS rate in the U.S. is 0.57 cases per 1000 infants; the rate in industrialized countries ranges from 0.09 to 0.80 per 1000 infants. The peak incidence of SIDS occurs at age 2 to 4 months.

SIDS is now believed to be caused by the interaction of multiple factors — a critical developmental period (first 12 months of life), underlying intrinsic vulnerability, and exogenous stressors. Intrinsic risk factors include male sex; race (Native American or black); polymorphisms in genes involved in neurotransmission, energy metabolism, autonomic function, and response to infection; prematurity; prenatal and postnatal exposure to smoke, alcohol, or drugs; and poverty (might reflect other stressors). Extrinsic risk factors are physical stressors that can cause asphyxia, including sleep position (prone or side positions), soft bedding, bed sharing, mild infections, bedclothes that cover the head, high temperature in the sleep environment, and sleeping on soft furniture.

Although the risk factors listed above for SIDS have been identified, the pathway that leads to death is still uncertain. Currently, the predominant theory is failure in the respiratory pathway described as follows:

• A life-threatening event causes asphyxia (e.g., rebreathing exhaled gases in the prone position, reflex apnea from activation of laryngeal receptors by gastric contents, obstructive apnea from regurgitation)

• Failure of arousal in response to asphyxia, preventing the infant from turning its head and recovering from the apnea

• Hypoxic coma resulting from continued asphyxia

• Bradycardia and hypoxic gasping

• Failure of autoresuscitation that results in death

In vulnerable infants, the various genetic, developmental, and environmental risk factors described above can affect the respiratory pathway at any stage, leading to fewer arousal periods, ineffectual gasping, or a failure of the autonomic nervous system to stimulate resuscitation.