Results show that the progression from sub-cortical activation to cortical arousal was depressed in smoke-exposed infants, who had lower proportions of full cortical arousals from sleep and higher proportions of sub-cortical activations than infants born to non-smoking mothers. The study also indicates that there is a dose-dependent relationship between cortical activation proportions and levels of infant urinary cotinine, a nicotine metabolite. Cortical arousals were lowest in babies with higher levels of smoke exposure.
According to senior investigator Rosemary Horne, PhD, scientific director of the Ritchie Centre for Baby Health Research at Monash University in Melbourne, Australia, decreased cortical arousals from sleep have been observed in victims of SIDS prior to death.
"Our study suggests that maternal smoking can impair the arousal pathways of seemingly normal infants, which may explain their increased risk for SIDS," said Horne.
According to the authors, SIDS is the third-leading cause of infant mortality in the U.S. Although the exact cause is unknown, research suggests that an impairment of the arousal process from sleep in response to a life-threatening situation is involved. Autopsies of SIDS victims have revealed brainstem abnormalities in key areas that are required for arousal and cardiorespiratory control.
The study involved 12 healthy, full-term infants born to mothers who smoked an average of 15 cigarettes per day. Their arousal responses during daytime sleep were monitored and compared with that of 13 healthy infants who were born to non-smoking mothers.
Daytime polysomnography was performed on each child on three occasions: at 2 to 4 weeks, 2 to 3 months and 5 to 6 months of age. Arousals were induced without compromising the infants' natural sleep cycles by delivering a pulsatile air-jet for five seconds through a hand-held cannula at the infants' nostrils.
The air pressure was increased incrementally until arousal criteria were met. Sub-cortical arousals were defined by body movements, heart rate increases and respiratory changes. Full cortical arousals involved a sub-cortical arousal that was accompanied by an abrupt change in electroencephalogram frequency.
According to the authors, 15 percent to 25 percent of pregnant women in Western countries smoke throughout pregnancy.