Stress : Clinical and Developmental Aspects of Salivary Cortisol in Infants

Abstract: A functional stress-response system is essential for survival at birth, as well as for health and further development. Altered cortisol response and hypothalamic-pituitary-adrenal system function may have both short and long-term effects on health and development throughout life. Cortisol secretion follows a circadian rhythm in adults. Data in the literature concerning basal cortisol levels is scant, with divergent results regarding the timeframe for establishment of cortisol circadian rhythm in children. Nevertheless, cortisol is often studied in stress-related research concerning preterm infants, full-term infants, and infants at high psychosocial risk.This thesis aimed to investigate at what age cortisol circadian rhythm develops in healthy full-term infants, preterm infants, and infants at high psychosocial risk and to identify whether such development is dependent on gestational or postnatal age. A secondary aim was to investigate whether either behavioral regularity or daily life trauma are associated with establishment of cortisol circadian rhythm. The last two interventional studies explored whether a) parental participation in the Hagadal daycare attachment program in one  study and b) oral administration of glucose during nasopharyngeal suctioning in the other study influenced development of salivary cortisol circadian rhythm and/or cortisol levels. The effects, if any, of the Hagadal daycare attachment program on caregiver sensitivity to infants were also investigated.The present thesis includes four original studies. Papers I, II, and III describes prospective, longitudinal studies extending over a year, including a survey of the cortisol levels and development of cortisol circadian rhythm in three infant groups. Paper III also included an intervention component addressing the possible effects of the Hagadal daycare attachment program. Paper IV describes a case-control study designed to generate paired baseline-response data concerning the effects of oral glucose administration during nasopharyngeal suctioning as an interventional procedure.Cortisol circadian rhythm in salivary cortisol secretion was similarly established at one month postnatal age in full-term infants and at one month corrected age in preterm infants, reflecting a process dependent on gestational age. This rhythm persisted throughout the first year of life in all infants and consolidated over time in healthy full-term and preterm infants, but not in infants at high psychosocial risk, who displayed higher variability in cortisol levels. The infants in paper IV had not yet reached one month of corrected age and therefore had not yet developed cortisol circadian rhythm at the time of the investigation. No correlation was found between development of cortisol circadian rhythm and either behavioral regularity or reported traumatic life events. This thesis presents data on salivary cortisol levels among three different groups of infants during the first year of life. Cortisol circadian rhythm among infants in study III evolved in response to parental participation in the Hagadal daycare attachment program, which increased caregiver sensitivity to infants. Study IV found that nasopharyngeal suctioning was not a sufficiently stressful stimulus to increase salivary cortisol or impact pain score. Oral glucose administration had no effect on salivary cortisol levels.This thesis concludes that cortisol circadian rhythm is already established in infants by one month of age, earlier than previous studies have shown, and further that this process is dependent on gestational age. The Hagadal daycare attachment program enhances parental sensitivity toward children, which helps to stabilize development of cortisol circadian rhythm.