Olfactory stimulation for promoting development and preventing morbidity in preterm infants

OBJECTIVES

This is a protocol for a Cochrane Review (intervention). The objectives are as follows: To evaluate the benefits and harms of olfactory stimulation with different odorants in the NICU for promoting development and preventing morbidity in preterm infants.

Olfactory sensory and perceptual evaluation in newborn infants: A systematic review

Fetuses are able to process olfactory stimuli present in the womb and continue to show a preference for these odors for months after birth. Despite the accumulated knowledge about their early ability to perceive odors, there is a lack of validated scales for odor response in newborns. The evaluation of reactions of the olfactory system to environmental stimuli in infants has been defined by methodological theoretical approaches of experimental and clinical assessment tools. These approaches are mainly based on psychophysical approaches and predominantly use behavioral and physiological measures. Examples can be found in studies describing early abilities of newborn babies for behaviors or heart rate variability showing memory of maternal food preferences or mother's breast milk. This systematic review aimed to determine whether validated odor assessment tools can be feasibly used in studies. Particularly in light of the current COVID-19 pandemic and evidence of associated olfactory impairment resulting from SARS-COV-2 infection, the study is also motivated by the need for tools to assess olfactory function in neonates.

Newborn crawling and rooting in response to maternal breast odor

A growing literature shows that perception and action are already tightly coupled in the newborn. The current study aimed to examine the nature of the coupling between olfactory stimuli from the mother and the newborn's crawling and rooting (exploratory movements of the head). To examine the coupling, the crawling and rooting behavior of 28 2-day-old newborns were studied while they were supported prone on a mobility device shaped like a mini skateboard, the Crawliskate®, their head positioned directly on top of a pad infused with either their mother's breast odor (Maternal) or the odor of water (Control). Video and 3D kinematic analyses of the number and types of limb movements and quantification of displacement across the surface revealed that newborns are significantly more efficient crawlers when they smell the maternal odor, moving greater distances although performing fewer locomotor movements. In addition, the newborns made significantly more head rooting movements in the presence of the maternal odor. These findings suggest that the circuitry underlying quadrupedal locomotion and exploratory movements of the head is already adaptable to olfactory information via higher brain processing. Moreover, the coupling between olfaction and the two action systems, locomotion and rooting, is already differentiated. As crawling enables the newborn to move toward the mother's breast immediately after birth and facilitates mother-infant interaction, the results of this study highlight the potential value of using maternal odors to stimulate mobility in infants at risk of motor delay and/or deprived of this odor when born prematurely.

A novel, short and easy-to-perform method to evaluate newborns' social olfactory preferences

Humans' early olfactory perception has been studied mainly within the framework of mother-offspring interactions and only a few studies have focused on newborns' abilities to discriminate body odors per se. The aim of this study was to develop a method to evaluate olfactory social preferences of infants at term-equivalent age. Twenty dyads of infants (10 born preterm and 10 born at term) at term-equivalent age and their mothers were included. We analyzed the behavioral reactions of infants to their mother's upper-chest odor (that bears social, non-food related information). The two impregnated gauzes and a control gauze were presented to the infants for 10 s each, in a random order. We compared two durations of gauze impregnation: 30 min and 12 h. This study reveals that mothers' upper chest emits sufficient olfactory information to induce reactions in infants born full-term or born preterm and that a short impregnation is preferable to evaluate their perception of body odors, notably for those born preterm.

Effect of environmental exposure to a maternally-learned odorant on anxiety-like behaviors at weaning in mice

Early sensory experience, such as exposure to maternal or other environmental factors, is considered to influence neurocognitive development and behaviors. In many species, exposure to odorants during pregnancy or lactation impacts the morpho-functional development of the olfactory circuitry with changes in olfactory sensitivity, feeding behavior and food preferences at birth or later. However, few studies have investigated the impact of a perinatal exposure to odorants on the anxiety-like behavior of animals to stressfull stimuli. Here, we exposed mice to heptaldehyde (HEP) during pregnancy and lactation and measured the anxiety-like behavior of their offspring to stress-inducing novel stimuli at weaning in presence or absence of odorants. We applied a combined social and maternal separation as a stressor and measured the anxiety-like behavior in an open field (OF) in presence of two odorants, HEP or α-pinene (AP) as a control odorant. Although the presence of the odorant during the social separation did not influence anxiety-like behavior, we found that, if mice born to non-odorized mothers exhibited a decreased exploratory behavior in the presence of both odorants, the effect was restricted to AP for the mice perinatally exposed to HEP. These results show that anxiety-like behaviors during a stress-inducing event could be reduced by the presence of a familiar odorant. We propose that the recall of an early olfactory experience could contribute to the improvement of animal welfare in various situations associated with husbandry practices.

fMRI-based Neuronal Response to New Odorants in the Newborn Brain

The sense of smell is one of the oldest and the most primitive senses mammals possess, it helps to evaluate the surrounding environment. From birth, smell is an important sensory modality, highly relevant for neonatal behavioral adaptation. Even though human newborns seem to be able to perceive and react to olfactory stimuli, there is still a lack of knowledge about the ontogeny of smell and the underlying central processing involved in odor perception in newborns. Brain networks involved in chemosensory perception of odorants are well described in adults, however in newborns there is no evidence that central olfaction is functional given the largely unmyelinated neonatal central nervous system. To examine this question, we used functional magnetic resonance imaging (fMRI) in the newborn to characterize cortical response to olfactory and trigeminal odorants. Here we show that brain response to odors can be measured and localized using functional MRI in newborns. Furthermore, we found that the developing brain, only few days after birth, processes new artificial odorants in similar cortical areas than adults, including piriform cortex, orbitofrontal cortex and insula. Our work provides evidence that human olfaction at birth relies on brain functions that involve all levels of the cortical olfactory system.

The human newborn's umwelt: Unexplored pathways and perspectives

Historically, newborns, and especially premature newborns, were thought to "feel nothing." However, over the past decades, a growing body of evidence has shown that newborns are aware of their environment, but the extent and the onset of some sensory capacities remain largely unknown. The goal of this review is to update our current knowledge concerning newborns' perceptual world and how ready they are to cope with an entirely different sensory environment following birth. We aim to establish not only how and when each sensory ability arises during the pre-/postbirth period but also discuss how senses are studied. We conclude that although many studies converge to show that newborns are clearly sentient beings, much is still unknown. Further, we identify a series of internal and external factors that could explain discrepancies between studies, and we propose perspectives for future studies. Finally, through examples from animal studies, we illustrate the importance of this detailed knowledge to pursue the enhancement of newborns' daily living conditions. Indeed, this is a prerequisite for assessing the effects of the physical environment and routine procedures on newborns' welfare.

The Analgesic Effects of Maternal Milk Odor on Newborns: A Meta-Analysis

OBJECTIVE

The objective of this study was to evaluate the analgesic effects of maternal milk odor on newborns.

MATERIALS AND METHODS

We searched the literature in PubMed, MEDLINE, CINAHL, EMBASE, Web of Science, and the Cochrane Central Register of Controlled Trials (CENTRAL) and collected all the randomized controlled trials (RCTs) investigating the effects of maternal milk odor versus scentless or other odors on procedural pain in newborns. The quality of included studies was assessed by the Cochrane Collaboration Risk of Bias tool. A meta-analysis was undertaken with the Review Manager 5.3 software and Stata version 11.0. Subgroup comparisons were prespecified according to the types of control groups.

RESULTS

Eight RCTs included a total of 453 participants. The results of meta-analysis showed that compared with the scentless group, the maternal milk odor group had lower pain scores during blood sampling (standardized mean difference, -0.81; 95% confidence interval [95% CI], -1.18 to -0.44; p < 0.001) and shorter crying time afterward (mean difference, -8.10; 95% CI, -15.46 to 0.73; p = 0.03). The maternal milk odor group had lower heart rate variability and oxygen saturation variability during and after a procedure, compared with both the scentless group and the vanilla group. However, no significant difference was identified in the mean heart rate and mean oxygen saturation in terms of the maternal milk odor group compared with amniotic fluid odor or mother's scent. The maternal milk odor group versus the formula milk odor group had shorter crying duration and lower levels of salivary cortisol after sampling.

CONCLUSIONS

Maternal milk odor appears to play an analgesic role in newborns. However, more high-quality studies are needed to confirm and quantitate the effect.

Adding odor: Less distress and enhanced attention for 6-month-olds

The effect of odor on cognitive and emotional processes has been studied in adults and children, but less so in infants. In this study twenty-seven six-month-olds were presented with a video while in either an odor (pine or baby-powder) or a no odor control condition. The video was a 92-s audiovisual presentation of a woman expressing happiness and sadness, with the order of emotion counterbalanced. Infant attention (looking time) and emotional expression (smiling, crying, mouthing) were coded. Infants looked longer in the presence of odor and expressed less crying and mouthing but more smiling behavior. Presence of odor markedly reduced infant emotional distress and increased attention, suggesting that the olfactory sensory system provides cues to infants that support mood regulation and maintain attention. These results have implications for optimizing infant environments for emotional health and cognitive development.

Familiarity breeds content? Soothing effect of a familiar odor on full-term newborns

OBJECTIVE

This study tested the effects of familiar and unfamiliar odors during a heel stick in full-term newborns.

METHOD

Forty-four newborns were exposed to vanillin (via their mother or via their crib) or no odor prior to a heel stick. On the day of the heel stick, infants were either exposed to a familiar odor, an unfamiliar odor, or no odor before, during, and after the procedure. Crying, grimacing, and oral movements were scored.

RESULTS

Infants exposed to a familiar odor displayed little distress and more oral movements during the procedure compared to the unfamiliar group. No advantage was found when infants were exposed to an odor learned via their mother compared to when the odor was learned via the crib. Exposure to an unfamiliar odor did not lessen distress compared to exposure to no odor.

CONCLUSION

A familiar odor is effective in significantly reducing crying and grimacing during a minor painful procedure. Olfactory support is a useful intervention that may potentially help minimize deleterious effects of neonatal pain.

La sensibilité olfactive du nouveau-né prématuré

This document reviews the main data relating to the structural and functional organisation of olfactory perception in the premature newborn. The chemoreceptive systems (main olfactory, trigeminal, vomeronasal and terminal systems) develop in different chronological orders but quite at very early stage during ontogeny. The premature newborn, despite being immature, has been shown to react to a wide variety of olfactory stimuli. Moreover, the infant seems capable of distinguishing odours of different qualities and intensities, memorising stimuli to which he is regularly exposed to, and categorising different odours based on their hedonic valence. An inventory of the olfactory stimuli to which the infant is regularly exposed to in the incubator is carried out. Several attempts to use pleasant and familiar odours to reduce stress due to separation of the infant from its mother, to promote oral feeding, to make medical procedures more acceptable, and more so, to reduce the respiratory instability of the premature infant, are described. If sustained attention is directed to the olfactory characteristics dwelling inside the incubator, the well-being, health and development of the premature newborn could be improved.

The calming effect of a familiar odor on full-term newborns

We assessed the effectiveness of an odor (familiar or unfamiliar) in soothing healthy full-term newborns undergoing a routine heel stick. Forty-four breast-fed newborns were randomly assigned to one of four groups: Before the heel stick, Group 1 was naturally familiarized with their mother's milk odor, Group 2 was familiarized with a vanilla smell, and Groups 3 and 4 did not receive any familiarization. During and after the heel stick, Group 1 was presented with their mother's milk odor, Group 2 was presented with the familiar vanilla, Group 3 was presented with an unfamiliar odor, and Group 4 was a control group. Infants' crying, grimacing, and head movements were analyzed before, during, and after the heel stick. Results show that infants who smelled a familiar odor (their mother's milk or vanilla) cried and grimaced significantly less during the recovery phase compared with the heel stick phase. Infants who were presented with an unfamiliar odor or no odor showed no significant changes during recovery. Moreover, infants who smelled their mother's milk exhibited significantly less motor agitation during the heel stick compared with the other groups. These findings indicate that smelling a familiar odor reduces agitation during the heel stick and diminishes distress after the procedure.

Olfactory stimulation prevents apnea in premature newborns

OBJECTIVE

Methylxanthines and doxapram are currently used to treat apnea of prematurity but are not fully effective and often present undesirable side effects. The present study examines whether exposure to an odor known to modulate the infant's respiratory rate could reduce the frequency of apneic spells.

METHOD

Fourteen preterm newborns born at 24 to 28 gestational weeks presenting recurrent apnea despite caffeine and doxapram therapy were exposed to a pleasant odor diffused during 24 hours in the incubator. Efficiency of the olfactory treatment was judged by comparing frequency and severity of apneas occurring during the day of odorization with that observed the day before (baseline) and the day after (posttreatment control). Apnea was defined as any complete cessation of breathing movements for >20 seconds, or less if associated with hypoxia or bradycardia.

RESULTS

Concerning all types of apneas, a diminution of 36% was observed and seen in 12 of 14 infants. Apneas without bradycardia were reduced (44%) during the day with odorization, and this diminution affected all the infants. The frequency of apnea with moderate bradycardia (heart rate between 70 and 90 beats per minute) was maintained while the frequency of apnea associated with severe bradycardia (heart rate <70 beats per minute) decreased strongly (45%) and affected all the infants. No side effects were observed.

CONCLUSION

The introduction of a pleasant odor in the incubator is of therapeutic value in the treatment of apneas unresponsive to caffeine and doxapram.

Olfaction in the fetal and premature infant: functional status and clinical implications

This article considers olfaction as a functioning source of information for the fetus and the neonate, born on term or prematurely. It aims to present how odors are involved in the sensory continuity between the prenatal and postnatal environments and how they influence the earliest adaptive responses of newborns in the realms of self-regulation, emotional balance, feeding, and social interactions.Finally, it evaluates odors as sensory means to ameliorate the physiologic and behavioral responses of preterm infants to the adverse impacts of separation from mother, nonoral feeding, or iatrogenic distress.