Early exposure to maternal voice: Effects on preterm infants development
Introduction
During intrauterine life the fetus experiences an extremely rapid cerebral growth and neurosensorial maturation. The quality of the experiences undertaken before birth may influence the fetal neurological development both in terms of structural and functional modifications.
Preterm infants are at high risk for developing adverse short and long-term developmental outcomes [1], [2]. Disorders have been linked not only to the immaturity of the cerebral structures but also to the sudden interruption of the physiological development of cerebral structures due to preterm birth [3], [4], [5]. Preterm infants actually complete their development being exposed to continous adverse environmental stimuli (light, noise, electromagnetic fields, drugs, inadequate manipulations, temperature etc.) during hospital stay [6], [7], [8], [9]. Indeed Neonatal Intensive Care (NICU) environment largely differs from that of the protective womb. In addition, the preterm infant is much more sensible and vulnerable than the term one. With regard to the development of the auditory system, exposition to NICU stimuli has been reported to cause an advanced progression of the infant's sensory systems maturation and to deeply affect the cortex functional organization [10].
It is widely acknowledged that the fetus perceives sounds and reacts to them with movements since the 26th–28th week of gestational age (GA) [11]. The sounds perceivable in the environment of a pregnant woman penetrate the tissues and fluids surrounding the fetal head, stimulating the inner ear through a bone conduction route [12]. The sound pressure into the amniotic fluid induces skull vibrations which are transmitted directly into the contents of the cranial cavity and from there into the cochlear fluids, presumably by fluid channels connecting them [13]. It can be therefore speculated that the exposure of preterm infants to maternal voice through bone conduction may mimic the prevalent method of operation of the fetal auditory system.
Developmental Care (DC) is any NICU intervention undertaken to improve neurodevelopmental outcome. It includes NICU design, nursing routines, nursing care plan, management of pain, feeding methods and parental involvement [14]. DC aims to provide an extrauterine environment which positively supports and influences the newborn development [15]. Indeed early interventions may significantly influence the well-being of the infant and his/her parents, reducing the negative effects of prematurity [2].
Maternal voice represents a unique source of sensory stimulation for the fetus and exerts a positive acustic stimuli for its correct development. The exposure to maternal voice within preterm infant's sound environment may therefore contribute to provide the infant a more comfortable and secure surrounding. Few studies addressed the effect of the exposure to maternal voice on preterm infant's development reporting inconsistent results. Some authors did report potentially positive developmental effects (i.e. the development of more adaptative responses such as a greater decrease in heart rates and higher oxygen saturation rates) whereas other did not [16], [17], [18], [19], [20], [21].
To our knowledge, there are no data available concerning the effect of the exposure to maternal voice administered by bone conduction on preterm infants development.
The aim of the present study was to investigate the effect of the exposure to maternal voice, administered by bone conduction, on preterm infants autonomic and neurobehavioral development. The hypothesis to be tested was that preterm infants exposed to the stimulation of maternal voice would show better autonomic and neurobehavioral parameters during the intervention period and a better neurobehavioral performance during the first six months of corrected age (CA) than preterm infants that have not been exposed to maternal voice.
Section snippets
Materials & methods
We conducted a prospective, longitudinal, explorative, case control study. Infants were scheduled to be prospectively followed up to the sixth month of CA. CA was calculated, up to 24 months of life, from the chronological age adjusted for gestational age that is, for the number of weeks different from the expected 40 weeks.
The study design was approved by the departmental ethics committee and written informed consent was obtained from all the infants' parents.
Study population
Of all the 405 consecutive infants admitted to NICU, Fondazione IRCCS, Ospedale Maggiore Policlinico, between 2008 and 2012, 87 infants entered the study. The inclusion criteria were: birth weight (BW) < 1500 g, being adequate for gestational age (AGA; birth weight ≥ 10 percentile according to Fenton's growth chart) [22], spontaneous breathing with saturation > = 95%, heart rate (HR) between 140 and 170 bpm during the last 48 h, normal cerebral ultrasound. The exclusion criteria were the presence of
Statistical methods
Descriptive data were presented as mean ± SD or n (%). The differences between the two groups were assessed by means of the T-Student test (for continuous variables) and the Chi Square test (for categorical variables). All obtained results were confirmed, when necessary, with the Wilcoxon–Mann–Whitney or the Fisher exact test.
Significance of multiple comparisons was adjusted by Bonferroni corrections.
All analysis were performed using the statistical software SAS 9.2 (SAS Institute Inc., Cary,
Results
Sixteen infants dropped out of the study either during hospital stay (n = 13 due to the occurrence of infections and cardiorespiratory instability) or after discharge (one due to the development of cerebral palsy and two because the parents failed to bring them to the scheduled follow up assessments) so that the study population was composed by 71 infants. The intervention group comprised 34 infants.
The two groups resulted homogeneous when compared by gender, ethnicity, GA and BW (Table 1).
All
Discussion
The findings of the present study indicate that the exposure to maternal voice filtered according to the Tomatis' method exerts a beneficial effect on preterm infants' maturation of autonomic functions and neurobehavioral outcome at term and at three months of CA. Indeed, with regard to the vital parameters, infants belonging to the intervention group showed consistently lower HR values while being exposed to maternal voice either during the single observations or throughout the whole
Conclusion
Our study provides evidence that hospitalized preterm infants benefit from the early exposure to maternal voice with regard to the autonomic maturation and the neurofunctional outcome. Further additional larger studies are desiderable in order to confirm the present findings.
Conflict of interest statement
Authors have no conflict of interest to declare. The follow-up program is funded through the National Health Service, with no outside funding or charitable contributions.
Acknowledgments
We are grateful to infants and families that participated to the study.
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