Maturation of auditory parameters in the preterm infant with particular reference to diet
This study was developed from the original protocol which was to examine the role of long chain polyunsaturated fatty acids (LCPUFA) in the maturation of the auditory system in the preterm infant. The particular dietary components under investigation were arachidonic (AA) and docosahexaneoic( DHA) acids. This study investigates the wider aspects and implications of the preterm birth on the maturation of the preterm auditory system. Testing was performed using Auditory brainstem response (ABR) on preterm infants in Hillingdon Hospital's neonatal intensive care unit (NICU). The recruitment criteria was <32 weeks gestational age (GA) or <1500g. A normative data set was produced using term infants (3742 weeks GA). Tympanometric testing was also introduced to assess middle ear (ME) function in both preterm and term infants. The susceptance and conductance components were recorded at frequencies between 226 and 2000Hz. This testing enabled normative data to be established for multi-component, multi-frequency tympanometric analysis. The effect of the preterm birth on tympanometric data was also examined. A total of 22 preterm infants were recruited onto the study. The maturational characteristics of ABR parameters relating to neural transmission and synaptic efficacy were investigated. The peripheral auditory system (PAS) and the lower brainstem region were found to have maturation in their neural transmission properties. The auditory nerve, however, displays constant neural transmission properties throughout the preterm period. Maturation of the synaptic efficacy (using the rate effect) of the PAS was not identified. However, both auditory nerve and lower brainstem regions display reductions in the rate effect. This was greatest in the auditory nerve. The effect of the preterm birth was assessed. The PAS showed lesser conductive properties for the preterm infant during the term period. This behaviour was confirmed by poor tympanometric data for the preterm infants. The auditory nerve showed the same transmission properties for both term and preterm infants. This indicates that the lack of maturation in this region is replicating the characteristics of the term infant developing in utero. The lower brainstem region would seem to be the most susceptible to delayed development in the preterm infant. The rate effect for the PAS and auditory nerve indicate that the extra-uterine environment has not been detrimental to the maturation of synaptic efficacy as measured by the rate effect. The lower brainstem region rate effect is slightly higher for the preterm infants. This may indicate that this region is the most susceptible to possible synaptic deficiency. There is indication of a gender effect for transmission properties in the preterm infant. This suggests superior properties for females in the central auditory system. It is possible that this also occurs on a synaptic level. Dietary analysis suggested that the preterm infants (on this study) fed formula milk enriched with a LCPUFA composition have comparable auditory function (as measured by ABR) with breastfed infants. It is concluded that breastmilk would be the diet of choice due to the additional medical benefits. Tympanometry was found to be well tolerated in both term and preterm neonate populations. Testing in enclosed style incubators in the NICU was successful. Normative characteristics for multi-component tympanornetry at various frequencies was established. The ear canal acoustic and mechanical properties were identified as a source of variability for tympanogram morphology in the neonate populations. The interaction of the neonate external auditory meatus (EAM) violates assumptions relating to numerical data. A lack of maturation was found for preteen data, there was no evolution to the normative term data over the period studied. The more complex nature of the preterm tympanometric data was still observed during the term period. This indicates that there is a difference between the maturational characteristics of the term infant and those for the preterm infant in the clinical environment. The PAS ABR characteristics were compared with tympanometric data collected from the preterm infants. The lack of maturation in the tympanometric data suggests that the major contribution to the ABR maturation during the preterm period is due to the cochlear transduction and basic synaptic delay components. In addition, that poorer ME function (as displayed in the tympanometric data) contributes to the lesser PAS ABR conductive properties in the preterm population by the term period.