Understanding the developmental course of the acoustic properties of the human outer and middle ear over the first 6 months of life by using a longitudinal analysis of power reflectance at ambient pressure.

Abstract

As shown by tympanometric and wideband acoustic immittance (WAI) measures, developmental changes that occur in the middle ear and the external auditory canal can affect the mechano-acoustic properties of the middle ear system during the first 2 years of life. However, these observed differences and the specific course of maturation have not been fully investigated. The overall goal of the current study was to define the time course and rate at which functional maturation of the middle ear occurs in human infants through wideband acoustic immittance (WAI). It was also within the scope of this study to establish normative data to characterize acoustic properties of the normal middle ear in infants from birth to 6 mo of age. Newborns were tested longitudinally at 1-mo intervals up to 6 mo of age for a total of six visits. All babies passed transient evoked otoacoustic emission screening on each visit. Tympanograms and WAI were recorded, and the distributions of WAI patterns were analyzed as a function of age. WAI was measured across a wide range of frequencies from birth to 6 mos of age in a cohort of 31 infants. Test-retest differences for WAI were also evaluated. Thirty-one newborns were recruited. A repeated-measure analysis of variance was conducted to investigate whether the variations observed in power reflectance at ambient pressure across the six visits were statistically different. All significant findings were subject to Greenhouse-Geisser correction for repeated-measures sphericity and inflated type-I error. RESULTS showed that power reflectance increased (closer to 1) at low frequencies (<400 Hz) and decreased (closer to 0) at high frequencies (>2000 Hz) as a function of age. There was very little change in power reflectance from 600 to 1600 Hz across the first 6 mo of life. Most group changes that were observed were also consistently shown in longitudinal changes observed within individual subjects. The overall maturation of the middle ear can result in a lower reflectance at higher frequencies and a higher reflectance at lower frequencies. This could be explained by known changes in ear-canal and/or middle-ear structures across this time period, which could lead to changes in mass and resistance. An increase in volume in the middle-ear cavity, reduction of middle-ear debris, and overall decrease in resistive elements might be contributing to these changes. Significant differences were observed between each visit, which could warrant the use of age-specific norms when applying WAI data to infants less than 6 mo of age.