Objective: The mature pattern of frequency distribution of synchronized spontaneous otoacoustic emissions (SSOAEs) has been reported to be bimodal in adults and children between 5 and 11 years of age; however, little is known about the distribution in neonates between 2 and 4 days after birth. Furthermore, overall differences in frequency distribution resulting from difference in sex and asymmetry between ears have not been carefully examined. The aim of this study is to determine the frequency distribution of SSOAEs in neonates at 2 to 4 days of age, evaluate the maturity of the pattern of distribution in this age group, and to evaluate the effects of differences in sex and asymmetry between left and right ears on the frequency distribution.
Methods: We evaluated 224 ears in 112 newborns (59 girls, 53 boys) whose ages ranged from 2 to 4 days. The SSOAEs were measured using ILO96.
Results: Most of the SSOAEs (86.5%) appeared at frequencies between 1.01 and 4.50 kHz. The overall frequency distribution of the SSOAEs showed a 'peak-valley-peak' pattern when plotted. Two peaks with maxima at 1.41-1.60 and 3.01-3.20 kHz were separated by a valley with a minimum at 2.41-2.60 kHz. Both girls and boys had approximate monomodal patterns in the distribution of SSOAEs. Significant sex-dependent differences were noted with more SSOAEs at the lower frequencies (<or=2 kHz) in boys (46.1%) than in girls (32.0%) (P<0.05) and more SSOAEs at the higher frequencies (2.51- 4.50 kHz) in girls (50.9%) than in boys (37.5%) (P<0.05). Both the right and left ears showed the 'peak-valley-peak' pattern that was similar to the overall distribution pattern. But, compared with the peaks measured in the left ears at 1.01-1.50 and 3.01-3.50 kHz, the peaks of the right ears at 1.51-2.00 and 2.51-3.00 kHz were much closer to the valley.
Conclusions: The overall distribution of frequency of SSOAEs in 2- to 4-day-old neonates had the similar mature 'peak-valley-peak' distribution pattern seen in adults. Significant sex-dependent differences of the SSOAEs frequency distributions have been found. However, only slight ear asymmetries of the SSOAEs frequency distributions can observed in this age group.