Ear effect and gender difference of spontaneous otoacoustic emissions in children with auditory processing disorder

Abstract

Spontaneous otoacoustic emissions (SOAEs) are found in most healthy ears, and can be used to measure the health of the cochlear structures and feedback mechanism. According to existing literature, right ears tend to exhibit greater numbers of SOAEs than left ears (Bilger et al., 1990) and females tend to show higher incidence of SOAEs than males (Moulin et al., 1993). The SOAE prevalence has not been extensively studied in children with Auditory Processing Disorder (APD), a disorder with unknown etiology that reduces one’s ability to process auditory information. This study examined the prevalence and ear advantage of SOAEs between genders in children diagnosed with APD. SOAEs were investigated in 19 children (7 girls and 12 boys) with APD and 2 4 typically developing children (14 girls and 10 boys) aged 7–12. Right ear advantage was more prevalent in control (71%) than APD subjects (42 %). However, over 30% more females exhibited a right ear advantage than males in each group. Although the results are not significant, our findings indicate that the lack of right ear advantage for SOAE is more prevalent in children with APD, particularly in males, suggesting that cochlear mechanisms or their control might be somehow affected in APD.

1. Introduction

Auditory Processing Disorder (APD) is a disorder in which children with normal peripheral hearing have difficulty processing speech. The issues are most pronounced in noisy environments and can manifest as difficulty understanding directions, reading and writing problems, and generally poor academic performance. Although a good deal of research has been done in the area, much is still unknown about APD. For example, there is much debate about the etiology of the disorder, namely whether the primary cause is a central auditory issue, a peripheral impairment, or both. Additionally, there is currently no gold standard for diagnosis. Instead, audiologists rely on a battery of subjective tests that are often based on a national standard, but that vary between institutions.

These gaps in the literature need to be filled in order to better understand and treat children with APD. This paper is part of a larger study which seeks to fill some of these gaps through information from a questionnaire and a battery of audiological and electophysiological tests.

This paper sought to delve deeper into one specific measure, spontaneous otoacoustic emissions (SOAE). SOAEs, acoustic signals emitted by the cochlea, are present in almost all infants’ ears and persist in many adults despite a decrease in prevalence with aging (Burns et al., 1992; Penner and Zhang, 1997). Originally thought to be a sign of cochlear pathology, SOAEs have since been found to indicate healthy cochlear structures and feedback mechanism. More specifically, they are a by-product of the function of the outer hair cells, which are responsible for fine frequency discrimination and amplification. According to existing literature, right ears tend to exhibit greater numbers of SOAEs than left ears (Bilger et al., 1990) and females tend to show higher incidence of SOAEs than males (Moulin et al., 1993), which is consistent with the general trend for more sensitive hearing in right ears and females (McFadden, 1993).

The purpose of this study was to compare the characteristics of the SOAEs in children with APD to an age-matched control group. We hypothesized that children with APD exhibit a different SOAE pattern than typically developing children.

2. Method

2.1. Participants

The APD group consisted of 23 children (14 boys; 9 girls) aged 7–12 years who were diagnosed with Auditory Processing Disorder. The control group was made up of 24 typically developing children (10 boys; 14 girls) aged 7–12 years. All children were screened with a pure-tone hearing test and a tympanogram prior to other testing. Participants were excluded if they had abnormal responses on either test.

2.2. Data Collection

SOAE

All testing was performed in a sound booth. SOAE data was collected using SmartEP (Intelligent Hearing Systems, Inc). Each ear was tested for 2 trials of 2000 sweeps, at a rate of 21.1/sec. Subjects watched a muted movie or played a game on an iPad during testing.

Other Testing

Additional testing was performed on each participant as part of a larger study. These tests included: OAE Suppression, Behavioral Speech Perception Testing, Dichotic Listening, Speech in Noise, and Auditory Evoked Potentials.

2.3. Data Analysis

After SOAE data was collected, the trial with the least noise was selected and peaks were picked from that run. SOAE peaks were chosen based on the following criteria: the signal was at least 6 dB greater than the noise, the signal was greater than −30 dB, and peaks occurred between 500 and 6000 Hz. The final selection of peak location was validated by two researchers.

Once peak selection was finalized, each participant’s SOAE was analyzed in the following ways. First, it was broken down into 500 Hz frequency bins to determine where peaks were the most common. Then the “advantage ear,” or the ear with more peaks, was determined for each subject.

3. Results

Figure 1 and Figure 2 show the proportion of subjects (in each group) who had at least one SOAE peak in the left and right ears, respectively. A greater proportion of APD than control subjects had at least one peak in the left ear in low frequency bins (see Figure 1). This pattern was not as prominent in the right ear (Figure 2). These differences were most notable at frequencies between 1000 and 2000 Hz.

Figure 1.

Proportion of Control vs. APD subjects who had at least one SOAE peak per frequency bin (500 Hz) in the left ear

Figure 2.

Proportion of Control vs. APD subjects who had at least one SOAE peak per frequency bin (500 Hz) in the right ear

Figure 3 shows the proportion of subjects within the group who exhibited a greater number of SOAE peaks in the right ear than the left ear. The ear difference that we expected to see (more peaks in the right ear than the left ear) appeared only in the females of the control group. About 50% of the males in both groups showed more peaks in the right ear.

Figure 3.

Proportion of subjects within each group who had a greater number of SOAE peaks in the right ear than the left ear

4. Discussion

We found that the SOAE patterns of children with APD seem to differ from those of typically developing children. More specifically, APD subjects tend to have more peaks at lower frequencies (500–1000 Hz), particularly in the left ear. These frequencies are very important for perception of speech sounds. Because the literature suggests that greater numbers of SOAEs are associated with better hearing sensitivity (McFadden, 1993), children with APD might have a hearing advantage in the left ear as opposed to typically developing children who have a “right ear advantage.” This right ear advantage is hypothetically linked to the fact that in almost all right handed and most left handed people, speech is processed predominantly in the left cerebral hemisphere (Kimura, 1961; Chi et al., 1977). It may be hypothesized that the SOAE left ear advantage in children with APD may somehow impair their auditory processing. We hypothesize that this discrepancy might be caused in part by the medial efferent olivocochlear system. More research is needed to further investigate this hypothesis.

Although we found differences in SOAE patterns between the APD group and control group, our sample size was not large enough for statistical significance. Our findings could have clinical significance, however, because the data suggest that in addition to central auditory involvement, children with APD may also have coexisting peripheral auditory deficits.

5. Conclusion

Our results show that the patterns of SOAEs in children with APD differ from those of typically developing children as seen in existing literature and in the control group of our study. These specific differences may help explain why children with APD have trouble processing speech and may lead to important diagnostic information in the future.