Comparison of Real-Ear to Coupler Difference Values in the Right and Left Ear of Hearing Aid Users

Effect of Real-Ear Adjusted Stimuli on Vestibular Evoked Myogenic Potential Variability in Children and Young Adults

OBJECTIVES

There is large variability in cervical and ocular vestibular evoked myogenic potential (c- and oVEMP) amplitudes. One potential source of variability is differences in ear canal shape and size. Real ear-to-coupler difference (RECD) values are used to measure the acoustic environment of an individual's ear canal. RECD may be a useful measure to calibrate air conducted VEMP stimuli, which are elicited at high intensities and may put patients at risk of unsafe sound exposure. A recommendation for avoiding unsafe exposure is to use a 125 dB SPL stimulus for individuals with an equivalent ear canal volume (ECV) ≥ 0.9 mL and a 120 dB SPL stimulus for individuals with a smaller ECV. The purpose of this project was to determine if using a stimulus calibrated in the ear using RECD values significantly reduces intra-subject and inter-subject VEMP amplitude variability. We hypothesized that using a RECD-calibrated stimulus would significantly reduce inter-subject amplitude variability but not significantly reduce intra-subject variability. We further hypothesized that an RECD-adjusted VEMP stimulus would better protect against delivering unsafe sound exposure compared to the method of using ECV alone.

DESIGN

Eleven children (4 to 9 years), 10 adolescents (10 to 18 years), and 10 young adults (20 to 40 years) with normal hearing, tympanometry, vestibular and neurological function participated. On all subjects, RECD was measured twice per ear to account for test-retest reliability. cVEMP and oVEMP were then recorded bilaterally with a 500 Hz tone burst at a traditional and an adjusted VEMP intensity level. The traditional intensity level was 125 dB SPL for individuals with ≥ 0.9 mL ECV and 120 dB SPL for individuals with ≤ 0.8 mL ECV. The adjusted intensity level was calculated by subtracting the average 500 Hz RECD measured values from the 500 Hz normative RECD value. This value was applied as a correction factor to a 125 dB SPL stimulus. Peak to peak amplitudes were recorded and used to calculate asymmetry ratios.

RESULTS

Young children had significantly smaller ECVs compared to adolescents and young adults. Young children had larger RECDs; however, this was not significant in post hoc analyses. The method of calibration had no significant effect on intra-subject variability for cVEMP [ F (1, 27)= 0.996, p = 0.327] or oVEMP [ F (1, 25)= 1.679, p = 0.206]. The method of calibration also had no significant effect on inter-subject amplitude variability for cVEMP [ F (1, 120)= 0.721, p = 0.397] or oVEMP [ F (1, 120)= 0.447, p = 0.505]. Both methods of calibration adequately protected against unsafe exposure levels. However, there were subjects with ECVs ≥ 0.9 mL who approached unsafe exposure levels from the ECV-calibrated stimulus, suggesting there may be rare cases in which a 125 dB SPL stimulus is unsafe, even for patients with larger ECVs.

CONCLUSIONS

The calibration method made no significant difference in intra- or inter-subject variability, indicating that the acoustic environment of the outer ear is not significantly contributing to VEMP amplitude variability. The RECD-adjusted stimulus is effective in protecting against unsafe exposure levels for two trials of both c- and oVEMPs. There may be instances where more than two trials of each test are required, which increases the effective stimulation level. Clinicians should be cautious when delivering VEMPs and not unnecessarily expose patients to unsafe levels of sound.

External and middle ear influence on envelope following responses

Considerable between-subject variability in envelope following response (EFR) amplitude limits its clinical translation. Based on a pattern of lower amplitude and larger variability in the low (<1.2 kHz) and high (>8 kHz), relative to mid (1-3 kHz) frequency carriers, we hypothesized that the between-subject variability in external and middle ear (EM) contribute to between-subject variability in EFR amplitude. It is predicted that equalizing the stimulus reaching the cochlea by accounting for EM differences using forward pressure level (FPL) calibration would at least partially improve response amplitude and reduce between-subject variability. In 21 young normal hearing adults, EFRs of four modulation rates (91, 96, 101, and 106 Hz) were measured concurrently from four frequency bands [low (0.091-1.2 kHz), mid (1-3 kHz), high (4-5.4 kHz), and very high (vHigh; 8-9.4 kHz)], respectively, with 12 harmonics each. The results indicate that FPL calibration in-ear and in a coupler leads to larger EFR amplitudes in the low and vHigh frequency bands relative to conventional coupler root-mean-square calibration. However, improvement in variability was modest with FPL calibration. This lack of a statistically significant improvement in variability suggests that the dominant source of variability in EFR amplitude may arise from cochlear and/or neural processing.

Real-Ear-to-Coupler Difference: Physical and Perceptual Differences

PURPOSE

The real-ear-to-coupler difference (RECD) is a recommended measure for accurate hearing aid fittings, especially for pediatric populations. However, for adults, many clinicians question whether it is necessary.

METHOD

Hearing aids were fit on two groups of 85 older adults seen at a Veterans Administration audiology clinic. One group was fit using RECD measurements, whereas the second group was fit with population-based average RECD values. The two groups had similar pure-tone hearing thresholds.

RESULTS

Like previous studies, there was little difference between the measured RECD for the right and left ears among the participants. Although the majority of the measured RECDs were within 1 SD of the mean, approximately 20% of those measured were outside of the normal range. It also was found that all participants produced lower (improved) Hearing Handicap Inventory for Elderly-Screening (HHIE-S) scores from pre- to postfitting, thus suggesting a reduction in self-perceived hearing handicap.

CONCLUSIONS

Despite having similar prefitting HHIE-S scores, those participants who had their hearing aids fit using measured RECD values had lower postfitting scores than the group that was fit with average RECD values. Furthermore, there was a significant difference between the groups on several questions of the International Outcome Inventory-Hearing Aids, suggesting that there was higher satisfaction with the fittings based on the custom RECD rather than the fittings based on the average RECD. This study demonstrated that, in addition to performing verification using real-ear measurements, accurate conversion of dB HL to dB SPL using personalized RECD likely improved hearing aid satisfaction.

The accuracy of standard audiometric hearing level thresholds in pediatric patients

OBJECTIVES

Standard audiograms provide decibels Hearing Level (dB HL) thresholds, which are referenced to normative values specified in decibels Sound Pressure Level in an acoustic coupler. Due to variability in external ear acoustics, the actual sound levels reaching the eardrum can vary across individuals. The real-ear to coupler difference (RECD) is a frequency-specific measurement of the difference between sound levels measured at the eardrum and in a coupler. Here, we compare the standard audiogram dB HL levels to RECD corrected hearing thresholds (dB RECHL) in children.

METHODS

Children who underwent standard audiometric and RECD testing were included. The dB RECHL was established and the differences between dB HL and dB RECHL (threshold error) was calculated. A threshold error >5 dB was considered significant.

RESULTS

A total of 166 children were included (mean age 12 years). Overall, 14% had normal hearing, 52% had conductive hearing loss and 27% had sensorineural hearing loss. Hearing threshold levels were overestimated by the standard audiogram compared to dB RECHL, at all frequencies (250-6000 Hz). In the lower frequencies and at 6000 Hz, 33-59% of patients were overestimated, with a threshold error up to 25 dB. In the mid frequencies, 33% were overestimated with a similar threshold error.

CONCLUSION

Standard audiogram thresholds overestimated hearing levels in children which may have clinical implications. This problem can be addressed by correcting thresholds with RECD. More studies are needed to assess the effect of correcting thresholds on hearing outcomes in children.

Extended bandwidth real-ear measurement accuracy and repeatability to 10 kHz

OBJECTIVE

Direct real-ear measurement to the 4-6 kHz range can be measured with suitable accuracy and repeatability. This study evaluates extended bandwidth measurement accuracy and repeatability using narrowband and wideband signal analysis.

DESIGN

White noise was measured in female ear canals at four insertion depths using one-third and one-twenty-fourth octave band averaging.

STUDY SAMPLE

Fourteen female adults with reported normal hearing and middle-ear function participated in the study.

RESULTS

Test-retest differences were within ±2 dB for typical frequency bandwidths at insertion depths administered in clinical practice, and for up to 8 kHz at the experimental 30 mm insertion depth. The 28 mm insertion depth was the best predictor of ear canal levels measured at the 30 mm insertion depth. There was no effect of signal analysis bandwidth on accuracy or repeatability.

CONCLUSIONS

Clinically feasible 28 mm probe tube insertions reliably measured up to 8 kHz and predicted intensities up to 10 kHz measured at the 30 mm insertion depth more accurately than did shallower insertion depths. Signal analysis bandwidth may not be an important clinical issue at least for one-third and one-twenty-fourth octave band analyses.

Comparison of Psychophysical and Physical Measurements of Real Ear to Coupler Differences

OBJECTIVES

The purpose of the study is to compare real ear to coupler difference (RECD) curves based on physical and psychophysical measures. For the physically measured RECD, the RECD was measured with real ear and coupler measurements for the ear simulator and HA1- and HA2 2-cc couplers. The psychophysically measured RECDs were derived from audiogram measures.

DESIGN

RECDs were measured in 19 normally hearing subjects. The coupler measurement was done with the probe microphone and the coupler microphone itself. Psychophysically measured RECDs were derived for all subjects by measuring the audiogram in sound field and with an ER-3A insert phone.

RESULTS

Reference data were obtained for the three coupler types. It was possible to derive the RECD curve with psychophysical methods. There was no overall statistical difference between the physically and psychophysically measured RECD curves for the HA2 2-cc coupler and the ear simulator. The standard deviation was, however, much higher for the psychophysically derived RECD, indicating that physically measured RECDs are more precise than psychophysically derived RECDs.

CONCLUSIONS

For the physical RECD measurements, the coupler microphone should be used for the coupler measurement. Physically measured RECDs were validated on group level by the reliable derivation of the RECD curve from audiogram measures.