Question
I am aware of many benefits of using insert earphones with children and with patients of all ages. My employer is concerned that threshold information obtained with inserts in small ear canals may not transfer well to hearing aid fitting, i.e., to accurate use of fitting algorithms and real ear measurements. Is there any potential downside related to this? What, if any, are the audiological (audiometric or fitting-related ) disadvantages of using insert earphones?
Answer
It is true there are many advantages to using insert earphones with children and patients of all ages. On the other hand, your employer has pointed out some valid concerns. There are potentially some audiometric and fitting-related disadvantages to using insert earphones if certain factors are not appropriately accounted for.
First of all, it is well known that the acoustic properties of infant ear canals are different from adults. This difference impacts the insert earphone calibration during audiometry. Insert earphones are calibrated using a set of reference equivalent sound pressure levels (RETSPL) for a 2cc coupler. These RETSPLs were derived from data collected from normal adult subjects. If the transducer is coupled to an ear that differs from the average adult, the calibration will no longer be valid. As a result, audiometric thresholds in dB HL will not be accurate. These inaccuracies are more significant the greater the ear canal size differs from the average adult (i.e. infants and children). A way to circumvent this problem without calibrating your audiometer for each individual patient is to convert dB HL thresholds to dB SPL at the eardrum using the real-ear-to-coupler difference (RECD). The RECD captures the individual ear canal acoustics of each patient and, when used in combination with the RETSPL, is a valid way to transform hearing thresholds in dB HL to dB SPL at the eardrum. Representing hearing thresholds in this way allows for individualized thresholds that are not derived from average adult data. Therefore, there is a potential disadvantage to using insert earphones for audiometry if dB HL thresholds are not converted to dB SPL using a measured RECD. Fortunately, there are prescriptive algorithms that compute this automatically when hearing thresholds in dB HL are entered and the type of transducer is indicated (i.e. DSL [i/o], NAL NL1).
The second potential disadvantage is related to the first downside described above. If audiometric thresholds in dB HL have not been individually converted to dB SPL at the eardrum, the inaccuracies in the thresholds will be reflected in the hearing instrument fitting. One goal of fitting hearing instruments to the pediatric population is to match the audiometric and amplification characteristics for the child in dB SPL. Since children cannot provide feedback about the fit of the hearing instrument, predictive electroacoustic procedures for fitting have been developed. These procedures require a description of the individual's ear canal acoustics. The RECD is used to more accurately represent the child's hearing thresholds as well as predict the real-ear hearing instrument performance across frequencies. It is feasible to compare audiometric and electroacoustic characteristics since both insert earphones and hearing instruments are calibrated using a 2cc coupler. However, if the RECD is not used, the performance of hearing instruments fitted to infants and children will be imprecise. The result could be over or underamplification which can have a significant impact on habilitative outcomes.
There are several benefits of the using insert earphones for adults and patients of all ages. However, one must be aware of the importance of supplementing the use of the RECD to individualize hearing thresholds and hearing instrument fitting, especially in the pediatric population. The following reference contains additional information concerning this important issue:
Seewald RC and Scollie SD 1999. Infants are not average adults: Implications for audiometric testing. The Hearing Journal 52(10): 64-72.
Dr. Richard Seewald holds a Canada Research Chair in Childhood Hearing at the National Centre for Audiology in London, Ontario, Canada. He is also a Professor in the School of Communication Sciences and Disorders, University of Western Ontario. For the past 25 years, Dr. Seewald's work has been focused on issues that pertain to the selection and fitting of amplification in infants and young children and is known internationally for his work in developing the Desired Sensation Level (DSL) Method for pediatric hearing instrument fitting.