Real-ear-to-coupler difference (RECD) measures take into account a person's unique ear canal or earmold characteristics in comparison to a coupler that simulates an average adult's ear canal. Using RECDs during pediatric hearing aid verification is very important, as infant and young children's ear canals vary significantly in size compared to an adult and the size of children's ear canals regularly change in size as they grow. RECD measurement is quick and can be completed for use in simulated real-ear verification measures in a couple minutes per ear with practice and appropriate distraction for the child.
Although average RECD values do exist for one-month age intervals through the age of 5 years, individual variability of age-related averages is high. Age-related averages do not take into account the unique acoustic characteristics of an individual child's ear or earmold, which can affect the real ear response. The use of age-related average RECD values result in hearing aid fittings matched farther from prescriptive targets compared to a measured RECD (McCreery, Bentler, & Roush, 2013).
In the following video, a probe tube is inserted into the ear canal of an 8-month-old child.
Note the use of these strategies in the following video.
Negative RECD values may result from several sources:
Figure 1. RECD responses for an intact (left panel) versus perforated (right panel) eardrum
Figure 2. Improper probe tube insertion
Figure 3. Blocked tube probe
Ideally, the same coupling method (earmold or insert earphone) will be used both for the assessment of hearing thresholds and the measurement of the RECD. However there are occasions when a mismatch may occur, e.g. insert earphones are used during assessment of audiometric thresholds and then the child's own earmolds are used to measure the RECD. In these cases, the coupling method in each step should be specified within the verification system so that appropriate correction factors may be applied.
See (Moodie et al., 2016) for further discussion on RECDs obtained using a child's personal earmolds.
Between-ear RECD values are highly correlated (Tharpe, Sladen, Huta, & McKinley Rothpletz, 2001). The measured RECD from one ear can be used to predict the second ear RECD, unless an obvious difference between ears is found (Munro & Howlin, 2010). If an ear malformation, middle ear effusion, or TM perforation is present, then neither an RECD from the contralateral ear nor an age-related average RECD should be used. In the case of obvious differences between ears, an RECD must be measured for each ear.
It is always recommended to measure an RECD for at least one ear. However, using age-related average values is better than ignoring the RECD completely or using aided sound field testing which provides no estimate of output or audibility.
Middle ear effusion and tympanic membrane perforations are the most common middle ear conditions that affect the RECD response. TM perforations were discussed above, with the effect being a negative RECD value in the low to mid frequency range. On the other hand, middle ear effusion with increased stiffness of the TM often creates larger positive RECD values across the frequency range tested (Martin, Westwood, & Bamford, 1996). If the perforation is longstanding, measure and use the RECD values for hearing aid fitting. Similarly, if the middle ear effusion is longstanding, use the RECD values for hearing aid fitting. If it is a temporary condition, you may wait until the middle ear is healthy to measure RECD or use a previously measured RECD from when the ear was healthy for the hearing aid fitting.
The RECD should be repeated whenever there is a change in the ear canal/earmold acoustics. The RECD should also be repeated when new hearing thresholds are measured. Most pediatric guidelines recommend that the RECD be repeated whenever the child receives new earmolds, which in infants may be as often as every 2-3 months when they are growing rapidly. McCreery & Walker (2017) recommend measuring RECD as part of hearing aid verification every three months in the first year of life, then every 6 months up until the age of 3 and annually thereafter in children with no risk factors of progressive hearing loss.
Martin, H. C., Westwood, G. F., & Bamford, J. M. (1996). Real ear to coupler differences in children having otitis media with effusion.
Br J Audiol, 30(2), 71-78.
McCreery, R. W., Bentler, R. A., & Roush, P. A. (2013). Characteristics of hearing aid fittings in infants and young children.
Ear Hear, 34(6), 701-710. doi:10.1097/AUD.0b013e31828f1033
McCreery, R. W., & Walker, E. A. (2017). Hearing Aid Verification for Children. In
Pediatric Amplification: Enhancing Auditory Access. San Diego, CA: Plural Publishing.
Moodie, S., Pietrobon, J., Rall, E., Lindley, G., Eiten, L., Gordey, D., . . . Scollie, S. (2016). Using the Real-Ear-to-Coupler Difference within the American Academy of Audiology Pediatric Amplification Guideline: Protocols for Applying and Predicting Earmold RECDs.
J Am Acad Audiol, 27(3), 264-275. doi:10.3766/jaaa.15086
Munro, K. J., & Howlin, E. M. (2010). Comparison of real-ear to coupler difference values in the right and left ear of hearing aid users.
Ear Hear, 31(1), 146-150. doi:10.1097/AUD.0b013e3181b8399b
Tharpe, A. M., Sladen, D., Huta, H. M., & McKinley Rothpletz, A. (2001). Practical considerations of real-ear-to-coupler difference measures in infants.
Am J Audiol, 10(1), 41-49.