One of the most crucial steps in the hearing aid verification process in children is to account for ear canal acoustics. The spectrum and intensity of sound in the ear canal not only varies across children of the same age, but also within an individual child as the ear canal grows over time (J Am Acad Audiol 2002;13:407-415). These acoustic changes mean that those of us who fit hearing aids in infants and young children are trying to hit a moving target.
Without an understanding of how the child's ear canal anatomy influences sound level in the ear, over- or under-amplification is likely to occur. Fortunately, verification techniques using probe microphone measures have been developed to individually estimate ear canal acoustics and integrate that information into the fitting.
One option for verification is to measure the output of the hearing aid in the child's ear canal using a probe microphone system. Because the hearing aid is coupled to the ear, this technique is known as in situ or real-ear verification, and it is the most accurate and realistic method for verifying the output of a child's hearing aid.
Real-ear verification requires that a child have enough head control and cooperation to sit for multiple verification measures with each hearing aid. In infants and even some older children, it is not possible to measure all these verification signals, including maximum power output with high-intensity swept pure tones.
In such cases, the real-ear-to-coupler difference (RECD) is a reasonable alternative. The RECD, which uses one quick measure of the child's ear canal acoustics through the earmold, is applied to hearing aid measurements in the test system coupler to estimate the response of the hearing aid in the child's ear.
When it is not possible to measure the RECD in infants and young children, age-related averages can be applied to verification measurements in the coupler (Trends Amplif 2005;9:199-226).
TEN TIPS FOR RECD USE
Keeping the following points in mind when using the real-ear-to-coupler difference will help give an accurate picture of hearing aid output in children.
1. Real ear is best. When the goal is to measure the output of the hearing aid in the child's ear, accept no substitutes. If a child has sufficient head control and cooperation, real-ear verification should be the goal.
2. Measured RECD is an accurate alternative. As previously mentioned, real-ear verification is not always practical or possible. In these cases, individually measured real-ear-to-coupler difference should be used.
3. Age-related average RECD is just an average. We know that children of the same age can have a wide range of RECD values. Age-related average real-ear-to-coupler difference should only be used if real-ear verification and measured RECD are not possible.
4. One measured RECD is better than none. In most people, RECD values for the right and left ears are within 3 dB of each other. This means that if a child will only cooperate for a single measurement, the RECD from one ear can be applied to the other. If there are obvious physical differences between the ear canals or the child has a unilateral tympanostomy tube, it is not advisable to use the RECD from the opposite ear.
5. Age-related average real-ear-to-coupler difference does not approximate the acoustics of an ear canal with a tympanostomy tube. Tubes change the acoustics of the ear canal, particularly at frequencies below 750 Hz. Average RECDs were collected on children with intact tympanic membranes. Using an average RECD for a child with a tube or perforation of the tympanic membrane may reduce the accuracy of verification.
6. RECD assumes an occluded ear canal. The real-ear-to-coupler difference is defined as the occluded response of the ear canal. For fittings with large earmold vents or an open ear canal, using an RECD to estimate the gain and maximum output of the fitting in a coupler could lead to inaccuracies. Real-ear verification is preferable in these instances.
7. Let the average RECD be your guide. One of the best ways to determine if a measured RECD is accurate is to compare it with the average real-ear-to-coupler difference for that child's age. Significant deviations of more than 5 dB from the average without an obvious reason, such as ear canal stenosis or a surgical modification of the ear canal, should be investigated.
8. Remember acoustics. The real-ear-to-coupler difference is a comparison between the child's ear and a 2 cm3 coupler that is designed to simulate an average adult ear canal. Except in cases of tympanostomy tubes or surgically-modified ear canals, children's ear measurements will almost always be higher than those of the coupler, leading to a positive RECD value.
Negative RECDs without an explanation might indicate a measurement error. Would a six-month-old ever have a lower response than a coupler designed to simulate an adult ear canal? Probably not, so if you see a negative RECD, try to determine the cause.
9. RECD is applied to hearing aid measurements and audiometric thresholds. Because the real-ear-to-coupler difference is collected as part of hearing aid verification, most clinicians understand how the RECD is applied to the output of the hearing aid. What may not be as apparent is how the RECD is applied to audiometric thresholds.
Insert earphones are calibrated in reference to a 2-cm3 coupler, and, when the dB HL thresholds are converted to dB SPL, the RECD is applied. This is important because the application of an RECD measured with an earmold to thresholds that were measured using an insert foam plug could lead to inaccuracies in estimating the audibility of the signal. One option is to measure thresholds using insert earphones coupled to the patient's earmolds.
Clinicians should consider the impact of the real-ear-to-coupler difference on thresholds, since most verification systems will not allow the specification of separate RECDs for assessment transducers and earmolds.
10. Measure RECD regularly. The RECD changes significantly during the first two years of life. The real-ear-to-coupler difference and hearing aid fitting should be updated every three months to ensure the best possible outcome. As the ear canal grows, the sound intensity level in the ear canal will decrease, potentially leading to reduced audibility of speech over time.
For children between three and five years of age, we recommend hearing aid verification with real ear or RECD every six months. After five years of age, annual follow-up is advised.
With frequent monitoring, we can ensure that children are hearing their best with hearing aids.