Fitting high frequency hearing losses with digital hearing instrument technology requires that the patient’s residual dynamic range be fully investigated (ie, frequency-specific thresholds and supra-thresholds must be defined).1-3 It is well known that severe recruitment may occur in these high frequencies. It is also becoming recognized that cochlear “dead regions” (areas in which there are no functioning hair cells) within the stimulated high frequency range may create distortion.4 Therefore, an acoustic signal can actually become disturbing to the patient and, therefore, be detrimental a successful fitting.4-6
Methodology for successfully fitting digital technology to high frequency losses should be modified according to the degree of the slope and the severity of the loss. For example, considerations for fitting mild, precipitous, high-frequency hearing losses will differ from the fitting of moderate-to-severe, high-frequency hearing loss. Also, the variable size (volume) of the ear canal will provide different sound pressure effects when the hearing instrument is inserted (eg, occlusion effect issues). The loss (or modification) of concha resonance should also be considered when making these choices.
When fitting mild, high frequency, sensorineural hearing loss, it should be recognized that these patients still have the ability to hear their own voice “naturally.” Once a digital hearing aid is fit to their ear canal, any subsequent acoustic event is perceived by their brain as “changed.” The physical presence of the hearing device becomes an “acoustic event” in itself, in addition to the modifications in sound pressure to their auditory system. Older analog WDRC technology was successful partially because the patient (when fitted appropriately) rarely received uncomfortably loud sounds, and a more full range of sound was perceived. The old WDRC technology was restrictive, in that the output and gain were always tied together with a fixed compression ratio strategy.
Digital hearing instruments, on the other hand, are generally wide dynamic range amplifiers. Digital technology provides us with the ability to modify—in bands or channels—the output and gain for frequency-specific areas as required by each patient’s residual auditory capacity. It is for this reason that one should never custom-fit digital hearing instruments on a person with high frequency hearing loss using the “old analog fitting methods” (ie, cut lows, employ filter earhooks, etc.)
When fitting mild high-frequency, precipitous losses, be certain to check supra-thresholds at 250 Hz and 500 Hz. The slope of the amplification in these areas can easily cause trouble if one follows the “old rules.” You need to program the hearing instrument sufficiently to overcome the excess low frequency energy generated by the patient’s own voice when a hearing instrument is inserted into their ear canal. You do need to program the instrument for sufficient gain to overcome the insertion loss. Gain should always be increased to overcome the insertion loss associated with the fitting. Rarely, do you find disturbing recruitment in the low frequencies with this patient population; therefore, added output by the amplifier will generally not be uncomfortable to the patient.
Venting strategies may also be implemented to fully achieve patient comfort and overcome any programming limitations. Of course, venting can lead to feedback issues. However, increasing the electroacoustic output in the (unstable) feedback area will allow the digital amplifier to more easily overcome this problem. Ultimately, this decision will depend on the specific type of feedback cancellation system that the hearing aid employs.
With current digital anti-feedback systems, we now have the ability to provide significant high-frequency gain and output to our patients. But, we need to fully assess the patient’s ability—or inability—to use this stimulation. With a cochlear “dead region” present, or with hearing loss thresholds greater than 90 dB, it may not be worthwhile to apply gain to those particular high frequency deficits. Instead, apply gain in the transition area of the hearing loss (ie, along the precipitous sloping portion of the audiogram)—especially when there are thresholds evident within this area—that often contains the most important speech cues (eg, 1000-2500 Hz).4-6
Summary
There are additional considerations when fitting moderate-to-severe high frequency hearing loss. The above-described procedures should be followed in most cases. However, with this loss configuration, any low-frequency reduction may actually reduce the patient’s ability to hear well and comfortably.
You need to be familiar with the noise reduction strategies of the digital hearing aid, and either activate or de-activate it as required. This should maintain proper low frequency gain and output. Of course, high frequency gain and output become much more critical in fitting the moderate-to-severe losses. Severe recruitment often presents itself in the high-frequency areas of these losses. High frequencies for these losses should be carefully assessed with frequency-specific supra-threshold testing and stimulated appropriately.
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Correspondence can be addressed to Larry Brethower at [email protected], or Jay B. McSpaden, PhD, at [email protected].
References
1. McSpaden JB, Brethower L. Fitting from the other side of the fence. The Hearing Review. 2003;10(7):16-18.
2. McSpaden JB, Brethower L. Supra-threshold testing: The nuts and bolts of modern fitting. The Hearing Review. 2003;10(12):22.
3. McSpaden JB, Brethower L. The necessity for identifying the patient’s residual auditory abilities. The Hearing Review. 2004;11(5):18-20.
4. Moore BCJ. Dead regions in the cochlea: Diagnosis, perceptual consequences and amplification for the fitting of hearing aids. Trends in Amplif. 2001;5(1), 1–34.
5. Venema T. Identifying cochlear dead spots. Hearing Professional. 2003;52(4):15-20. Also see: The Hearing Review. 2005;12(3):58-65.
6 Venema T, McSpaden JB. Cochlear dead spots. Hearing Professional. 2004; 53(2):19-22. Also see: The Hearing Review. 2005;12(3):65-72.
