Research suggests that patients with one or two cochlear dead regions between 1,000 and 4,000 Hz should be prescribed a broadband prescriptively appropriate hearing aid response. Several studies recommend how to go about diagnosing dead regions and modifying the prescription of hearing aid gains.
One suggested limiting high-frequency gain in moderate hearing loss or greater (HJ 1999;52:10; see FastLinks), but a more commonly cited recommendation suggested that high-frequency gain should be limited above 1.7 times the start frequency of the dead region(Ear Hear 2004;25:98.)Diagnosing a dead region at 2,000 Hz, for example, would result in limiting high-frequency bandwidth at 3,400 Hz (1.7 x 2,000).
Mechanical vibration of the basilar membrane is not transduced appropriately in and around the dead region to elicit electrical stimulation of the auditory nerve. The effective result of listening to amplified sound within those cochlear dead regions has been described as information overload (Trends Amplif 2001;5:1) and is thought to have a distorted perception by the patient with hearing loss.
It is not possible to confirm the presence or absence of dead regions based on audiometric configuration and hearing loss severity. Several authors have suggested that dead regions most commonly appear at audiometric thresholds above 60 or 70 dB HL. (J Am Acad Audiol 2009;20:251; Ear Hear 2007;28:231.) Prevalence of dead regions in individuals with thresholds exceeding these levels may range from 29 percent to 84 percent. (J Am Acad Audiol 2009;20:251; J Am Acad Audiol 2005;16:600)
Implications of High-Frequency Cochlear Dead Regions for Fitting Hearing Aids to Adults with Mild to Moderately Severe Hearing Loss
Cox RM, Johnson JA, Alexander GC. Ear Hear 2012;33:573.
This study from the University of Memphis recruited 18 participant pairs matched for age, degree, and slope of hearing loss. The experimental half of this sample was selected because of the presence of at least one dead region as measured by the threshold-equalizing noise test; the control half of the sample was not diagnosed with a dead region. (Ear Hear 2004;25:98.) Those diagnosed with dead regions were distributed as one participant with four dead regions, one participant with three dead regions, five participants with two dead regions, and 11 participants with one dead region.
All participants were fit unilaterally with a hearing aid that featured two different memories — one programmed to match broadband NAL-NL1 targets and a the other matching the same NAL-NL1 targets through 1,000 Hz and rolling off smoothly into the higher frequencies. A four-week field trial followed the hearing aid fitting. Participants completed field diaries during the last two weeks of the trial and returned to the lab for testing of speech recognition in quiet and in noise and an interview about their experience.
Both groups, in quiet and in noise, with and without dead regions, demonstrated improved speech recognition in the broadband NAL-NL1 hearing aid fitting when compared with the low-pass fitting condition. Participants without dead regions showed greater negative effects of limiting high-frequency gain, but both groups benefited to some degree with the broadband NAL-NL1 fitting when compared with the low-pass fitting.
Subjective preference ratings during real-world listening suggested that both groups favored listening with the NAL-NL1 fitting. Each participant was interviewed at the study's conclusion about his experience. Twenty-three participants indicated a preference for listening with the NAL-NL1 prescription and 11 participants preferred the low-pass fitting. Most often the NAL-NL1 memory was preferred for speech clarity while the primary reason for selecting the low-pass memory was that the NAL-NL1 memory was too loud.
The combined strength of observations made with objective laboratory data and the subjective measures of preference are of particular value. The participant groups showed small, positive speech recognition benefits with the broadband NAL-NL1, but their subjective data indicated strong, consistent preferences for the same memory. These observations highlight the utility of leveraging subjective evaluations as a complement to well controlled laboratory measures.
The outcomes suggested that patients with one or two dead regions falling between 1,000 and 4,000 Hz will perform better and likely prefer a broadband hearing aid fitting, similar to NAL-NL1, over a band-limited, low-pass hearing aid fitting. The authors concluded that patients diagnosed with one or two dead regions should be prescribed a broadband prescriptively appropriate hearing aid response. This is a tangible, evidence-based clinical recommendation that can be easily integrated into clinical practice.
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