A cochlear dead region is caused by non-functioning inner hair cells and/or neurons. Sometimes neighbouring hair cells, normally tuned for other frequencies, are able to pick up sounds in a dead region. Clinically, this will present as a threshold on a traditional pure tone audiogram. However, this can be very misleading as this may of course not be the real threshold. It is therefore very difficult to diagnose dead regions by a normal hearing test. The threshold equalizing noise (TEN) test was developed to overcome this problem. TEN measures pure tone thresholds in a special masking noise.
If dead regions are present this may have important implications for fitting hearing aids. There may in fact be little or no benefit from hearing aid amplification well inside the dead region (Moore 2009).
A dead region at a particular frequency is indicated when:
A masked threshold is at least 10 dB or more above the level of the TEN AND masked threshold is at least 10 dB above the non-masked threshold.
Some behaviors that could indicate the presence of a dead region
- Severe to profound hearing loss
- Absolute threshold at a specific (suspected) frequency is 70 dB HL or greater
- Steeply sloping hearing loss
- Complaints of distortion
- Extremely poor speech discrimination
Once a dead region has been identified, the clinician can then:
- Fit the most appropriate hearing aid
- Counsel the patient and manage expectations regarding how they can expect to benefit from the hearing aids
- Determine if a patient would be a good candidate for cochlear implants (eg if extensive dead regions are discovered)
Read more about the TEN test in the Threshold Equalizing Noise quick guide.
Read more about dead regions Moore's article: www.hearingreview.com