Pure Tone Audiometry - Dead Regions and Their Effect On Audiometric Results

Dead Regions and Their Effect On Audiometric Results

When the IHCs or part of the basilar membrane are damaged or destroyed, so that they no longer function as transducers, the result is a ‘dead region’. Dead regions can be defined in terms of the characteristic frequencies of the IHC, related to the specific place along the basilar membrane where the dead region occurs. Assuming that there has been no shift in the characteristic frequencies relating to certain regions of the basilar membrane, due to the damage of OHCs. This often occurs with IHC damage. Dead regions can also be defined by the anatomical place of the non-functioning IHC (such as an “apical dead region”), or by the characteristic frequencies of the IHC adjacent to the dead region.

Dead regions affect audiometric results, but perhaps not in the way expected. For example, it may be expected that thresholds would not be obtained at the frequencies within the dead region, but would be obtained at frequencies adjacent to the dead region. Therefore, assuming normal hearing exists around the dead region, it would produce an audiogram that has a dramatically steep slope between the frequency where a threshold is obtained, and the frequency where a threshold cannot be obtained due to the dead region.

However, it appears that this is not the case. Dead regions cannot be clearly found via PTA audiograms. This may be because although the neurons innervating the dead region, cannot react to vibration at their characteristic frequency. If the basilar membrane vibration is large enough, neurons tuned to different characteristic frequencies such as those adjacent to the dead region, will be stimulated due to the spread of excitation. Therefore, a response from the patient at the test frequency will be obtained. This is referred to as “off-place listening”, and is also known as ‘off-frequency listening’. This will lead to a false threshold being found. Thus, it appears a person has better hearing than they actually do, resulting in a dead region being missed. Therefore, using PTA alone, it is impossible to identify the extent of a dead region (See Figure 7 and 8).

Consequently, how much is an audiometric threshold affected by a tone with its frequency within a dead region? This depends on the location of the dead region. Thresholds at low frequency dead regions, are more inaccurate than those at higher frequency dead regions. This has been attributed to the fact that excitation due to vibration of the basilar membrane spreads upwards from the apical regions of the basilar membrane, more than excitation spreads downwards from higher frequency basal regions of the cochlea. This pattern of the spread of excitation is similar to the ‘upward spread of masking’ phenomenon. If the tone is sufficiently loud to produce enough excitation at the normally functioning area of the cochlea, so that it is above that areas threshold. The tone will be detected, due to off-frequency listening which results in a misleading threshold.

To help to overcome the issue of PTA producing inaccurate thresholds within dead regions, masking of the area beyond the dead region that is being stimulated can be used. This means that the threshold of the responding area is sufficiently raised, so that it cannot detect the spread of excitation from the tone. This technique has led to the suggestion that a low frequency dead region may be related to a loss of 40-50 dB.Terkildsen K. Hearing impairment and audiograms. Scand Audiol. 1980;10 Suppl:27-31. Cited in: Moore, BC. Dead Regions in the Cochlea: Diagnosis, Perceptual Consequences, and Implications for the Fitting of hearing aids. Trends Amplif. 2001;5:1-34.Thornton AR, Abbas PJ. Low-frequency hearing loss: Perception of filtered speech, psychophysical tuning curves, and masking. J Acoust Soc Am. 1980;67:638-43. Cited in: Moore, BC. Dead Regions in the Cochlea: Diagnosis, Perceptual Consequences, and Implications for the Fitting of hearing aids. Trends Amplif. 2001;5:1-34. However, as one of the aims of PTA is to determine whether or not there is a dead region, its may be difficult to assess which frequencies to mask without the use of other tests.

Based on research it has been suggested that a low frequency dead region may produce a relatively flat loss, or a very gradually sloping loss towards the higher frequencies. As the dead region will be less detectable due to the upward spread of excitation. Whereas, there may be a more obvious steeply sloping loss at high frequencies for a high frequency dead region. Although it is likely that the slope represents the less pronounced downward spread of excitation, rather than accurate thresholds for those frequencies with non-functioning hair cells. Mid-frequency dead regions, with a small range, appear to have less effect on the patient’s ability to hear in everyday life, and may produce a notch in the PTA thresholds.