Correlation Between Thresholds Obtained Using PTA and Hearing Disability
PTA is described as the gold standard for assessment of a hearing loss.Sindhusake D, Mitchell P, Smith W, Golding M, Newall P, Hartley D, et al. Validation of self-reported hearing loss. The Blue Mountains Hearing Study. Int. J. Epidemiol. 2001;30:1371-78. But, how accurate is PTA at classifying the hearing loss of an individual, in terms of hearing impairment and hearing disability? Hearing impairment is defined by the World Health Organisation (WHO) as a hearing loss in one or both ears. The degree of hearing loss is classified as mild, moderate, severe or profound.http://www.who.int/mediacentre/factsheets/fs300/en/index.html . The results of PTA are a good indicator of hearing impairment.
Hearing disability is defined by the WHO as a reduction in ability to hear sounds in quiet and background noise (compared to people with normal hearing), which is caused by a hearing impairment.http://www.dohc.ie/publications/pdf/hearing.pdf?direct=1 . Several studies have investigated whether self-reported hearing problems (via questionnaires and interviews) were associated with the results from PTA. The findings of these studies indicate that in general, the results of PTA correspond to self-reported hearing problems (i.e. hearing disability). However, for some individuals this is not the case. Therefore, the results of PTA cannot be used to ascertain an individuals hearing disability.Hietamen A, Era P, Henrichsen J, Rosenhall U, Sorri M, Heikkinen E. Hearing among 75-year old people in three Nordic localities: A comparative study. Int. J. Audiol. 2004;44:500-08.Uchida Y, Nakashima T, Ando F, Niino N, Shimokata H. Prevalence of Self-perceived Auditory Problems and their Relation to Audiometric Thresholds in a Middle-aged to Elderly Population. Acta. Otolaryngol. 2003;123:618-26.
Hearing impairment (based on the audiogram) and auditory handicap (based on speech discrimination in noise) data was reviewed by Reinier Plomp. This led to the formulation of equations, which described the consequences of a hearing loss on speech intelligibility. The results of this review indicated that there were two factors of a hearing loss, which were involved in the effect on speech intelligibility. These factors were named Factor A and Factor D. Factor A affected speech intelligibility by attenuating the speech, whereas Factor D affected speech intelligibility by distorting the speechPlomp, R. Auditory handicap of hearing impairment and the limited benefit of hearing aids. J. Acoust. Soc. Am. 1978;63:533-49..
Speech recognition threshold (SRT) is defined as the sound pressure level at which 50% of the speech is identified correctly. For a person with a conductive hearing loss (CHL) in quiet, the SRT needs to be higher than for a person with normal hearing. The increase in SRT depends on the degree of hearing loss only, so Factor A reflects the audiogram of that person. In noise, the person with a CHL has the same problem as the person with normal hearing (See Figure 10).
For a person with a SNHL in quiet, the SRT also needs to be higher than for a person with normal hearing. This is because the only factor that is important in quiet for a CHL and a SNHL is the audibility of the sound, which corresponds to Factor A. In noise, the person with a SNHL requires a better signal-to-noise ratio to achieve the same performance level, as the person with normal hearing and the person with a CHL. This shows that in noise, Factor A is not enough to explain the problems of a person with a SNHL. Therefore, there is another problem present, which is Factor D. At present, it is not known what causes Factor D. Thus, in noise the audiogram is irrelevant. It is the type of hearing loss that is important in this situation.
These findings have important implications for the design of hearing aids. As hearing aids at present can compensate for Factor A, but this is not the case for Factor D. This could be why hearing aids are not satisfactory for a lot of people.
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