NIHL-Shape of the audiogram- part 2

Marshall Chasin
October 13, 2015

The “shape” or configuration of the sensori-neural hearing loss, at least in worker’s compensation board claims for noise exposure, is one of the base elements for deciding whether a hearing loss is indeed a noise induced hearing loss.  Yet, not all audiograms of workers exposed to high levels of noise (or music) have a “notched” audiogram with the greatest hearing loss being in the 3000-6000 Hz frequency region.

Dr. David Barrs, MD, who is now with the Mayo Clinic in Rochester, MN, did some interesting work earlier in his career. Dr.  Barrs and his colleagues found that only 37% of those workers who were exposed to noise and had a sensori-neural hearing loss had an audiometric notch in the 3000-6000 Hz frequency region.

They attributed this to a number of potential reasons such as presbycusis which over time reduced the 6000 Hz and 8000 Hz acuity such that the upper frequency end of the notch was flattened out.  Nevertheless, not having an audiometric notch does noise prevent a diagnosis of noise exposure, and conversely, having an audiometric notch does not mean that the hearing loss was noise induced.

As we will see below, the studies about the various proposed reasons for why the greatest hearing loss is in the 3000-6000 Hz region reads as an historical novel with contributions from as far back as 1934.  Here is a synopsis of the studies about the various potential reasons about the NIHL notch.  Several explanations have been proposed for this audiometric notch over the past almost 80 years but to date, there is no one accepted explanation- it is a frequently observed audiometric shape (but not the only noise induced shape).  In the following cases, the internet has not been gracious enough to provide us with links, historical or otherwise:

Crow, Guild, and Polvagat, 1934 have shown that part of the explanation may be related to a poorer blood supply in the cochlea that corresponds to the 3000-6000 Hz region; 

Bohne, 1976 showed  that there is a greater susceptibility for damage of the supporting structures of the cochlear hair cells in the 3000-6000 Hz region;

Hilding, 1953, and Schuknecht and Tonndorf, 1960 showed that the orientation of the stapes footplate in the middle ear, into the inner ear is such that the primary force vector aims towards those cochlear hair cells in the 3000-6000 Hz region, with the effect of eventual failure because of the constant hydro-mechanical action; and

Tonndorf, 1976, and Caiazzo and Tonndorf, 1977, have shown that permanent noise exposure has its greatest effect approximately one half octave above the peak frequency of the noise spectrum, and since the “peak” is typically that of an adult’s ear canal resonance (at 2700 Hz), the greatest hearing loss will be the 4000-6000 Hz region. 

Despite the age of these references, there are no more modern data that contradict these findings or suppositions.

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