The Simplistic Nature of Asymptotic Hearing Loss

I love seeing 15 year old kids for a musicians’ hearing assessment. These are generally extremely talented and hard working young people who are doing everything they can to assure themselves of a life long career in some aspect of music and the arts.  Even at age 14 they are aware that the road ahead will not be smooth, with many bumps and turns, and even if they do everything “correctly”, this does not guarantee even a modicum of success.

I also love seeing 50 year old kids for a musicians’ hearing assessment. These are generally extremely talented and hard working musicians who are doing everything they can to assure themselves of a livelihood in the music industry.  And even at age 50, they are still acutely aware of the road ahead and have already experienced their share of bumps and turns in their careers, despite doing everything correctly.

Other than the obvious difference in their ages, there are also differences in what I say and do during a musicians’ hearing assessment when I see them…. or at least there “were” differences in what I said and did…

Below is a set of curves, from ISO 1999, one of several large data sets that are available, that show an interesting, and well-studied, characteristic- the hearing change from year to year in the first few years of musicians’ (or industrial workers’) careers can be large, but this change gradually decreases to the point that larger audiometric changes are no longer observed. It’s almost as if the  hearing mechanism becomes calloused and more immune to the effect of music or noise exposure.  Some researchers have referred to this as an “asymptotic” hearing loss, as if the degree of hearing loss approaches some asymptotic value.

From ISO1999 the noise exposure at averaged at 2000 Hz, 3000 Hz, and 4000 Hz for varying exposure levels from 85 dBA to 100 dBA. Note the “asymptotic” or “slowing down” of the change after 5-10 years
From ISO1999 the noise exposure and averaged at 2000 Hz, 3000 Hz, and 4000 Hz for varying exposure levels from 85 dBA to 100 dBA. Note the “asymptotic” or “slowing down” of the change after 5-10 years

Previous blogs have been based on the concept of auditory toughening where it appeared as if the cochlear mechanism was toughened up against high sound level insults. Researchers such as Henderson, Campo, Subramanian, and Spongr feature in this area of study.

The bottom line is that we don’t really know what is happening in the cochlea.   We do know (mainly from the work of Sharon Kujawa, Charlie Liberman, and Jos Eggermont) that despite there being a lessening in the cochlear susceptibility over time, including even TTS, that there can be permanent and increasing neural pathologies, which when combined with normal aging, can become quite significant years down the road.

So, as a clinical audiologist what is my responsibility?

At one point I would “read the riot act” to the younger musicians and counsel them about hearing loss prevention strategies, moderation, and hearing protection, but be more “relaxed” with those who have a few more years under their belts. After all, the ear does seem to be less susceptible over time.

 

But the work of Kujawa, Liberman, and Eggermont clearly shows that while the cochlea may remain relatively healthy as assessed with pure tones (such as on an audiogram), the neural structures downstream are a different story.

Neural degradation is much more difficult to demonstrate in a clinical environment- especially when the appointment is only 30-45 minutes long. Certainly changes in wave I ABR can be assessed, but given the time constraints of a typical clinical environment, this will typically not be done.  One can argue that in a perfect world it should be done but at some point a clinical decision needs to be made about what is important (at least for educational and counseling concerns) and what has less importance.

The data in the figure from ISO 1999, and which has been replicated by others such as Ulf Rosenhall, are important but no longer feature prominently in my clinic. At one point I would  have a laminated sheet showing this graph- actually the one I had was from Rosenhall- but I have decided to “hide”  this information from my clinical population.

I am not making a clinical “error of omission” but I do feel that these data are quite misleading. Pure tone audiometric results are just overly simplistic, and while I once felt that TTS was just a “benign” characteristics of music or noise exposure and not useful, I also feel that showing a graph depicting asymptotic hearing loss is simplistic and not the true story.

About Marshall Chasin

Marshall Chasin, AuD, is a clinical and research audiologist who has a special interest in the prevention of hearing loss for musicians, as well as the treatment of those who have hearing loss. I have other special interests such as clarinet and karate, but those may come out in the blog over time.