In part 1 of this blog series several issues were touched upon such as the relative uselessness of using gross measures such as puretone testing to determine the true nature of hearing loss from loud music or loud noise. In short, puretone hearing loss thresholds (and also otoacoustic emission (OAE) testing) are rather gross, albeit convenient, measures of hearing loss. By the time that one notes a puretone hearing loss there has been a lot of cochlear (and possibly neural) damage.
Although the pathophysiology was not discussed in part 1, you can just Google the phrase “glial excitotoxicty” and that will bring you to reams of articles about a major route of hearing loss that is a component is both temporary hearing loss (TTS) and permanent hearing loss (PTS).
In short (or is too late….), high levels of noise or music (and also, interestingly enough, high levels of stress), increase the concentration of Glutamate- a neuro-transmitter substance found ubiquitously in the mammalian auditory system. And this high level of Glutamate can be ototoxic to the hearing mechanism. The damage is related to an increase in calcium (Ca+) influx to the cells which functions to depolarize and destroy the cell. Cell death, or “apoptosis” can occur.
How’s that for a one-year course in the physiology of the auditory system crammed into one paragraph!
Since it does take a lot of cochlear damage before puretone hearing thresholds are altered, using puretones or even OAEs can be simplistic.
We now turn our attention to an interesting article from a 2015 issue of the International Journal of Audiology called Short-term music-induced hearing loss after sound exposure to discotheque music: The effectiveness of a break in reducing temporary threshold shift
In this excellent, but difficult to interpret article, the Dutch researchers examined both puretone hearing thresholds and OAEs before and after exposure to 2 hours of discotheque music in two groups of subjects- one group was exposed to 2 hours non-stop disco music and the other group was exposed to one hour of disco music, followed by a one hour break, and then a second hour of disco music. (This of course is a rather odd study since the research was performed in 2014 and I don’t even know where they found a disco in 2014! The last one I was at was back in 1973 with my high school girlfriend, Marilyn…. I wonder where she is now?).
But back to the study…
Using a standard TTS paradigm, the researchers carefully measured the puretone thresholds and OAEs just before the music exposure and then after the exposure.
When all was said and done, there really weren’t any measureable differences between the two groups. The rest period away from loud music didn’t really show up as a factor in this research.
The authors did however conclude that “The advice to use chill-out zones is still valid, because this helps to reduce the duration to the exposure.” Initially when one looks at the results, one cannot help but question that conclusion- no measureable effect of taking a break, yet still concluding that taking a break is a good thing.
While this may seems contradictory (and perhaps sloppy), I would agree with the authors. This study is very difficult to interpret.
If I were a peer reviewer of this article prior to publication, I would not have been so supportive of this conclusion; after all they provided no data to support this claim, but I am glad that I was not a peer reviewer (and would have loved to have been privy to some of the editorial discussion around this point). This borders on advocacy rather than scholarly work.
These researchers are right and they are wrong. It is true that the tools used to assess cochlear damage were blunt tools but the article is important in that it demonstrates the bluntness of these tools as well as the corollary- just because no TTS was measured, doesn’t mean that there was no damage. (Actually this is the law of contra-positives, for those philosophy majors out there).
This corollary is quite important. Clinically when we say that a person has “normal hearing” or that there has been no change since a previous hearing loss, this is simplistic and perhaps we are doing a disservice to our clients. These people may be subject to significant noise and/or music exposure (although probably not disco in 2017) and we are using a sledge hammer to get at something that is very subtle, but potentially very important.
As discussed in part 1 of this blog series, the work of Jos Eggermont, Sharon Kujawa, Charles Liberman and others, have shown that there can be significant neural damage despite having normal puretone thresholds and normal OAE results- namely “hidden hearing loss”.
The conclusion by the Dutch researchers in this article are most likely based on this knowledge and we are at a time where “evidence based knowledge” may not be the best approach. To even inform our clients that taking a break is not needed when in noise or music, is wrong, despite the fact that articles such as this show no measureable evidence to support this.
Until clinically feasible assessment procedures and research tools become available, we are truly just scratching the surface of hearing loss. The tools do not need to be complex- even the use of ABR found in many, but not all, audiology clinics can be quite useful.
Reduced amplitude wave I results can be indicators of hidden hearing loss and the ratio of the amplitudes of the summating potential to the action potential (SP/AP) may be very instructive in identifying hidden hearing loss.
It would have been interesting if the Dutch researchers would have then examined differences (before and after exposure for the two groups) some neural measures (such as wave I in ABR). I suspect that they did know that there may have been differences which is why they concluded that a rest period would still be useful despite no measureable differences with puretone and OAE results.
Clinically I still expound the benefits of a rest period from loud noise or loud music to my clients and given that this Dutch article did not find any benefit, my clinical strategy is to simply hide this article (but not their conclusions) from my clients!