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

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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. 

Gross measures such as removing the legs of frogs to see if they can no longer jump can be simplistic.

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!

There have been a number of studies over the years trying to gauge the benefits, if any, of moderation and taking breaks away from loud noise or music. The short answer is that we are not really sure, but possibly. How is that for fence sitting?

Intuitively it does make sense that reducing the overall “time weighted” average exposure of loud music or loud noise would reduce any eventual permanent hearing loss, but this line of research is very difficult to pursue. 

After all, it’s not just the peak or the average sound levels that we are concerned with but the durations of these levels as well.  here is nothing wrong with being subjected to a few seconds of 100 dBA but there is something wrong with being subjected to 100 dBA over many hours- it’s not just the sound level, but the duration as well.

Now we get to the complicated part – how do we measure this effect, if indeed it does exist? 

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Traditional measures of temporary hearing loss (also known as Temporary Threshold Shift or TTS) measure a person’s hearing threshold before and after an exposure – the difference being a measure of TTS. This is typically performed with puretones and the result can be frequency dependent.  But measures of puretone acuity have been shown to be simplistic.  One can argue that by the time there is a measureable puretone loss in acuity, a lot of cochlear damage has already occurred. And this can be further complicated if this last sentence was to be rewritten as “One can argue that by the time there is a measureable puretone loss in acuity, a lot of cochlear and/or neural damage has already occurred.”

Puretone testing is easy to perform and gives pretty results we can see on an audiogram, but in the larger scheme of things, is a rather insensitive measure of neural or central auditory function. Puretone testing however can provide some measure of cochlear acuity, but for short exposures, tends to exhibit only a temporary shift.

Otoacoustic emissions (OAEs) have been shown to provide information on “function” and this takes into account some elements of acuity and also how the cochlea actually performs. While this has been a major step forward, and may provide different, but comparable information than puretones, OAEs still are rather blunt and gross measures.

These last couple of paragraphs demonstrate that this is not a straightforward question- Does taking a break from loud noise or music really work? We need to design and choose tools that can truly answer this question.

There are clinically expeditious tools that can be used that can be used but these tend to be more time consuming and are not found in all audiology clinics. “Evoked response” tools, such as ABR, seek to assess the neural damage or changes that occur as a result of loud noise or music.  Actually tools such as ABR can assess a wide range of issues and pathologies and not just the effects of loud music or noise.

The work performed by researchers such as Jos Eggermont, Sharon Kujawa, Charles Liberman, and many others in the last 15 years, demonstrate that while the cochlea can return to normal function and acuity after loud noise and music exposures, there may be permanent neural structures “downwind” that do not return to normal.

That is, the puretone thresholds and the OAEs can return to a pre-exposure, or even normal state, there can be permanent neural damage that remains hidden. The popular media has quite astutely referred to this as “hidden hearing loss” and as clinical audiologists, we are just now starting to scratch the surface of this area of study.

It is quite possible that regardless of the results of well-controlled experiments that use puretone threshold acuity or OAEs, much damage remains that is “hidden” from the person and the audiologist. 

In part 2 of this blog series, an article will be reviewed that deals with some of these issues and some suggestions will be provided that may allow us to pry into the realm of hidden hearing loss.