Hidden hearing loss, also known as “cochlear synaptopathy”, is a phrase used to refer to neural dysfunction where there is still good cochlear (or sensory) function.  It is the sensory function that typically yields the measurement of hearing sensitivity on an audiogram or hearing test.  People with normal cochlear function would have a normal audiogram but may report that they have limited speech understanding in noisier social situations.

It is “hidden” in the sense that on standard audiology tests, everything appears to be normal, despite complaints from the clients.

Courtesy of www.psypost.com

Part of this stems from the limitations of standard audiology tests.  It is not a normal clinical procedure to perform neurological testing such as ABR or other evoked response measures, unless there are more significant complaints.  Even in the case of an asymmetry between the ears, only an MRI is performed, rather than looking for other neurological difficulties.  In the case of “cochlear synaptopathy”, it is the synapse between the inner hair cells in the cochlea and the VIIIth auditory nerve that is part of the problem.  This is not something that can be picked up on MRIs or CT scans.

Traditionally we have referred to hearing loss from sources such as noise exposure, music exposure, and aging (presbycusis), as “sensori-neural” hearing losses.  This suggests some vagueness. We don’t really know whether the hearing loss is sensory or whether it is neural.  It is assumed that it is one or the other or both, and for most purposes, this is fine. A sensori-neural type of hearing loss is generally not medically treatable in any event, and many require some form of rehabilitation such as hearing aid amplification.

This is further exacerbated by the fact that there are two types of inner hair cell nerve endings- those nerve fibers that respond for soft sounds (and these provide hearing threshold information on audiograms) and those that only respond for higher level sounds such as speech and music. One can have a dysfunction of only the higher threshold nerve fibers with intact lower threshold nerve fibers.  The audiogram would be normal but there would be some hearing dysfunction for higher level sounds.

The phrase “cochlear synaptopathy” has another problem. Audiology programs need to add an entire year of study for their students just to learn how to pronounce it!

We are just now scratching the surface of hidden hearing loss. ost of the studies have been performed on animals and while they are good models for study, they are not humans.  Since 2011 there have been some post-mortem studies on human temporal bones demonstrating that, at least in these cases, there is some synaptic pathology but normal cochlear sensory anatomy.  This suggests that cochlear synaptopathy does indeed occur in humans, but its prevalence is not known.

And we don’t have any reliable neurological measures. Much has been written about looking at the various amplitudes of some neurological measures such as ABR (wave I/wave V ratios), or even the ratio of the SP/AP, but each of these approaches have some limitations.

Finally, what do we do about it?

There are no therapies that can re-establish synaptic function (despite what Star Trek fans may believe). Perhaps, hearing aids or other assistive listening devices that can provide slight high frequency amplification in hopes of improving the signal to noise ratio may be useful, but at this point, we are not sure.

Cochlear synaptopathy is hard to pronounce, hard to assess, and its prevalence is not well defined.

This blog was originally published in Canadian Audiologist, Vol. 1, Issue 3, 2014 which is the official e-journal of the Canadian Academy of Audiology.  We thank the publishers for permission to reprint.

Sandra Teglas, PhD.

Sandra Teglas holds BM, MM, and PhD degrees from UNCGreensboro. At UNCGreensboro, Dr. Teglas was Program Coordinator with the Music Research Institute in the School of Music. She is published in the Journal of Band Research, Music Performance Research, Medical Problems of Performing Artists, and The International Journal of Audiology. Her presentations include the International Conference of Music Perception and Cognition (Bologna, Italy), National Conference of Music Perception and Cognition, Music Educator’s National Conference, North Carolina Music Educators State Conference, North Carolina – American Choral Directors Association, and Music in Lifelong Learning Symposium. Dr. Teglas has returned to public schools where she teaches K-5 music, and uses ukulele instruction with grades 3-5. 

 

 

 

By Sandra Teglas, PhD

 

In the Music Research Institute (MRi) at the University of North Carolina Greensboro, we seek any and all approaches to educate about, and get musicians to use hearing protection. We have little difficulty convincing musicians, that in certain music situations, they need to use hearing protection (we let the decibels and sound doses tell the story). However, getting musicians to use hearing protection is quite a challenge. For reasons most of us understand, one being risking less than optimal performance, musicians resist wearing earplugs. Enter the option of using acoustic shields, screens, and baffles.

Just as the individual musician controls the use of earplugs, nearly all of the commercially available acoustic shields are designed for individual use. Many of the designs allow for changing height and angle of the shield. Shields seemed a useable alternative, so we decided to acquire and test a few models. The models tested included the Manhasset Acoustic Shield, the Wenger Acoustic Shield, and shields that were custom made for the North Carolina Symphony (base of a Manhasset stand with tri-panel plexiglass as the shield, seen in Figure 1).

 

Results – Shields Work (in most cases)

When we studied sound doses using shields (for individuals – not as distant barriers or sound reflection), the results were mixed, as results usually are. In short, the shields (all three models) reduced sound levels/doses for individuals in some, but not all environments.

The head of the shield-protected musician must remain < 4 inches, and closer is better. At distances greater than 4 inches from the shield, protection is diminished to the point that the shield no longer functions as desired. It isn’t easy for a musician to sit still, but if they choose to use an acoustic shield, that is what they need to do.

When musicians were seated close to each other, and in relatively small rehearsal venues, the sound levels/doses were greater when using baffles/shields than when not using them. Without sufficient space between the shield-protected musician and other sources of sound, shields functioned as reflective surfaces directing more sound to ears of the user. In small venues, musicians should use earplugs, because they simply cannot get away from the sound sources.

 

What the Musicians Prefer

Musicians reported that they preferred the relatively small, and angled or wrap around design. They felt that they could “get inside” the protected area.

The shield of choice was the custom-designed North Carolina Symphony shield (Figure 1).

Figure 1. North Carolina Symphony shield with dose Badges, shield-protected and not shield-protected, for sound-level measurement.

Because of the maneuverability of the Wenger Acoustic Shield (Figure 2)  musicians preferred its stand base. However, the Plexiglass shield was larger than the musicians preferred. 

The Manhasset Acoustic Shield (Figure 3) doubles as Conductor’s Stand, so it is quite large (66 X 60 cm). In local orchestras, I have witnessed musicians use three or more of these as a wall-type barrier between brass and strings, and percussion and strings. They also use them to reflect sound for the horns. As of yet, I have not seen any local musicians using these as individual sound shields.

 

Figure 2. Wenger Acoustic Shield.

Figure 3. Manhasset Acoustic Shield.

Other Commercially Available Shields

Although the North Carolina Symphony shield is not available for purchase, the K&M-11900 Sound Insulation Stand is similar in design (Figure 4). The RAT Acoustic Screen provides positional options similar to that of Wenger, and the Kolberg Sound Screen provides the largest shield surface of 100 X 60 cm.

­The Wilde & Spieth Padded Acoustic Shield and Goodear Acoustic Shield (Figure 4) appear as if they would be effective, and attractive. However, the shields are not transparent, thus some orchestras may think them to be an obstacle for musicians, and possibly detract from the experience of the audience.

Figure 5. Wilde & Spieth Padded Acoustic Shield and Goodear Acoustic Shield.

The Amadeus Acoustic Shield (Figure 6) is designed as a double layer shield with a perforated layer acting as a sound diffuser. This model is fixed to the frame of chairs specific to the Amadeus company, and allows for no position options.

 

Figure 6.  Amadeus Acoustic Shield.

 

Considerations

As seen above there exist commercially available acoustic shields, screens, and baffles. However, when working with musicians, it is important to discuss environments in which shields are appropriate, whether specific shields will allow proper performance posture, and when earplugs would the better option. And please remind musicians that the ‘best’ hearing protection is the one they will use.

 

 

 

 

References

 

  1. Libera, R., D.M.A. (2009). Shielding a Musician: A Case Study on the Effectiveness of Acoustic Shields in Live Ensemble Rehearsals.

 

  1. Libera, R. & Mace, S. (2010). Shielding Sound: a Study on the Effectiveness of Acoustic Shields. Journal of Band Research, Vol. 45, No. 2

 

  1. http://www.wengercorp.com/stands/acoustic-shield.php

 

  1. http://shop.manhasset-specialty.com/p/acoustic-shield?pp=24

 

  1. http://produkte.k-m.de/en/product?info=54&xd609e=mq1cjjac3hr98tlb5m4kf5lac2

 

  1. http://www.ratstands.com/rat_acoustic_stands.html

 

 

  1. http://products.kolberg-percussion.com/en_GB/150/product/4586.html#

 

  1. http://www.wilde-spieth.de/index.php/sound-insulation-element.html

 

  1. http://symphonyinternational.net/products/ohs/

 

  1. http://www.amadeus-equipment.co.uk/acoustic-shields/