A recent news report noted that loud sounds may not only not be damaging, but may actually be protective. The way the story broke however was a bit disingenuous and the headline was simplistic and slightly sensationalist. The title was “Concert Cacophony: Short-Term Hearing Loss Can Be Protective,” but this only tells one side of a well-known story. I guess that not all journalists can be audiologists as well. Before reading on, I would still recommend anyone who exposes themselves to loud noise or music to wear hearing protection. To think that this recommendation is no longer valid is simplistic and erroneous.
A recent article in the journal Proceedings of the National Academy of Sciences (PNAS) showed that for louder sounds, Adenosine Triphosphate (ATP) was released which causes a temporary hearing loss by binding to a receptor in the cochlea of mice. In experiments performed with mice that did not have the receptor, no temporary hearing loss was noted, but these same mice showed permanent hearing loss when exposed to very intense levels of noise. Indeed, the report goes on to say that there are two families in China with a genetic mutation in which there is no ATP receptors in the cochlea. Members of these families are reported to suffer greatly from hearing loss if exposed to noise.
I have a problem with this. My gripe is not with the researchers, but with the lay journalists covering the story. The story begins with the sentence: “Contrary to conventional wisdom, short-term hearing loss after sustained exposure to loud noise does not reflect damage to our hearing.” Clearly this research is exciting, but the protective mechanisms of temporary threshold shift (or TTS) have been reported before, and the proper coverage would have been to say that this is one more element to show the ear may protect itself from loud noises. There is no “conventional wisdom” that states that TTS has no benefit.
Physiologically, TTS is related to several factors: 1. genetic, and this current study with ATP receptors may be part of that puzzle; 2. Mechanical, where there is a temporary disconnection between the tectorial membrane and some hair cells… sounds like a fuse to me which is known to be protective of our electrical systems in our house and in machinery; 3. Bio-chemical, where high levels of the neurotransmitter substance Glutamate are known to be toxic.
To say that everything in TTS is mediated genetically and that there are no other biochemical or mechanical factors is simplistic. The research is great; the reporting on it is less than stellar. I can see people rushing out to concerts and feeling that no hearing protection is required. I guess I don’t need to wear my hearing protection when using a chain saw or at the gun range (this last item would be for our American readers since Canadians don’t know how to use guns). Yes, short-term exposure can be protective, but it can also cause permanent peripheral and central pathologies as well.
Nevertheless, this is a really interesting finding, but one that needs to be viewed in the context of the effects of an entire range of insults that the cochlea may receive over a lifetime. Some physiological mechanisms can be protective and others not.
And it gets even more complicated as we will see in part 2 next week. We will review the literature on “auditory toughening” from the 1980s and early 1990s. Pre-exposure to some sounds may limit pure tone acuity damage; and pre-exposure to other sounds may increase pure tone acuity damage…. But that’s for next week.