Today was the first time that I was ever late for work. I was busy reading a fascinating article about the latest information about the possible protective role(s) of the efferent pathway in the auditory system, and I lost track of time. This is nothing new and there have been a number of articles over the past decade (many from the same laboratory at the Eaton-Peabody Laboratory at Massachusetts Eye and Ear Infirmary) about the role of the efferent (and afferent) systems in the ear. The article is Efferent Feedback Minimizes Cochlear Neuropathy from Moderate Noise Exposure by Stephane Maison, Hajime Usubuchi, and M. Charles Liberman. http://www.jneurosci.org/content/33/13/5542.
Many articles have been published about cochlear pathology that results from more intense sounds that may not show up on an audiogram. There may have been a temporary threshold shift (TTS) that has resolved, and still a number of subtle pathological changes may have occurred in the cochlea and/or the neurological pathways.
This article addresses whether these and other cochlear pathologies can occur even if there is no audiometric indication of this and also if the exposure is to everyday softer sounds such as restaurant noises. The researchers, led by Dr. Maison, described the cochlear pathologies as cochlear neuropathy, which they describe as a loss of auditory nerve fibres. The possible implications include having a reduced ability to communicate effectively in noisy social environments.
While “cochlear neuropathy” is a great sounding phrase (and should be used at parties to make you the belle of the ball), the implications can be far reaching.
An aspect of this study is that Dr. Maison and colleagues surgically removed the efferent (olivo-cochlear) efferent system in mice, and found that the number of auditory nerve fibres was greatly increased. This clearly demonstrates the protective mechanism of the efferent feedback system in the ear (assuming that we are mice) at levels that are typically found in the environment where there is no measurable shift in audiometric thresholds. Stating this backwards, by the time one observes an audiometric threshold loss, there is a lot of cochlear pathology already. The group of researchers noted that there already exist non-invasive forms of analyses that can provide information about those who perhaps are more susceptible to hearing loss from noise (and music). No “clinical” tool yet exists but this is something to watch for down the line.
Clearly this has implications for all of us (assuming that the results in mice are transferrable to those of us who wear shoes and brush our teeth). Going to a rock concert can create a temporary hearing loss or TTS that may last for 16-18 hours, but we assume that once the hearing thresholds have returned to normal, there is also normal cochlear function. It seems that this is not the case after all. Even attending the local coffee shop may create some cochlear neuropathy for some, which when combined with a lifetime of routine noise and music exposure can significantly reduce communication in noisy environments down the line.
Although we don’t have the clinical tools yet to assess individual susceptibility to noise and music exposure, we are very confident that the audiogram is not the first place to look. The assessment of Otoacoustic Emissions (OAEs) has some role, but modifications of the test setup may be required to allow us to estimate possible damage and to delineate those people who are the most likely to suffer from cochlear neuropathy.
In industry we use a fence criterion of 85 dBA for 40 hours a week as the maximum permissible exposure before action such as hearing protection and/or environmental controls are implemented. This is also the case for the various music exposure regulations and guidelines commonly used around the world. However, these were all based on the observation that prolonged exposure can be measured audiometrically (in the 3000-6000 Hz region) for prolonged exposure over 80 dBA. I am not sure of the decision history of OSHA or NIOSH or any of the other regulatory jurisdictions around the world, but I would presume that any “fence” for action be implemented around the 80-85 dBA level was based on the blunt measure of audiometry.
If we knew then what we know now about cochlear pathology, I am sure that there would have been significant pressure to either use a non-audiometrically based “fence” or that the fence for action would have been significantly lower.
I tell my clients about the 80/90 rule when it comes to MP3 usage- 80% volume for 90 minutes a day and that should provide about 50% of the daily exposure. Perhaps we are doing patients a disservice and we should be telling them about the 60/90 rule (60% volume for 90 minutes a day) or be even less conservative?
Clinically I spend a significant amount of time performing education around recreational noise and the effects of industrial strength noise. Perhaps we should now ramp it up and bring our counseling up to 2013 standards.