Noise Protection Health Effects Reduction
2000 International Symposium
CDR Keith S. Wolgemuth, MSC, USN
Naval Submarine Medical Research Laboratory
Groton, CT
I attended the NOPHER (Noise Protection Health Effects Reduction) 2000 International Symposium on Noise Induced Hearing Loss at Cambridge University, United Kingdom 7-10 July 2000. Wow, TAD assignments like this are not offered on a regular basis so I jumped at the chance to go! My thanks to my former research supervisor, Colonel Richard Kopke, MC, USA, at the Department of Defense Spatial Orientation Laboratory, and the Office of Navy Research, for procuring funding allowing me to go. This was a unique opportunity to hear presentations on cutting edge research related to noise protection and hearing conservation. Scientists from the United Kingdom, Norway, Sweden, Finland, Denmark, Poland, France, the United States, Israel, the Czech Republic, and the Netherlands presented their work. As if the presentations were not enough, Robinson College, at Cambridge, provided very delicious, gourmet meals. Include a Shakespeare performance at a nearby college (Much Ado’ About Nothing) and time in cozy, British pubs and you have all the ingredients for a most excellent travel experience.
My goal with this essay is to summarize the work of these international investigators, including some of the Navy’s top investigators (Colonel Kopke, Lynn Marshall, Ph.D. and her team at the Navy Sub Lab), to help keep us all up to date with future directions of hearing conservation. Slowly, but steadily, strides are being made to improve how we protect military and non-military personnel from the effects of hazardous noise. NOPHER is a European Commission, concerted action group project devoted to protection against noise. This was their sixth in a series of international meetings focused on the auditory effects of noise exposure. A lot was packed into two and a half days of presentations. I will do my best to highlight what was important and may have practical applications to future hearing conservation practices.
Protective Role of Efferent Auditory System
Day 1 started off with presentations pertaining to the role of the cochlear efferent system in noise-induced hearing loss (NIHL). We know that high noise levels (excluding blast types of injuries that we know also cause mechanical damage) stress out the outer hair cells (OHCs) by causing them to “over metabolize” or work too hard. This results in the OHCs entering into a “death cycle” and they become nonfunctional. Some evidence also exists that noise can also cause swelling and degeneration of the afferent VIIIth nerve terminals that attach below the inner hair cells (IHCs). Some investigators believe that the auditory efferent system, nerves traveling from the medial and lateral olivary regions of the brainstem to the outer hair cells of the cochlea, may play in role a person’s susceptibility to NIHL. When sound is processed by the VIIIth nerve and reaches the superior olivary region of the auditory system, signals are then sent down via the efferent system to the OHCs. Then the OHCs exhibit their mechanical movements that enhance basilar membrane movement and allow the IHCs to be more sensitive in processing frequency and intensity properties Otoacoustic emissions, both transient (TEOAEs) and distortion product (DPOAEs) types, involve the measurement of the mechanical movement of the OHCs as driven by the efferent system. Don Henderson, of the State University of New York, Buffalo, Hearing Research Center (This is where CDR Nanci Hight, MSC, USN earned her Ph.D.) presented findings indicating when the chinchilla’s efferent auditory system is stimulated with electric shocks, temporary threshold shift (TTS) can be reduced for some noise conditions. TTS was induced in chinchillas placed within two groups, those with the efferent nerves intact and those with their efferent nerves sectioned so signals could not reach the OHCs. Those with the efferent nerves cut developed more hearing loss and greater loss of OHCs than animals whose efferent system was left intact.
Dr.s Jean-Luc Puel and Jerome Ruel of France think they have found a neurochemical basis for a “protective efferent effect.” Dopamine (DA) is released by olivary efferent nerves and prevents over neurochemical stimulation of the inner hair cells. Thus, the IHC suffers less damage. CDR Hight could explain this better than I, since her background in bio-chemistry using chemical treatments to restore/protect the inner ear is vastly superior to mine. This is interesting stuff in that it suggests we have some “natural protection” within our auditory nervous system and reminds us of how important a role the efferent “feedback” nervous system and the OHCs both play in auditory processing.
OAEs as a Screening Tool for NIHL
Dr. Mark Lutman and associates the University of Southhamptom, U.K., and our very own Dr. Lynn Marshall of the Navy Submarine Medical Research Laboratory, Groton CT, talked about their work in using OAE testing as a screening tool for NIHL. OAEs are dependent on normal outer hair cells for their generation. Since this is the site in the human inner ear that is most susceptible to noise-induced damage, there has been a lot of interest in the application of OAEs to NIHL screening. Mark Lutman compared the test-retest reliability and sensitivity to measuring changes in hearing for TEOAE and DPOAE otoacoustic emissions and pure tone audiometry. He found that both types of OAE tests were twice as sensitive as pure tone audiometry in detecting changes in hearing thresholds. Being objective measures, he suggests they offer the promise of improved monitoring for NIHL. Lynn Marshall and her team have also been examining the sensitivity of OAEs to detect sub-clinical hearing damage (noise-induced changes to the inner ear but no accompanying hearing loss), pre-clinical damage (the damage will eventually turn into actual hearing loss), and whether or not OAE measures can reliably detect temporary threshold shift (TTS), permanent threshold shift (PTS), and susceptibility to TTS and PTS. Lynn and her staff are analyzing longitudinal data from a four year study, doing a study on aircraft carrier personnel in Norfolk, and working with COL Kopke and myself in San Diego assessing the Marine Corps recruit population. So far, it appears that DPOAEs appear to be more sensitive for detecting NIHL in individuals, but the results from the three studies mentioned are not completely analyzed yet.
Dr. Attias from Israel has also been doing some related work and like Dr.s Lutman and Marshall, believes OAEs are more sensitive in revealing subtle cochlear damage before audiometry does. I believe OAEs are going to play a bigger role in hearing conservation programs in the future in conjunction with audiometry. It is a more objective test than audiometry, something that may be an important addition in testing fatigued Marine Corps recruits on processing 1 day in boot camp or flightdeck sailors who worked a 12 hour shift the day and night before! Otoacoustic emission testing is not a replacement for the audiogram. Although emissions do correlate with hearing testing, they do not actually measure hearing.