Antioxidants: An Antidote for Noise-Induced Hearing Loss?

By Jess Dancer, EdD

Editor: We're learning that anti-oxidants can be effective in cases of noise induced hearing loss. Here's Jess Dancer with a great review of the work being done in this area. This article is republished with Jess' kind permission.

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Thirty million Americans are exposed to hazardous noise levels on a regular basis, and 10 million have developed permanent noise-induced hearing loss.1 At present, hearing conservation programs in the workplace employ the noise standards specified under the Occupational Safety and Health (OSHA) Act of 1970. The OSHA standards mandate that hearing conservation programs be established for workers who are exposed to noise that exceeds 85 dBA over an 8-hour workday. A standard hearing conservation program emphasizes education, hearing testing, lowered noise levels, and hearing protection such as earplugs or earmuffs when necessary.

Noise Effects on Cochlea

Without ear protection, intense noise exposure can damage or destroy the cochlear hair cells that are the key to hearing function. Especially vulnerable are the outer hair cells, which act as a "cochlear amplifier" to increase hearing sensitivity. In addition to the mechanical destruction of hair cells, excessive noise can cause metabolic "exhaustion" of the cells, leading to their "burn-out". As a result of these destructive mechanical and metabolic processes, damaged molecules called free radicals are released which can further spread the damage by stealing critical components from other molecules such as fat, protein, or DNA.

Normally, free radicals are controlled by antioxidants, which are nutrients found naturally in the body and in plants such as fruits and vegetables and in vitamins A, C and E. Are antioxidants an antidote to noise-induced hearing loss? Are we at a time when drugs can be used to prevent or minimize noise-induced hearing loss, or, as one headline in Wired News stated, "Pop a Pill, Save Your Hearing?" Read on.

Search for Chemical Treatments

For over 30 years, researchers have searched for chemical treatments to reduce or eliminate the damaging effects of noise on hearing. These noise effects can be temporary, lasting only a few hours, or can lead over time or instantaneously to permanent sensory/neural hearing loss. As early as 1969, the protective effects of cytochrome C and adenosine-triphosphoric acid were studied in acoustic trauma, and dozens of chemical studies have appeared in the medical literature since then.2

For your reading pleasure, here are the chemicals tested in the past 5 years alone: NMDA receptor blockage, AfGF, magnesium supplements, riluzole, LFGF, ALCAR, carbamathione, MCT, caroverine, alpha-tocopherol, GEE, R-PIA, GDNF, NT-3, CNTF, salicylate, allopurinol, ebselen, peptide inhibitors, superoxide dimutase, reservatol, leupeptin, odaravone, carnosine, NAC, adenosine triphospate, vitamin B12, vitamin E, ascorbic acid, DEX, caroverine, glutacoid receptor antagonist, vitamin C, Src-PTK inhibitors, trolox, calcineurin inhibitors, immunosupppressants, edaravone, GGA, and carbogen.

Antioxidants an "Appealing" Treatment

To understand how all these chemicals work on the cochlea is a daunting task for most audiologists. Fortunately, a recent 2005 article by Lynch and Kil in Drug Discovery Today cuts through all these chemical possibilities and hones in on the three most promising compounds for the prevention and treatment of noise-induced hearing loss in humans.3 In the article, the authors suggest that antioxidants are an "appealing early intervention step in prevention of cellular damage to the cochlea."

The three compounds, which can be orally administered, are as follows:

a. Magnesium-a trace mineral, is required for several hundred different functions of the body, including energy metabolism and protein synthesis. It also helps maintain normal muscle and nerve function. Eating a wide variety of legumes, nuts, whole grains and vegetables will help meet daily dietary needs, and supplements are available for those with magnesium deficiency, which is associated with hearing loss. Oral magnesium intake has been found to reduce both permanent and temporary threshold shifts in humans.4,5

b. N-acetylcysteine (NAC)-an amino acid and antioxidant, is a precursor to the critical antioxidant glutathione, which promotes cell detoxification and acts directly as a free radical scavenger. Glutathione is found in fresh and frozen fruits and vegetables, fish and meat. Asparagus, avocado and walnuts are particularly rich dietary sources of glutathione, which is also available as a dietary supplement. NAC is FDA-approved as a mucous dissolving agent and to protect the liver in cases of Tylenol overdose; it is not FDA-approved as a treatment for noise-induced hearing loss. The compound is available as a dietary supplement and forms the basis of the Hearing Pill, marketed by the American BioHealth Group.6

On this group's Web site, the American BioHealth Group states that the pill is..."indicated for helping the remediation of the mechanisms that lead to permanent hearing loss, either before or shortly after noise trauma, after diagnosis by a qualified physician." The product is meant to be used with external hearing protection, as part of a "comprehensive hearing protection program." Although clinical trials involving humans are underway, the Web site cautions that..."Statements or information on this site have not been evaluated by the Food and Drug Administration," and..."No products are intended to diagnose, treat, cure or prevent any disease." The pills can be ordered for $34.95 per bottle.

c. Ebselen-the enzyme ebselen mimics glutathione peroxidase (GRx), a selenium compound which acts as a multifunctional antioxidant to promote healthy cells and tissues. Selenium, a trace mineral found in most rocks and soils, is needed in small amounts for good health. GRx is actively decreased in the cochlea after noise exposure, and the depletion of the GRx1 gene confers increased susceptibility to noise damage. Ebselen has been shown to protect guinea pig and rat cochleas from noise-induced hearing damage and is currently undergoing clinical evaluation in humans.7,8

Although no drug is as of yet has been approved by the FDA for the prevention or treatment of noise-induced hearing loss, Lynch and Kil of Sound Pharmaceuticals Inc.9 predict in their 2005 article that safe and effective drugs could be approved as the first generation of otoprotectants within the next five years.3 Perhaps a silver bullet is in sight-at last-especially now that Sound Phamaceuticals recently completed Phase I clinical testing of SPI-1005, which is an oral compound under development for the prevention and treatment of noise-induced hearing loss.9

References

1. Noise-Induced Hearing Loss, accessed online at www.nidcd.nih.gov/health/hearing/noise.asp

2. Janokwski W, Ziemski Z, Cyrulewska-Orlowska J. (1969). Protective effect of cytochrome C and adenosinotriphosphoric acid in acoustic trauma. Otolaryngologia Polska, 23(2): 133-40. [Article in Polish]

3. Lynch ED, Kil J. (2005). Compounds for the prevention and treatment of noise-induced hearing loss. Drug Discovery Today, 10(19): 1291-8.

4. Attias J, Weisz G, Almog S, Shahar A, Wiener M, Joachims Z, Netzer A, Ising H, Rebentisch E, Guenther T. (1994). Oral magnesium intake reduces permanent hearing loss induced by noise exposure. American Journal of Otolaryngology, 15(1): 26-32.

5. Attias J, Sapir S, Bresloff I, Reshef-Haran I, Ising H. (2004). Reduction in noise-induced temporary threshold shift in humans following oral magnesium intake. Clinical Otolaryngology Applied Science, 29(6): 635-41.

6. The Hearing Pill (2004), accessed online at www.thehearingpill.com/index.asp?PageAction=Custom&ID=11.

7. Pourabakht A, Yamasoba T. (2003). Ebselen attenuates cochlear damage caused by acoustic trauma. Hearing Research, 18(1-2): 100-8.

8. Lynch ED, Gu R, Pierce C, Kil J. (2004). Ebselen-mediated protection from single and repeated noise exposure in rat. Laryngoscope, 114(2): 333-7.

9. Sound Pharmaceuticals Inc., accessed online at www.soundpharmaceuticals.com