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.
~~~~~~~~~~~~~~~~~
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
Jess Dancer is professor emeritus at the University of Arkansas at
Little Rock.
