In the not-too-distant future, medication may reverse the effects of hearing loss from acoustic trauma, according to Ben J. Balough, a Navy captain and otolaryngologist at the Naval Medical Center San Diego.
Balough is leading research on a "hearing pill"—a special formulation of N-acetylcysteine (NAC) on which the Navy conducted a double-blind, placebo-controlled clinical trial in 2004 ("A Magic Pill?", The ASHA Leader, June 14, 2005).
The clinical trial found that when compared to the placebo, NAC reduced permanent hearing loss in the ear closest to the source of acoustic trauma. NAC has also shown potential in reversing other symptoms of acoustic trauma, such as tinnitus and balance disorders.
The U.S. Department of Defense is providing $2.5 million for more clinical trials, according to Balough, who was involved in the earlier study. The trials will include higher dosing of NAC in search of a more beneficial effect, he said. The Navy also is seeking to package the supplement in an actual pill form instead of previous formulation of an effervescent tablet mixed with water.
Approximately 2,000 Marine recruits will participate in the clinical trials. One of the trials will involve identical noise exposure for recruits in tightly controlled environments. The other trial will monitor active-duty service members living and working in various settings on ships and in field training, where they are exposed to different noise levels. All trials will screen out individuals with any prior hearing loss or history of noise exposure.
Military Noise Research
"The Army in particular has done hearing conservation work for a long time," Balough said, noting that such research dates back to the early 1900s. In the early to mid-1990s, research developed about how and why noise affects nerve endings in the ear.
"The understanding of why noise causes hearing loss, biochemical changes in the ear, and how we can apply a pharmacologic intervention—that work has been going on for the past 12 to 15 years," he said.
The research revealed that mechanical trauma to nerve endings is not the sole cause of hearing loss. Scientists hypothesize that oxidative stress and the ways the body responds to increased metabolic demands also play a role.
"An example [of oxidative stress] is taking Tylenol," Balouh said. "If you take too much, it creates a toxic oxidative molecule—a free radical—that causes cellular death." Acoustic trauma also causes oxidative stress, and "we’ve learned about free radical scavengers and other drugs like NAC that help prevent the oxidative stress of noise-induced hearing loss," he said.
Despite the success of the 2004 trials, recruits are not yet taking the supplement. "We’re still investigating NAC," Balough said, adding, "While it has approval for certain applications, part of our current research is finding the right dose and developing a pill size."
A commercial company, however, has rolled out a hearing supplement that was evaluated at the Naval Medical Center San Diego. Known as Hearing Health, the supplement is designed to reduce tinnitus and provide cellular protection against hearing impairment and balance disorders. It was introduced by Premier Micronutrient Corporation (PMC, Nashville, Tenn.). The supplement, which contains NAC and other vitamins and nutrients, is being marketed as a daily brand. It is not undergoing review by the Food and Drug Administration.
Balough noted that the Navy has a cooperative research and development agreement with PMC but is not currently doing clinical work with the company.
"We looked at a small group [of recruits] for PMC’s formulation. In the future we may do more with the company," he said. "We have an agreement with Premier and other companies, basically looking at anything we can find that will be useful for [hearing loss]."
The results of the 2004 NAC study were presented at the Combined Otolaryngologic Spring Meeting (COSM) in April 2007, he said. The paper has been accepted for publication by Otology and Neurotology and is in review.
"I believe that someday—soon, I hope—we will have approved pharmacological agents to help with the problem of hearing loss," he said.
Ben J. Balough, a Navy captain and otolaryngologist at the Naval Medical Center San Diego, can be reached at BJBalough@nmcsd.med.navy.mil.