Tinnitus isn’t the only effect of blast exposure on the auditory system; research led by Marjorie Leek, PhD, and Frederick Gallun, PhD, has shown that those exposed to high-intensity blasts are more likely to experience problems with central auditory processing, or the delivery of sound information from the inner ear to the brain’s auditory cortex. Blast-related problems in neural connections, they hypothesize, may be responsible for difficulties processing rapid sequences of sounds and in understanding speech in a noisy environment. This was a significant finding: Standard hearing tests, which focus on the ear and don’t evaluate neural connections, had indicated normal hearing in many veterans who had complained of hearing difficulties in noise. The team also established that these processing deficits were long-term, lingering years after veterans’ exposure to blasts.
Another investigator looking at the effects of blast exposure is Melissa Papesh, PhD, AuD, who is building on previous NCRAR studies suggesting that people who have been exposed to blasts may have a difficult time understanding speech when competing noise is coming from a different location. Papesh and Gallun are both in the early stages of exploring rehabilitation options for blast-exposed veterans with auditory processing disorders.
Age and hearing loss are also known factors in auditory processing, and several NCRAR researchers are examining the ways in which they affect hearing, both at the functional and physiological levels: Michelle Molis, PhD, recently reported on the effects of hearing loss and aging on word recognition, and Curtis Billings, PhD, is at work measuring the brain waves of veterans in response to noise and speech. “He’s finding that in older listeners, there’s an effect in brain activity that’s different from younger listeners,” said Feeney. “These give us an objective measure of the brain function of these individuals, and establish a link to the deficits they have in their perceptions of speech and noise.”
Sometimes hearing loss isn’t directly attributable to age, but to external factors whose incidence may increase with age. Dawn Konrad-Martin, PhD, and Marilyn Dille, PhD, have spent several years collaborating on investigations related to two factors known to be associated with hearing loss: diabetes and medications. It’s long been known that people with diabetes have higher rates of hearing loss, though it’s not yet known exactly why. Because about 1 out of every 4 U.S. veterans has diabetes, NCRAR studies of these associations have focused on discovering linkages that could reveal new avenues for intervention. A study published in the spring of 2016, for example, revealed that diabetes is associated with slower neural conduction to the auditory cortex – to a much higher degree than age alone – and that this damage occurs early in the diabetes disease process.
Dille and Konrad-Martin also lead NCRAR efforts to investigate the problem of ototoxicity, a line of investigation first established by Stephen Fausti, former NCRAR director. Simply put, ototoxicity is the tendency of some drugs, such as antibiotics, diuretics, antiseptics, and chemotherapeutic agents, to literally poison the ear – particularly the cochlea or auditory nerve – and cause hearing loss. NCRAR researchers have developed a small handheld device, the OtoID, that will allow an individual to monitor his or her hearing before each dose of the damaging drug is administered; Feeney uses the example of a cancer patient in a treatment unit receiving an IV drip of cisplatin, a known ototoxin. OtoID, which is patented by the VA, can send a message directly to the audiologist after the test is completed, offering real-time updates and improving the odds of preserving a patient’s hearing function while effectively treating their cancer.
“And over long periods of time, being exposed to loud noise, we think there could be an effect on more complicated signals. Maybe you hear tones okay, but you might have a deficit in hearing a more complex signal, requiring more of those eighth nerve fibers to respond – say, speech. A lot of this right now is theory. …”
In 2014, a team led by Konrad-Martin and Dille also published a “Proposed Comprehensive Ototoxicity Monitoring Program for VA Healthcare (COMP-VA),” a plan for implementing a monitoring regimen throughout all levels of VA health care. Dille is spearheading the next steps to be taken in gathering the data and forming the partnerships needed to go agency-wide with this initiative, which has the potential to mitigate hearing loss for tens of thousands of veterans.
Konrad-Martin was among the first researchers to investigate a new and emerging area of study in the field of audiology: “hidden hearing loss,” or, simply speaking, a form of hearing impairment that can’t be measured by a conventional audiogram. Hidden hearing loss may be associated with the function of the eighth cranial (vestibulocochlear) nerve, which transmits sound information from the inner ear to the brain. Konrad-Martin, who has demonstrated age-related deficits in the “first wave” of activity transmitted from the eighth nerve to the brain (the auditory brainstem response, or ABR), has helped to guide the work of another NCRAR researcher, Naomi Bramhall, PhD, who recently discovered significant differences in the ABR of veterans with a history of loud noise exposure.