Many people have trouble understanding conversations in noisy situations. Scientists are beginning to understand why.
The problem is sometimes called “hidden hearing loss”: Standard hearing tests don’t measure it, and sufferers are often told their hearing is normal. But the distress they feel struggling to discern what others are saying in crowded restaurants and business meetings is real.
Now there’s growing evidence that the causes of problems processing speech amid noise are different than the causes of problems hearing sound. Scientists believe exposure to loud noises can erode the brain’s ability to listen selectively and decode words, without causing traditional hearing damage. Difficulty understanding speech amid noise can set in long before traditional hearing loss.
“This is something we’ve recognized for a long time—and this research tells us why it’s happening,” says Anne Oyler, associate director for audiology at the American Speech-Language-Hearing Association. “Audiologists will have to start actively looking for this disorder.”
Hearing loss in adults is usually associated with damage to the tiny hair cells that line the inner ear and transfer sound signals to nerve fibers that lead to the brain. Aging, trauma and noise exposure can all cause those hair cells to deteriorate.
New research suggests that the synapses connecting the hair cells to nerve fibers are even more vulnerable and suffer permanent damage long before the hair cells deteriorate, bringing about the difficulties in selective listening.
While there aren’t immediate remedies, there is a glimmer of hope that the damage might be reversible. In a study published in Nature in April, University of Michigan researchers were able to regenerate cochlear synapses in mice that had been exposed to loud noises by injecting them with neurotrophin-3, a protein that stimulates nerve growth.
In a landmark study in 2009, scientists at Massachusetts Eye and Ear Infirmary, led by otolaryngologist Charles Liberman, showed that mice exposed to loud noises lost up to 50% of those synapses even though their hair cells soon returned to normal.
Earlier in September, the same researchers showed that such damage occurs in humans as well. In a study published in the journal PLOS One, they compared a group of college-age music students regularly exposed to loud sounds for four to six hours with another group studying quieter subjects. Both groups tested normal on standard hearing tests and were equally able to understand words in quiet environments. But the high-risk students fared significantly worse on tests of understanding speech-in-noise. Tests with electrodes showed that their auditory nerves had a diminished response to sound stimulation compared with the low-risk group.
“We believe this is the first evidence of hidden hearing loss in humans—but it is just a first step,” said Stéphane Maison, the study’s lead investigator. He and his colleagues are hoping to develop more precise tests to measure damage to cochlear synapses and diagnose it definitively.
Exactly how such damage, called cochlear synaptopathy, compromises the ability to understand speech amid noise isn’t fully understood. Dr. Liberman likens sound signals arriving at the brain to a high-resolution photo, with nerve connections like thousands of pixels creating a clear picture. If some of those nerve connections die, the brain gets a lower-resolution image and may lose the ability to distinguish where sounds are coming from and who is speaking.
The researchers think cochlear synaptopathy may help explain tinnitus, the persistent buzzing or ringing some people hear, as well as hyperacusis, which is an increased sensitivity to unpleasant sounds such as a baby crying or a siren. With fewer signals from the auditory nerve fibers reaching it, the brain may generate the buzzing to fill in the vacuum, or turn up its internal volume, making sounds that already loud become intolerable.
Audiologists are well aware that many patients have trouble understanding speech in noisy situations. “It’s the primary reason people walk in the door,” says Joanna Roufos, an audiologist with Manhattan Audiology in New York.
Besides giving standard hearing tests, many audiologists now use speech-in-noise tests that ask patients to pick out and repeat words or entire sentences amid a variety of background sounds and voices. There is little consensus, though, about what to do when such tests confirm a problem with understanding speech in noise. Many factors, including aging, cognitive ability, memory and conventional hearing loss, all play a role. Some patients are sent for an evaluation of an auditory-processing disorder, although such specialists may be more accustomed to dealing with children.
“The jury is still out as to how hidden hearing loss manifests in humans and how we might diagnose it,” says James W. Hall III, an expert in auditory processing disorders at Salus University in Elkins Park, Pa. “We don’t have enough information yet for it to be part of routine diagnosis of hearing problems.”
Many patients who have speech-in-noise difficulties also have traditional hearing loss. Audiologists say both can be improved with hearing aids, especially newer models with directional microphones and technology that damps background noise.
But hearing aids can’t return hearing to normal, experts caution. “The number one complaint of people who wear hearing aids is that they have trouble hearing in noise, and that’s not going to change,” says Dr. Oyler.
A variety of assistive listening devices can also help patients hear what they want to hear in specific situations. With a personal frequency-modulation (FM) system, for example, the person speaking uses a microphone and the listener wears a wireless Bluetooth receiver. Other devices transmit the audio feeds in theaters, concerts, meetings, churches or even home TV directly to listeners without amplifying background sounds.
Many people who struggle with speech in noise are reluctant to take such remedial steps, however. Hearing-aid companies often say it takes people seven years, on average, to seek treatment after they first suspect a hearing issue.
Audiologists urge sufferers to get evaluated, even if the solutions aren’t perfect. “Some people feel they can fake their way through conversations,” says Sarah Sydlowski, an audiologist at Cleveland Clinic and board member of the American Academy of Audiology. “But it can be extremely taxing to go through a busy workday and be constantly struggling to understand.”