Millions of Americans face a silent, or at least muffled, future as several recent studies show the prevalence of hearing loss is increasing among young and old alike.
One study led by researchers at Johns Hopkins University School of Medicine found that nearly a fifth of all Americans age 12 and older have hearing loss in at least one ear severe enough to make communication difficult.
Based on hearing tests done as part of national health surveys conducted between 2001 and 2008, the researchers estimated that about 30 million Americans, or 12.7 percent of the population, had hearing loss in both ears, and 48 million, or 20.3 percent, in at least one.
Testing used the World Health Organization's definition for hearing loss -- not being able to hear sounds of 25 decibels or less in the speech frequencies (30 decibels is a soft whisper, 50 a quiet conversation).
Dr. Frank Lin and colleagues reported in the Archives of Internal Medicine last November that the prevalence of hearing loss nearing doubled with every decade of aging, but that women and blacks are less likely to have hearing loss at any age.
The researchers said they're not sure why the two groups seem less susceptible to hearing loss, but speculate that estrogen in women and the melanin pigment in darker skin could have some sort of protective effect on damage to the inner ear -- something they plan to explore in future studies.
Other estimates based on population surveys indicate hearing loss among adults has doubled in the last 20 years. Researchers have found two-thirds of all Americans age 70 and over have significant levels of hearing loss.
Experts say there are many causes for the hearing loss across generations, but say the chief reasons are a noisier world in general from things like machinery and traffic, increased use of more and louder audio devices throughout the day and night, and an aging population that's at greater risk.
Scientists have also found that people with heart disease, diabetes, chronic kidney disease and Alzheimer's disease are all at greater risk for hearing loss. At the same time, recent studies have shown that hearing loss can contribute to other medical problems, including dementia. The Johns Hopkins team reported in February that people with hearing loss aged 40 to 69 were three times more likely to suffer falls than people the same age with normal hearing.
And hearing loss among adolescents is at least as widespread as among adults and may be increasing at a faster pace, according to another round of research based the national health surveys.
A team led by Dr. Josef Shargorodsky reported in August 2010 in the Journal of the American Medical Association that 15 percent of 12-19-year-olds had any degree of hearing loss when surveys were done between 1988 and 1994, while the percentage went up to 19.5 percent in surveys taken in 2005-06.
A small study presented at a scientific meeting in San Diego last month, scientists with the House Research Institute in Los Angeles said 72 percent of 29 teens that attended a rock concert experienced reduced hearing ability based on a test done afterward.
Researchers measured sound levels averaging more than 98 decibels during the concert -- noise that would have violated federal workplace standards in about half an hour. They offered earplugs to all the teens, but just three took them.
The testing showed reduced function in the tiny hair cells in the inner ear that are mainly used to pick up soft or low-level sounds by converting vibrations into electrical signals. Although it's thought the cells can usually recover within a day or so, scientists also know that repeated exposure to loud noise can leave the hairs permanently damaged.
Some recent animal studies suggest that a single exposure to loud noise may result in permanent damage to the hearing nerve connections to the hair cells.
Scientists are aggressively looking for drugs and other methods to repair or restore the damaged hearing hair cells, and several novel approaches have been reported progress in recent months.
One involves using gene therapy to regenerate the hair cells, which has so far shown limited success only in immature mice. The second approach uses stem cells, most likely drawn from the patient, to regenerate the hair cells, but also requires the immune system to be manipulated to aid in nerve cell repair or connection with the damaged cells.
Most of the stem cell work has also been done in mice, but one trial involving a small group of children is underway at several institutions in the U.S.
(Contact Lee Bowman at BowmanL@shns.com. Distributed by Scripps Howard News Service.)