Of all our senses, hearing is the most vulnerable. It can diminish or disappear in anyone, at any age, not just from illness or genetic conditions but from simple exposure to loud sounds or the passage of time.
The snail-shaped cochlea contains about 16,000 hair cells that convert sound waves into electrical impulses. These then travel to the brain via the auditory nerve. Most hearing loss occurs when these hair cells are destroyed, usually due to loud sounds, infection, aging, or genetic disorders.
The cost of hearing loss may dramatically increase in coming years because of the rapidly aging populations in the United States and many other nations.
—Approximately 36 million Americans (17 percent of the population) have some form of hearing loss.
—Two to three of every thousand children in the United States are born deaf or hearing impaired.
—About 15 percent of Americans between 20 and 69 (26 million people) have hearing loss that may have been caused by loud sounds. In children aged 6 to 19, 12.5 percent (approximately 5.2 million) are believed to be similarly affected.
—More than 30 percent of people over the age of 65 and 47 percent over the age of 75 have hearing difficulties.
—Experts fear the popularity of portable music players may lead to an increase in hearing loss in coming years.
Fortunately, brain researchers supported by the National Institutes of Health and other global funding agencies have made significant strides in understanding the causes of hearing loss and developed methods for treating even severe cases.
Hearing loss occurs when infection, injury, disease, or trauma prevent the ear from receiving sound information and sending it to the brain. Often, this is the result of damage to the 16,000 tiny sensory hair cells in the cochlea of the inner ear. If the loss happens early in life, a person may never learn how to speak and may suffer from social isolation.
Sometimes, no medical treatment is needed or wanted; sign language and lip-reading skills give many people near-normal lives. Youths can attend specialized schools for the deaf and receive support and guidance from an active "deaf culture."
For those who desire treatment, several options have been developed, based on basic research and its application. People with partial hearing loss, for instance, commonly turn to hearing aids. Government scientists estimate, however, that only a fifth of people who would benefit from hearing aids actually use them.
Even those with complete damage to hair cells in the inner ear can still hear. A device known as a cochlear implant can bypass hair cells and directly stimulate the auditory nerve to provide a limited form of hearing. As of April 2009, the worldwide population of users is estimated at about 188,000.
Cochlear implants are not used for moderate or minor hearing loss, which are more common than severe cases, because the devices require completely blocking off sound from the ear. Recently, researchers have developed a hybrid implant, which combines cochlear implant and hearing aid technology to preserve low-frequency hearing while restoring high-frequency hearing loss.