Hearing Loss
Understanding hearing loss becomes much easier once we grasp the process of hearing. When a sound wave enters an ear, vibrations travel across a membrane known as the ear drum, through the tiny bones in our inner ear to a pea-sized, fluid-filled bony structure called the cochlea. Containing 15 000 microscopic ‘hair cells’, each vibrating to a different sound, or frequency, the cochlea is our main hearing organ. When pressed or moved, hair cells, like piano keys, send sound signals to our brains.
Once hair cells die, or hearing starts to fail, we lose some of the sounds in the world around us.
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As we age, hair cells die. This is natural and unavoidable. For reasons we can’t explain, the hair cells corresponding to the high frequencies often die first. We might notice when the sibilant sounds in speech, such as ‘s’, ‘f’ and ‘sh’, become harder to distinguish and we might lose the sounds of waves lapping, leaves rustling and birds singing. By the age of 60, more than half the population will be aware of at least some loss of hearing. Many people assume that those with hearing loss experience a world with the volume turned down. In fact, hearing loss can be experienced very differently depending on its causes; sometimes with loss of frequencies; sometimes with distortion of sound; sometimes with less ability to separate, or attend to, specific sounds.
Noise-induced hearing loss is caused by a combination of loudness of sound and length of exposure. The louder the sound (measured in decibels), the less exposure is needed to suffer a hearing loss. Sounds loud enough to cause ringing in the ears afterwards are loud enough to cause hearing damage. Prolonged exposure to loud noise will kill hair cells. A single very loud sound (such as a gun shot or explosion) can cause instant hearing loss. Earplugs can be worn or loud noises avoided to prevent noise-induced hearing loss.
The treatment of hearing loss depends on the cause. Bacterial
infections of the middle ear can be treated with antibiotics; blockages
of the outer and middle ears can be cleared; surgery can repair damaged
ear drums, and bones of the middle ear affected by otosclerosis (a
hereditary condition where bone grows around one of the bones of the
middle ear) can be replaced by artificial bones.
At present, age-related hearing loss cannot be cured, but a hearing
aid may help. There are many different types of hearing aids and an
audiologist will advise which one is most suitable. For the profoundly
deaf, a cochlear implant may assist. This device transmits sound
directly into the auditory nerve via electrodes surgically implanted
into the cochlea. Recipients of a cochlear implant will need to learn
how to marry the electronic sounds produced by the implant with lip
reading skills.
We are exploring the potential of adult stem cells to replace
damaged hair cells. We now know that adult stem cells from the brain
can generate new nerve cells thus dispelling the long-held belief that
nerve cells could not be regenerated.
A readily accessible and abundant source of adult stem cells is the
lining of the nose. These stem cells constantly replace cells that
enable us to smell. Olfactory stem cells, as they are known, have the
potential to be used for a range of stem cell therapies. They can also
be taken from one’s own body thus eliminating the problems of tissue
rejection.
Garvan scientists have successfully converted adult olfactory stem
cells are into hair cell-like cells and, using fine microsurgical
techniques, have successfully injected the stem cells them into the
cochlea of deaf mice . We will now test for the recovery of hearing
with a non-invasive hearing test, called the auditory brainstem
response (ABR), which is routinely used in humans. This will determine
if the injected stem cells have converted into hair cells and can form
functional connections to the brain to repair hearing loss.
News
How nasal stem cells might prevent childhood deafness
MEDIA RELEASE:
11 Feb 2011
Researchers at Garvan have shown for the first time in mice that nasal stem cells injected into the inner ear have the potential to reverse or restore hearing during early onset sensorineural hearing loss.
Further Information
Better Hearing Australia - A non-profit, self-help organisation that provides an Australia-wide community support service of rehabilitation and help for Australia's hearing impaired.