In Australia, it is estimated that just over 20% of the adult population suffer from hearing loss and that this number exceeds 50% for those over 65 years of age (Access Economics Report 2006). In children, hearing loss impairs speech and language development, which in turn undermines academic achievement. In adults, it has a negative impact on employment opportunities and social functioning. It can cause social isolation that develops into depression and early onset dementia.
Hearing is one of our special senses that allows us to detect vibrations in air and perceive them as sound. Different kinds of vibrations produce different sounds, and these sounds can represent danger, communication signals, or music. Sound vibrations are collected by our external ears, travel to the eardrum, and are transmitted to the inner ear by the three tiny bones of the middle ear. The inner ear, also called the cochlea, is our hearing organ.
Approximately 15,000 microscopic sensory ‘hair’ cells are arrayed in four longitudinal rows along an elastic membrane within the cochlea. Each cell vibrates to a different frequency, from low frequencies at one end of the membrane to high frequencies at the other. When stimulated, hair cells activate auditory nerve fibres that carry sound signals to our brains. Brain circuits process the signals to yield a percept of the sound.
The cochlea is an extremely delicate structure. Disease, head trauma, ageing, drugs, and loud noise can cause hair cells to die. When hair cells die, they are gone forever and as they die, our hearing likewise diminishes. For reasons we can’t explain, ageing typically causes hair cells corresponding to high frequencies to die first. We might notice this loss when consonant sounds of speech, such as ‘s’, ‘f’ and ‘th’, become difficult to distinguish. Many people wrongly assume that hearing loss can be treated by simple amplification. In fact, hearing loss has many adverse symptoms: speech comprehension in a noisy background is lost; tinnitus or ringing of the ears often emerges; and sometimes there is severe loudness distortion.
Types of hearing loss
In adults, hearing loss may be gradual, as in age-related hearing loss. Sudden loss can occur from other factors such as viral infections of the inner ear or head trauma. Hearing loss falls into two broad categories:
- Conductive Hearing Loss – this ailment is caused by damage or a blockage to the outer and/or middle ear and can be acquired or congenital. Loss of loudness is the key symptom. Causes may include perforated eardrums, middle ear infections (‘glue’ ear), blockage of the ear canal by impacted wax, infection, or foreign objects, and otosclerosis (a hereditary condition where bone grows around one of the bones of the middle ear).
- Sensorineural Hearing Loss – this condition is caused by damage or malfunction of the cochlea or the auditory nerve, which can result from ageing, excessive noise exposure, diseases such as meningitis, and head injury. Loss of loudness and lack of clarity are the key symptoms. One in 1000 children are born with a congenital form of sensorineural hearing loss. Causes may include an inherited hearing loss, premature birth, lack of oxygen, damage from viruses such as rubella, and jaundice.
How is it diagnosed?
An audiologist or clinician will always take a person’s family and medical history. After that, a series of hearing tests are generally performed to determine the type of hearing loss.
Sounds will be transmitted through headphones (air conduction) and through the skull (bone conduction) at different frequencies and volumes. Speech and word recognition tests should also be carried out in quiet and in noise.
The results of hearing tests are plotted on a chart called an audiogram, which will show the levels at which various frequencies become audible for each ear. The audiogram will inform the audiologist whether or not the person is likely to benefit from wearing a hearing aid.
Can we prevent hearing loss?
The most common source of hearing loss is exposure to loud sounds. The louder the sound and the longer the exposure, the greater the damage to the inner ear. Sounds that cause ringing in the ears kill hair cells. A single very loud sound (such as a gun shot or explosion) can cause instant hearing loss. To minimise noise-induced hearing loss, we should avoid loud noise or wear earplugs.
How is hearing loss treated?
The treatment of hearing loss depends on the cause. Conductive hearing loss is usually treatable. 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 can be replaced by artificial bones.
At present, sensorineural hearing loss cannot be cured, but a hearing device may help. For moderate hearing loss, there are different types of hearing aids and an audiologist can 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 integrate the electronic sounds produced by the implant with lip reading skills.
What research is Garvan doing in this area?
Degeneration of hair cells in the inner ear occurs with ageing and as a result of excessive noise exposure. Since hair cells are not replaced in humans, the sensory deprivation is permanent and irreversible. Garvan’s Hearing Research Group is conducting research along two separate paths. One line explores the potential for preventing noise-induced hearing loss through chemical intervention. A second examines abnormal brain changes consequent to hearing loss.
These changes are implicated in the production of tinnitus or ringing of the ears, an impaired ability to understand speech in noisy environments, and difficulty using hearing devices such as hearing aids and cochlear implants. As hearing loss cannot be remedied by amplification alone, we need to understand how brain circuits change so that treatment strategies actually address such alterations. This kind of information can contribute to a modification of current hearing aid devices for better outcomes.
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