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Health, Knowledge

>Hearing loss : Sound and the ear

>Sound consists of vibrations of air in the form of waves. The ear is able to pick up these vibrations and convert them into electrical signals that are sent to the brain. In the brain, these signals are translated into meaningful information, such as language or music with qualities like volume and pitch. The volume of sound is measured in decibels (dB).
The ear

The ear consists of three parts: the outer ear, the middle ear and the inner ear (See the diagram Cross section of the ear).

The outer ear is the visible part of the ear on either side of the head and includes the ear canals that go into the head. The fleshy parts of the outer ear act as “collectors” of sound waves, which then travel down the ear canal to the eardrum. This is a thin membrane of tissue that separates the outer ear from the middle ear.

The sound waves cause the eardrum to vibrate. This vibration is passed on to the middle ear, which consists of three small bones called the “ossicles”, which amplify and conduct the vibrations of the eardrum to the inner ear. These are the smallest bones in the human body.

The inner ear consists of a structure called the cochlea, which is shaped like a snail’s shell. The cochlea, which is full of fluid, contains tiny cells called hair cells. Vibrations from the ossicles pass through a small window in the cochlea, and the fluid transmits the movements to the hair cells. The movement of these hair cells generates an electrical signal that is transmitted to the brain through the auditory nerve.

Illustration showing the outer, middle and inner ear
The outer, middle and inner ear
Causes of hearing loss
There are many possible causes of hearing loss. These can be divided into two basic types, called conductive and sensorineural hearing loss.

Conductive hearing loss is caused by anything that interferes with the transmission of sound from the outer to the inner ear. Below are some possible causes of conductive hearing loss.

* Middle ear infections (otitis media).
* Collection of fluid in the middle ear (“glue ear” in children). For more information, please see the separate Bupa factsheet Ear infection.
* Blockage of the outer ear, most commonly by wax.
* Otosclerosis, a condition in which the ossicles of the middle ear harden and become less mobile.
* Damage to the ossicles, for example by serious infection or head injury.
* Perforated (pierced) eardrum, which can be caused by an untreated ear infection, head injury or a blow to the ear, or from poking something in your ear.

Sensorineural hearing loss is due to damage to the pathway that sound impulses take from the hair cells of the inner ear to the auditory nerve and the brain. Below are some possible causes.

* Age-related hearing loss (presbyacusis) . This is the natural decline in hearing that many people experience as they get older. It’s partly due to the loss of hair cells in the cochlea.
* Acoustic trauma (injury caused by loud noise) can damage hair cells.
* Certain viral or bacterial infections such as mumps or meningitis can lead to loss of hair cells or other damage to the auditory nerve.
* Ménière’s disease, which causes dizziness, tinnitus, and hearing loss.
* Certain drugs, such as some powerful antibiotics, can cause permanent hearing loss. At high doses, aspirin is thought to cause temporary tinnitus – a persistent ringing in the ears. The antimalarial drug quinine can also cause tinnitus, but it’s not thought to cause permanent damage.
* Acoustic neuroma. This is a benign (non-cancerous) tumour affecting the auditory nerve. It needs to be observed and is sometimes treated with surgery.
* Other neurological (affecting the brain or nervous system) conditions such as multiple sclerosis, stroke, or a brain tumour.

Mixed hearing loss is a combination of conductive and sensorineural hearing loss.

Diagnosing hearing loss
In adults, hearing loss may be very gradual, as in age-related hearing loss, or it can be very sudden, as in some viral infections of the inner ear. If you, your friends or your family think that your hearing is deteriorating, you should see your GP.

If you experience hearing problems, there are a range of tests available, usually at an audiology clinic of the local hospital or health centre. A number of different professionals may be involved in testing and treatment of hearing loss.

* an ear, nose and throat (ENT) specialist, also called an otolaryngologist
* an audiovestibular physician or paediatrician (doctor specialising in hearing problems)
* an audiologist (a specialist in the testing of hearing and fitting of hearing aids)

When examining a person with hearing loss, a doctor will want to know how the hearing loss has developed and what sort of problems it causes
.
He or she will also perform a physical examination. A special electronic device with headphones (an audiometer) and other equipment is used to test the degree of hearing loss (See Hearing tests).
Hearing tests
Whispered speech test. Your doctor may use this as a basic screening test by whispering words behind you and asking if you can hear anything.

Tuning fork test. Different tuning forks can be used to test hearing at a variety of frequencies.

Pure tone audiometry. An audiometer produces sounds of different volumes and pitch (frequencies) . During the test, you are asked to indicate, usually by pushing a button, when you hear a sound in the headphones. The level at which a person cannot hear a sound of a certain frequency is known as their threshold.

Hearing loss is measured in decibels hearing level (dBHL). A person who can hear sounds across a range of frequencies at 0 to 20dB is considered to have normal hearing. The thresholds for the different types of hearing loss are as follows:
Mild

21-40 dB
Moderate

41-70 dB
Severe

71-95 dB
Profoundly deaf people, who cannot hear sounds quieter than 95dB, often communicate using sign language and lip reading. However, cochlear implants or hearing aids can now provide an alternative, allowing oral communication (See Treating hearing loss).

If a sensorineural cause is suspected, a number of tests can be performed to pinpoint where the problem lies.

Otoacoustic emissions. This measures the responses the cochlea makes to sounds produced by a probe placed in the outer ear.

Auditory brainstem response. This measures the activity of the cochlea, auditory nerve and brain when a sound is heard.
None of these hearing tests are uncomfortable.
Other tests

If the cause of the hearing loss seems to be due to a brain abnormality, a magnetic resonance imaging (MRI) scan of the head may be recommended. For more information, please see the separate Bupa health factsheet, MRI scan.
Treating hearing loss
The treatment of hearing loss depends on the cause. If you have a bacterial infection of the middle ear, it can be treated with antibiotics; blockages of the outer and middle ear can be cleared; damaged eardrums can be repaired surgically; and ossicles affected by otosclerosis can be replaced with artificial bones. Some causes of sensorineural hearing loss can also be improved. For example, an acoustic neuroma can be removed surgically.
If there is no cure for the hearing loss (as with age-related hearing loss), a hearing aid for one or both ears usually helps most people, whether the hearing loss is the result of conductive or sensorineural problems. Many different types of hearing aid are available, and your audiologist will advise you as to which type best suits your needs. More information is also available from RNID (see Further information).

When a hearing aid does not give enough amplification, as with profound deafness, a cochlear implant – sometimes known as a bionic ear – may help.
This device transmits sound directly into the auditory nerve via electrodes which are surgically implanted into the cochlea. The results of a cochlear implant vary between people, and it is hard to tell how useful it will be before it is implanted.

Although the sounds you hear tend to be of a buzzing or electronic nature, it can be very helpful when used in combination with lip reading. It may also let you hear the volume of your own speech and so makes conversation easier. Some people with cochlea implants find that they can enjoy music.

Cochlear implants can be particularly valuable for deaf children if they are implanted before the age of two, when the foundations for language skills are laid.

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