How do we hear?
Sound waves are created when air
vibrates. To hear, the ear must change sound into electrical signals which the
brain can interpret. The outer part of the ear (the pinna) funnels sound waves
into the ear canal. When sound waves reach the eardrum they cause it to
vibrate. Vibrations of the eardrum cause the tiny bones in the middle ear to
move too. The last of these bones, stapes, passes on the vibrations to the
fluid-filled chamber called the cochlea. When the cochlea receives the
vibrations, the fluid inside it moves. As the fluid moves it causes the special
sensory cells to create an electrical signal. This electrical signal is sent to
the brain. Special areas in the brain receive these signals and translate them
into what we know as sound.
Your
ears create electrical signals that represent an extraordinary variety of
sounds. For example, the speed at which the eardrum vibrates varies with
different types of sound. With low-pitched sounds the eardrum vibrates slowly.
With high-pitched sounds it vibrates faster. This means that the special hair
cells in the cochlea also vibrate at varying speeds. This causes different
signals to be sent to the brain. This is one of the ways we are able to
distinguish between a wide range of sounds
What is audiometry?
Audiometry is the medical word used
to mean hearing tests. Audiometry checks whether there are problems with any of
the different mechanisms that allow us to hear sound.
What can cause hearing loss?
Damage to any part of the ear can
cause a hearing loss. If there is a problem in the ear canal or the middle ear,
this causes what is known as a conductive hearing loss. In conductive hearing
loss, the movement of sound (conduction) is blocked or does not pass into the
inner ear. There are several causes of conductive hearing losses and this may
result in a permanent or temporary hearing loss. A common cause in children is
the build-up of fluid in the middle ear space, otherwise known as glue ear. For
more information see separate leaflet called Glue Ear.
What causes glue ear?
The cause is probably due to the
Eustachian tube not working properly. The balance of fluid and air in the
middle ear may become altered if the Eustachian tube is narrow, blocked, or
does not open properly. Air in the middle ear may gradually pass into the
nearby cells if it is not replaced by air coming up the Eustachian tube. A
vacuum may then develop in the middle ear. This may cause fluid to seep into
the middle ear from the nearby cells.
Some children develop glue ear after
a cough, cold,
or ear infection when extra mucus is made. The mucus
may build up in the middle ear and not drain well down the Eustachian tube.
However, in many cases glue ear does not begin with an ear infection
If the fluid-filled chamber called
the cochlea or the hearing nerve is not working properly this causes what is
known as a sensorineural hearing loss. Usually this means that hair cells in
the cochlea are not working properly or there is a problem with the hearing
nerve so that some or all sounds are not being sent to the brain. Sensorineural
hearing losses are usually permanent. They can be mild, moderate, severe or
profound and affect one or both ears.
It is also possible for sensorineural
and conductive hearing losses to occur together in a mixed hearing loss.
How does audiometry work?
There are various types of tests that
can be carried out to check hearing ability. They vary according to who is
being tested and why. Babies are obviously not able to say when they have heard
sound, so special methods are used. In children the principles of testing may
be the same, but the way in which the tests are carried out may be varied to
get the most accurate results. There are also additional tests which help to
check how well the middle ear and the brain are working in the hearing pathway.
The most common tests of hearing are
described below.
Testing
newborn babies
The otoacoustic emissions (OAE) test
is a quick, simple and painless way to screen newborns for hearing loss. A
small earpiece containing a microphone and a mini-loudspeaker is placed in the
ear. The loudspeaker makes clicking sounds in the ear. These are passed to the
fluid-filled chamber called the cochlea. If the cochlea is working normally, it
responds sending a sound back to the ear canal. This is detected by the
microphone. The test is so sensitive that even a slight hearing loss can be
detected and if there is a good response then no further checks are needed.
Sometimes, the response cannot be
detected when the test is done. This could point toward hearing problems but
initially it is more likely to be due to other factors. This could be because
the baby is unsettled, the room was noisy or there was some fluid left in the
ear after birth. The test will usually be repeated and if there is still not a
good response then it will be followed up with another type of test called an
automated auditory brainstem response (AABR) test.
In an AABR test a small earphone
plays clicks into the baby's ear. If the baby can hear the click, a signal in
the hearing nerve can be measured from sensors that are placed on the baby's
skin. The loudness of the clicks is set to a particular level. If this does not
produce a response, further diagnostic testing will be required.
Both OAE and AABR testing are best
done when the child is asleep, as the response to be detected is very small and
can be hidden if there is a lot of movement.
Testing
in babies and young children
In young children a technique called
visual reinforcement audiometry is used. In this test the child hears sounds,
usually through speakers in the testing room. When the child hears the sound
and turns their head towards it they are given a reward. Usually this is a
visual reward such as the flashing lights of a toy. The person testing the
child's hearing continues reinforcing this behaviour with a reward every time
the child turns towards a sound. Then the person carrying out the test begins
to assess the child's hearing by seeing if they respond to different types of
sound. By doing this it is possible to find the quietest sound the child can
hear.
Different variations of this
reward-based test are used as a child gets older and finds it easier to
communicate.
Testing
in older children and adults
Testing in adults mainly uses a
technique called pure tone audiometry. This uses a machine called an audiometer
to play a series of tones through headphones. The tones vary in pitch (frequency,
measured in hertz) and loudness (intensity, measured in decibels).
The health professional conducting
the test will control the volume of a tone and reduce its loudness until you
can no longer hear it. Then the tone will get louder until you can hear it
again. You signal by raising your hand or pressing a button every time you hear
a tone, even if the tone you hear is very faint. The health professional will
then repeat the test several times, using a higher-pitched tone each time. Each
ear is tested separately.
The results of the test are plotted
on a special graph called an audiogram which helps to show the pattern of any
hearing loss.
What is audiometry used for?
Audiometry is used in a number of
different circumstances including:
·
To evaluate possible hearing loss in
anyone who has noticed a persistent hearing problem in one or both ears or has
had difficulty understanding words in conversation.
·
When determining the type and amount
of hearing loss (conductive, sensorineural, or both).
·
Screening babies and young children
for hearing problems that might interfere with their ability to learn, speak,
or understand language.
·
To screen for hearing loss in people
who are repeatedly exposed to loud noises or who are taking certain antibiotic
medicines, such as gentamicin.
·
What should I do to prepare for
audiometry?
·
Usually very little preparation is
needed for audiometry. If you are known to have wax in your ears you may have
this removed before the test so that it does not interfere with the results.
·
Let the person doing the test know
if you have had, or the child being tested has had, a recent cold or ear
infection, as this may interfere with the results.
·
Are there any side-effects or
complications from audiometry?
·
These are very safe tests;
complications arising from these tests are extremely rare.
SYED IRFAN ABID BUKHARI
0333-6366260
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