Hearing and Hearing Loss

 Ears are amazing organs.

The ear is sensitive enough that it can detect sound vibrations in air which are so weak

that the air molecules move less than the diameter of an atom!

The ear consists of three main parts which we call the outer, middle and inner ears,

collectively these three parts are known as the peripheral auditory system.

 

The Outer Ear

The visible part of the outer ear is called the pinna. Also part of the outer ear is the

auditory canal which is a hollow tube that carries sound to the eardrum.

Although many think of the outer ear as a simple funnel for collecting sound in reality the

specialised shape and position of our outer ears allows us to pinpoint the direction of

sounds coming from the side of us or above and below us.

At the end of the auditory canal is a small membrane about 1cm in diameter called the

tympanic membrane (or eardrum)

It seals off the delicate structures of the middle and inner ears to prevent foreign matter

and bacteria from damaging them. It's main function is the efficient transfer of sound

vibrations to the middle ear.

 

The Middle Ear

The inner surface of the tympanic membrane is part of the middle ear. It is attached to a

chain of three bones (ossicles) called the malleus, incus and stapes. The non latin names

given to these bones is the hammer, anvil and stirrup since their shape resembles these

objects.

The function of the ossicles is to transform sound vibrations in air to pressure waves in

the fluid filled chambers of the inner ear.

Because the fluids in the inner ear are much denser than air, without the middle ear

ossicles most of the sound entering the ear canal would be reflected straight back out of

the ear. There is resistance to the transfer of energy which is more accurately described

as impedance. The middle ear can be considered as an impedance matching transformer which

uses the mechanical advantage of levers as a primary method of ensuring efficient transfer

of energy to the inner ear fluids.

 

The Inner Ear

The inner ear is the most complex and sophisticated part of the auditory system. The

structure within the inner ear which is responsible for the sense of hearing is called the

cochlea. Many of the structures within the cochlea share similarities with the balance

organs of the inner ear.

The cochlea is a helically shaped bony structure that resembles a snail in appearance. It

has three  fluid filled channels which run in parallel around the axis of the cochlea.

The last of the middle ear bones, the stapes, acts like a piston, and pushes on the fluid

in the first channel, the scala vestibuli, through an opening in the base of the cochlea

called the oval window thus setting up a pressure wave. The pressure wave travels through

the perilymph fluid within the channel towards the other end of the cochlea, the apex.

At the apex, the scala vestibuli connects through an opening called the helicotrema to the

third channel which is called the scala tympani. This conducts the pressure wave back

towards the towards the base of the cochlea which terminates in another membrane called the

round window. The round window is flexible and bulges out to allow the wave of fluid to

flow unimpeded.

The fluid filled channel sandwiched between the scala vestibuli and scala tympani is called

the scala media. Seperating the scala meda and the scala tympani is a flexible membrane

called the basilar membrane.

The basilar membrane houses the organ of corti which is made up of thousands of tiny inner

hair cells, so called because they have hairlike structures called stereocillia which

project from the top into the fluid of the scala media called endolymph.

As pressure waves travel through the scala vestibuli to the scala tympani they cause the

basilar membrane to vibrate at different points along it's length which displaces the hair

cells and causes their stereocillia to bend. There are also more numerous hair cells called

outer hair cells which also part of the organ of corti. They are a very important part of

the hearing process as they amplify the movement of the basilar membrane for soft and

moderate sounds.

These also contain stereocilia but these are embedded in a gelatinous membrane called the

tectorial membrane. They enhance the movement of the basilar membrane by expanding and

contracting in synchrony with the fluid pressure wave. This causes  shearing forces between

the tectorial membrane and the basilar membrane which sharpens its vibratory response.

The inner hair cells are the true transducers of sound. The bending of their stereocillia

causes ion channels to open in the cell surface which stimulates the production of

neurotransmitter. The neurotransmitter initiates firing along the auditory nerve to the

auditory centres of the brain.

 

What are the main causes of permanent (inner ear) sensory hearing loss?

Sensory hearing loss is a type of hearing loss in which the root cause lies in the inner

ear (i.e. the cochlea).

By far the biggest cause of permanent sensory hearing loss is the gradual ageing of the

auditory system know as presbyacusus. It is generally accepted the the main cause of

presbyacusis is hair cell damage within the cochlear. Other inner ear structures can be

involved such as the stria vascularis which maintains the ionic concentration of cochlear

fluids.

The second most common cause of permanent hearing loss is noise induced hearing loss. Most

people associate noise induced hearing loss with occupations where hazardous noise exposure

is always a threat such as construction, engineering or the military. A recent Australian

government study found that 13% of young people in Australia receive an annual dose of

leisure-related noise which exceeds that allowed on a building site. Loud music from MP3

players, rock concerts and nightclubs were the main noisy activities damaging the hair

cells within the cochlear and causing irrecoverable hearing loss.

 

What are the main causes of conductive (middle ear) hearing loss?

Conductive hearing loss happens when there is a problem conducting sound waves anywhere

along the route through the outer ear, eardrum and middle ear (ossicles) to the inner ear

(cochlea).

Glue ear is by far the most prevalent cause of conductive loss in childhood. Peak incidence

of glue ear in the UK is at 2 and 6 years of age. At least one episode of glue ear occurs

in 27% of our child population. Glue ear is often preceded by an acute middle ear infection

or upper respiratory tract infection. The Eustachian tube which runs from the back of the

nose to the middle ear cleft and normally ventilates the middle ear, becomes blocked or

restricted.

This causes a pressure drop in the middle ear cleft due to the absorbtion of oxygen by the

middle ear mucosa which then cannot be replenished by the Eustachian tube. The middle ear

mucosa becomes inflamed causing fluid and/or mucous to middle ear cavity which prevents the

ossicles of the middle ear and the eardrum from vibrating in response incoming to sound.

Glue ear can cause up to 60 decibels of hearing loss but it is generally considered a

temporary affliction although some children with chronic glue ear go on to suffer

complications well into adulthood. See  http://www.patient.co.uk/doctor/Fluid-in-the-Middle-Ear-and-Glue-Ear.htm

In adults a simple case of wax blockage in the outer ear can sometimes cause relatively

mild conductive hearing loss.

 Ear wax production is very variable between individuals but it is thought to  have a

protective purpose in the it traps any foreign material before it can get further into the

ear to do potential damage. It is also reported to have an antiseptic effect but this is

debateable. Ear wax is acidic thus generally though of as anti microbial but there are many

compounds within it that can serve as microbial nutrients such as fats, amino acids and

trace elements.

Deep seated, impacted wax which totally occludes the ear canal can cause a significant

conductive hearing loss. Wax is only produced in the outer third of the ear canal. This

sort of problem usually occurs when people poke things in their ears and push the wax

farther in.

 

Mixed hearing losses

Frequently a person can experience two or more types of hearing loss.For example a person

with a sensory hearing loss may acquire an acute infection which may cause an additional

conductive hearing loss albeit temporary.

Some people are diagnosed with permanent mixed hearing loss from birth. This may be caused

by a failure of more than one part of the ear to develop properly.

Otosclerosis is an acquired inherited hearing loss which is caused by abnormal bone growth

which can fixate the footplate of the stapes in the oval window preventing transmission of

sound. Sometimes the round window is involved which impairs the passage of sound pressure

waves through the cochlea. Otosclerosis is predominantly a conductive type of hearing loss

but sclerotic lesions can damage delicate structures within the inner ear leading to a

mixed hearing loss. In older adults this might be additional to the normal ageing effects

on the inner ear.

 

It is estimated that nine million people have some form of hearing problem in the UK but less than half actually do something about it. Hearing loss can affect people of all ages and is usually of gradual onset. As the hearing deteriorates we are often aware that we are missing the softer sounds of everyday life.

 

Are you one of those who suffer in silence?

 

If any of the following affect your quality of life you may benefit from a hearing test then please contact us: