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Hearing loss can be divided into 3 types:
SENSORI-NEURAL
CONDUCTIVE
MIXED
SENSORI-NEURAL HEARING LOSS
‘Sensory’ refers to the cochlea which is the inner ear
‘Neural’ refers to the hearing nerve which takes the messages from the cochlea into the brain.
It is not usually possible to determine whether it is the ‘sensory’ bit or the ‘neural’ bit which is at fault.
Throughout life many things can damage these nerve endings including:
- loud noise
- infections
- drugs
- trauma
- genetic (inherited) conditions
However the commonest cause of damage is just the of passage of time. As we all get older these nerve endings(hair cells) degenerate and do not replace themselves as most other types of cells in the body do. We start with about 30000 hair cells and can afford to loose quite a few before starting to notice problems.
As the hair cells start to be lost speech starts to sound quieter because more energy(i.e. more volume) is needed to stimulate the remaining hair cells. The commonest cause of sensori-neural hearing loss is age related hearing loss where the ability to hear high tone sounds often goes first.
Audiogram of a SENSORI-NEURAL hearing loss
The black circles (right ear) and the black crosses (left ear) represent what the person can hear through air conduction.
In other words this is their day to day hearing ability.
The sound is going down the ear canal, through the ear drum, through the hearing bones, into the inner ear and along the nerve up to the brain. The black circles and crosses represent the whole hearing pathway and do not help identify the area of the hearing loss.
The red triangles represent the bone conduction hearing level. When the bone vibrator is pressed onto the skull and the hearing level tested the sound goes straight to the nerve. The ear canal(outer ear) and the ear drum and the hearing bones (middle ear) are all bypassed.
BONE CONDUCTION MEASURES THE NERVE FUNCTION (INNER EAR AND HEARING NERVE).
In the above audiogram the red triangles show us that the nerve function is not working properly (above the 25dB level would be normal).
Because the black line is the same as the red line it tells us that there is no added hearing loss from the outer or middle ear – so all the hearing loss is due to an inner ear or nerve(sensori-neural) problem.
The Audiogram below show you the volume of every day sounds. If you plot your hearing level on this audiogram you will not be able to hear any of the sounds above the line.
The purple area is known as the SPEECH BANANA. It is the volume of normal conversational speech and it shows that some parts of speech are quieter. than others
Why does this hearing loss make it difficult to understand speech?
Some of the consonants in speech are not only high tone but are also spoken with a quieter volume than the the vowels.For an elderly person who can only just hear the high tones in a quiet background if someone speaks to them in a noisy background they will no longer be able to hear the high tone parts of speech. In other words they will not be able to hear the consonants (can’t hear difference between ‘fat’ and ‘sat’ or ‘ship’ and ‘chip’ or ‘kiss’ and ‘this’.)) They will know someone is talking but will not be able to understand the speech. Using a hearing aid to boost the volume of the frequencies they can not hear may over come the problem. However as the hearing loss becomes worse a hearing aid may become less effective. The reason for this is explained below.
Discrimination
The very large number of ‘hair cells’ (nerve endings) that people are born with allows people to detect very small differences between the different frequencies of sound. Speech is a combination of sounds of different frequencies. Different words are a combination of different frequencies. If you lose a lot of the hair cells you will no longer be able to hear the difference between different frequencies – they will sound the same and this will make it very difficult to understand speech even though you may be able to ‘hear’ it. This is called a ‘loss of discrimination’ and a hearing aid can not return the ability to discriminate.
CONDUCTIVE HEARING LOSS
Sound waves are normally ‘conducted’ (in other words they ‘travel’) to the inner ear via the ear canal, ear drum and hearing bones.
A conductive hearing loss is when there is a problem with the sound being ‘conducted’ along this route. So anything that blocks the ear canal, affects the vibration of the ear drum or affects the chain of hearing bones will cause a ‘conductive ‘hearing loss.
A conductive hearing loss can often be repaired surgically. We can clear a blocked ear canal or repair a damaged ear drum or repair or replace damaged hearing bones.
Audiograms of CONDUCTIVE hearing loss
The black circles and crosses represent what the person can actually hear. The half red squares on the right show what the person could hear if the sound was being ‘conducted’ properly through to the inner ear. This audiogram suggests that there is a problem with either the ear canal, ear drum. middle ear space or hearing bones.
If we can fix the problem with the ‘conduction’ pathway then the patient will also be able to hear sound between the black circles and the red half squares. This will return the patients hearing to normal because the red half squares are in the normal range of hearing (less than 25dB)
MIXED HEARING LOSS
This is when a patient has a ‘sensori-neural ‘ hearing loss AND a ‘conductive hearing loss’. In this case correcting the conductive hearing loss will only make the hearing as good as the level of the sensorineural hearing loss.
Audiograms of MIXED hearing loss
The difference with the audiograms above is that the red half squares (the bone conduction) is not normal. The inner ear is not working properly.
The person can still only hear sounds that are loud enough to fall below the black circles.
If we could completely fix the conductive part of the hearing loss the hearing would be better but not normal. The patient would still have a hearing loss but not as bad as before.
