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Hearing 2

 

A. Otosclerosis:round window otosclerosis

Otosclerosis is a disease whereby bone accumulates in the middle ear. When this accumulates around the ossicles, then everybody will understand that this will impair their movements, leading to deafness. In some cases however, bone accumulation also occurs around the round window. This also causes deafness. The question is why?

 

The inner ear can be described as a single tube which starts at the oval window and ends at the round window. This tube is filled with fluid. If the membrane of the oval window is pushed inwards, by a sound vibration, then the round window membrane, at the other end of the tube, is pushed outward. This is because fluid is not compressible.

The opposite happens when the oval membrane is pulled outwards, then the round membrane will be pulled inwards. During normal hearing, this push and pull will occur very quickly and the fluid will oscillate left and right (thereby creating vibrations in the basilar membrane; not shown in this diagram).

But if the round window is closed (by otosclerosis) and therefore fixed, then the fluid inside the tube cannot be pushed or pulled. Then it is no longer possible to push or pull the membrane of the oval window. And therefore, the fluid inside no longer vibrates and can no longer push & pull the basilar membrane and stimulate the organ of Corti.

 

Therapy: removal of the bone accumulation.

 

 

 

 

B. Air and Bone Conduction:

diagram of air and bone conduction

Air Conduction:

The usual way of hearing is by conduction of sound waves through the air to our ears. That is how we listen to music, to talks, to my lectures, etc.

But, when we, ourselves, talk, then we also listen to ourselves in a different way, namely through bone conduction.

 

Bone Conduction:

As shown in the diagram, sound waves are generated in the larynx and propagate from the mouth through the air to the ears. But they also propagate through the skull to the ears. This is called bone conduction. Therefore, we hear ourselves talking through both the air and the skull.

Everybody else hear us talk only through the air. There is a difference. The propagation and the frequency of the sound through the skull is influenced by the bone. Therefore we hear ourselves talk in a different way from anybody else. The only time you really hear your own voice is when you hear it back from a recording on a taperecorder, an iPod, or something like that. And then; nobody likes their own voice!

 

C. Deafness (Hearing Abnormalities):

Deafness

can be caused by an abnormality in the outer ear, the middle ear or the inner hear. If there is a problem in the outer ear or in the middle ear, then there is a problem in the conduction of the sound waves. Therefore, these problems are called Conduction abnormalities which result in Conduction Deafness.

Abnormalities in the cochlea (the inner ear) or the nerves are called Nerve deafness or Sensorineural deafness.

 

Audiogram.

An audiogram is a measurement of how well we can hear. This is tested by producing sound at different frequencies and at different amplitudes and plotted in a graph. Furthermore, we can test, separately, air and bone conduction.

In air conduction, sound is perceived through the outer ear, the middle ear and the inner ear.

In bone conduction, sound is perceived by creating sound waves through the skull, thereby bypassing the outer and the middle ear and only stimulating the inner ear.

This distinction makes it possible to differentiate between conduction deafness and nerve deafness (see below).

 

 

D. Normal Audiograms:

Audiogram.

An audiogram is a graph that plots the threshold levels for hearing sounds measured at different frequencies. These levels are expressed in decibels (dB) on a logarithmic scale. Zero dB represents threshold for normal healthy people. These levels are measured at different frequencies, usually from 125 to 8000 Hz. If a patient requires more than 20 dB to hear a particular frequency, this is considered a significant loss.


The audiogram is measured for both ears separately (with earphones) and by comparing air conduction and bone conduction. Bone conduction is induced with a vibrator placed against the mastoid (the bone behind the ear).

 

Decibel.

is the unit of sound on a logarithmic scale. It is actually the ratio between the actual sound and the threshold for sound which is set at 0 dB. A whisper produces a sound of 20-30 dB, a conversation 40-50 dB, a vacuum clearner produces 80dB noise, a truck rumbles at 90 dB, and a civil defense siren blasts at 130 dB which is close to the threshold of pain.

 

 

 

Normal audiogram of the left and the right ear. Normal audiogram of air conduction and bone conduction.

 

E. Abnormal audiograms:

 

Presbycusis:

In this patient, the hearing for higher frequencies is impaired (worse than normal) whereas it is ok at the middle and lower frequencies. Both the air and the bone conduction have declined at the high frequencies. This implies that the problem is located in the inner ear; the cochlea or the nerve. Usually, this is due to the wear and tear in the organ of Corti (= old age!).

gun shot audiogram

 

Gun-shot damage:

In this patient, air and bone conduction is impaired at a specific frequency; in this case at about 4000 Hz. Since bone and air conduction are both equally affected, it is a sensorineural deafness. A damage at one frequency means that a small portion of the basilar membrane is damaged; usually by being exposed repeatedly to a high volume noise of a specific frequency; for example the cocking of the head and the ear close to the barrel of a rifle (hence the name!). Nowadays, such damages often occur when people work in environments where there is a lot of (industrial) noise such as airports but also disco's etc.

middle ear sclerosis

 

Otosclerosis:

In this patient, bone has accumulated in the middle ear thereby impeding the movements of the ossicles. This is a conduction problem as there is no problem with the cochlea. So the measured bone conduction is normal but his conduction is impaired. The same type of pattern could be obtained in the case of an otitis media (= middle ear inflammation).

impacted wax audiogram

 

Impacted Wax:

In this case, the external auditory canal is blocked by old and hardened wax. This is a typical conduction problem. The bone conduction is therefore normal. A ruptured tympanic membrane would produce a similar audiogram.

 

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