• Nalaka de Silva

Understanding Audiology



Please note that the following is a general guideline only. For a full assessment, exclusion of any other underlying cause for your symptoms and an individualised treatment approach, you will need to be seen by a qualified specialist.



Understanding Tympanometry


Definition

Tymponometry is The dynamic measurement of middle ear admittance as pressure in the ear canal is raised and lowered


Use

To look at middle ear status,



Instrument

Performed with three channel audiometer

Sound-220 Hz at 85DB-children 1000 Hz

Microphone (to pick up -return sound hence work out admittance)

Pressure (to monitor P in the ear)



Results

Note that in tymponmetry, Y axis is the conductance of sound, X axis is the pressure applied the the tympanic membrane.

Acoustic admittance – a general term expressing the ease with which sound energy flows through a system










Type A Curve

If the tympanic membrane (TM) is in a neutral position, the admittance of sound (Y axis) will be maximum at 0 pressure applied to the TM (X axis). Negative pressure applied will suction the TM and positive pressure will push the TM reducing admittance. Hence in the normal ear, the bell curve of sound admittance peaking at "0" pressure is called type A curve


Type B Curve

If there is fluid behind TM or a perforation, the tympanic membrane will not move with any positive or negative pressure applied to it. Fluid makes the TM stiff, and perforation lets the pressure distribute to either side of the TM hence un affecting its position, as a results conductance. As a result a flat curve is achieved called a B curve. B high means there is a high volume noted by the tympanometer (volume of the ear canal + the middle ear) hence a perforation. B low means low volume is noted, i.e. ear canal volume only: hence OME.


Type C curve

In Eustachian tube dysfunction, the middle ear develops negative pressure due to air absorption against a closed tube. The position of the TM in this situation is suctioned inwards. Application of negative pressure will pull the TM outwards hence to a central position maximising conductance. Further negative pressure will pull the TM outward further reducing conductance. Hence in the bell curve that peaks on negative pressure is called a type C curve.






Understanding Pure Tone Audiometry

Whilst understanding a pure tone audiometry to its fullest requires a good knowledge on how its performed, all physicians should be able to read a basic audiogram.

Pure tone audiometry is testing the right ear and left ear independently for bone and air conduction for specific frequencies. Its performed in a sound proof room. Patients need to be corporative, and usually of the ages five and above.




Frequencies tested (X axis)

Frequency or pitch is measured in Hertz (Hz). Frequencies range from low-pitch to high-pitch and read from left to right on the audiogram along X axis. Ones used most often during testing are 250, 500, 1000, 2000, 4000 and 8000 Hz.

Sound Intensity or volume (Y axis)

The sound intensity is measured in decibels (dB). The intensity relates to how loud or soft a sound is. Markings along Y axis represents increasing intensity level. Please note that the softest sounds are at the top loudest sounds at the bottom. Intensity at which each frequency is heard is marked using different symbols as below.

Right Ear / Left Ear

In audiometry air conduction thresholds are done first. Right air conduction thresholds are marked as an "O" for each frequency tested. Left is make with an "X".


Air conduction difference over 50 db between R and L ears may cross stimulate the non test ear. Hence if there is such a difference between the ears, the better ear may be masked/ or distracted whist testing the poorer ear with sufficiently loud sounds to obtain a response.


Next responses obtained are during bone conduction testing. The right ear thresholds are graphed with < and the left ear with > or. Bone conduction test stimulates both ears together (due to skull vibration). This is unless one is distracted or masked. Right masked bone conduction is depicted with " [ " and Left masked bone conduction with " ] "











Following are some examples and interpretations.






Example 1




Type A temps both middle ears are normal


Right air conduction "O" is along 20 db i.e. normal.

Left ear air conduction is also along 20 db i.e. normal left ear

Unmasked bone conduction is depicted as > is normal, this really does stimulate both ears masked individual bone were not needed due to normal hearing at all levels

speech is 80% correct at 30 db, and 100% correct at 50 db in both ears i.e. as expected

Interpretation : Normal Audiogram








Example 2




Tymps: type B, volume is 1.9 and 1.4 i.e. low volume. This is consistent with middle ear pathology i.e. OME


Right air conduction marked " X "is 60 db at 250 Hz, going down to average around 40 db

Left air conduction marked " O " is around 60 db for most frequencies

Due to abnormal air conduction findings, individual ears bone conduction has been tested and marked

Both sides masked bone conduction levels are around 20 db i.e. Normal bone conduction in both ears.

Hence there is an airborne gap bilaterally


speech is 60% correct around 30-40 db and around 90% correct around 60 Db- as expected


Interpretation: OME leading to bilateral conductive hearing loss.







Example 3






Left Type A tymp ie Normal middle ear status, Right B with a very high volume: i.e. Right has either a perforation or a large mastoid cavity with a stiff graft.


Left air conduction " X " is around 20 db or less i.e. normal

Right air conduction has been masked (due to the big gap between the sides) and is around 60DB.

Masked right bone conduction " [ " is also around 60.

The left bone conduction was only done at 500 Hz and rest were not necessary as left air conduction was essentially normal.


speech on the left 100% correct at 40 db, on the right the patient gets 100% correct at 70-80 db i.e. aid able


Interpretation: essentially normal left ear, R sensory loss with a high vol B curve: either a perforation or a mastoid cavity. Left side is aid-able.








Example 4



Type A tymps

Left air conduction starts normally and slopes down to about 40 db in high Hz

Right masked air conduction is around 60-70 db

Right masked bone conduction around 60 db

Left unmasked bone conduction mirrors left air conduction


Interpretation: left normal to mild sloping sensory hearing loss (no air bone gap)

Right severe mixed hearing loss (50 db bone conduction loss, and 2O db air-bone gap bring air conduction down to 70 db)




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