Tympanometry
Purpose
Tympanometry is an essential component of the audiologic evaluation. Use of tympanometry allows the clinician to have an objective measure that contributes to the understanding of middle ear function.
Admittance is a measures of the ease with which sound energy is transferred into the middle ear space. Tympanometry involves measuring admittance while varying pressure in the ear canal relative to atmospheric pressure. when the eustachian tube is functioning properly, the air trapped in the middle ear space tends to be maintained at atmospheric pressure. When the middle ear system is functioning optimally, the greatest admittance of sound energy into the middle ear system occurs at atmospheric pressure. When there is pathology that distrubs function of the eustachian tube, the greatest admittance of sound energy into the middle ear system may occur at a different pressure level (positive relative to atmospheric pressure, or more commonly, negative relative to atmospheric pressure). In some cases, for example when the middle ear is filled with fluid, there is little admittance of sound energy into the middle ear system. These effects can be observed on the tympanogram.
Admittance is a measures of the ease with which sound energy is transferred into the middle ear space. Tympanometry involves measuring admittance while varying pressure in the ear canal relative to atmospheric pressure. when the eustachian tube is functioning properly, the air trapped in the middle ear space tends to be maintained at atmospheric pressure. When the middle ear system is functioning optimally, the greatest admittance of sound energy into the middle ear system occurs at atmospheric pressure. When there is pathology that distrubs function of the eustachian tube, the greatest admittance of sound energy into the middle ear system may occur at a different pressure level (positive relative to atmospheric pressure, or more commonly, negative relative to atmospheric pressure). In some cases, for example when the middle ear is filled with fluid, there is little admittance of sound energy into the middle ear system. These effects can be observed on the tympanogram.
Equipment
Tympanometer
Procedures/Instruction for Tympanometric Measurement
To perform tympanometry, an immittance probe is inserted into the opening of the ear canal. An airtight seal must be obtained in order to vary pressure in the ear canal. The immittance meter probe has an air pump to adjust air pressure in the ear canal, a monometer (instrument for measuring pressure) to measure air pressure, a microphone to measure the intensity of sound in the ear canal, and a loudspeaker to deliver a probe tone.
In order to measure immittance, a probe tone is presented continuously to maintain a fixed intensity level in the ear canal. the probe tone most commonly used for adults is 226 Hz, and for infants, 1000 Hz. As the probe tone is being presented, some of the sound energy is entering the middle easr system, and some of the sound energy is being maintained in the ear canal. The total sound evergy in the ear canal is measured with the microphone.
The amount of sound energy admitted to the middle ear system is related to the air pressure on either side of the tympanic membrane. When sound energy is transferred from one area to another where the air pressures are relatively equal, much of the energy is admitted into the next area. so, when the air pressure is equal on both sides of the tympanic membrane, the sound energy most easiiy travels into the middle ear space, and the sound pressure level (SPL) in the ear canal becomes lower.
Alternatively, when pressure in the ear canal is greater than or less than the pressure in the middle ear space, less sound energy is transferred. Compared to an equal pressure condition, much more sound energy is maintained in the ear canal, resulting in a higher SPL. the process for measurement of the admitted sound energy to the middle ear space can be conceptualized as follows. When performing tympanometry, the loudspeaker delivers a probe tone of known intensity into the ear canal. The microphone picks up the remaining auditory signal. A device known as an automatic gain control compares the difference in the electrical signal for the loudspeaker and the microphone and continuously adjusts the level of the probe tone coming from the loudspeaker so that the sound pressure level in the ear canal remains constant. By comparing the difference between the amplified probe tone signal and the sound pressure level picked up by the microphone, it can be determined how much sound energy was admitted into the middle ear space. Admittance of sound pressure into the middle ear space is measured in units of millimhos (mmho), millimeters of water (mm H20) or milliliters (mL).
While this sound measurement process is occurring, the air pump works to make a very positive pressure in the ear canal space. Then, the air pressure is decreased until the pressure is very negative in the ear canal space. (Alternatively, pressure may be increased from negative to positive). By doing this, the admittance of sound energy into the middle ear space can be measured over a wide range of air pressures. Air pressure in the ear canal is measured in decapascals (daPa). The range of pressures measured on most tympanometers is approximately -400 daPa to +200 daPa.
The typanogram is a graph of admittance of sound energy as a function of sound pressure in the ear canal. The air pressure is plotted on the abscissa. The admittance (in mmho, mm H20, or mL) is plotted on the ordinate.
By introducing substantially positive pressure (+200 daPa) into the ear canal, the tympanic membrane and ossicular chain essentially become rigid structures. Very little admittance of sound energy into the middle ear space occurs. In the case of a normally functioning middle ear space, as the pressure is continuously decreased a greater amount of sound energy is admitted into the middle ear space. Eventually, when the pressure in the ear canal equals the pressure in the middle ear space, the amount of admitted sound energy reaches a peak level. Then, as the pressure in the ear canal becomes negative relative to the pressure in the middle ear space, the admittance of sound energy into the middle ear space begins to decrease. We expect that when the middle ear is functioning optimally, the greatest amount of admittance will occur at about atmospheric pressure (0 daPa on the tympanogram).
In order to measure immittance, a probe tone is presented continuously to maintain a fixed intensity level in the ear canal. the probe tone most commonly used for adults is 226 Hz, and for infants, 1000 Hz. As the probe tone is being presented, some of the sound energy is entering the middle easr system, and some of the sound energy is being maintained in the ear canal. The total sound evergy in the ear canal is measured with the microphone.
The amount of sound energy admitted to the middle ear system is related to the air pressure on either side of the tympanic membrane. When sound energy is transferred from one area to another where the air pressures are relatively equal, much of the energy is admitted into the next area. so, when the air pressure is equal on both sides of the tympanic membrane, the sound energy most easiiy travels into the middle ear space, and the sound pressure level (SPL) in the ear canal becomes lower.
Alternatively, when pressure in the ear canal is greater than or less than the pressure in the middle ear space, less sound energy is transferred. Compared to an equal pressure condition, much more sound energy is maintained in the ear canal, resulting in a higher SPL. the process for measurement of the admitted sound energy to the middle ear space can be conceptualized as follows. When performing tympanometry, the loudspeaker delivers a probe tone of known intensity into the ear canal. The microphone picks up the remaining auditory signal. A device known as an automatic gain control compares the difference in the electrical signal for the loudspeaker and the microphone and continuously adjusts the level of the probe tone coming from the loudspeaker so that the sound pressure level in the ear canal remains constant. By comparing the difference between the amplified probe tone signal and the sound pressure level picked up by the microphone, it can be determined how much sound energy was admitted into the middle ear space. Admittance of sound pressure into the middle ear space is measured in units of millimhos (mmho), millimeters of water (mm H20) or milliliters (mL).
While this sound measurement process is occurring, the air pump works to make a very positive pressure in the ear canal space. Then, the air pressure is decreased until the pressure is very negative in the ear canal space. (Alternatively, pressure may be increased from negative to positive). By doing this, the admittance of sound energy into the middle ear space can be measured over a wide range of air pressures. Air pressure in the ear canal is measured in decapascals (daPa). The range of pressures measured on most tympanometers is approximately -400 daPa to +200 daPa.
The typanogram is a graph of admittance of sound energy as a function of sound pressure in the ear canal. The air pressure is plotted on the abscissa. The admittance (in mmho, mm H20, or mL) is plotted on the ordinate.
By introducing substantially positive pressure (+200 daPa) into the ear canal, the tympanic membrane and ossicular chain essentially become rigid structures. Very little admittance of sound energy into the middle ear space occurs. In the case of a normally functioning middle ear space, as the pressure is continuously decreased a greater amount of sound energy is admitted into the middle ear space. Eventually, when the pressure in the ear canal equals the pressure in the middle ear space, the amount of admitted sound energy reaches a peak level. Then, as the pressure in the ear canal becomes negative relative to the pressure in the middle ear space, the admittance of sound energy into the middle ear space begins to decrease. We expect that when the middle ear is functioning optimally, the greatest amount of admittance will occur at about atmospheric pressure (0 daPa on the tympanogram).
Interpretation/Site of Lesion
The Impact of Middle Ear Function on Tympanometric Outcomes
Middle ear pathology often causes changes to the function of the admittance of sound energy to the middle ear system. Typically, the result is that there is less admittance of sound energy into the middle ear space under normal atmospheric conditions. For example, at the onset or offset of otitis media, the pathophysiology of the condition often creates a situation where there is significant negative pressure in the middle ear space.
As mentioned previously, the greatest amount of admittance of sound energy to the middle ear system occurs when the pressure on either side of the tympanic membrane is relatively equal. When significant negative pressure exists in the middle ear space, then the greatest admittance will occur when the pressure in the ear canal is significantly negative.
In cases of otitis media, fluid may fill the middle ear space. In this case, there is very little admittance of sound energy into the middle ear space, regardless of the ear canal pressure. Because there is very little difference in the amount of admittance at any pressure level, the resulting graph appears to be relatively flat.
The type of tympanometry described above is known as single-frequency tympanometry. There are other types of tympanometry, but single frequency is currently the most commonly used clinically.
As mentioned previously, the greatest amount of admittance of sound energy to the middle ear system occurs when the pressure on either side of the tympanic membrane is relatively equal. When significant negative pressure exists in the middle ear space, then the greatest admittance will occur when the pressure in the ear canal is significantly negative.
In cases of otitis media, fluid may fill the middle ear space. In this case, there is very little admittance of sound energy into the middle ear space, regardless of the ear canal pressure. Because there is very little difference in the amount of admittance at any pressure level, the resulting graph appears to be relatively flat.
The type of tympanometry described above is known as single-frequency tympanometry. There are other types of tympanometry, but single frequency is currently the most commonly used clinically.
Tympanometric Static Admittance
Tympanometric static admittance is determined by the height of the peak on the tympanogram. This measure is an indication of the amount of admittance of sound energy into the middle ear space.
Tympanometric Peak Pressure
Tympanometric peak pressure is the pressure level at which the peak of the tympanogram occurs. This measure is an indication of the pressure level at which the greatest admittance of sound energy occurs. From this, we can infer whether the pressure in the middle ear space is positive or negative relative to the pressure in the ear canal.
Tympanometric Width
Tympanometric width is a measure of the width of the tympanogram measured at half of the static admittance from the peak to the admittance at +200 daPa. Certain pathologies, such as fluid in the middle ear, can increase tympanometric width.
Equivalent Ear Canal Volume
Equivalent ear canal volume is a measurement of the volume of air in front of the probe in cubic centimeters (cc) or milliliters (mL) when the ear canal is pressurized to +200 daPa.
In most adult cases where there is an intact tympanic membrane, the volume of air in the ear canal in front of the probe is around 1.0 cc. In children and infants it is typically smaller. This measurement is useful for determining whether there is an opening in the tympanic membrane (either due to perforation or to patent pressure equalization tubes), which allows measurement of the entire ear canal and middle ear system, demonstrated by an abnormally large volume. Typically, when this is the case, the tympanogram shape is flat or otherwise abnormal.
In most adult cases where there is an intact tympanic membrane, the volume of air in the ear canal in front of the probe is around 1.0 cc. In children and infants it is typically smaller. This measurement is useful for determining whether there is an opening in the tympanic membrane (either due to perforation or to patent pressure equalization tubes), which allows measurement of the entire ear canal and middle ear system, demonstrated by an abnormally large volume. Typically, when this is the case, the tympanogram shape is flat or otherwise abnormal.
Typanometric Shape
Immittance at the Plane of the Eardrum
For diagnostic purposes we are mainly concerned with the immittance of the middle ear because it provides information about
We must removed the outer ear component from the total admittance value at the probe tip to get an undistored representation of the middle ear's admittance at the eardrum. In other words, removing the effect of the ear canal moves the measurement location from the end of the probe tip to the plane of the tympanic membrane. We can achieve this goal by taking advantage of the fact that total admittance is simply the sum of the admittances of the outer ear and the middle ear.
The pressure change is accomplished using the pressure pump connected to one of the tubes in the probe tip. The rationale for this tactic is that the heightened air pressure puts the eardrum under so much tension that it acts like a hard wall, so that essential no sound energy can be transmitted into the middle ear. This strategy prevents the probe tip from measuring the admittance of the middle ear. In this case, we say that the middle ear has been excluded from the measurement. Hence, the admittance obtained under these conditions comes from the outer ear alone
We now know the total admittance (of the outer ear and middle ear combined) from the first measurement and the admittance of the outer ear from the second measurement. The third step to figure out the previously unknown middle ear admittance by simply subtracting the outer ear admittance from the total admittance.
- middle ear pathologies
- middle ear muscle contractions
We must removed the outer ear component from the total admittance value at the probe tip to get an undistored representation of the middle ear's admittance at the eardrum. In other words, removing the effect of the ear canal moves the measurement location from the end of the probe tip to the plane of the tympanic membrane. We can achieve this goal by taking advantage of the fact that total admittance is simply the sum of the admittances of the outer ear and the middle ear.
The pressure change is accomplished using the pressure pump connected to one of the tubes in the probe tip. The rationale for this tactic is that the heightened air pressure puts the eardrum under so much tension that it acts like a hard wall, so that essential no sound energy can be transmitted into the middle ear. This strategy prevents the probe tip from measuring the admittance of the middle ear. In this case, we say that the middle ear has been excluded from the measurement. Hence, the admittance obtained under these conditions comes from the outer ear alone
We now know the total admittance (of the outer ear and middle ear combined) from the first measurement and the admittance of the outer ear from the second measurement. The third step to figure out the previously unknown middle ear admittance by simply subtracting the outer ear admittance from the total admittance.
Tympanometry involves the measuring of the acoustic admittance of the ear with various amounts of air pressure in the ear canal. We can control the amount of air pressure in the ear canal because the probe tip makes a hermetic seal with the ear canal, and one of its tubes is connected to an air pump and manometer. The amount of air pressure is expressed in terms of decapascals (daPa) or of millimeters of water pressure relative to the atmospheric pressure in the room where the test is being done. Hence, 0 daPa implies that the pressure in the ear canal is equal to the atmospheric pressure, positive pressure (+100 daPa) means that the ear canal pressure is greater than atmospheric pressure and negative pressure (-100 daPa) means it is less than atmospheric pressure. This information is shown on a diagram called a tympanogram with admittance in mmhos or (equivalent volume in ml) on the y-axis and pressure in daPa on the x-axis. Notice that atmospheric pressure is in the middle with positive pressure increasing to the right and negative pressure increasing to the left.
Procedures/Instruction
- A steady low frequency tone at 70dB SPL is presented through the probe speaker and the probe. microphone picks up whatever sound bounces back off from the TM
- Place the probe into the ear of measurement by pulling superior and posterior on the pinna, placing the probe, letting go of the pinna and finally letting go of the probe
- Tell the patient going to feel some pressure changes in your ear
- Tell the patient to please be as still as possible while the test is administered
- Tell the patient to please let you know if you feel any discomfort and i will discontinue the test immediately
- Press measurement to measure the tympanogram
- Record the measurements on the audiogarm
Interpretation/Site of Lesion
The shape, a combination of the height and location of the tympanometric peak, has long been used to describe the tympanogram. The typically used shape types are as follows:
Type A: Normal peak height and normal peak pressure
Type B: Flat. This type of tympanogram is typically seen with middle ear dysfunction characterized by the addition of mass to the system, such as fluid behind the tympanic membrane. This type can also occur in the presence of cerumen impaction.
Type C: Negative peak pressure. this type of tympanogram typically is seen with eustachian tube dysfunction.
Type As: Abnormally low peak height and normal peak pressure. This type of tympanogram is consistent with an increase in the stiffness of the middle ear mechanism; often seen in otosclerosis.
Type Ad: Abnormally high peak height and normal peak pressure. This type of tympanogram is consistent with a decrease in the stiffness of the middle ear mechanism.
Type A: Normal peak height and normal peak pressure
Type B: Flat. This type of tympanogram is typically seen with middle ear dysfunction characterized by the addition of mass to the system, such as fluid behind the tympanic membrane. This type can also occur in the presence of cerumen impaction.
Type C: Negative peak pressure. this type of tympanogram typically is seen with eustachian tube dysfunction.
Type As: Abnormally low peak height and normal peak pressure. This type of tympanogram is consistent with an increase in the stiffness of the middle ear mechanism; often seen in otosclerosis.
Type Ad: Abnormally high peak height and normal peak pressure. This type of tympanogram is consistent with a decrease in the stiffness of the middle ear mechanism.
Management
Works Cited
Gelfand, Stanley A. Essentials of Audiology. Thieme, 2016.
DeRuiter, Mark, and Virginia Ramachandran. Basic Audiometry Learning Manual. Plural Publishing Inc., 2017
DeRuiter, Mark, and Virginia Ramachandran. Basic Audiometry Learning Manual. Plural Publishing Inc., 2017