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Tympanometry in Just Seconds
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To begin testing, the probe, fitted with an
appropriately sized eartip, is positioned at the opening of the ear canal. For
operator convenience, the probes of the Auto Tymps are equipped with either
signal lights (GSI 38) or an easy to read display (GSI 37) to keep the operator
informed of the test status at all times. Even inexperienced instrument users
can obtain reliable results with confidence. When a tight seal is obtained, a
known quantity of sound energy is introduced to the ear. By introducing a known
amount of sound energy to the ear, a measurement of the energy not transmitted
into the middle-ear system is recorded. The amount of sound energy transmitted
is equal to the amount of sound energy introduced, minus the amount of sound
energy that returns to the probe microphone. The amount of energy transmitted is
directly related to the compliance of the system. Compliance (labeled in
cm3 of equivalent volume) indicates the amount of mobility in the
middle ear. For example, the more energy returned to the probe, the less energy
admitted, and thus a low compliance measurement. Low compliance measurements
indicate a stiff or obstructed middle ear (See Figure 3). Of course, the
opposite is true for a high amount of energy transmission, indicating a flaccid
or highly mobile system. |
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| FIGURE 3: Serous Otitis Media, Fluid-Filled Middle Ear |
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| At this point, the probe introduces a pressure of +
200 daPa to the middle ear canal. Decapascals, or daPa is a measurement of air
pressure; whereby, 1.02mm H20 = 1.00 daPa. This positive pressure
forces the tympanic membrane inward and the approximate ear canal volume is
recorded. This volume gives a baseline from which the compliance curve is drawn.
The pressure is now varied in the negative direction, while constantly
monitoring the compliance of the system. The pressure continues toward the
negative direction until a pressure peak has been detected and -100 daPa has
been reached or until a pressure of -400 daPa is present in the ear canal,
whichever comes first. Once the pressure is equalized on both sides of the
tympanic membrane, the point of peak compliance is stored, as is the pressure at
which peak compliance occurs. |
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| A tympanogram is a graphic representation of the
change in compliance (Y or vertical axis) of the middle-ear system as air
pressure (X or horizontal axis) is varied. Valuable diagnostic information is
obtained from both the graphic and quantitative data (See Results
Section). |
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| FIGURE 4: A Normal Tympanogram |
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| Some screening instruments, such as the GSI 38 Auto
Tymp, offer an additional capability - acoustic reflex response testing. An
acoustic reflex is elicited by presenting a very loud stimulus to the auditory
pathway. During acoustic reflex testing, the stimulus is presented to the ear
canal either through a probe or an earphone (insert phone). This stimulus then
travels through the middle ear to the cochlea. From the cochlea, frequency and
intensity information is transmitted via the 8th nerve to the brainstem where a
determination is made as to whether or not the intensity of the stimulus is high
enough to elicit a reflex response. If it is, a bilateral response occurs, ie.:
the right and left 7th nerves innervate their respective stapedial muscles
causing them to contract. As these muscles contract, they stiffen their
respective ossicular chains. This stiffening of the ossicular chains reduces the
compliance of each middle ear system. As in tympanometry, a probe tone is used
to measure this decrease in compliance. Thus, the reflex response can be
measured in either ear with only one ear directly receiving the tone stimulus. A
tone is presented to the ear at 85 dB HL to test the acoustic reflex, if a
reflex does not occur, the HL is increased by 10 dB HL increments until a reflex
of 0.05 cm3 change in compliance is measured due to a stiffening of
the stapedial muscle. |
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| FIGURE 6: GSI Auto Tymp |
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| The operator may select IPSI or CONTRA, which
dictates the ear which receives the stimulus. For example, if the probe is
inserted in the left ear and the acoustic reflex of the left ear is to be
tested, the user then selects IPSI. This is known as ipsilateral testing. It is
necessary to utilize the earphone (insert phone) if the opposite ear is to be
tested. This is known as contra lateral testing. In this case, the signal is
presented to the earphone (insert phone) ear and the reflex is measured via the
probe ear by simply preselecting the desired buttons. ipsilateral and
contralateral testing can be done sequentially at any designated frequency, with
the GSI 38 Auto Tymp. |
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| Reflex test results can be displayed and printed in
three different ways: reflex dB HL plus curve; reflex dB HL only; or reflex
yes/no (See Figures 7, 8, & 9). |
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| FIGURE 7: Reflex dB HL with Curve |
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| FIGURE 8: Reflex dB HL |
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| FIGURE 9: Reflex Yes or No |
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