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Tympanometry and Gradient
Tympanometry provides an objective means for determining the amount of mobility present within
the eardrum and the ossicular chain. It is, however, important to keep in mind the fact that the amount
of mobility present within the ossicular chain may be camouflaged by a scarred or thickened eardrum.
Acoustic energy, commonly referred to as the probe tone (226 Hz or 1000 Hz) is introduced into a
hermetically sealed ear canal by means of a loudspeaker located within the probe. The intensity of
this tone is monitored via a microphone, also located within the probe box. Measurements are taken
at fixed time intervals.
As pressure within the ear canal is varied, the eardrum is subjected to varying degrees of stress which
alters the mobility of the eardrum. Maximum mobility will occur when the pressure on both sides of
the eardrum are equal. Changes in mobility of the eardrum tend to produce changes in the probe tone
level within the ear canal. Probe tone intensity changes indicate the amount of sound energy entering
the middle ear.
Compliance is calculated based on these measurements. Since the sound pressure level of the probe
tone within the ear canal varies as a function of mobility, it is possible to record these changes in
mobility as a function of pressure. While the recording is visualized in the horizontal direction (X-axis)
as a function of differential pressure across the eardrum, the tracing also moves in the vertical
direction (Y-axis) as a function of mobility or admittance of the middle ear system. A graphic
presentation of this information is known as a tympanogram
The point of the tympanogram which represents the point of maximum compliance is the compliance
peak of the tympanogram. The air pressure (pressure at the peak) where this compliance peak occurs
approximates the pressure within the middle-ear system, since maximum mobility is only possible
when there is little or no pressure difference between the ear canal and the middle-ear space.
Compliance using a 226 Hz probe tone is measured with respect to the ability of an equivalent volume
of air to conduct sound and the scientific quantity used is cm
3
. Compliance using the 1000 Hz probe
tone is measured in mmhos as it is not an ear canal compensated measurement. Air pressure is
measured in deca Pascals (daPa).
NOTE
:
1.02 mmho = 1.0 daPa
The presence of a pathological condition which interferes with the mobility of the tympanic
membrane, the ossicular chain, or the air pressure within the middle-ear space can be detected during
tympanometry.
•
If the air pressure within the middle-ear space becomes negative due to a blocked Eustachian
tube, tympanometry measures this negative pressure and its effect on middle-ear
compliance.