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, 678 Hz or 1000 Hz) is introduced into a hermetically sealed ear canal by means of a loudspeaker located within the probe box. 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 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.
Admittance 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.
Two Component Tympanometry
Two approaches have been used to describe middle ear function - impedance and admittance. The components of impedance are resistance and reactance. Acoustic resistance refers to the amount of sound energy dissipated within the middle ear system due to friction and cochlear consumption. The viscoelastic properties of tendons and ligaments, the viscosity of perilymph and the mucosa, and the narrow air passages of the middle ear contribute to resistance. Reactance refers to the amount of energy stored and then reflected from the middle ear system. The reactive component is composed of mass and stiffness. The ossicles and perilymph in the cochlea contribute as mass components of reactance. The ligaments, tendons, tympanic membrane, and enclosed air contribute to the stiffness of the system.
It is not possible to measure impedance directly because it is an intangible quantity that must be arrived at inferentially. Therefore, Admittance “Y,” the reciprocal of impedance, is generally measured instead. The two components of Admittance are Conductance “G” which is the reciprocal of resistance, and Susceptance “B” which is the reciprocal of reactance.
The response of the middle ear system to an acoustic stimulus such as the probe tone may be measured as a response of the system as a whole, or measured as the response of each component which contributes to the total response. A measurement of the total response provides an “overview” of how the middle ear system responds to the probe tone without providing any information about the status of each component. If the tympanic membrane is normal, a measurement of the total response at 226 Hz provides sufficient data to assess middle ear function. However, if a tympanic membrane abnormality exists, it camouflages or masks the true status of the middle ear (i.e. an otosclerotic condition may be masked by a hyperflaccid eardrum). In this case, a measurement of each component will yield more detail regarding the total state of the middle ear system. This is particularly true with probe tone frequencies of 678 Hz or above.
With a probe tone frequency of 226 Hz, the normal middle ear system acts as a stiffness controlled system. The resistive (conductance) component contributes very little to the total system response. Thus, a measurement of Susceptance (primarily compliance in this case) will be very close to the total admittance measurement at 226 Hz. (“Y” approximately equals “B”). Since the compliance of the middle ear can be calibrated with respect to the compliance within an equivalent volume of air in ml or cc, it is also possible to measure admittance at 226 Hz in ml or cc. However, this is only true at 226 Hz. When higher probe tone frequencies are used, the middle ear no longer acts like a stiffness controlled system. Mass components become more significant. Notching will appear in the tympanogram as resonance is approached providing valuable diagnostic information. At the resonance frequency of the middle ear system stiffness and mass are equivalent. At frequencies above resonance the middle ear system acts as a mass controlled system. Middle ear pathologies alter the resonant point of the system. A measurement of the contribution of each component at the higher probe tones provides valuable information about the status of the middle ear system. Admittance in such cases is measured in acoustic mmhos. (1 acoustic mmho = 10-8 m3/Pa.s)