The normal middle ear maximally transmits energy at a specific peak pressure and frequency. This equilibrium is disrupted when the components that make up the middle ear (ME) are changed. The ME can be described by the elements of mass, stiffness and friction elements. Changing any of these elements will cause the resonance of the ME system to change in a predictable fashion. Admittance in simple terms is the ability of a system to transmit energy (inverse of impedence). Susceptance (B) is a measure of how easily energy flows through a system based on the elements of stiffness and mass. Conductance (G, inverse of friction) is the ability of the system to transmit energy without loss, this value changes with frequency. Multi-frequency Tympanometry measures middle ear functioning by quantifying the: Resonant frequency (RF) when total susceptance (B)is 0, Frequency corresponding to when susceptance equal to conductance which is B=G (F45), theVan Huyse pattern of B and G using 678 Hz pure tone, and typical tympanograms using higher frequency stimuli.
RF and F45 are measured using the MFT function of the Tympstar 33, using a sweep pressure and then a sweep frequency procedure. This is done to find first the middle ear’s peak pressure then use this pressure value to measure B and Gs. The B and G values are then plotted so that RF and F 45 can be determined. In general, pathologies that increase the overall stiffness of the middle ear (otosclerosis) will cause an increase in RF and F45 values to an abnormal level. Pathologies that increase the overall mass of a system (fluid) will cause these values to shift to a lower frequency value.
Rule 1: Frequency = SQRT (stiffness)
Rule 2: Frequency = SQRT (1/mass)
RF norms: Adults > 18, 90% range: 650-1400 Hz.
F45 norms: Adults > 18 90% range: 450-1000 Hz
The Van Huyse pattern is measured by selecting B and Gs and using a 678 Hz pure tone stimulus.
Abnormal patterns include:
- Greater than 5B3G pattern.
- G peaks that are wider than B peaks.
- G width greater than 100 daPa