Tympanometry

It is based on a simple principle,
i.e. when a sound strikes tympanic membrane, some of
the sound energy is absorbed while the rest is reflected.
A stiffer tympanic membrane would reflect more of
sound energy than a compliant one. By changing the
pressures in a sealed external auditory canal and then
measuring the reflected sound energy, it is possible to
find the compliance or stiffness of the tympano-ossicular
system and thus find the healthy or diseased status of
the middle ear.
Essentially, the equipment consists of a probe which
snugly fits into the external auditory canal and. has three
channels; (i) to deliver a tone of 220 Hz, (ii) to pick up
the reflected sound through a microphone and (iii) to
bring about changes in air pressure in the ear canal
from positive to normal and then negative. By
charting the compliance of tympano-ossicular system
against various pressure changes, different types of graphs
called tympanograms are obtained which are diagnostic of
certain middle ear pathologies.
Types of tympanograms:
Type A Normal tympanogram.
Type As Compliance is lower at or near ambient air
pressure. Seen in fixation of ossicles, e.g. otosclerosis
or malleus fixation.
Type AD High compliance at or near ambient pressure.
Seen in ossicular discontinuity or thin and lax
tympanic membrane.
Type B A flat or dome-shaped graph. No change in compliance
with pressure changes. Seen in middle
ear fluid or thick tympanic membrane.
Type C Maximum compliance occurs with nega tive
pressure in excess of 100 mm of H20. Seen in
retracted tympanic membrane and may show
some fluid in middle ear.
Testing function of eustachian tube. Tympanometry has
also been used to find function of eustachian tube in cases
of intact or perforated tympanic membrane . A negative
or a positive pressure (- 200 or + 200 mm of H20) is created
in the middle ear and the person is asked to swallow
5 times in 20 seconds. The ability to equilibrate the pressure
indicates normal tubal function. The test can also be
used to find the patency of the grommet placed in the
tympanic membrane in cases of serous otitis media.