FUNCTIONAL EXAMINATION OF THE EAR
In bilateral deafness it is customary to use a battery of tests, e.g. (a) Bekesy
self-recording audiornetry, (b) SISI (short increment sensitivity index) loud-
ness discomfort level, (c) tone decay level, and (d) acoustic impedance
a. Bekesy Audiometry. In this type of audiometry the frequency of the tests
signal is continuously altered from low to high frequency. The patient
controls the intensity of the signal so that it fluctuates about the threshold
and the level is plotted by a pen. This not only gives a record of the threshold
Fig. 149. Diagram shoving loudness balance test. (.After Dix, Hallpike and Hood.)
but it has been shown that in cochlear lesions the excursions of the pen
around threshold become much smaller than when the hearing is normal. In
lesions of the auditory nerve the plotted threshold may be lower than that
demonstrated by normal pure tone audiometry due to abnormal adaptation.
b. SISL In this test the patient's ability to distinguish small fluctuations in
intensity of a tone is measured. In cochlear deafness smaller increments than
normal may be distinguished.
c. Tone Decay Level. In cochlear deafness loud sounds may cause discomfort
at a lower level above threshold than in normally hearing subjects and a
quiet continuous tone may cease to be heard after an abnormally short
duration of exposure.
d. Acoustic Impedance Measurements. A tone played in to the ear will be
partly absorbed by the sound-conducting apparatus and partly reflected
outwards from the surface of the drum. The maximum absorption will
occur when the contour of the drum is normal with equal pressure on either
side. This absorption or impedance of the drum can be measured. By altering
the pressure in the external meatus until maximum absorption by the drum is
demonstrated, the pressure in the middle ear can be measured, e.g. Eustachian
obstruction where the middle ear pressure is reduced. Characteristic patterns
can be shown in secretory otitis media, adhesions in the middle ear, ossicular
disruption and otosclerosis. Alterations of the contour of the drum can also
be demonstrated when the stapedius and tensor tympani muscles contract.
The stapedius muscle can be made to contract by a loud sound and the
intensity of sound required to induce contraction can be used as a measure
of recruitment and it also provides a rough objective measurement of the
threshold of hearing. Demonstration of the stapedius reflex can be used to
show the level of a facial paralysis. Contraction of the tensor tympani muscle
is produced by stimulation of the sensory part of the trigeminal nerve and
the usual stimulus is cold air applied to the cornea.