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.

Pure Tone Audiometry

An audiometer is an electronic device which produces
pure tones, the intensity of which can be increased or
decreased in 5 dB steps (Fig. 4.2). Usually air conduction
thresholds are measured for tones of 125,250,500,1000,
2000 and 4000 and 8000 Hz and bone conduction
thresholds for 250, 500, 1000 and 2000 and 4000 Hz.
The amount of intensity that has to be raised above the
normal level is a measure of the degree of hearing impairment
at that frequency. It is charted in the form of a
graph called audiogram. The threshold of bone conduction
is a measure of cochlear function. The difference in
the thresholds of air and bone conduction (A-B gap) is
a measure of the degree of conductive deafness. It may be
noted that audiometer is so calibrated that the hearing of
a normal person, both for air and bone conduction, is at
zero dB and there is no A-B gap, while turning fork tests
normally show AC > BC
When difference between the two ears is 40 dB or
above in air conduction thresholds, the better ear is
masked to avoid getting a shadow curve from the nontest
better ear. Similarly, masking is essential in all bone
conduction studies. Masking is done by employing
narrow-band noise to the non-test ear.

Uses of pure tone audiogram

(i) It is a measure of threshold of hearing by air and
bone conduction and thus the degree and type of
hearing loss.
(it) A record can be kept for future reference.
(iii) Audiogram is essential for prescription of hearing
aid.
(iv) Helps to find degree of handicap for medicolegal
purposes.
(v) Helps to predict speech reception threshold.

Recruitment

Recruitment is a phenomenon of abnormal growth of loudness.

The ear which does not hear low intensity sound begins to
hear greater intensity sounds as loud or even louder than
normal hearing ear. Thus, a loud sound which is tolerable
in normal ear may grow to abnormal levels of loudness in
the recruiting ear and thus becomes intolerable. The
patients with recruitment are poor candidates for hearing
aids. Recruitment is typically seen in lesions of the
cochlea (e.g. Meniere's disease, presbycusis) and thus
helps to differentiate a cochlear from a retrocochlear
sensorineural hearing loss.
Alternate binaural loudness balance test is used to detect
recruitment in unilateral cases. A tone, say of 1000 Hz, is
played alternately to the normal and the affected ear and
the intensity in the affected ear is adjusted to match the
loudness in normal ear. The test is started at 20 dB above
the threshold of deaf ear and then repeated at every
20 dB rise until the loudness is matched or the limits of
audiometer reached. In conductive and neural deafness,
the initial difference is maintained throughout while in
cochlear lesions, partial, complete or over-recruitment
may be seen

Tuning Fork Tests: Rinne, Weber, ABC, Schwabach’s Test

The clinically useful tuning fork tests include:
(a) Rinne test. In this test air conduction of the ear
is compared with its bone conduction. A vibrating tuning
fork is placed on the patient's mastoid and when he
stops hearing, it is brought beside the meatus. If he still
hears, AC is more than Be. Alternatively, the patient is
asked co compare the loudness of sound heard through
air and bone conduction. Rinne test is called positive
when AC is longer or louder than Be. It is seen in
normal persons or those having sensorineural deafness. A
negative Rinne (BC > AC) is seen in conductive deafness.
A negative Rinne indicates a minimum air-bone
gap of 15-20 dB.
A prediction of air-bone gap can be made if tuning
forks of 256, 512 and 1024 Hz are used.
• A Rinne test equal or negative for 256 Hz but positive
for 512 Hz indicates air-bone gap of 20-30 dB.
• A Rinne test negative for 256 and 512 Hz but
positive for 1024 Hz indicates air-bone gap of
30-45 dB.
• A Rinne negative for all the three tuning forks of
256, 512 and 1024 Hz, indicates air-bone gap of
45-60 dB .
Remember that a negative Rinne for 256, 512 and
1024 Hz indicates a minimum AB gap of 15, 30, 45 dB
respecti vel y.
False negative Rinne. It is seen in severe unilateral sensorineural
hearing loss. Patient does not perceive any sound
of tuning fork by air conduction but responds to bone
conduction testing. This response to bone conduction is,
in reality, from the opposite ear because of transcranial
transmission of sound. In such cases, correct diagnosis can
be made by masking the non-test ear with Barany's noise
box while testing for bone conduction. Weber test will
further help as it gets lateralised to the better ear.
(b) Weber test. In this test, a Vibrating tuning fork
is placed in the middle of the forehead or the vertex and
the patient is asked in which ear the sound is heard.
Normally, it is heard equally in both ears. It is lateralised
to the worse ear in conducti ve deafness and to the better
ear in sensorineural deafness. In weber test, sound travels
directly to the cochlea via bone. Lateralisation of sound
in weber test with a tuning fork of 512 Hz implies a conductive
loss of 15-25 dB in ipsilateral ear or a sensorineural
loss in the contralateral ear.
(c) Absolute bone conduction (ABC) test. Bone
conduction is a measure of cochlear function. In ABC
test, patient's bone conduction is compared with that of
the examiner (presuming that the examiner has normal
hearing). External auditory meatus of both the patient
and examiner should be occluded (by pressing the tragus
inwards), to prevent ambient noise entering through AC
route. In conductive deafness, the patient and the examiner
hear the fork for the same duration of time. In sensorineural
deafness, the patient hears the fork for a
shorter duration.
(d) Schwabach's test. Here again BC of patient is
compared with that of the normal hearing person (examiner)
but meatus is not occluded. It has the same significance
as absolute bone conduction test. Schwabach is
reduced in sensorineural deafness and lengthened in
conductive deafness.