Balasubramanian M.S. D.L.O.
Synonyms: Tympanometry, acoustic immitance test.
The primary purpose of impedence audiometry is to determine the
status of the tympanic membrane and the middle ear. It is also
otherwise known as Tympanometry or acoustic immitance test. The
secondary purpose of this investigation is to evaluate the acoustic
reflex pathway which include the 7th and 8th cranial nerves and the
brain stem. This test should not be used to assess the sensitivity
of hearing and the results of this test should always be viewed in
conjunction with the results of pure tone audiogram.
Impedence audiometry is a measurement of energy or air pressure
which involves the external auditory canal, the ear drum, ossicluar
chain, stapedius muscle, cochlea, 7th cranial nerve, 8th cranial
nerve and the brain stem. This test is affected by the mass,
mobility and resistance systems of the external and middle ear
The following tests have been included under the battery of
2. Eustachean tube function
3. Tests to identify perilymph fistula
4. Acoustic reflex threshold
5. Acoustic reflex decay
These tests can be used to identify the following pathologies
involving the peripheral and central portions of hearing.
a. Middle ear effusion
b. Ear drum perforations including patency of eustachean tube
d. Hypermobile ear drum
e. Eustachean tube dysfunction
f. Glue ear
h. Ossicular discontinuity
g. Acoustic neuroma
h. Facial nerve function
i. Hearing loss
j. Brain stem disorders
Tympanometry: Measures the sound reflected from the ear drum
while the pressure of the external canal is varied by the operator.
It aids in the assessment of outer ear, middle ear and the
eustachean tube. This test should not be performed in infants below
the age of 7 months because the suppleness of the cartilage of the
external canal may produce misleading results.
Procedure: First the probe is inserted into the external auditory
canal till a airtight seal is obtained. Probe tone is presented
typically at 226Hz into the ear canal while the air pressure of the
canal is altered between +200 and - 400 decapascals. The maximum
compliance occurs when the pressure of the external auditory canal
and the middle ear becomes equal. Only at this pressure maximal
acoustic transmission occur through the middle ear. The compliance
peak therefore indicates therefore indicates the pressure of the
middle ear implying efficacy of the eustachean tube function. The
height of the compliance peak indicates the modbility / stiffness of
the tympanic membrane or the middle ear cavity.
The term static compliance indicates the height of the
tympanogram at its peak, and it is the measurement of the moblility
of the whole system.
Classification of tympanograms:
The classification system introduced by Jerger is commonly used
to classifiy various types of tympanograms. Other systems have been
proposed, but none of them are in common use.
Type A curve: Suggests normal middle ear function. The compliance
peak occur between -150 - +100 dapa. The value of compliance ranging
between 0.2 - 2.5 millimhos. This type of curve is also known to
occur in early stages of otosclerosis
showing Jerger Type A impedence curve.
Type As curve: is a shallow curve suggesting a
stiffened middle ear system. Compliance peak occurs at -150 - + 100
dapa. The compliance value is less than 0.2 mmhos. This curve is
commonly found in patients with glue ear, stiffened ear drum, or
showing Jerger Type As curve
Type Ad curve: is a deep curve suggests a
flaccid ear drum or middle ear system, ossicular disruption. Usually
ossicular disruption gives a compliance higher than the recording
parameters (infact the recording goes off chart). The compliance
peak occurs between -150 - + 100 dapa. The compliance value is more
than 2.5 mmhos.
showing Jerger Type Ad curve: Note the compliance value is so high
that the curve goes off the chart.
Type B: is a flat curve with no
compliance peak. This Type B curve must always be interpreted in
conjuction with the ear canal volume. Average ear canal volume in
children ranges between 0.42 - 0.97 ml, while in adults it ranges
between 0.63 - 1.46 ml.
Type B curve with normal ear canal
volume suggests otitis media.
Type B curve with small canal volume
suggests that the ear canal could be occluded by the presence of
wax, or the proble of the impedence audiometer has not been properly
Type B curve with large canal volume
suggests that there could be perfortaion of the ear drum. This curve
is caused due to a patent pressure equalisation
showing Jerger Type B curve
Type C curve: This curve suggests a significant
negative pressure in the middle ear, or eustachean tube dysfunction.
Compliance is recordable but the peak compliance occur at less than
showing Jerger Type C curve.
Assessing eustachean tube functioning
by impedence audiometry:
The function of the eustachean tube
can easily be assessed by reading the tympanograms. Type A
tympanograms reflect a normal middle ear function which is only
possible in the presence of a normally functioning eustachean tube.
Similarly Type C tympanograms indicate significant negative pressure
in the middle ear implying that the eustachean tube is blocked. If
there is tympanic membrane perforation a Type B curve will be
produced. In this situation the eustachean tube function cannot be
assessed using a tympanogram. However an indirect assessment of the
pressure equalisation function of the eustachean tube can be made by
increasing the probe pressure in the external ear canal, asking the
patient to swallow then assessing whether the eustachean tube is
able to clear the increased pressure applied to the external ear
Testing for the presence of absence of
This can be indirectly assessed by the
presence of intense giddiness along with nystagmus when the external
canal pressure in increased by increasing the probe pressure. This
sign is also known as the Hennebert's sign. This sign is manifested
only in the presence of perilymph fistula.
Eliciting acoustic reflex thresholds:
This is a measure of the stapedial muscle reaction to exposure to
high intensity sounds. When the stapedius muscle contracts in
response to sound it stiffens the osscicles and the ear drum
altering the compliance values which can be measured using an
impedence audiometer. The recording is ideally made at a single
pressure setting i.e. the pressure which shows the maximum
compliance. The reflex on the opposite side also is tested since it
is a bilateral reflex. The sound frequencies used to test this
reflex are 500, 1000, 2000 and 4000Hz. For screening purposes it is
sufficient if 1000Hz is used.
The acoustic reflex cannot be recorded
in patients with a type B tympanogram. It also cannot be recorded in
patients with severe profound sensorineural hearing loss. The reflex
may be attenuated in the presence of conductive deafness. Using this
test it is possible to assess the whole of the acoustic reflex
pathway. If the pathway is affected at central level then
ipsilateral recordings will be normal with absent contralateral
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