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 cavities.
The following tests have been included under the battery of impedence audiometry:
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.
Figure showing 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 otoscleorsis.
Figure showing Type As Jerger 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.
Figure showing Jerger Type Ad curve
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 placed.
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 system
Figure showing Type B Jerger 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 -150 dapa.
Figure showing Type c Jerger 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 canal.
Testing for the presence of absence of perilymph fistula:
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 acuoustic reflexes.