41 Impedance Audiometry
41.1 Physics and Physiology
The middle ear and mastoid air cells communicate with the nasopharynx via the eustachian tube. As a closed system, air is being continually absorbed by the lining mucosa to create negative middle ear pressure. Swallowing allows the eustachian tube to open by the pulling action of tensor levi palatini and salpingopharyngeus. The levator levi palatini and tensor tympani complete the list of four muscles that open and close the eustachian tube. Air in the nasopharynx is at atmostpheric pressure and therefore on swallowing air flows up the eustachian tube from the nasopharynx to the middle ear and will equalise middle ear pressure. Sound transmission from the external to the inner ear is optimal when the compliance of the middle ear system is maximal, that is, when the pressure in the middle ear is equal to the pressure in the external auditory meatus.
Compliance (or admittance) is the measure of this system to allow the passage of sound energy through it, and is inversely related to impedance. Impedance is the resistance to the passage of sound energy. The mass, stiffness and frictional resistance of the medium through which the sound wave travels contribute to the impedance, which at low frequency is stiffness dominated. Strictly speaking, compliance is the reciprocal of stiffness, so impedance measurements at low frequency are usually referred to as the compliance.
41.2 Basic Principles
Impedance audiometry consists of three tests:
1. Tympanometry.
2. Acoustic reflex testing.
3. Static compliance (rarely used).
All three tests work on the same basic principle. The test probe consists of a sound producer, a sound receiver and a device for altering the air pressure within the external auditory meatus (EAM). The probe has a soft plastic or rubber tip to allow an airtight seal in the EAM. A test tone is emitted (226 Hz, 70 dB) into the EAM, of which some will be absorbed (admitted) by the middle ear system (drum and ossicles) and some reflected. The reflected sound energy is measured by the probe microphone. The compliance (the amount of sound absorbed by the middle ear system) can be determined either by measuring the reflected sound level in the ear canal or more commonly by measuring the amount of energy required to keep the sound level constant at varying ear canal pressures. The compliance will be maximal when the ear canal pressure is equal to middle ear pressure, when there is no pressure differential across the tympanic membrane. A tracing of the compliance as ear canal pressure alters allows this and other parameters to be determined. The tone frequency of 226 Hz is used as at this sound frequency the middle ear system is stiffness dominated, and compliance (measured in mmho) is directly proportional to the closed air volume and so can predict ear canal volume (1 mmho = ~ 1 mL). In very small infants (usually < 6 months old), the system is hypercompliant and so a 1-kHz tone is often used instead.
41.3 Tympanometry
This test is the most commonly used aspect of impedance audiometry and is particularly useful in evaluating children with otitis media with effusion. Here compliance is measured continuously while the pressure in the EAM is automatically varied from +200 to −400 daPa. This gives a graphical result which can be classified into one of three groups (
Fig. 41.1).
• Type A: Maximal compliance occurs when the pressure in the EAM is between +50 and −100 daPa. A normal maximal middle ear admittance or compliance value is between 0.3 and 1.6 mL. A low value for maximal compliance (type As) indicates stiffness of the middle ear system as in tympanosclerosis or otosclerosis. A high (type Ad) or unrecordable peak of compliance indicates excess mobility of the middle ear system as in ossicular discontinuity or atelectasis.
• Type B: A low-value flat or horizontal compliance trace occurs, implying persistently low compliance. This is usually taken to indicate fluid in the middle ear cavity, and in young children (< 7 years) with glue ear can be correlated with audiometric hearing loss. A type B tympanogram will also occur in the presence of a perforation in the tympanic membrane, but the ear canal volume will be large (> 6 mL) because it is measuring that of the middle ear cleft too. It can occasionally be useful in confirming this diagnosis or to test the patency of a ventilation tube.
