Tympanometry is an objective, non-invasive test used to assess the functional status of the middle ear system. This quick procedure provides information about the mobility of the eardrum and the condition of the tiny bones that transmit sound vibrations. Unlike a standard hearing test, tympanometry does not measure hearing sensitivity but evaluates how effectively sound energy travels through the middle ear space. The test is a routine part of a comprehensive audiological evaluation for patients of all ages.
Mechanical Function of the Test
The tympanometry procedure relies on a specialized instrument called a tympanometer, which uses a small, soft probe tip inserted into the ear canal. This probe is designed to create an airtight seal, which is necessary to control and measure the pressure environment accurately. The probe contains three functional components: a miniature speaker, a microphone, and an air pump. The speaker emits a continuous, low-frequency tone, typically 226 Hertz, toward the eardrum.
The air pump systematically changes the air pressure within the sealed ear canal, sweeping from positive to negative pressure. As the pressure changes, the eardrum’s ability to move, or its acoustic compliance, is altered. The microphone simultaneously measures the sound energy reflected back from the eardrum. When the eardrum is stiff, more sound is reflected; when compliant, more sound is absorbed and transmitted to the middle ear.
The core principle is that the eardrum vibrates with the greatest compliance when the air pressure in the ear canal equals the pressure in the middle ear cavity. By recording the eardrum’s movement across a range of pressures, the tympanometer pinpoints the pressure level where maximum mobility occurs. This data gathered during the pressure sweep is then plotted on a graph called a tympanogram.
Conditions Identified by Tympanometry
Tympanometry results offer insight into several conditions affecting the middle ear’s ability to transfer sound. One common issue identified is Otitis Media with Effusion, or “glue ear,” where fluid collects behind the eardrum. The presence of fluid severely restricts the eardrum’s movement, resulting in an abnormal measurement.
The test assesses the function of the Eustachian tube, which connects the middle ear to the back of the throat to equalize pressure. Dysfunction of this tube can lead to a buildup of negative pressure in the middle ear space. Tympanometry can suggest the presence of a hole or perforation in the tympanic membrane.
The test can help differentiate other problems, including scarring on the eardrum from past infections or issues with the tiny bones in the middle ear not moving correctly. It is frequently used to determine the source of conductive hearing difficulties.
Interpreting the Tympanogram Results
The tympanogram is the graphical output of the test, plotting air pressure (daPa) on the horizontal x-axis against acoustic compliance (mL) on the vertical y-axis. The shape and peak location of this curve are categorized into distinct types, each correlating to a specific middle ear status.
Type A Tympanogram
A Type A tympanogram represents a normal, healthy middle ear system. This curve features a sharp, distinct peak close to 0 daPa, indicating that the pressure in the middle ear equals the outside atmospheric pressure. This confirms the eardrum is moving freely and the middle ear is functioning correctly.
Type C Tympanogram
A Type C tympanogram is characterized by a peak shifted significantly into the negative pressure region, typically below -100 daPa. This negative pressure indicates Eustachian tube dysfunction, suggesting the tube is not opening efficiently to ventilate the middle ear. This result is often seen during the initial or resolving stages of a cold or ear infection.
Type B Tympanogram
The Type B tympanogram presents as a flat or rounded line with no discernible peak, showing very little eardrum movement regardless of the pressure applied. This shape is highly suggestive of fluid filling the middle ear cavity, which immobilizes the eardrum. A flat line can also occur if there is a hole in the eardrum or a patent ear ventilation tube.
To distinguish between fluid and a perforation, the tympanometer measures the equivalent ear canal volume. If the volume is normal alongside the flat Type B tracing, it confirms fluid in the middle ear. If the volume is abnormally large, it suggests the probe is measuring the volume of both the ear canal and the middle ear space, which only happens if the eardrum is not intact.