Tympanometry is a quick, objective diagnostic test used to assess the function of the middle ear and the mobility of the eardrum (tympanic membrane). Unlike a standard hearing test, this procedure does not measure hearing ability but provides quantitative information about the physical status of the sound-conducting pathway. This information helps healthcare professionals determine if hearing loss is caused by a middle ear problem. This article explains the mechanics of the measurement, the conditions it identifies, the patient experience, and how the resulting graph is interpreted.
The Mechanics of Middle Ear Measurement
Tympanometry measures the admittance or compliance of the eardrum, which reflects how freely the eardrum and the tiny bones of the middle ear move. The test uses a specialized device called a tympanometer, which has a probe with three channels. One channel emits a low-frequency sound tone, typically 226 Hertz, directed at the eardrum.
A second channel acts as an air pump, systematically changing the pressure within the sealed ear canal from positive pressure, through zero, to negative pressure. The third channel is a microphone that measures the sound energy reflected back from the eardrum. When the pressure in the ear canal matches the pressure in the middle ear, the eardrum is most flexible, absorbing the most sound energy and reflecting the least amount back.
The device plots the eardrum’s mobility against the air pressure changes, creating a graph known as a tympanogram. This graph reveals two primary metrics: eardrum compliance and middle ear cavity pressure. Sound transmission is most efficient when pressure is equalized on both sides of the eardrum. By sweeping the pressure, the tympanometer finds the point of maximum mobility, which indicates the middle ear pressure.
Specific Conditions Tympanometry Helps Identify
The primary use of tympanometry is to detect fluid buildup, known as otitis media with effusion, behind the eardrum. Fluid restricts the eardrum’s ability to move, which the test registers as a flat or non-mobile reading. This condition is common in children and frequently causes temporary hearing loss.
The test also assesses the function of the Eustachian tube, which connects the middle ear to the back of the nose and throat to equalize pressure. If the tube is not ventilating properly, a vacuum forms in the middle ear, pulling the eardrum inward. Tympanometry identifies this by showing the eardrum is maximally compliant only at a significantly negative pressure.
The procedure can also identify a perforation, or tear, in the eardrum, often due to infection or trauma. If perforated, the probe cannot create a sealed cavity, and the measured ear canal volume is abnormally large because the measurement includes the middle ear space. The test can also provide clues about the integrity of the ossicular chain (the three tiny bones that transmit sound vibrations). For example, an unusually high compliance reading may suggest disconnected ossicles or an abnormally flaccid eardrum.
Tympanometry objectively measures the ear canal volume before pressure changes, which helps identify blockages. An extremely small canal volume often indicates earwax completely blocking the probe. Conversely, a very large volume without a peak may point to a patent pressure equalization tube or an eardrum perforation. The results, combined with a physical examination, guide appropriate treatment.
What to Expect During the Examination
The tympanometry examination is non-invasive, painless, and typically takes only a few seconds per ear. The audiologist first visually inspects the ear canal to ensure it is clear of foreign objects or excessive earwax that could interfere with the seal. A small, soft-tipped probe is then gently placed into the ear canal, similar to an earplug, until an airtight seal is achieved.
Once sealed, the patient hears a low-pitched tone and feels a momentary change in air pressure, similar to pressure changes during an airplane’s ascent or descent. The patient must remain still, avoid talking, and refrain from swallowing during the brief test, as these actions can disrupt the pressure measurement. The process is automated, and the resulting data is instantaneously plotted onto a graph, providing immediate diagnostic information.
Decoding the Tympanogram Results
The tympanogram plots eardrum compliance (vertical axis) against air pressure in the ear canal, measured in decapascals (daPa) (horizontal axis). Professionals classify the results into distinct types corresponding to specific middle ear conditions by analyzing the shape and location of the peak. The most common classification system involves three main types: A, B, and C.
A Type A tympanogram is considered normal, characterized by a single, sharp peak near the zero daPa mark. This peak indicates that middle ear pressure equals atmospheric pressure, and the eardrum and middle ear system are moving freely to transmit sound. Variations exist, such as Type As (shallow peak) suggesting reduced eardrum mobility, or Type Ad (deep, high peak) suggesting a hypermobile eardrum.
A Type B tympanogram is a flat line with no discernible peak, indicating the eardrum is barely moving regardless of the applied pressure. This lack of movement is most often a sign of fluid filling the middle ear space, which impedes vibration. A flat line can also be caused by complete ear canal obstruction (earwax) or a perforated eardrum, which is differentiated by examining the measured ear canal volume.
The Type C tympanogram features a peak shifted significantly to the negative side of the graph, typically below -100 daPa. This negative pressure peak signifies the eardrum is pulled inward due to a vacuum in the middle ear, indicating Eustachian tube dysfunction. This finding is common during the early or resolving stages of a cold or ear infection when the middle ear struggles to equalize internal pressure.