Tympanometry is a type of hearing test that is used to establish the cause of conductive deafness. During the test, a probe that contains a tone generator, a microphone, and an air pump is introduced into the outer-ear canal. The air pressure in the ear is varied and tones are played into it. The tone pattern reflected from the eardrum and received by the microphone shows if the eardrum is moving normally. Tympanometry is a particularly useful technique for use in children because it does not rely on a response from the person being tested.
Tympanometry - technical
Synonyms and related terms are: Acoustic admittance; Acoustic immittance; Acoustic impedance; Admittance; Immittance; Impedance; Tympanogram
Tympanometry is a clinical test measuring tympanic membrane (TM) mobility and is graphically displayed as a tympanogram. Tympanometry provides information on acoustic admittance, which is a measure of the ease with which energy flows through the middle ear system, and impedance, which is the opposition to this flow (Hall and Mueller 1997). The tympanogram is a measure of middle ear function and can help differentiate many pathologic conditions. The measurements of acoustic admittance at various pressure points are graphed to form a tympanogram (Katz 2002).
Normal hearing depends on the function of the outer, middle, and inner ear. Tympanometry tests the function of the outer and middle ear only. The normal middle ear system consists of the TM, three ossicles (Malleus, Incus, Stapes), and an air-filled cavity. The TM “catches” acoustic energy and transfers that energy to the ossicles, which in turn transmits it into the inner ear. The transfer of vibrations from an external source to the inner ear is dependent upon the connection of the TM to the ossicles and their mobility. When the air pressure on both sides of the TM is equal the system functions optimally.
The ▶hearing evaluation typically includes ▶audiometry, acoustic reflex testing, ▶otoacoustic emission testing (OAE), and tympanometry and together with history and otoscopy provides the most comprehensive clinical evaluation of disorders of the ear and hearing. Tympanometry is the part of the hearing test which measures the mobility or compliance of the TM. A normal eardrum moves easily when subject to pressure changes and as such is very compliant. A pathologic TM typically has an abnormal compliance and moves less efficiently. Although tympanometry does not directly provide a measure of hearing level it indicates whether middle ear function is normal or abnormal. Various patterns of compliance suggest different pathologies that may affect the TM, outer ear, or middle ear and provide information about the cause of hearing loss (Table 1).
Principle of Tympanometry
A tympanometer is a piece of special equipment required to conduct tympanometry. The machine includes a pressure pump, manometer, and tone generator. It also includes a graphic display, often a small printer, and flexible extension which houses the ear probe. The test is conducted by sealing the ear canal with a probe that contains three parts. The first component contains an air pump and manometer to modify and record the pressure, the second component has a tone probe to deliver a low-frequency sound, and the third portion contains a microphone to measure the sound pressure reflecting off the TM. The air pump and manometer are coupled to the probe by a tube to pressurize the ear canal. These pressure variations change the stiffness of the TM. During these pressure variations the miniature loudspeaker delivers a tone to the ear while the microphone picks up the acoustic signal and converts it to an electrical signal that is used to measure the acoustic immittance. The pressure at which the most sound is reflected indicates the greatest compliance. The normal TM is most compliant and vibrates best when the pressure on both sides is the same (Musiek and Rintelmann 1999).
The procedure is begun by examining the ear for any abnormalities of the outer ear, TM, or middle ear. After the patient is instructed on the procedure and a clean and appropriate sized probe tip is placed, the probe tip is inserted into the external ear canal so it fills the meatus and has an airtight seal. Once the test has begun, the patient will hear a pure tone, called a probe tone stimulus, and experience air pressure changes in their ear. The test results are displayed on a tympanogram which reflects the TM’s response to the probe tone with changes in air pressure. The entire test requires less than 1 min.
The most commonly used probe tone uses a low frequency of 226 Hz (Katz 2002). When using a low-frequency probe tone such as 226 Hz, the volume of the air medial to the probe tip and stiffness of the middle ear system is assessed. The most common pathology affecting stiffness is ▶middle ear effusion. Tympanometry using a low-frequency probe tone is the procedure of choice in screening children for ▶middle ear effusion. A variation using high-frequency probe tones (i.e., 800 Hz) can provide information regarding the mass characteristics of the middle ear, particularly the eardrum and ossicles. Hypermobile eardrums, atrophic scarring, and ossicular discontinuity are several middle ear conditions that can display distinct characteristics when using a high-frequency probe tone (Katz 2001). Although high-frequency tympanometry is beneficial in identifying mass-related middle ear pathology, this procedure is not often used because the measurements are complex and not as easily understood as 226 Hz tympanometry. High-frequency tympanometry using 1,000 Hz probe tone is the test of choice for infants (from birth to 3 months of age). This frequency is more accurate and reduces variability. Although the exact reason for the variable tympanometric patterns in newborns is unclear, it is evident that low-frequency tympanometry is not effective for detecting middle ear pathology in neonates.
After the tympanogram is complete the results must be interpreted as normal or abnormal. If findings are abnormal, bone conduction ▶audiometry should be completed for additional information regarding possible middle ear pathologies that could be associated with the tympanogram type. A referral to an Ear, Nose, and Throat (ENT) physician may be warranted pending results of the tympanogram (Musiek and Rintelmann 1999).
Tympanograms are classified by the height and location of the tympanometric peak and generally fall into one of three patterns. A type A tympanogram indicates normal middle ear and Eustachian tube (ET) function. A Type A tympanogram has a normal peak height and location on the pressure axis, consistent with normal eardrum mobility and mobility of the ossicles. Two subtypes exist; type Ad which consists of a Type A tympanogram with a high peak (ossicular disarticulation or a flaccid TM) and type As which has a low peak (ossicular fixation). A type B tympanogram indicates poorTM mobility without a compliance peak and is considered a “flat” tympanogram. A flat type B tympanogram may indicate several clinical conditions including ▶middle ear effusion, TM perforation, patent ▶tympanostomy tube, or cerumen impaction (Katz 2002). If a type B tympanogram is associated with a normal ear canal volume it usually indicates middle ear fluid or a non-patent▶tympanostomy tube. If a type B tympanogram is associated with a large ear canal volume it is generally associated with a TM perforation or a patent ▶tympanostomy tube. A Type C tympanogram peak is shifted toward the negative pressure region and occurs in the presence of negative middle ear pressure, which can be a result of ▶ET dysfunction, ▶middle ear effusion, or a ▶cholesteatoma (Fig. 1).
Tympanometric shapes can be further distinguished by analyzing the quantitative value of the tympanogram’s static acoustic admittance, width/gradient, peak pressure, and volume. Static acoustic admittance describes the height of the tympanogram measured at the plane of the TM (Katz 2002). The primary use for this measurement is to identify pathologic conditions since certain pathologies can increase or decrease the height of the tympanogram. For example, conditions that stiffen the middle ear system reduce the height of the tympanogram (i.e., fluid or granuloma tissue in the middle ear). Pathologies that add mass (or reduce stiffness) to the middle ear system, such as ossicular discontinuity or atrophic scars on the TM, increase the height of the tympanogram. Although acoustic admittance is useful in identifying certain disease processes, multiple middle ear pathologies may be difficult to rule out solely based on admittance of the tympanogram. For example, any highadmittance pathology at the eardrum (scarring of the eardrum) could mask low-admittance pathology of the ossicular chain (▶otosclerosis) or other portions of the middle ear.
Tympanometric width (TW) or gradient is the steepness of the slope near the peak of the tympanogram (Musiek and Rintelmann 1999). The sharpness of the tympanometric peak may also be used to identify middle ear pathologies, which can either widen or narrow the TW. An abnormally narrow width may be associated with atrophic scarring of the eardrum, ▶tympanosclerosis, and ossicular/stapes fixation (Ivey 1975; Katz 2002). A wide TW may be indicative of ▶middle ear effusion.
Tympanometric peak pressure (TPP) is the ear canal pressure where the peak of the tympanogram occurs and the point at which the pressure in the ear canal equals the middle ear pressure (Katz 2002). TPP is commonly used to evaluate middle ear pressure. ▶ET dysfunction (blocked vs. patulous ET) can result in significant negative or positive middle ear pressure due to the effects of the status of the ET on the diffusion of gases in the middle ear. Sniffing in children can produce significant negative middle ear pressure due to the evacuation of air through the ET from the negative pressure generated in the nasopharynx (Musiek and Rintelmann 1999). Positive middle ear pressure has been seen in patients with ▶acute otitis media (Ostergard and Carter 1981; Margolis and Nelson 1992).
Tympanometry using a 226 Hz probe tone is useful in estimating the volume of air in front of the probe (Shanks et al. 1992). A flat tympanogram with large ear canal volume is consistent with an opening in the TM such as a perforated eardrum or patent ▶tympanostomy tube. Normal ear canal volume is typically consistent with an intact eardrum. However, a perforation cannot be ruled out with normal ear canal volume because ears with past or present middle ear disease may have smaller middle ear/mastoid volumes than normal ears (Musiek and Rintelmann 1999).
Reasons for a normal or reduced volume in the presence of a TM perforation and active disease include displacement of air volume from fluid, inflammation, granulation, ▶cholesteatoma, and fibrosis in the middle ear. Active diseases may also obstruct the mastoid air cell system, reducing the overall volume. Therefore, normal ear canal volume in the presence of a flat tympanogram is indicative of an intact eardrum but could result from a non-patent ▶tympanostomy tube or a perforation with active middle ear disease. A significantly large ear canal volume is consistent with a perforation with aerated middle ear/mastoid space (Shanks et al. 1992).
The quantitative components associated with the analysis of a 226 Hz tympanogram, including static acoustic admittance, TW (gradient), TPP, and ear canal volume, assist in establishing appropriate norms for analysis and improved consistency of tympanometric data across clinics (Musiek and Rintelmann 1999). Gender and age play a role in the variability of these values and must be considered clinically (Table 2).
Tympanometry is indicated in the assessment of suspected or known disorders of the TM, middle ear system, and hearing. It verifies middle ear pathologies and confirms conductive components found on audiograms when performed in conjunction with case history, otoscopy, and audiometric evaluations.
Contraindications to tympanometry include: a foreign body in the external ear canal, discomfort of the ear (i.e., otalgia due to severe ▶otitis externa or herpes zoster oticus), draining ear, and a recent eardrum ormiddle ear surgery (i.e., myringoplasty, tympanoplasty, and ▶stapedectomy). The volume on tympanograms may be appropriately abnormal on surgically modified external or middle ears (i.e., large ear canal volume with a ▶canal wall downmastoidectomy). Extra care should be taken when a vestibular dysfunction from trauma (perilymphatic fistula) is suspected due to the vertigo/ nausea that occurs with air pressure changes.
Advantages of Tympanometry
Tympanometry is a quick and noninvasive test which can be easily performed on patients of any age, from infants to adults. Testing time is typically less than a minute. Tympanometry is an objective measure which does not require a behavioral response from the patient. With these objective measures, tympanograms provide a quick and effective way to monitor eardrum mobility, chronic ▶middle ear effusion, ET function, ▶tympanostomy tube function, and confirm TM perforations. Many tympanometers have the capability to print the test results for proper documentation and reporting. Tympanometry has current procedureal codes (CPT) and is reimbursable (Hall and Mueller 1997).
Disadvantages of Tympanometry
A disadvantage of tympanometry is the variability in results when using a low-frequency probe tone versus a high-frequency probe tone. For example, high-frequency probe tone tympanograms recorded on ears with ossicular discontinuity or post-▶stapedectomy produced broad undulating peaks, whereas the 226 Hz tympanograms recorded on the same ears recorded single- peaked Type A patterns (Katz 2002). Since 226 Hz tympanometry is most commonly used in clinic, massrelated middle ear pathologies may not be clearly identified on tympanograms. Inexperienced operators may also misinterpret tympanometric data and overlook pertinent information obtained on the graph.
Tympanograms are typically not sufficient in identifying middle ear pathologies in isolation. Since its sole purpose is to measure the mobility of the eardrum and ossicular chain, tympanometry needs to be completed with other procedures including case history, otoscopic examination, and audiometric exam for a more comprehensive view of the system to better identify middle ear pathologies. Tympanometry may also be difficult to assess in children who are not able to tolerate the probe tip in their ears. This is pertinent when the child has a history of middle ear pathology and needs to have their middle ear assessed.
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