Tympanometry

Ear, Nose & Throat Journal, Nov, 2007 by Mohamed Hamid, Kenneth H. Brookler

A 50-year-old man presented with left ear hearing loss, fullness, and tinnitus. The patient's normal tympanogram (shape A, as shown in figure 1) and the presence of ipsiand contralateral reflexes ruled out otosclerosis in favor of inner ear conductive hearing loss. Computed tomography (CT) of the temporal bone (figure 2) showed dehiscence of the superior semicircular canal. Without tympanometry, the management of this case would have been different, and this patient might even have been misdiagnosed with otosclerosis and undergone inappropriate stapes surgery rather than having undergone CT in the plane of the superior semicircular canal.

Tympanometry involves measurement of the function of the middle ear. This mechanoacoustic system is composed of mechanical springs (tympanic membrane, ligament, tendons, and muscles), acoustic springs (middle ear air volume), mechanical mass (ossicles), and middle ear volume. Tympanometry measures the mechanoacoustic properties of the external-middle ear system in terms of acoustic immittance (expressed in mm [H.sub.2]O or equivalent volume), which is the vector sum of the resistive (lost energy) and reactive (transmitted energy) components. The reactance of the acoustic immittance is frequency-dependent, with more pronounced effects of mass and stiffness at high and low frequencies, respectively. The typical tympanogram instrument uses a probe to deliver a low-frequency puretone signal, 220/226 Hz at intensities of 85 to 95 dB SPL (sound pressure level), via a sealed tip at the entrance of the ear canal. Acoustic immittance is measured (in equivalent volume) as a function of varying air pressure (+200 to -300 deka Pascals [daPa]) in the external ear canal. The resulting graph is called a "tympanogram."

The tympanogram is analyzed in terms of its shape, acoustic admittance (mm [H.sub.2]O/ml), peak pressure (daPa), width (daPa gradient), and external ear canal/middle ear volume (ml). The peak of the tympanogram is at the point of maximum transmission of sound while applying air pressure to the external ear which, for a normal ear, occurs at or near atmospheric pressure (0 daPa). As the pressure varies (either more negative or more positive), the tympanic membrane stiffens (decreased reactance), causing a reduction of sound transmission and acoustic energy through the external ear-middle ear system.

[FIGURE 1 OMITTED]

The shape of the tympanogram has been classified into five types (figure 1) that are correlated with normal middle ear function and various middle ear dysfunctions: types A, B, C, [A.sub.d], and [A.sub.s]. Type A is the typical normal tympanogram with peak pressure at or near 0 daPa. The type B tympanogram is relatively flat and is commonly seen with middle ear effusion, cerumen occlusion, tympanic membrane perforation, or a probe sealed against the canal wall. The type C tympanogram has a normal peak (admittance) at a substantially negative air pressure. This pattern is characteristic of malfunction of the eustachian tube. Type [A.sub.d] has an increased admittance at or near 0 daPa and is usually seen with ossicular disarticulation or atrophic scarring of the tympanic membrane. Type A has a reduced admittance at or near 0 daPa and is usually seen with tympanosclerosis or ossicular fixation in the middle ear (stiffness pattern), such as in otosclerosis.

[FIGURE 2 OMITTED]

Analysis of the tympanogram shape is usually subjective, and it is important that other objective parameters (usually automatically calculated by most instruments) are used and compared with normative data (for children or adults) to interpret the tympanogram fully. It is also important that tympanograms are interpreted in conjunction with careful otoscopic examination, especially in cases of flat or high-volume tympanograms. Care also should be taken in interpreting tympanometry in infants, as their external ears are still "compliant" and a normal response may be misleading. High-frequency tympanometry may be useful in these cases.

Additional measurements may be made during tympanometry, especially the acoustic reflex and its decay. The probe is used to deliver several frequencies (500 Hz, 1 to 4 KHz) at 80 to 90 dB HL (hearing level) (above the hearing threshold) to illicit and record the ipsi- and contralateral acoustic reflex. It is graphed as an increase in the stiffness of the middle ear system (decreased admittance). The reflex is mediated via the VIIth and VIIIth cranial nerves and may be valuable in assessing peripheral and central auditory functions. For example, a lower stapedial reflex threshold is commonly seen in recruitment and an absent reflex in otosclerosis. Of importance is the presence or absence of the acoustic reflex in cases with conductive hearing loss. In such cases, the presence of an acoustic reflex essentially rules out otosclerosis and points to "cochlear conductive loss" commonly seen in superior semicircular canal dehiscence, large vestibular aqueduct, and Meniere's disease. An acoustic reflex decay of more than 50% is suggestive of retrocochlear pathology. There has been a gradual disinterest in tympanometry and stapedial reflex testing since the emergence of auditory brainstem response audiometry and magnetic resonance imaging. However, the results of tympanometry are not redundant and can be of great help in the differential diagnosis of hearing disorders.