In the clinic we have been able to detect and to quantify conductive hearing losses for at least two centuries. Tuning-fork tests and pure-tone audiometry, however, require subjective responses from a patient. Moreover, the results of these tests typically do not identify the etiology of middle-ear disease. The importance of knowing etiology, of course, increases as surgical procedures become more sophisticated. In the first half of the 20th century, investigators reasoned that abnormalities within the middle-ear transmission system might be reflected in acoustic conditions at the lateral surface of the tympanic membrane. The first clinical tests involved the measurement of static acou stic impedance (Za) in the external auditory canal (Schuster, 1934). Static Za reflects conditions that exist at the tympanic membrane while a patient is engaged in everyday activities.
Static Acoustic Immittance at the Tympanic Membrane
In 1886, Heaviside introduced the concept of impedance to electric-circuit theory. In 1919, Webster extended this tool to the analysis of mechanical and acoustic systems. In simplest terms, aural acoustic impedance is the total opposition that exists within the ear canal to an acoustic wave. Aural acoustic admittance (Ya), the reciprocal of Za, can be thought of as the ease with which an acoustic wave moves through a medium or system.
Acoustic impedance and acoustic admittance are complex quantities. This means that they are composed of two interrelated parts. An analysis of these two components allows us to estimate the interaction between mass, stiffness, and friction within the middle ear. The term acoustic immittance refers collectively to Za, to Ya or to both quantities.
Beginning in 1957, Zwislocki published a series of studies on Za at the tympanic membrane of subjects with normal hearing and of patients with middle-ear disease. These research efforts led to the development of a commercial acoustic-impedance bridge in 1963.
In summary, these early measurements showed that aural acoustic-impedance values were: 1) lower than normal with ossicular discontinuity; 2) higher than normal with clinical otosclerosis; and 3) very much higher than normal with acute inflammatory and chronic diseases of the middle ear. In most cases, differential diagnosis of middle-ear disease improved when both components of Za or Ya were available (Lilly, 1973).
Dynamic Acoustic-Immittance Measurements
In the second half of the 20th century, many clinical studies focused on changes in acoustic immittance in response to known stimuli. These dynamic measurements involved the use of acoustic signals to contract the middle-ear muscles (the acoustic reflex) or air-pressure changes in the external auditory meatus (tympanometry).
Tympanometry with single-frequency probe tones
Early work in tympanometry used a probe tone of only one frequency. Analysis of the resultant recordings typically was based upon the morphology of the tympanogram. At a conference in 1962, Lidén catalogued the tympanometric patterns that accompany common middle-ear problems. He assigned the letters A, B, C, and D to these patterns. Variations upon these morphologic categories still are used today.
With tympanometry, as with static acoustic-immittance measurements, the differential diagnosis of some middle-ear problems often is more accurate when both components of Za or Ya are available for analysis. A mathematical model developed by Vanhuyse and his colleagues (1975) is especially useful for the analysis of problem tympanograms.
Tympanometry with more than one probe frequency
Tympanometry with low-frequency probe tones provides useful diagnostic information for children and adults with disorders of the tympanum, the tympanic membrane, and the Eustachian tube. Unfortunately, low-frequency, single-component tympanometry, can produce spurious patterns in neonates (Holte et al., 1991) and is relatively insensitive to many lesions that affect the ossicular chain (Lilly, 1984).
The clinical utility of tympanometry with probe tones of more than one frequency (multiple-frequency tympanometry) first was demonstrated by Colletti (1976). In general, over the last 30 years, research and clinical experience has shown that the resonance frequency of the middle-ear transmission system: increases with clinical otosclerosis; decreases with ossicular discontinuity; and changes in predictable ways following middle-ear surgery (Hunter and Margolis, 1992).
Clinically, unless one is working only with neonates, the need for multiple-component, multiple-frequency tympanometry exists for fewer than 20% of all patients evaluated. Still, these techniques are invaluable for the differential diagnosis of: 1) fixation of the lateral ossicular chain from fixation of the stapes; 2) profound mixed hearing losses; 3) clinical otosclerosis from disruption of the ossicular chain; 4) hypermobility of the incudostapedial joint; and 5) congenital ossicular fixation in children. These techniques also are useful: 1) for the evaluation of post-operative problems that lead to a recurrence of conductive hearing losses following middle-ear surgery; 2) for patients with "objective" tinnitus aurium; 3) to help with the interpretation of otoacoustic-emission data from neonates; 4) as part of the test battery for auditory neuropathy (auditory dys-synchrony); and 5) to increase the amplitude of acoustic-immittance changes during acoustic-reflex measurements.