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Tympanometry in Clinical Practice Janet Shanks and Jack Shohet

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Tympanometry in Clinical Practice Janet Shanks and Jack Shohet P1: OSO/UKS P2: OSO/UKS QC: OSO/UKS T1: OSO Printer: RRD LWBK069-09 9-780-7817-XXXX-X LWBK069-Katz-Hood-v1 October 1, 2008 11:59 CHAPTER Tympanometry in Clinical Practice Janet Shanks and Jack Shohet HISTORY AND DEVELOPMENT tic impedance instrument to allow for variation in ear-canal ± OF TYMPANOMETRY pressure over a range of 300 mm H2O and described the first“tympanogram”asauniformpattern“. withanalmost Two “must read” articles on the development of clinical tym- symmetrical rise and fall, attaining a maximum at pressures panometry are Terkildsen and Thomsen (1959) and Terkild- equaling middle ear pressures” (p. 413). They further noted sen and Scott-Nielsen (1960). Their interest in estimating that, “. the smallest impedance always corresponded ex- middle-ear pressure and in measuring recruitment with the actly to the zone of maximal subjective perception of the test acoustic reflex had a profound effect on the development tone” (p. 413). In other words, the probe tone was the most of clinical instruments. Each time I read these articles, I am audible and the tympanogram peaked when the pressure was struck first by the incredible amount of information in the equal on both sides of the eardrum. articles, and second, by the lack of scientific data to support In addition, these authors recognized that although the their conclusions. Most amazing of all, however, is that the measure of interest was the acoustic immittance in the plane principles presented in these two articles have stood up for of the eardrum, for obvious reasons, the measurements had nearly 50 years, and provide the basis for commercial instru- to be made in the ear canal. They introduced the clinical ments and tympanometry procedures still in use today. procedure used to compensate for the volume of air en- These articles laid the foundation for the use of hard- closed between the probe tip and the eardrum. “Under con- walled calibration cavities and the term “equivalent volume ditions where the ear drum is under considerable tension, of air”,compensation of ear-canal volume, and the selection the impedance volume measured is dominated by the vol- ofasinglelow-frequencyprobetone.Youshouldbesurprised ume of the ear canal space itself. With decreasing tension of to read that the probe tone “frequency of 220 cps was cho- the ear drum the influence of the middle ear space gradu- sen partly at random” (Terkildsen and Scott-Nielsen, 1960, ally increases, attaining a maximum under conditions where p. 341). A low-frequency probe tone was preferred because the pressures in both spaces are identical”.“. the difference the current day microphones were nonlinear at high frequen- between highest and lowest impedance volumes as obtained cies and the probe tone level could be increased without elic- by variations of the pressure, is thought to some extent to iting an acoustic reflex. In other words, the selection of a 220 indicate the vibrating characteristics of the individual ear Hz probe tone was made without any consideration of its drum” (p. 414). The difference between the highest and low- diagnostic value in evaluating middle-ear function. Finally, est impedances, therefore, was attributed to the middle-ear these articles also set the precedent for measuring only the system, absent the effects of the ear canal. Here, they laid the magnitude of complex acoustic immittance rather than both basis for estimating ear-canal volume and calculating peak magnitude and phase angle. Phase angle measurements were compensated static acoustic admittance. abandoned because the middle ear is so stiffness controlled Instead of reporting tympanograms in acoustic ohms, at 220 Hz that phase angle did not vary considerably in either Terkildsen and Thomsen (1959) reported them in terms of normal or pathological middle ears. an equivalent volume of air with units in milliliters (ml) or The focus of Terkildsen and Thomsen (1959) was on es- cubic centimeters (cc or cm3). This precedent was followed timating middle-ear pressure. They adapted an electroacous- because at low frequencies such as 220 Hz, the ear functions 157 P1: OSO/UKS P2: OSO/UKS QC: OSO/UKS T1: OSO Printer: RRD LWBK069-09 9-780-7817-XXXX-X LWBK069-Katz-Hood-v1 October 1, 2008 11:59 158 Section II ■ Physiological Principles and Measures ho) 5 B 15 B 226 Hz 678 Hz 4 12 ANCE (mm 3 9 B B 2 6 FIGURE 9.1 Uncompensated acoustic susceptance (B) and 1 B 3 B conductance (G) tympanograms USTIC ADMITT G G 0 0 recorded at 226 Hz (A) and 678 Hz (B) in calibration cavities with ACO -300 -200 -100 0 100 200 -300 -200 -100 0 100 200 volumes of 0.5, 2.0, and 5.0 cm3. A EAR-CANAL PRESSURE (daPa) B like a hard-walled cavity of air. Although the use of equiva- multiple admittance components [e.g., acoustic admittance lent volumes is acceptable at low frequencies, its use is not magnitude (Ya), phase angle (ϕ), Ba, and Ga]. These in- appropriate at high frequencies where the middle ear is no struments are extremely versatile and have the added con- longer stiffness dominated. Instead of simplifying measure- venience of being able to store commonly used tympanom- ments, the term equivalent volume of air continues to be a etry protocols and several records of patient data. By the end source of confusion. of this chapter, you will hopefully recognize several applica- Based largely on these early studies, the electroacous- tions for the under utilized high frequency options on these tic immittance systems developed in the early 1960’s mea- instruments. sured only the magnitude of acoustic impedance for a single, low-frequency probe tone of 220 Hz. The first generation of CALIBRATION instruments, like the popular Madsen ZO70, did not incor- porate an automatic gain control (AGC) circuit to maintain ANSI S3.39 requires that three calibration cavities (0.5, 2.0, 3 the sound pressure level of the probe tone at a constant level. and 5.0 cm ) be provided with each instrument. Measur- As a result, the amplitude of the tympanogram, reported in ing the acoustic admittance of these calibration cavities pro- arbitrary units from 1 to 10, was partially dependent on ear- vides a convenient introduction to tympanometry estimates canal volume. The sound pressure level of the probe tone of ear-canal volume, compensated for ear-canal volume ver- and the height of the tympanogram were higher in small susuncompensatedtympanograms,andflattympanograms. ear canals than in large ear canals even when the acoustic Figure 9.1 shows acoustic susceptance (B) and conduc- immittances of both middle ears were identical (see Shanks, tance (G) tympanograms in the three calibration cavities for 1984: Figure 4). Not surprisingly, quantifying the height of 226 Hz (Panel A) and 678 Hz (Panel B) probe tones. For the tympanograms in arbitrary units was not useful clinically. smallest cavity at 226 Hz, the B and G tympanograms are To circumvent this problem, the next generation of in- straight lines at 0.5 and 0 acoustic mmho, respectively. Simi- 3 struments incorporated AGC circuits to maintain a constant larly, for the 2.0 and 5.0 cm cavities, G at 226 Hz remains at probe tone level in ear canals of all sizes. Measurements no 0 acoustic mmho and B is equal to the volume of the cavities. longer were reported in arbitrary units but rather in ab- Because G=0 in these calibration cavities, admittance mag- solute physical units, the acoustic mmho. In 1970, Grason nitude (Y) also would be identical to B (see Eq. 8.5) and equal Stadler introduced a new instrument (Model 1720) with two to the volumes of the calibration cavities. At 678 Hz, the re- probe-tone frequencies, 220 and 660 Hz, and two admit- sults are the same except B is increased by a factor of 3 to 1.5, tance components, acoustic susceptance (Ba) and conduc- 6.0, and 15.0 acoustic mmho while G remains at 0 acoustic tance (Ga). Feldman (1976b) used this instrument to collect mmho in all three cavities. You should further notice that the 220 and 660 Hz tympanograms from ears with a variety of B tympanograms slope slightly upward as pressure decreases. middle-ear pathologies, and clearly demonstrated the ad- This is expected because the acoustic susceptance (Ba)ofan vantage of a high-frequency probe tone in evaluating mass enclosed volume of air increases slightly as the density of air related pathologies of the middle ear. Despite findings such decreases (Beranek, 1954; Lilly and Shanks, 1981). as these, high-frequency tympanometry has never become Hard-walled cavities are used for calibration because routine. they can be constructed as an ideal acoustic element. That is, The latest generation of computer-based instruments at sea level, an enclosed cavity of air with certain constraints (e.g., Virtual, Model 310; Grason Stadler TympStar, Version placed on the radius and length of the cavity can be mod- 21; Madsen OTOflex 100) continued to offer multiple fre- eled as pure acoustic compliance (or inversely, stiffness) with quency probe tones (e.g., 226, 678, and/or 1000 Hz) and negligible mass and resistance up to approximately 1000 Hz (Beranek, 1954; Lilly and Shanks, 1981; Margolis and Smith, 1 Grason Stadler, a division of VIASYS Healthcare, Inc. supported this en- 1977; Moller, 1960, 1972; Rabinowitz, 1981; Shanks and deavor by providing test equipment for research by the second author. Lilly, 1981; Shaw, 1974). The low-frequency probe tone was P1: OSO/UKS P2: OSO/UKS QC: OSO/UKS T1: OSO Printer: RRD LWBK069-09 9-780-7817-XXXX-X LWBK069-Katz-Hood-v1 October 1, 2008 11:59 Chapter 9 ■ Tympanometry in Clinical Practice 159 increased from 220 to 226 Hz in the 1987 standard because sion from the ear canal to the cochlea reminds us of two im- under standard atmospheric conditions, the acoustic ad- portant constraints.
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