Tympanometry is a common technique used by audiologists to give quantitative information on the mobility of the middle ear, which can help determine if a middle ear disorder is present.
The units of tympanic measurement are determined by the impedance and admittance of the middle ear system. Impedance is the measurement of the "opposition of flow of energy into a system" , where acoustic impedance (acoustic ohms) is "the complex ratio of effective sound pressure averaged over a surface to the effective volume velocity through it" . Admittance is considered to be the reciprocal of impedance, hence is considered to be the measurement of the compliance of flow into a system . The acoustic admittance (acoustic mmhos) is the ratio of "the effective volume velocity through a surface to the effective sound pressure averaged over the surface" . The term acoustic immittance is given to the testing of acoustic admittance and impedance in relation to middle ear function.
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Within the ear there are three components that determine the acoustic admittance:- compliance, mass and resistance .
The compliance of the ear (compliant susceptance, BS) is the admittance that is presented by the spring or stiffness element of the middle ear. The spring components of the middle ear include: the tympanic, round and oval membranes, ligaments and tendons, and air within the middle ear .
The mass elements of the middle ear (the ossicles, pars flaccida of the tempanic membrane and perilymph) contribute to the mass susceptance (BM), hence the amount of energy that flows through the middle ear mass .
Susceptance is the imaginary component of admittance and acoustic susceptance is "the admittance offered by the acoustic compliance and acoustic mass elements" .
Through the middle ear there are a number of elements that contribute to the friction component of acoustic admittance. These friction components include: the viscosity of the perilymph and the movement of the tendons, ligaments and membranes throughout the middle ear .
Friction determines the amount of acoustic energy that is dissipated as it moves through the middle ear, also known as conductance. Conductance (G) is the real component of admittance and is defined by the ability for energy to flow through the middle ear (ASHA Working Group, 1988; Van Camp et al., 1986). The overall components of Admittance are summarized in the equation below:
Y = G + jB
Similar to the equation for admittance, the equation for impedance suggests that there is an imaginary and real component.
ZÂ =Â RÂ +Â iX
Acoustic resistance (R) is the real component of impedance and is defined by the 'loss' or dissipation of energy (due to friction) as a sound wave passes through a medium. This friction within the middle ear is due to mechanical friction of the ossicles, membranes and ligaments and the acoustic resistance due to fluid viscosity .
Acoustic reactance is the imaginary part of impedance and is determined by its ability to oppose the "energy flow due to storage" (Stach, 2003). Like admittance, the reactive component is subdivided into the mass (XM) and compliant (-XC) elements. The compliant elements include: the closed volumes of air in the external acoustic meatus and the middle ear cavity, which have the ability to expand and compress. The mass elements are made up of the ossicles and membranes in the middle ear . Seeing that impedance and admittance are reciprocals, it means that the equations can be interchangeable where Y=Z-1 and phase (ï¦) of impedance is the reciprocal of admittance () . Hence, to convert impedance to admittance components:
Like wise, the admittance components can be expressed in impedance components:
Figure 1: Phasor representation of components of admittance (Adapted from .
The phase relationship of Impedance and Admittance can be replicated using vectors (Figure 1). Plotted on the Y-axis is the total susceptance (Admittance), which is comprised of the sum of the mass and compliance elements. The velocity of a compliance element leads applied force by 90Â° and the mass element lags by 90Â°. Since the compliance element of susceptance (BS) is leading the force it plotted on the positive Y-axis and as the mass element lags behind the force it is plotted on the negative Y-axis. Hence, when the total susceptance is positive, the conductance is dependent on the compliance of the system. Conversely, if the total susceptance is negative, the conductance of sound energy through the middle ear is mass dependent . Conductance (G) is represented by the X-axis and is an indicator of the 'in phase components of admittance . From the vector the total admittance can be determined by plotting the total susceptance against the total conductance.
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A tympanometer is used to measure the admittance of the middle ear, which is comprised of: a probe, a pneumatric system, an acoustic immittance measurement system, an acoustic activator system and a recording device . Within the probe that is inserted into the ear canal there are three ports, including; the loudspeaker which produces the pure tone, the port for the pneumatic system that controls pressure in the ear canal and the microphone that reads the change of sound pressure within the cavity. The pneumatic system alters the pressure in the ear canal and is measured in decaPascals (dPa); this means that tension at the tympanic membrane can be altered. The acoustic-immittance measurement system is responsible for converting the microphone voltage units into immittance units. The acoustic-reflex activator system provides the pure tone through the loudspeaker to the tympanic membrane. Finally, there is a recording device that determines the immittance values from the measurement system and plots them as a function of air pressure .
Prior to testing it is important to calibrate the tympanometer to ensure that the results are consistent and reliable. Air cavities are used in calibration as enclosed volumes of air have known impedances and can be used as a standard of measurement (Lilly & Shanks, 1981). The enclosed volume can be seen as pure compliant reactance as the mass and resistive elements are at zero (Shanks, 1984). The ANSI standard states that the three cavities sizes are 0.5, 2.0 and 5.0 cm3 are to be used as the enclosed volume sizes (ASH Working Group, 1988; ANSI, 1987). During 226 Hz testing on the 0.5 cm3 cavity, the conductance and susceptance tympograms are flat lines. The susceptance is at 0.5 mmho and the conductance is at 0 mmho, supporting that it is pure compliant reactance (Shanks & Shohet, 2009). The standard suggests that measurements should be with Â±5% or Â±0.1 acoustic ohms (ASHA Working Group, 1988; ANSI, 1987) At sea level, the 226 Hz pure tone projected into a 1 cm3 enclosed volume has am impedance of 1000 acoustic ohms and admittance of 1 mmho (Shanks, 1984; Lilly & Shanks, 1981). Hence, the purpose of using 226 Hz during low frequency tympanometry (as opposed to the 220 Hz tone from Terkildsen & Nielsen, 1960) helps for ease of calibration (Shanks & Shohet, 2009).
The standard practice to measure acoustic immitance magnitude of the middle ear employs low frequency (220-226 Hz) tympanometry , which was first suggested by Terkildsen & Nielsen . In the ear system, susceptance is altered with frequency, where the mass susceptance is proportional to the projected frequency and the compliance susceptance is inversely proportional to frequency . This can be replicated using the two equations for impedance (inverse of admittance):
Where M is mass, ï² is medium (air) density, c is velocity of sound and V is volume of the ear canal (Margolis & Hunter, 1999). When mass is added to a system (at the tympanic membrane), it has a large effect on the high frequency (as opposed to the lower frequencies) resulting in a downward sloping hearing loss. Conversely, when the tympanic membrane is stiffened it results in an upward sloping hearing loss. This is due to the lower frequencies being affected more than the higher frequencies by the compliant reactance (Margolis & Hunter, 1999). At 226 Hz, the total susceptance is positive (compliance controlled) and as the frequency increases, the total susceptance becomes negative (mass controlled) . The conductance operates independent of frequency, hence, adjusting frequency does not affect the conductance as the force and velocity are in phase .
When the mass and compliance susceptance are equal meaning that the total susceptance is 0 mmhos and the phase angle of admittance of the middle ear is at 0Â°, the resonance of the middle ear can be found (Shanks & Shohet, 2009; Margolis & Goycoolea, 1993). The resonance of the ear can be defined as the "condition of peak vibratory response obtained on excitation of a system that can vibrate freely." (Stach, 2003). The resonant frequency is the "frequency at which a secured mass will vibrate most when set into free vibration" (Stach, 2003) and frequencies below the frequency are compliance controlled and are mass controlled when above the resonant frequency (Margolis & Hunter 1999). The resonant frequency is determined using multiple frequencies to illustrate the point of transition where the susceptance is going from a notched susceptance peak and single conductance peak tympanomgram (3B1G) to a notched susceptance and conductance tympanogram (3B3G) (Margolis & Hunter, 1999). Resonance has been reached when the notch has reached the corrected ear canal volume. When the susceptance notch is above the corrected ear canal volume, the middle ear system is stiffness controlled and when below, it is mass controlled (Wiley & Fowler, 1997).
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Another way of identifying the resonant frequency is the point at which the compensated susceptance reaches 0 (Margolis & Hunter, 1999). By determining the resonance of the middle ear, a clinician can diagnose certain middle ear pathologies as certain pathologies can alter the resonance. For instance, otosceloresis is characterised by having increased stiffness meaning that the resonance frequency can shift higher (Van Camp & Vogeleer, 1986). Conversely, ossicular discontinuity in the middle ear means that there is a greater mass and a lowering of the resonance frequency (Feldman, 1964). The normal adult resonance frequency can range from 800-2000 Hz with a 1135 average when measured with the sweep frequency method and 630-1400 with an average of 990 using the sweep pressure method (Margolis & Goycoolea, 1993).
When performing tympanometry at low frequency (226 Hz) the following information can be gathered:- static admittance, tympanometric width, tympanometric peak pressure and the equivalent ear canal volume.
Static admittance is the admittance in the ear canal at a specific air pressure and is an indicator how mobile the tympanic membrane is (Keefe & Fenney, 2009; Stach, 2003). During testing, the ear canal pressure is decreased from 200 daPa, which means that the tension of the tympanic membrane is decreased and the membrane as at its most compliant. Admittance is at its peak when the pressure between the ear canal and the middle ear are the same. As the ear canal pressure decreases to atmospheric pressure (0 daPa), the pressure in the ear canal becomes equal to that of a normal middle ear and the admittance is at its maximum (Bess & Humes, 2003). For adults the average static admittance is 0.79 with a range of 0.3-1.7 mmho (Margolis & Goycoolea, 1993). To determine the immitance of the lateral side of the tympanic membrane (compensated static acoustic admittance), the acoustic admittance of the outer ear needs to be measured. To remove the ear canal contributions, the static admittance of the outer ear is subtracted from the total admittance to give the static admittance of the middle ear (ASHA Working Group, 1988; Stach, 2003).
To determine the sharpness of the tympanic peak, the tympanic width is used, which is the air pressure at the half height of the tympanogram from peak to tail (Stach, 2003). Originally the gradient was used as the measure for the tympanometric peak (Brooks, 1968) but tympanic width is considered the more reliable measurement (DeJonge, 1986) and with an increase in tympanometric width comes no relative lowering in static admittance (Koebsell & Margolis, 1986). Normal adult values for tympanic width range from 51-114 daPa with an average of 77 daPa (Margolis & Heller, 1987).
The tympanometric peak pressure (TPP) is the "air pressureâ€¦at which the peak of the tympanogram occurs" (Stach, 2003) where the normal values lie within ï‚± 50 daPa . The TPP is used as a value to indicate the middle ear pressure (Terkildsen & Thomsen, 1959) and can also illustrate eustachian tube function (Lazo-Saenz et al., 2005; Margolis & Hunter, 1999). The ex vacuo theory helps demonstrate the Eustachian tube function by suggesting that when gases are absorbed into the mucosa of the middle ear it generates a negative pressure which pulls the tympanic membrane inward. When the Eustachian Tube is not functioning correctly (i.e such as the precursor to acute otitis media), the negative pressure within the middle ear continues to build thus maintaining the membrane in an inward setting (Magnuson, 1983).
The measurement of the static admittance, tympanometric width and tympanometric peak pressure help in the qualitative classification of the different tympanometric shapes described by Liden (1969) and Jerger (1970). The classification system has four main types of tympanometric shapes, being: A. B, C and D type tympanograms. Type A tympanograms are characterised by being within the normal range of static admittance and normal tympanometric peak pressure (Margolis & Hunter, 1999). Type A tympanograms are further segregated into type AS and AD, where AS indicated that the static admittance is shallow and the AD has a large static admittance (Jerger et al., 1972). Where AS are usually correlated with otosclerosis and AD are connected with tympanic membrane scarring or ossicular discontinuity (Shanks & Shohet, 2009). Type C tympanograms are described as having a peak that has a negative peak pressure and Type B tympanograms are flat with no apparent peak. Type B tympanograms are usually found in cases of middle ear effusion such as chronic otitis media (Shanks & Shohet, 2009).
Ear canal volume is also measured during low frequency tympanometry, which is useful in the diagnosis of eardrum perforations and patency of tympanostomy tubes (Margolis & Hunter, 1999). During immitence testing the ear canal volume is the volume between the probe and the tympanic membrane (Lindeman & Holmquist, 1982). When there is a perforation in the ear drum the volume of air measured would include the volume of the middle ear space and contigous mastoid cells (Margolis & Hunter, 1999), which is reported to range from 2 cm3 to 22 cm3 (Molvaer et al., 1978). As the normative upper limit for an adult is at 2 cm3 it is apparent when a subject has a perforated eardrum (Margolis & Heller, 1987; Margolis & Hunter, 1999). Measuring the ear canal volume is important in determining the resonance of the middle ear, as acoustic susceptance and phase angles can only be determined once the measures are adjusted by removing the ear canal contributions (Shanks et al., 1993; Stach, 2003). As proposed by Terkildsen & Thomsen (1959), the compensation for the ear canal contributes can be determined by increasing the pressure (+ 200 daPa) within the ear canal to the point that the tympanic membrane is at its most tense. At this point, "the impedance of the middle ear is toward infinity" (Shanks et al., 2003) and the impedance measured is the ear canal contribution (Terkildsen & Thomsen, 1959). This can be shown on the tympogram as the admittance at the tail (+200 daPa) of the curve. The contribution of the ear canal can be demonstrated by trying to measure the ear canal volume using the 226 Hz and 678 Hz tones measuring acoustic suseptance tympanograms. Shanks and Lilly (1981) indicated that the ear canal volume decreased by 22% and 31%, at 226 Hz and 678 Hz, respectively.
There are reports that indicate that low frequency tympanometry fails to indicate when there is ossicular discontinuity (Browning et al., 1985). When the frequency of the probe tone is increased from the low frequency (226 Hz) to a high frequency of 668 Hz, it alters the admittance in the plane of the tympanic membrane (YTM). At 678 Hz, the YTM has moved toward conductance and away from susceptance when compared to the 226 Hz phase vector (Shanks & Shohet, 2009). This is reflected by comparing the uncompensated acoustic admittance (Ya), susceptance (Ba) and conductance (Ga) tympanograms, as the vector has moved from the susceptance contribution it means that there is deviation between the peaks of the acoustic admittance and the susceptance. As probe tone frequencies increase there vector moves beyond conductance (X-axis) toward the mass susceptance. By performing tympanograms at higher frequencies it helps differentiate middle ear pathologies, as the admittance is varied by different elements of the middle ear (i.e the compliance, conductance or mass elements) when the frequency is adjusted *** (Shanks & Shohet, 2009). For instance, by performing a tympanogram at 660 Hz, it has reported having benefits in helping diagnose certain middle ear pathologies such as ossicular disruption . By performing mulitfrequency (250-2000 Hz) tympanograms there has been an improvement in the detecting ability of tympanometry for otosclerosis (Shahnaz & Polka, 1997) and ossicular discontinuity (Funasaka & Kumakawa, 1988). When the frequency is further raised from 678 Hz the tympanograms of conductance, susceptance and admittance become more complicated (Appendix 1). Alberti & Jerger first reported that when comparing the peaks of a 200 Hz tone to an 800 Hz that the peak goes from a V-shaped peak to a W-shape (Alberti & Jerger, 1974). These complex arrangements were first described by Vanhuyse et al. (1975) where the Vanhusye model was established to graphically indicate the different arrangements (Appendix 1).
The Vanhuyse model is based on converting impedance measurements from Moller (1965) into admittance measurements, so that susceptance, admittance and conductance could be measured (Margolis & Hunter, 1999). The resistance was considered to be decreasing as the air pressure increases from negative to positive (Vanhuyse et al., 1975; Margolis & Hunter, 1999) and the reactance was a symmetrical parabolic function present at 0 daPa (Vanhuyse et al., 1975; Shanks & Shohet, 2009). The conditions were shifted from being in a stiffness controlled to a mass controlled state, where the acoustic resistance was kept constant and the acoustic reactance was shifted (Vanhuyse et al., 1975; Shanks & Shohet, 2009). At each significant shift in the acoustic reactance, the susceptance and conductance tympanograms were plotted (Vanhuyse et al., 1975; Shanks & Shohet, 2009). The patterns were differentiated by the amount of extrema in the susceptance and conductance tympanograms (Vanhuyse et al., 1975; Margolis & Hunter, 1999). The 1B1G type tympanogram is defined by the susceptance, conductance and admittance tympanograms all being single peaked. This occurs as the acoustic reactance remains negative when the ear canal pressure is adjusted (Vanhuyse et al., 1975; Margolis & Hunter, 1999). The 3B1G is characterised by having a single peaked conductance tympanogram and susceptance peak with a notch (3 extrema) and the admittance is still single peaked (Vanhuyse et al., 1975; Appendix 1). The notching is thought to occur as the reactance is closer to zero, meaning that the absolute value of reactance is less than resistance but as the pressure increases the reactance is greater than the resistance (Vanhuyse et al., 1975 as cited in Margolis & Hunter, 1999). In the 3B3G pattern there is a notch in the susceptance, conductance and admittance tympanograms. This occurs as the reactance becomes positive suggesting that the system is becoming mass controlled, where it's less than resistance at low pressures (Margolis & Hunter, 1999). The 5B3G type has double notch in the susceptance tympanogram and a single notch in the conductance and admittance tympanograms. This is thought to occur as the "reactance is postive and greater than resistance at low pressures and negative at high pressures" (Margolis & Hunter, 1999). With the classification system produced by Vanhuyse et al. it meant that multifrequency techniques could be used to further help in the diagnosis of middle ear pathologies.
The sweep frequency technique was developed by Funasaka et al. (1984), which was a technique that measures the difference in sound pressure and phase with a range of frequencies. The technique involves a frequency tone that shifts from 220 to 2000 Hz, where the sound pressure and phase difference are measured at 0 and -200 daPa ear canal pressure. The difference between sound pressure and phase difference at 0 and -200 daPa are then plotted as sound pressure curve and phase difference curve (Funasaka at al., 1984). To determine the resonant frequency the point at which the compensated susceptance reaches 0 is used as an indicator and is illustrated by the point of the sound pressure curve that crosses the x-axis (Funasake at al., 1984; Shanks & Shohet, 2009). The normal adult resonance frequency can range from 800-2000 Hz with a 1135 average when measured with the sweep frequency method (Margolis & Goycoolea, 1993). These values increase with increased stiffness (such as ossicular fixation) where values are greater than 1880 Hz. With an increase in mass (such as ossicular discontinuity) values are less than 720 Hz (Funasaka & Kumakawa, 1988; Shanks & Shohet, 2009). The Grason Stadler TympStar Version 2, uses the TPP measurement as opposed to the 0 daPa (Shanks & Shohet, 2009). The procedure involves a frequency sweep from 250 Hz to 2000 Hz at the start pressure of 200 daPa. A low frequency (226 Hz) tympanogram is performed to estimate the TPP and a second sweep is performed at the TPP (GSI Reference Instruction Manual, 2003; Shanks & Shohet, 2009). Like the Funasaka technique the phase difference and sound pressure curves are measured by the difference between starting (200 daPa) and peak pressure (TPP) (Shank & Shohet, 2009).
With the use of tympanometry we can understand how the middle ear is responding to sound. Using low frequency tympanometry the static admittance, tympanometric width, tympanometric peak pressure, tympanometric shape and the equivalent ear canal volume can be determined which help the Audiologist diagnose certain middle ear pathologies such as: otosclerosis, otitis media, ossicular discontinuity and tympanic perforations. There has been increased interest in using multifrequency tympanometry as it gives information of a range of frequencies, hence, helping define the resonant frequency to further certify if a patient has a middle ear pathology.
Part 2 - Measurement of Middle Ear Function using Tympanometry
Tympanometry is used on a routine basis to evaluate to evaluate the middle ear function by measuring the acoustic admittance. The standard clinical practise involves the use of low frequency tympanometry, which gives information on the shape of the tympanogram, amount of acoustic admittance and ear canal volume. Though, using higher frequencies have potential benefits in helping diagnose certain middle ear pathologies such as ossicular disruption . Hence, by utilizing information from a wide range of frequencies an audiologist can gather more data to make a more substantial conclusion on the nature of a middle ear pathology.
Multifrequency tympanometry gives audiologists useful information on the resonant frequency of the middle ear. As stated prior, the resonant frequency is the frequency at which compliance and mass elements are equal (Shanks & Shohet, 2009; Margolis & Goycoolea, 1993). Depending on the amount of stiffness or mass within the middle ear (due to middle ear pathology) it can shift the resonant frequency. Hence, by performing the multifrequency tympanometry the information gathered from the resonant frequency is of diagnostic value. Other qualitative information can come from the observation of the conductance, susceptance and admittance tympanograms at the various frequencies. By comparing each of the patterns with the noramative patterns defined by Vanhyuse et al. (1975) an audiologist can decipher the transitional frequency (from 3B1G to 3B3G) which is useful in the evaluation of middle ear function.
The purpose of this study is to obtain data on the middle ear of a normal hearing adult using low frequency tympanometry and multifrequency techniques such as the sweep frequency tympanometry. By gathering information using these techniques it the function of the middle ear can be evaluated but also the effectiveness of the tests can tested. Hence, to test this function, information on the static admittance, resonant frequency and shape of the tympanograms (in low and multifrequency tympanometry) needs to be gathered.
For this study, a 22 year old Caucasian male with normal hearing was used as a subject. The subject has no history with any middle ear diseases, head injuries, tinnitus or vestibular system dysfunction. Pure-tone testing done prior to testing indicated that the subject was within normal limits (15 dBHL) and otoscopy was performed to ensure the ear canal was unoccluded and the tympanic membrane was intact; which it was. All tympanometric testing was performed on a calibrated Grason-Stadler TympStar Version 2 tympanometer. Prior to testing, the calibration of the tympanometer was verified using the 0.5, 2.0 and 5.0 cm3 enclosed air cavities.
Single frequency tympanometry was performed first, using 226, 668 and 1000 Hz as the probe frequencies. Testing was performed on the left ear using a pressure sweep of 50 daPa/sec. At these frequencies admittance, susceptance and conductance tympanograms were plotted (Figure 2). From these tympanograms the static admittance, tympanometric width, tympanometric peak pressure and the equivalent ear canal volume were determined from the 226 Hz tympanogram. Also, the shape of the tympanogram was determined to test if was a type A, B, C or D. Further more, the Vanhuyse model was used to define the shapes of the susceptance, conductance and admittance tympanograms at the various frequencies.
Sweep frequency tympanometry was used to confirm and substantiate the findings of the single frequency tympanograms. By keeping the air pressure in the ear canal constant and sweeping the frequency from 250 to 2000 Hz the admittance, the phase difference and sound pressure curves could be determined at the various frequencies. Initially, a low frequency tympanogram is performed to estimate the TPP of the middle ear. The phase difference and sound pressure curves are estimated by measuring the difference in susceptance and phase at the start pressure (+200 daPa) and at the TPP. From these values they are plotted as difference (B or ï±) on the Y-axis and frequency on the X-axis.
The resonant frequency is an important measure as it is an indicator of whether the middle ear is mass or stiffness controlled, as it is the point at which the mass and stiffness elements are equal. This is determined from the sound pressure (compensated susceptance) curve as the point at which the change in sound pressure crossed the X-axis (reaches 0 mmho). This is illustrated by being the transition frequency at which the 1B1G becomes a 3B1G type curve. Hence, after the resonant frequency was determined using the sweep frequency technique, a susceptance tympanogram was performed at the resonant frequency to illustrate the notching.
To ensure that the results were consistent, all testing was performed at two different occasions using different GSI TympStar machines.
To assess the function of the middle ear, the static admittance, tympanometric width, tympanometric peak pressure, shape of tympanometric curve and the equivalent ear canal volume needed to be determined using low frequency tympanogram. 226 Hz was used as the probe tone and the admittance, susceptance and conductance tympanogram were measured (top panel of Figure 2).
The shape of the 226 Hz admittance tympanogram is a Type A, indicating that the ear drum is not retracted (Type C) nor does the tempanic membrane have decreased mobility (Type B). The shape is almost a Type AS as the static admittance is at 0.4 ml, being on the lower end of the normal range (0.3) (Margolis & Heller, 1987). This is supported by the tympanic width being large (115 daPa), where it is at the limit of the normal range (125 daPa in Wiley et al., 1996). Both the TPP (being -15 daPa) and the ear canal volume (being 1.5 cm3) are within the normal ranges as the TPP is close to 0 daPa and that the 1.5 cm3 is under the upper normal limit of 2.2 cm3 (Wiley et al., 1996). The susceptance and conductance tympanograms both have a single peak, indicating that they have a 1B1G vanhyuse pattern. From the low frequency tympanograms it can be concluded that the subject is within the normal limits for most elements of measurement and the shape coincides with the normative shapes (Appendix 1), indicating that the middle ear is functioning normally.
The 668 Hz tympanograms (middle panel of Figure 2) coincide with the normative shapes of conductance, susceptance and admittance typanograms (Appendix 1). With the increase in frequency, came an increase in conductance, represented by the greater peak, where notching did not occur. The susceptance tympanogram became shallower compared to the normative shape, though it still illustrated a clear peak. The fact that there are two single peaks indicates that they still have a 1B1G pattern and still haven't reached the resonant frequency. The admittance tympanogram, like the susceptance tympanogram, demonstrates a clear curve although it is shallower than expected (Appendix 1).
At 1000 Hz, the typanograms (lower panal of Figure 3) start becoming abnormal when compared to the noramative shapes (Appendix 1). At 1000 Hz, there are no definitive peaks in both the susceptance and admittance tympanograms, so there is no indication whether they are notched. The conductance peak still shows a single peak format and is larger than the 668 Hz conductance peak. Without the information on the susceptance notching it cannot be determined whether the tympanograms are a 1B1G or 3B1G type but seeing the conductance is still single peaked it implies that the shape has not reached the 3B3G shape and hasn't reached the resonance frequency.
Figure 2: The single frequency tympanograms of the subject performed at 226 (top panel), 668 (middle panal) and 1000 Hz (lower panal). The admittance at the plane of the probe (Ya) tympanograms and tempanic membrane (YTM) are on the left, the susceptance (B) tympanograms are in the centre and conductance (G) tympanograms are on the right.
Seeing that the single frequencies could not determine when the 3B3G pattern had occurred, the resonant frequency could not be determined. With the use of multifrequency tympanometry, the resonant frequency and phase difference can be determined (Figure 3 middle panel). The initial 226 Hz tympanogram was used to confirm the TPP, so that the peak pressure could be determined for multifrequency tympanometry (Figure 3 top panel). Once the resonant frequency was determined, a tympanogram was performed at that frequency so that the notching in the typanograms could be illustrated.
The initial 226 Hz tympanogram (Figure 3 top panel) showed that the static admittance was 1.9 ml where the peak was achieved at -15 daPa (TPP). Using the TPP, the multifrequency was performed (automatically) at -15 and +200 daPa at the range of frequency from 250-2000 Hz.
From the sound pressure curve of the multifrequency tympanograms, the resonance frequency can be determined by being the point at which the change in susceptance (B) becomes 0 or cross the X-axis (Figure 3 middle panel). At 700 Hz, the susceptance first crossed the X-axis, indicating that 700 Hz is the resonant frequency. This value is slightly outside the normal values of 800-2000 Hz (Margolis & Goycoolea, 1993), indicating that the middle ear system must be mass controlled. This value is supported by the phase difference being -14ï‚° at 700 Hz.
To further substantiate that 700 Hz is the resonant frequency, a susceptance tympanogram was performed at 700 Hz to determine whether notching has occurred (Figure 3 lower panel). Interestingly, from the susceptance tympanogram, it appears that notching has not occurred, indicating that the typanograms are not a 3B1G type (nor 3B3G) yet and may still be a 1B1G. From the shape of the tympanogram it means that it cannot be certain if 700 Hz is the resonant frequency.
Figure 3: The Multiple frequency tympanometry of the subject. The 226 Hz tympanogram used to determine the TPP (top panel). The Multiple frequency change in sound pressure (ï„B) and phase difference (ï„ï‘) curves are presented in the middle panels. Once the resonant frequency at tympanogram was performed at that frequency (700 Hz), seen in the lower panel.
In this study, I have successfully demonstrated how a lot information can be gathered from the single 226 Hz tympanogram but with multifrequency tympanometry using the sweep technique more conclusive information can be gathered about certain middle ear pathologies.
From the information gathered from the 226 Hz tympanogram (Figure 2 top panel), it suggests that the subject has a normal functioning middle ear. The static admittance of the middle ear was 0.4 ml, which was just within the 90% range of static admittance being 0.27-1.38 ml (Margolis & Heller, 1987). The large tympanic width of 115 daPa further substantiated that the tympanogram was shallow but within normal limits (as the width was within 35-125 from Wiley et al., 1996). The TPP was within normal limits as it was -15 daPa and close to 0 daPa. The ear canal volume was 1.5 cm3, which was also normal as the result was with upper limit of ear canal volume norm (2.2 cm3 in Wiley et al., 1996). Seeing the shape of the tympanogram demonstrated a Type A shape, which supports that the static admittance, tympanic width and tympanic peak pressure are within the normal limits. The susceptance and conductance tympanograms both showed single peaks thus demonstrating a normal 1B1G type tympanogram at 226 Hz. From the information given at 226 Hz, it indicated that the subject has normal middle ear function at this frequency and suggests that the subject does not have a retracted eardrum (Type C tympanogram) or fluid within the middle ear that could impede the movement of the ossicles (Type B tympanogram).
The tympanograms (Figure 2 middle panel) performed at 668 Hz showed a 1B1G type tympanogram as no notching had occurred in the conductance and susceptance tympanograms, which suggested that the resonant frequency had no been reached. By comparing the shape of the admittance and susceptance tympanograms to the results of Shanks et al. (1987) it was observed that the tympanograms were shallow compared to the normative but still showed a 1B1G type. The conductance tympanogram demonstrated a peak that was similar to the norm (Shanks et al., 1987).
The 1000 Hz admittance and susceptance tympanograms (Figure 2 lower panel) were abnormal when compared to the shapes from Shanks et al. (1987). No visible peaks could be determined from the susceptance and admittance tympanograms, whilst the conductance had a distinct peak. Hence, it cannot be concluded whether the tympanogram is notched (3B1G) or a single peak.
From the single frequency tympanograms, only the 226 Hz provided consistent, reliable information that demonstrated the middle ear of the subject was functioning correctly. Seeing the 1000 Hz did not illustrate a 3B3G type tympanogram, it can be suggested that the resonant frequency has not been reached.
From the multifrequency tympanograms performed using the sweep frequency techinique (Figure 3), it was determined that the resonance frequency was at 700 Hz where the phase difference was -14ï‚°. From these values it can be suggested that the middle ear system may be mass controlled, as the phase difference was negative. The resonant frequency was outside the normal ranges estimated by Margolis & Goycoolea (1993) being 800-2000 Hz but within the ranges measured by Shahnaz & Polka (1997) that were 630-1120 Hz. It was important to note that the changes in sound pressure were fairly flat and were close to zero until 1500 Hz. The change of susceptance crosses the X-axis once more at ~1250 Hz, which could be the resonant frequency. This theory of a higher resonant frequency is supported by the lack of notching when a tympanogram was performed at 700 Hz, as it indicates that the middle ear has not reached resonant frequency yet.
In summary of the information provided from the tympanograms performed using the single frequency and multifrequency techniques, it can be concluded that with the 226 Hz frequency tympanometry that the subject has a normally functioning middle ear. With multifrequency testing, the resonance frequency and phase difference indicate that the middle ear is mass controlled, though this was not confirmed, as the susceptance tympanograms did not demonstrate notching.
The resonant frequency results suggest that they are not entirely reliable in a clinical setting. If one were to perform a tympanometry experiment on the subject once more, providing conductance and susceptance tympanograms whilst performing the sweep of frequencies it would prove beneficial in substantiating the resonant frequency. Though, there are a number of reasons why this notching may not have occurred as expected and why the reasonance frequency appeared to be quite low compared to the norms, which included: the rate and direction of pressure changes, the number of consecutive pressure sweeps and correct calibration of the tympanometer.
It is recognized that with an increase in pump pressure speed when doing the pressure sweep comes an increase in the amplitude of the admittance tympanogram. This variable was suggested by Margolis & Heller (1987), where they demonstrated that when testing was performed at 200 daPa/sec and increased to 400 daPa/sec they noticed that there was a 10-14% increase in the amplitude of the admittance tympanogram (Shanks & Shohet, 2009). Seeing that all pressure speed in during testing was performed at 50 daPa/sec, it could be suggested that with an increase in pump speed, the notches could be seen better as there would be an attenuation of the tympanogram amplitude. Hence, for future testing the pump speed could be increased to 200 daPa/sec to see if the notches can be observed.
Another factor to consider is whether the GSI Tympstar Version 2 tympanometers were calibrated correctly, as one machine was nearing it's calibration due date (September 2011) and the other was past it's calibration date. The use of the calibration cavities indicated that tympanometer conformed to the ANSI standards (1987) but for future experiments a tympanometer that was properly calibrated could provide more consistent results.
When observing the tympanograms from Figures 2 and 3, it would appear that there is more information on the positive ear canal axis that is not present when the range is from (+200 daPa to -400 daPa). Hence, for future experiments it would be beneficial to provide a larger range of pressure sweep by starting at +400 daPa.
Although this subject may not be the exemplar model of how tympanometry is effective in middle ear diagnosis, many studies have demonstrated advantages of using single frequency and multifrequency tympanometry to diagnose low and high impedance.
Otitis Media with effusion (OME) is a condition where there is an increase in fluid within the middle ear cavity, which by making the middle ear system more mass controlled increases the impedance of the system. Depending on the stage of progession of OME, varies the appearance of the tympanograms. With 226 Hz single frequency testing, a typical tympanogram shows a Type B pattern or flat tympanogram, where there is no visible peak and the tympanic width is not measurable (Paradise et al., 1976). When there is increased mass due to the OM touching the ossicles and decreased compliance from reduced air volume, it can give a rounded and broader tympanogram as opposed to the flat tympanogram (Margolis & Hunter, 1999; Paradise et al., 1976). Margolis and Hunter (1999) recommended that higher frequency testing is needed to show the impedance changes between the stages of OME (Margolis & Hunter, 1999). During chronic OME the tympanic membrane can become retracted, as there is a negative middle ear pressure. This is represented on the tympanogram as being a Type C pattern where the static admittance is normal, but the tympanic width is large and the TPP is very negative (Paradise et al., 1976; Margolis & Hunter, 1999) When multifrequency tympanometry is performed, the tympanograms at all frequencies are flat and there is a drop in resonance frequency, which support the argument that the middle ear system in mass controlled (Vlachou et al., 1999).
Another high impedance pathology of the middle ear is otosclerosis, which is characterized my the "remodelling of bone, by resorption and new spongy formation around the stapes and oval window, resulting in stapes fixation and related conductive hearing loss" (Stach, 2003). With 226 Hz tympanometry, a patient with otosclerosis has a low static admittance but has an overlap when compared to the normative values making it very difficult to diagnose (Jerger et al., 1974; Margolis & Hunter, 1999). This is thought to be due to the site of lesion being present at the footplate, meaning that the impedance input into the cochlea is very high but tympanic membrane impedance is not effected significantly (Margolis & Hunter, 1999). Multifrequency tympanometry is suggested to have some benefit in diagnosing otosclerosis (Shahnaz & Polka, 1997). Their results suggested that the static admittance and tympanic width were similar between the normal groups and otosclerosis groups but there was a statistically significant difference for the resonance frequency and F45ï‚° (admittance phase angle at 45ï‚°) (Shahnaz & Polka, 1997). Margolis and Hunter (1999) recommend that a combination of absent acoustic reflexes, a PTA that indicates conductive hearing loss, a normal 226 Hz tympanogram, a normal eardrum appearance and a increased resonance frequency, should all be considered when diagnosing otosclerosis, due to tympanometry not being conclusive enough for accurate diagnosis (Margolis & Hunter, 1999).
Ossicular discontinuity is pathology in which there is disarticulation at the incus, which is commonly due to chronic OM (Shanks & Shohet, 2009). This disarticulation is caused by the decalcification of the long process of the incus, meaning that the incudostapedial joint is connected only by fibrous bands (Terkildsen, 1976 in Shank & Shohet, 2009). Using 226 Hz tympanometry, ossicular discontinuity results in a large static admittance (Type AD) that has a very narrow TW (Hunter & Margolis, 1992). Using multifrequency tympanometry, Margolis & Hunter (1999) determined that the resonant frequency of a patient with ossicular discontinuity was 355 Hz, where the susceptance notch of the 1B1G was beyond the resonance frequency (Margolsi & Hunter, 1999). From these results, it can be suggested that the middle ear system is mass controlled and the disarticulation causes the tympanic membrane to move more freely as the static immitance was so large.
It is important to note that when analysing tympanogram values it is significant to recognize that it is not a reflection of the energy getting to the inner ear through the middle ear. It is a representation of the amount of energy that is passing through the tympanic membrane to the middle ear cavity. The decoupled portion of the tympanic membrane and ossicular joints absorbs the flow of energy, meaning that not all is passed to the cochlea (Shanks & Shohet, 2009). Hence, tympanometry is not an accurate reflection of a person's hearing sensitivity but how a portion of their ear is responding to sound.
Another point to consider is that tympanometry helps us define whether a system is mass controlled or stiffness controlled but does not tell us what structures are contributing to the to these elements. Therefore, as an audiologist we are limited in what we can diagnose from tympanometry and we require more information from the test battery.
In summary, this study has demonstrated the benefit of why 226 Hz is used as the standard for gathering information about the function of the middle ear. When compared with the 668 Hz and 1000 Hz tympanograms more reliable information can be gathered, where the results suggested the subject has normal middle ear function. With multifrequency tympanometry, more information about the resonant frequency can be gathered. Thus, with the combination of these two techniques in clinical practice an audiologist can make a more conclusive diagnosis of a middle ear pathology and whether a middle ear is mass controlled or stiffness controlled.