MIDDLE EAR PHYSIOLOGY

MIDDLE EAR PHYSIOLOGY

ÞCONCEPTS OF IMPEDENCE

ÞIMPEDENCE MIS-MATCHING

ÞTRANSFORMER  ACTION

ÞCLINICAL TESTING OF MIDDLE EAR

ÞPHYSIOLOGY OF EUSTACHIAN TUBE

ÞTESTING OF EUSTACHIAN TUBE

 

INTRODUCTION:

The middle ear (ME) is a semi-rigid biological gas pocket, which is closed most of the time. The eardrum, and especially the pars flaccida (PF), are displaced if a pressure difference exists between the ME and ambient, making the gas filled cavity partly non-rigid. Under normal conditions, the Eustachian tube (ET) remains closed, but it will open spontaneously to equilibrate large pressure differences. During deglutition, the peristaltic movements of the ET walls regularly inject small boluses of air into the ME cavity. Gas exchange processes through the ME mucosa cause slow pressure variations, and external events such as blowing the nose or taking an elevator can cause sudden and large pressure changes. At hearing threshold, the cochlea detects eardrum motions with amplitudes of less than 0.1 nm, while quasi-static pressures in everyday situations can generate eardrum motions a factor of millions larger. At 120 dB sound level, pressure amplitude is about 10 Pa, while quasi-static pressure variations of several kPa occur in everyday situations.

     The cochlea itself is protected from static overloads because for very low frequencies fluid can move freely at the helecotrema from the scala vestibule to the scala tympani. The large static loads however push the stapes far from its equilibrium position and cause large deformations of the round window. The fact that large static pressures are an inherent part of daily life, and that pressures up to at least 100Pa hardly influence hearing, suggests that ME mechanics protect the round window and the annular ligament of the stapes from these quasi-static pressure overloads. Although it is generally accepted that there is a correlation between ME pressure deregulation and important pathologies of the ME, the underlying basic processes still elude us, so more quantitative insight is needed in the several mechanisms of ME pressure regulation. To understand the mechanisms and effects involved with quasi-static high amplitude pressure variations, we need to know how ME pressure varies in normal circumstances, investigate both fast and slow regulation mechanisms, and determine the effect of such pressures on middle ear mechanics.

                       

1.1 Pressure in the normal ME:

To understand the effect and importance of the different regulatory mechanisms, it is first of all necessary to know how M.E pressure varies over time, but little quantitative data are available. ME pressure can be assessed in two fundamentally different ways: indirect estimation from tympanometric recordings, and direct pressure measurements in the middle ear or the Mastoid.  The tympanometric approach is easy, and as it can be performed in any clinical setting with standard equipment, indirect measurement has, however, several important drawbacks. First of all, a basic assumption is that the magnitude of the impedance of the ME mechanical system is at a minimum when M.E pressure equals the ear canal pressure. This assumption is not necessarily true: it may well be that sound transfer is actually better when the ear is at a slight over or under pressure. Furthermore, recording a tympanogram takes at least several seconds, so the time resolution of the method is inherently very low, and in practice one can only perform the test a limited number of times without discomfort to the patient. To include the pressure regulation effect of the eardrum, it is, however, necessary to measure ME pressure with the eardrum intact. To detect changes in fast and small pressure buffering mechanisms, a good pressure measuring resolution, combined with a small sampling interval is necessary. We have developed a system which meets these requirements, and we will show preliminary results.

Pressure regulation by pars flaccida deformation:

     Deformation of the eardrum leads to changes of the ME volume and can therefore (partly) compensate the effect of external pressure changes. It is a very fast reacting mechanism and especially the soft PF may play an important role. The PF is commonly involved in many pathologies of the ME, but its actual function still remains unclear. Traditionally, the PF is seen.

As a regulator of static ME pressure: the easily bending membrane can reduce pressure by changing the ME volume. However, only few quantitative measurements of this deformation as a function of ME pressure are available.

  As we studied earlier the primary structures of the middle ear are the tympanic membrane, the ossicles and the entry to the cochlea, the oval window. These are the players in one of the important evolutionary drama of the auditory system.

                     

 MIDDLE EAR TRANSFORMER MECHANISM

The ratio between the impedances of the cochlear fluids and the air is approximately 4000:1 .To find out how much energy would be transmitted from the air to the cochlea without the middle ear, we apply simple formula

                T = 4r/(r+1)²

T – Transmission and r is the ratio of impedance.

This result is approximately 0.001 .In other words only about 0.1% of air bone energy  would be transmitted to the cochlea, while about 99.9% would be reflected back. This corresponds to a 30 dB drop going from air to cochlea

               The middle ear thus step up the level of air bone sounds to overcome the impedance miss match between the air and the cochlear fluids ,early place theory held that the middle ear transformer was the source of various non linearitites in hearing such as perception of combination of tones. These distortion products of the middle ear’s hypothetical non linear response were ostensibly transmitted to the cochlea, where the non linearitites were analyzed according to the place principle as though they were present in original signal .However Waver and Lawrence have demonstrated that the middle ear performs its function with elegant linearity. And we must accordingly regard it as a linear transformer.

Components of the middle ear transformer, viewed as a system of pistons connected by a folder

Lever. A, area; p, sound pressure; l, length. Subscripts: d,eardrum; m, manubrium of the malleus; i, long crus of the incus; s, stapes footplate.

Components of the middle ear transformer, viewed as a system of pistons connected by a folder

·         Lever.

·         A: Area

·         Ps: Sound pressure.

·         L: Length.

·          Subscripts:

Ø  d: Eardrum.

Ø  m: Manubrium of the malleus.

Ø  i: Long crus of the incus.

Ø  s: Stapes footplate.

Sound reaches the ear by way of the air ,a gas ,.On the other hand the organ of corti is contained within the cochlear fluids which are physically comparable to sea water .This difference between these media is of considerable important to hearing .Air offers less opposition to the flow of sound energy or impedance than does the fluids ..fluid’s impedance  is greater than that of the air ,there is a impedance mismatch at the boundary between them .The middle ear system serves as an impedance matching transformer that makes it possible for the sound energy to be efficiently transmitted from the air to the cochlea.

INCUDO MALLEAL JOINT

EAR DRUM    (DECOUPLED)

INCUDO STAPEDIAL JOINT

  As in any other system, the impedance of the middle ear is due to its stiffness, mass and resistance.

Figure is the block diagram of the middle ear with respect to its impedance revised from Zwislocki’s earlier model .The first box represents the middle ear cavities which contribute significantly to the stiffness of the system .The next two boxes ear drum/malleus and eardrum (decoupled) should be thought together .The former represents the proportion of sound energy transmitted from the drum to malleus. It includes the inertia of the malleus, the elasticity of the drum, tensor tympanic muscle, and the malleus ligaments and the friction caused by the strain on these structures. Eardrum decoupled is the proportion of energy diverted from the system when then drum vibrates independently (decoupled from the malleus) which occurs particularly at high frequencies .The test box labeled ‘incus is the effective mass of the incus and the stiffness of its supporting ligaments .The energy lost at two ossicular joint is represented by incudo malleus joint and incustapeal joint which shunt energy off the main lane of the diagram. The last box shows the effects of the stapes, cochlea, and round window in series .The attachment of the stapes as well as the round window membrane contribute to the stiffness components .Most of the ears resistance to due to cochlea .Zwislocki has pointed out that a major effect of this resistance is to smooth out the response of the middle ear by damping the free osscilation of the ossicular chain.

FACTORS CONTRIBUTING TO THE MIDDLE EAR MECHANISM   

Several factors contribute to the transformer function of the middle ear .They includes

(1) Areal ratio of the ear drum to the oval window.

(2) Curved membrane mechanism of the drum.

(3) Lever action of the ossicular chain.

(1) AREAL RATIO OF THE EAR DRUM TO THE OVAL WINDOW

Since the periphery of the drum membrane is fixed and rigidly fastened to the tympanic annulus ,something less than the total area must be regarded as effective in transmitting vibration .

  We know that pressure is equal to force F per unit area or P =F/A. If we therefore exert the same pressure over two areas ,One of which is five times larger than the other ,then the pressure of the smaller surface will be five times greater than the other.

 The area of human eardrum is roughly 64.3mm²,while that of the oval window is about 3.2mm².Thus the drum : oval window ratio is about 20: 1 .If we assume that ossicles act as a simple rigid connection between the two membrane ,the there would be a pressure application by a factor of 20:1 going from the ear drum to the oval window. So we must realize that although the force exerted on the drum membrane is the same as the force exerted on the stapes foot plate, the pressure at the stapes is increased by a factor equal to the ratio of the effective areas.

 

P1 = F1/A1              P2 =F1/A2      =   P1A1/A2

Thus pressure is the product of the ratio of the two areas .Using an ossicular chain lever ratio of 1.31 to 1 as obtained by Dahman the cobined ratio provided by the transformer action of the middle ear is the product of the ossicular chain ratio and effective areal ratio.

         

EAR DRUM MECHANISM

Qualitative description of drum membrane movement are difficult to obtain simply because of the minute displacement that occur in response to sound .A sound intensity level that are nearly painful, the displacement at low frequencies may amount to about one –tenth of the milli meter, but a comfortable sound intensities (56 DB SPL) displacement is on the order of 5 A⁰ .( 1 A⁰ is equal to 10^-8 cm )

Helmholtz suggested that the ear drum contributes to the effectiveness of the middle ear transformer by lever action according to the curved membrane principle.  The Dum’s rim is firmly attached to the annulus and curves down to the attachment of the malleus, which is mobile as in the figure

The curved membrane principle (see text).  Adapted from Tonndorf and Khanna (1970), by permission of Ann. Otol.

A given force increment f thus displace the membrane with a greater amplitude than it displaces the manubrium .Since the products of force and distance  (amplitude of displacement )on both legs of the lever are equal( F1D1=F2D2),the smaller distance travelled by Manubrium is accompanied by a much greater force .In this way ,Helmholtz proposed that lever action of the ear drum would result in amplification of force to ossicles.

   

 Bekesy result for a 2000 Hz probe tone in the form of equal displacement contours. For frequencies up to approximately 2000Hz ,the drum moved as a stiff plate or piston ,hinged superiorly at the axis of the ossicles .The greatest displacement occurred inferiorly .Bekesy attributed the drum’s ability to move in this manner ,without significant deformation to a highly elastic and loose fitting fold at its inferior edge .Above about 2400Hz the membrane’s stiffness broke down and movement of the Manubrium lagged behind that of the membrane rather than being synchronized with it. At high frequencies the pattern broke up and membrane vibrated segmentally. The stiffly moving portion of the drum had an area of 55 mm² out of a total area of 85 mm². This area constitutes an effective area for the eardrum of about 55 mm2/85 mm2=65% of its total area. Using Bekesy’s values, the ratio of the area of the entire eardrum to the area of the oval window would be 85 mm²/3.2 mm²=26.6 to

1.     However, applying the eardrum’s 65% effective area to the overall ratio results in an effective area ratio of 26.6×0.65=17.3 to 1.

2.    

3.7 Vibration patterns of the cat’s eardrum in response to 600 Hz tone. From Tonndorf and Khanna

In 1968 Tonndorf and Khanna studied the pattern of membrane vibration using time averaged holography. Time-averaged holography is an optical method that reveals equal-amplitude (or isoamplitude) contours as alternating bright and dark lines on a vibrating membrane. These contours show that the eardrum actually does not move as a stiff plate. Instead, there are two areas of peak displacement revealing a buckling effect in the eardrum vibration pattern that is consistent with Helmholtz’s curved-membrane concept. This mode of vibration is seen up to about 1500 Hz. The pattern becomes more restricted at higher frequencies, and increasingly complex sub patterns occur in the vibrations as frequency rises above 3000 Hz. The curved Membrane principle contributes to the middle ear transformer ratio by a factor of 2.0 based upon the cat data. If we accept an area ratio of 34.6 to 1 for the cat, then the middle ear transfer ratio as of this point becomes 34.6×2.0=69.2 to 1. This value must be multiplied by the level ratio of the ossicles to arrive at the final transformer ratio of the middle ear.

OSSCULAR LEVER ACTION

  Helmholtz propose  that non linear distortions are introduced by the ossicular chain, and are largely due to what he conceived of as a cogwheel articulation between the malleus and incus .This situation would allow for relative movement in one direction at the malleo incincudal joint. The resulting distortion will simulate the cochlea at places corresponding to the receptors to the frequencies as though they were present in the original signal. Barany proposed that except during intense stimulation these two bones are rigidly fixed at the malleo incudal joint and move as a unit in response to the sound stimulation.

              

The nature of stapes movement in cadavers in response to stimuli presented at

(a)   moderate levels and (b) very intense levels. From Bekesy (1936).

 Bekesy has reported that the stapes moves differently in response to moderate and intense stimulation in human cadavers .At moderate intensities,the stapes foot plate rocks with a piston like motion in that oval window ,with greater amplitude anteriorly. Intense stimulation result in rotation of the foot plate around its longitudinal axis substantially reduces the energy transmitted to the cochlea which most likely serves as a protective device.

The axis of rotation of the ossicular chain and the ossicular lever mechanism. Based in part on drawings by Barany (1938) and Bekesy (1941)

Measurements using advanced optical techniques have revealed that the motion of the malleus is frequency-dependent, so that its vibratory pattern is essentially in one dimension below 2500 Hz, but involves an elliptical path in all three dimensions, as opposed to having a single axis above 2500 Hz (Decraemer et al., 1991, 1994).

 The ossicular chain rotates around in axis corresponding to a line through the long process of the malleus and the short process of incus .The ossicular chain is delicately balanced around its centre of gravity so that the inertia of the system is minimal. The osicles act as a lever about their axis .Lever ratio is in the order of 1.3 in humans and 2.2 in cats, because of the interaction of the curvature of the drum and the length.

MIDDLE EAR OSSICLE INERTIA

The three middle ear ossicles are suspended by several ligaments, the TM and two muscle tendons. This becomes a mechanical mass-spring system that, due to inertial forces on the masses, constitutes different vibration than the surrounding bone during BC stimulation. This difference motion between the stapedius footplate and the cochlear promontory provides a stimulation input to the cochlea  The importance of the ossicles inertia component can be estimated by comparing the stapes footplate motion at threshold stimulation by AC and BC; this was done in  and some of those results are presented in Fig 2.

Fig.  Stapes footplate (solid line) and malleus umbo (dotted line) motion at BC threshold stimulation compared with the motion at AC threshold stimulation.The vibration of the umbo and footplate at threshold vibration is 5 to 15dB lower with BC stimulation compared with AC stimulation at frequencies between 0.2 and 1.5 kHz. This result indicates that middle ear inertia is not an important contribution to BC perception at these frequencies. At frequencies above 4 kHz, the comparison is   valid since the stimulation is probably no longer through the oval window. However, the similarity between the ossicle threshold motion between 1.5 and 3.5 kHz indicates that the inertial effects of the ossicles can contribute to BC sound perception within this frequency range. Also, lesions and manipulations of the ossicles provide results indicative of possible importance of ossicle inertia at the mid-frequency range [10].

MIDDLE EAR MICROMECHANICS

    Only modern techniques using laser Doppler vibrometry (LVD) ,squid technique ,Mossbauer effects ,or hydrophone sound pressure measurements in the cochlea allow an investigation of acoustic middle ear mechanics in physiologic sound pressure dimension below 100 dB.

 Vibration in the physiologic range of acoustic middle ear function are too small to be visible with optical methods .Even the most powerful optical microscopes are unable to scan below1 μm because of the wavelength of light .Considering that the amplitude of stapes is approximately 20 to 30nm at 1 Pa (corresponding to 94 dB), a vibration of middle ear structures which is visible with optical methods has no correlation to the motion in physiologic sound conduction.

The middle ear transport the sound into the inner ear fluid .For the transport of this tiny amount of vibrational energy, the ossicular chain vibrates as a unit as one solid body. The vibration mode of tympanic membrane and the stapes can be considered as piston like. This motion however suggests that a fixed rotational axis of the ossicular chain (which previously was assumed to act as a lever mechanism in amplifying sound pressure) doesn’t exist for sound induced vibration. Modern experiments demonstrates that the position and orientation of the rotational axis changes with every frequency .A fixed rotational axis, however is characteristic of the movements induced by atmospheric pressure variation.

 The ossicular joints are functionally fixed during sound transport. Therefore a columella transmit sound as effectively as the articulated chain .The ossicular joints of the mammalian middle ear do not seem to be necessary for the acoustic function of the middle ear.

        As a sensitive pressure receptor, the middle ear is exposed constantly to the changing static pressure of the environment .These changes of ambient air pressure induce huge but slow unidirectional displacement of the tympanic membrane and the malleus of as much as 1nm .With such displacement the joint between the malleus and incus start to glide .Because of the intricate construction of joints and the supporting ligaments, the incus is forced predominantly upward or downwards. Because of the joint between the incus and stapes can glide, the stapes and consequently the inner ear are uncoupled from the extensive displacement of the tympanic membrane and the malleus .The maximal piston like inward or outward movement of the stapes never exceeds 10 to 30 μm. Regardless of the pressure in the external ear canal.

The micromechanics of the ossicular chain at static air pressure variation can be explained by the figure (4). An inward movement of malleus handle, as caused by difference in air pressure across the tympanic membrane or by the pull of a tensor tympani muscle, resulting in out ward and upward movement of the malleus head. The anvil, which is supported only at its short process will be pushed upwards. This movement results in an upward and forward rotation of the anvil’s process. The stapedial muscle is stretched by this movement.

 The change in the direction of motion with in the ossicular chain explains the vertical position of the incudo- stapedial joint .its orientation ,perpendicular to the piston like acoustic vibration ,concurs with the upward and downward movements of the anvil with variation in atmospheric pressure in the plane of the joint’s surface.

 This mode of motion within the ossicular chain also explains Von Bekesy‘s findings that, at very loud noise .This sound pressure level is definitely in the atmospheric range, when the joints glide and the incus move upward and downward .Von Bekesy himself remarked this movement at the incudo-stapedial joint. In his original drawing a arrow can be seen pointing upward at the long process of the incus .He did not pursue this finding however.

        An unimpeded gliding in the ossicular joints is the pre requisite for this mechanism. The joint cartilage is highly differentiated, and the construction of the joint is similar to the big joints of the body to guarantee only the smallest frictional resistance .In a scanning electron microscopy, even in the 2000 X magnification, the surface of the malleus joint appears to be totally smooth. In ossicualr chain molecular sized amplitude of the sound vibration are far too small to provide this movement. Furthermore the joints are functionally fixed for the transmission of the sound .A gliding the in the joints would result in a loss of transport of acoustic energy and in the non linear distortion of sound transmission.

            This movement of the ossicles induced by the contraction of the two muscle in the middle ear,the tensor tympanic muscle and the stapedius muscle .The acoustical function of these muscles is still obscure .The explanation that they protect the inner ear by an acoustic reflex is no longer considered valid : sound energy must enter the inner ear before it can elicit the acoustic reflex; Furthermore, an evolutionary need to develop an inner ear protection against high –energy noise is not so evident, because such loud sounds do not occur in a natural environment. The suggested protection obtained by stapedius muscle contraction is also implausible; considering its effect on sound transport: sound transmission is attenuated only in the unharmful low frequency of 250 Hz to 500 Hz for approximately 10 to 20 dB.The transmission of the higher dangerous frequencies of 2 to 4 kHz will actually be increased by few decibels. This shift of resonance of the middle ear is caused by stiffening  of the annular ligaments as the result of the muscular pull and the tilting of the stapedial supra structure .According to Accommodation theory ,they function of the stapedius muscle in accomplishing this resonance shift offers a survival advantage.

         This acoustic effect, however cannot explain the function of the antagonistically acting tensor tympanic muscle .In humans this muscle does not react at all to the acoustic stimuli and its contraction does not influence sound transmission significantly. It is conceivable that the middle ear muscle accomplish a non acoustic purpose, comparable to the action of the muscles of skeleton: the muscles by their contraction move the joints and thus maintain the circulation of the synovial fluid, which is necessary for lubrication and nutrition of the hyaline cartilage of the joints.

     A further finding reinforces this theory of non acoustic function of the middle ear muscle: if sound pressure is the physiologic stimulus for a contraction of the stapedius muscle, the muscle would be expected to atrophy in a deaf ear. In cochlear implant surgery, however stapedius reflex control reveals a perfectly contracting muscle even in the ears that have been deaf for decades .Therefore it can be assumed that the function of the middle ear muscle is to preserve the regular joint function of the ossicular chain.

      The middle ear anatomy shows that the two muscles do not pull directly against each other, as would expected from the arrangement of other antagonistic muscle.(eg.the muscular biceps or muscular triceps brachii)The tendons of the middle ear muscles are aligned perpendicularly to each other .This orientation consequence of the change of the direction of motion with in  the ossicular chain : a pull of tensor tympanic muscle will result in a stretching of the stapedius muscle and vice versa.

      The non acoustic construction and function of the middle ear is also apparent in the arrangement of the stapes and its muscle .Former theories assumed an axis running through the posterior pole ,which would result in the complete foot plate tilting out wards at a contraction of the stapedial muscle .This mode of motion ,however would cause a significant drag at the perilymphatic fluid .Specific measurement in fresh temporal bones showed that the foot plate rotates around the central axis at the pull of the stapedial muscle. This mode of motion ,which is caused by the uniform elasticity of the annular ligaments, result in an outward movement of the anterior pole and an inward displacement of the posterior half of the foot plate .The opposite movement of the poles neutralize their effect on the cochlear fluid but a generate a maximal movement within the incudo-stapedial joint.

       Another puzzling detail is encountered when the middle ear construction is considered from a non acoustic point of view .The arch of the stapedius supra structure is asymmetric in more than two third of all humans stapes. There is no obvious acoustic reason for this construction .The piston like vibration of the stapes demands a symmetric supra structure for optimal acoustic quality.

ROLE OF ROUND WINDOW

The middle ear space or tympanic cavity is an air filled cavity, when the TM vibrates the ossicles transfer the vibrations to the oval window while the secondary TM of the round window is set into vibration because of the vibrations of the vibrating air column in the tympanic cavity, which causes movement of the fluid in scale vestibule (oval window) and scale tympani (round window). The basilar membrane which lies in between the two fluid columns is set into vibration but in opposite direction. The amplitude of vibration in the scale tympani is lesser because dampening of energy (air), so the effective movement of the basilar membrane will be in downward direction.

Hughson and Crewe utilized electrical activity of cochlea and auditory nerve as measure of hearing. They placed cotton pledgets on the round window, the electrical potentials increased when compared to normal conditions. If the walls of the round window and oval window were rigid, then there would be no vibration of the cochlear fluids or basilar membrane. However, round window is not rigidly fixed, but partially protected from air borne sound so some improvement in hearing occurs due to the presence of round window.

MIDDLE EAR MUSLE FUNCTION

The stapedius muscle and the tensor tympanic muscles are pinnate muscles i.e, consists of many short fibres directed obliquely to impinge on a tendon at the mid line because of which the muscles have greater capacity for excerting tension with little linear displacement.

The tendons of the muscles are elastic to serve two purposes:

1) Dampening the vibrations of the ossicles

2) Render the muscular retractions less sudden in onset and slower

A second unique feature is that the muscles are enclosed within body cavities which reduce the interaction of muscular vibration with sound transmission by generating sub harmonics and also decreasing the mass of the ossicles.

Contraction of the tensor tympanic muscle draws the malleus medially and anteriorly which is at right angles to the movement of the ossicular chain and increases the stiffness of the TM. Contraction of the stapedius muscle exerts force on the head of the stapes and draws it posteriorly at right angles in the right angles to the rotation of the ossicular chain.

Both the muscles act at directions opposite to each other and the rotation of the ossicular chain.

The acoustic reflex changes the mechanical properties of the transmission system of the ME and the mechanical resistance, which is explained by theories as follows:

Ø  The intensity control theory supposes that the muscles contracts in response to certain critical levels of sound intensity to protect the ear.

Ø  The accommodation theory states that the muscular contraction acts as a dampening mechanism which selectively absorbs sound energy at certain frequencies there by selectively increasing the sensitivity of the ear.

Ø  The fixation theory states as the ossicular chain rocks forth and back during the transmission of the sound, the muscle contraction prevents the movements from surpassing certain critical limits.

Ø  The labyrinthine pressure theory states that movement of stapes results in increase and decrease of impedance respectively.

Tympanic muscles support the ossicular chain because, when cut the ossicular chain becomes loose and flaccid. Instead increased tension due to muscle contraction raise impedance which reduces low frequency transmission.

This describes that the ME acts as a linear system at least up to 100dBSPL and higher in some species. In the studies which form the foundation of the conclusion was done on animals whose muscle system was deactivated. ME muscle reflex is non linear, so when awake the function of these ME muscles comes into action which clearly states that the ME would be linear up to 60-70dBSPL which is the acoustic reflex threshold. Consequently the action of the ME muscles will make the whole system nonlinear.

This was experimented using the carefully constructed model of ossicles (Stulhman), which describes the source of the non linearity as the malleus-incudal joint. When the joint is rigid the function is linear but when the articulation becomes loose to ratio of the movement of the malleus to the incus becomes 2:1 instead of 1:1 which results in non linearity. This dislocation acts as a protective mechanism against large inward pressures.

Though the malleus-incudal joint acts as a one of the source of nonlinearity, its not the only factor. The other factors contributing to the nonlinearity includes the elasticity of the TM, ligaments. Low frequencies are mostly the strongest component of the signal so dampening of this law is considered as a protective function.

NON-LINEARITY OF THE MIDDLE EAR

The relative straight forward description of a vibratory system is valid only if the system is linear. To study the transfer function of the system the first step is to check if the system is linear. Mundie 1963 measured acoustic impedance of guinea pigs at 100 and 130dBSPL, the results indicate that the ME is nonlinear.

Fisher et al 1967 measured ear drum movement in cadaver ears with capacitor probe and demonstrated that at 250Hz amplitude versus SPL function is linear between 64 and 104dBSPL, however noticeable nonlinearity at 114dBSPL.

Dollas and Linnel 1966 demonstrated that sub harmonics appear above 110dBSPL with real thresholds, indicating the presence of nonlinearity.

Rubinstein et al 1966 measured the displacement magnitude of the stapes footplate in cadaver ears. They found proportional increase in amplitude up to 104dBSPL at 3 frequencies.

Guinan and Peake 1967 using stroboscopic elimination studies the stapes displacement and found that it is proportional up to 130dBSPL for frequencies below 1500Hz and that and even wider linear range might exist at higher frequencies.

MIDDLE EAR RESPONSE

Bekesy reported that the resonant frequency of the middle ear is in the 800-1500 Hz region. Mollar found the major resonance peak of the middle ear to be about 1200 Hz region with a smaller resonant peak around 800Hz .This effect is due to the middle ear mechanism itself, which is stiffness controlled below the resonant frequency .There is virtually no reactance between 800 and 6000Hz, indicating that the energy transmission from the drum to cochlea is maximally in this range..Positive reactance takes over at higher frequencies as a result of the effective mass of the drum and ossicles .So sound transmission through the middle ear to be frequency –dependent with a emphasis on the mid frequencies.

              According to umbo and stapes displacement measurements in temporal bone and live humans, in some 30% of ears tympanic membrane doesn’t produce smooth frequency response over the important hearing frequencies .Measurement of umbo displacement ( Richard  L.and Goode M.D.,1995 ) for a constant sound pressure level (SPL) at the TM at 22 frequencies between 200 and 6000Hz have shown peaks (+) and valleys (-) of more than 10 dB ,particularly near 3000Hz. Testing at a larger number of frequencies would be expected to show more peaks and valleys .This is possibly the result of previous injury ,both major and minor ,to the TM and perhaps to the ossicle .It may also be an aging effect. These peaks and valleys in middle ear sound transmission would not be expected to provide excellent sound fidelity. The majority of the ears will have a relatively smooth frequency response. Higher sound intensities the middle ear muscles will have function of smoothing out the peaks and valleys by dampening ossicular vibration. It s well known that however that the middle ear muscles have essentially no acoustic function in the human above 1000Hz .This of course ,is the major frequency range involved with presbyacusis.Ear drum becomes increasingly inefficient above 1000Hz and that there is also translational movements.

ACOUSTIC IMMITTANCE

    Acoustic immitance is a general term referring to either acoustic impedance or acoustic immitance .Opposition to the transfer of acoustic energy is called acoustic impedance .If we measure in terms of the resulting energy flow as a result of the transfer of air pressure changes we would use the term acoustic admittance .As the sound flow increases from the system ,such as the human middle ear ,the acoustic admittance will increase and the acoustic impedence offered by the system will decrease in a reciprocal manner.

BASICS OF MEASUREMENTS

  At least three primary subsystem are fundamental to any instrument used for acoustic immittance measures in human ears .The measurement of of acoustic immittance is performed by introducing an acoustic (probe)signal to the ear and measuring sound pressure level (SPL) of the ear canal .These measurements are based on the acoustic measurement principle .This accounts for the small ‘a’ subscript for admittance (Y) and impedance (Z) measures .The probe signal introduced in into the ear canal is usually a tone ,and the  SPL of the tone serves as an indirect index of acoustic admittance or impedance .The SPL of the probe signal measured at the probe tip is directly proportional to the acoustic impedance offered by the ear at that particular point. higher the measure SPL of the probe tone ,higher the acoustic impedance represented by the ear under measurement ,conversely the higher the SPL of the probe tone ,lower the acoustic admittance offered by the ear under test.

 

Analysis system ( Acoustic immittance measures are recorded )

 

The probe contains opening connected to three basic sub systems of the instruments:

(1) An air pump used to introduce air pressure in the ear canal and a manometer for   monitoring the air pressure.

(2) A miniature loud speaker used for the introduction of a probe tone.

(3) A microphone with an analysis system used to monitor the sound pressure level of the probe tone.

ACOUSTIC IMPEDENCE

Acoustic impedance is a general term referring to the total opposition to sound flow offered by the system .The middle ear system ,however is composed of different mechanical structures that react to a force (such as an input sound pressure) in variety of ways .A given acoustic impedance measure is determined by the complex ratio of the applied force  (sound pressure) to the velocity (or sound  flow).The manner by which different structures oppose sound flow differs in a complex fashion across components of the middle ear .The volume of the external auditory canal ,the tympanic membrane ,the interconnected cavities of the middle ear, the ossicular chain and the coupling of the stapes foot plate to the oval window of the cochlea all contribute in a complex fashion to the overall acoustic impedence measured at the probe tip .The acoustic impedance measured at the tympanic membrane is controlled by the mass of the middle ear ossicles ,stiffness of the ossicular ligaments and muscles ,stiffness of the tympanic membrane and round window membrane ,the stiffness of the air contained in the tympanum ,the mass and friction that result from air movement within the tympanum and finally the impedance  (primarily resistance )offered by the coupling of the stapes foot plate to the cochlea at the oval window. Acoustic impedance at the TM ,then is determined by different masses ,stiffness, and frictions, In terms of these components to the overall opposition to the flow of energy ,the components may be categorized as in phase components  (those that occur simultaneously with the applied force) and out of phase components (those that lead or lag the applied force ).

ACOUSTIC ADMITTANCE (Y_a)

   The acoustic admittance and the acoustic impedance are reciprocal measures. The vast majority of commercially available clinical instrument provide for acoustic admittance measures primarily due to simpler mathematic and engineering principles associated with acoustic admittance measures as opposed to acoustic impedance measures. The same measurement principles we discussed under acoustic impedance also are fundamental to acoustic admittance measures. The specification of dissipative and storage components and computational methods, however, differ for the 2 different measurement systems.

Whereas acoustic impedance represents the opposition to the flow of acoustic energy, acoustic admittance represents the ease of sound flow. The measurement unit for acoustic impedance is the acoustic ohm; acoustic admittance measures are expressed in acoustic mhos. Acoustic admittance measures in human ears are relatively small in magnitude and accordingly most clinical measures of acoustic admittance are expressed in acoustic milli ohms.

As Y_a and Z_a are reciprocals, the phase angle associated with each term is of the same magnitude but of opposite direction. If the acoustic impedance phase angle is negative, then the acoustic admittance phase angle (Ø_y) is positive.

The acoustic admittance is also determined by both in phase (real) and out of phase (imaginary) components contributing to the flow of acoustic energy the real component of acoustic admittance is acoustic conductance (G_a) and the imaginary component is acoustic susceptance (B_a). The acoustic susceptance is of 2 types’ i.e. compliant acoustic susceptance and mass acoustic susceptance. The reactive elements acoustic admittance is plotted on the imaginary (j) axis in the same manner as a acoustic reactances. Because of the reciprocal relation between acoustic admittance and impedance the positive and negative imaginary axis on which reactive components of admittance are plotted are reversed relative to those for impedance. Specifically mass acoustic susceptance (-jB_a) is plotted on the negative axis and the compliant acoustic susceptance (jB_a) is plotted on the positive axis. All the acoustic admittance and impedance are simple reciprocals acoustic reactance and susceptance are not. Acoustic reactance and susceptance will be reciprocals, if and only if, the acoustic resistance is 0. However with acoustic resistance and reactance values are finite in the case of impedance measure of human ears. Thus, there is not a simple reciprocal relation between acoustic reactance and acoustic resistance. Rather, a given acoustic conductance or acoustic susceptance is determined by the value of acoustic resistance and real.

PHYSIOLOGY OF EUSTACHAIN TUBE

Poe and Ilmari had reported the use of DSVE to analyze ET motion using rigid and fiber optic nasal endoscope, introduced at nasal pharyngeal orifice of the ET. Video recording taken during rest. Swallowing and yawning were analyzed in slow motion.

Four consistent sequential dilatory movements were noted in all normal subjects: (1) The palate elevated and the lateral pharyngeal wall and medial cartilaginous lamina moved medially.

(2) The lateral pharyngeal wall moved laterally.

(3) Dilation of the tube began with lateral movements of the lateral tubal wall, beginning at the nasopharyngeal orifice and propagated smoothly towards the isthmus.

 (4) Opening of the cartilaginous part of isthmus completed the tubal dilation process.

The physiologic functions of the Eustachian are as follows:

·         Ventilation or pressure regulation of the middle ear.

·         Protection of the middle ear from nasopharyngeal secretions and sound pressures.

·         Clearance or drainage of middle ear secretions into the nasopharynx.

·         Ventilation or pressure regulation

The normal Eustachian tube at rest is collapsed, with perhaps slight negative middle ear pressure. Repeated opening of the Eustachian tube actively maintains normal atmospheric pressure.

The Eustachian tube opens upon swallowing or yawning by contraction of the tensor veli palatini muscle. Defective tensor veli palatini muscle function in cleft palate results in Eustachian tube dysfunction. The role of the levator veli palatini muscle is unclear. Its contribution in opening the Eustachian tube has been questioned.

Eustachian tube ventilatory function is less efficient in children than in adults. In addition, repeated upper respiratory tract infections and enlarged adenoids in children further contribute to the increased incidence of middle ear disease in children. However, as children grow, Eustachian tube function improves as evidenced by the reduced frequency of otitis media from infancy to maturity.

Normally, the Eustachian tube opens frequently, stably maintaining the middle ear pressure between +50 mm and -50 mm H₂ O. However, pressures above and below this range do not necessarily indicate middle ear disease.

About 1 ml of air or gas may be absorbed from the middle ear in 24 hours. The mastoid cell system is thought to function as a gas reservoir for the middle ear.

·         Protection

The Eustachian tube is closed at rest. Sudden loud sounds are thus dampened before reaching the middle ear through the nasopharynx.

The Eustachian tube drains normal secretions of the middle ear by the mucociliary transport system and by repeated active tubal opening and closing, which allows secretions to drain into the nasopharynx.

A derangement in the closed middle ear system, such as tympanic membrane perforation or after mastoid surgery, sometimes results in reflux of nasopharyngeal secretions into the tube and can cause otorrhea.

Similarly, forceful nose blowing creates high nasopharyngeal pressure and may force nasopharyngeal secretions into the middle ear. Laryngopharyngeal reflux (LPR) was recently implicated in the etiology of otitis media with effusion (OME). Al-Saab et al (2008) demonstrated the presence of pepsinogen in 84% of middle ear effusions (MEEs) at concentrations 1.86 to 12.5 times higher than that of serum.

Conversely, a relative negative middle ear pressure, as occurs in aircraft or scuba diving descent, may lock the Eustachian tube. This leads to stagnation of secretions, and effusion collects in the middle ear as otitic barotraumas evolves. Inflation of the Eustachian tube by the Valsalva maneuver or by politzerization can break the negative pressure in the middle ear and clears the effusion.

The middle ear is also protected by the local immunologic defense of the respiratory epithelium of the Eustachian tube, as well as its mucociliary defense (clearance). A pulmonary immunoreactive surfactant protein has been isolated from the middle ears of animals and humans. It is thought to have the same protective function in the middle ear.

·         Clearance or drainage

Drainage of secretions and occasional foreign material from the middle ear is achieved by the mucociliary system of the Eustachian tube and the middle-ear mucosa and muscular clearance of the Eustachian tube, as well as surface tension within the tube lumen.

The flask model proposed by Bluestone and his colleagues helps to better explain the role of the anatomic configuration of the Eustachian tube in the protection and drainage of the middle ear.  In this model, the Eustachian tube and middle ear system is likened to a flask with a long narrow neck. The mouth of the flask represents the nasopharyngeal end, the narrow neck represents the isthmus, and the middle ear and mastoid gas cell system represents the body of the flask.

Fluid flow through the neck depends on the pressure at end, the radius and length of the neck, and the viscosity of the liquid. When a small amount of liquid is instilled into the mouth of the flask, the liquid flow stops somewhere in the narrow neck due to the narrow diameter of the neck and the relative positive air pressure in the chamber of the flask. However, this does not take into consideration the dynamic role of the tensor veli palatini muscle in actively opening the nasopharyngeal orifice of the Eustachian tube.

FLASK MODEL

The flask model proposed by Bluestone and his colleagues helps to better explain the role of the anatomic configuration of the ET in the protection and drainage of the middle ear. In this model, the ET and the middle ear is likened to be a flask with a long narrow neck. The mouth of the flask represents the nasopharyngeal end, the narrow neck represents the isthmus and the middle ear and the mastoid gas cell system represents the body of the flask. Fluid flow through the neck depends on the pressure at either end, the radius and the length of the neck, and the viscosity of the liquid. When a small amount of liquid is instilled into the mouth of the flask, the liquid flow stops somewhere in the narrow neck due to the diameter of the neck and due to the +ve pressure in the chamber of the flask. However, this does not take into consideration the dynamic role of the TVP in actively opening the nasopharyngeal orifice of the ET.

TUBAL CLOSURE AND OPENING MECHANISM

v  TUBAL OPENING MECHANISM:

As a result of muscular activity the cartilaginous part of the tube will open. The Tensor Palitini is considered as the main dilator while the levator veli palitini muscle forming the part of the bottom of the tube supports the opening. The Tensor Tympani may also play a role in the tube opening mechanism. As stated above, the Tensor Tympani is attached to the neck of the malleus and contraction of the muscle slightly increases the middle ear pressure by the medial placement of the tympanic membrane. The origin of this muscle from the cartilage of the tube as well as the adjoining bone may well influence the opening of the tube at this point. This is also supported by the fact that the tube begins to open from the middle ear end. The contraction of the tensor tympani may facilitate tubal opening.

Another mechanism for the opening of the tube must be taken into account. If the intratympanic pressure for some reason exceeds 100-150mm of H₂O above the ambient pressure, the ET may open spontaneously without any muscular activity is involved. This holds for an increases in air pressure applied from the pharyngeal end of the tube, exemplified in tube or applying +ve pressure in the nasopharynx as in valsalva’s maneuver.

v  TUBAL CLOSING MECHANISM:

In contrast to the opening of tube, closure is exclusively a passive phenomenon. When the muscles relax or when a static pressure no longer keeps the tubal walls separated, the tubal lumen collapses .Ashan showed that the closure of the tube starts at the nasopharyngeal end. In this way it is possible that small air volume are forced into the middle ear during the closing of the tube.cc

 

INFANT VS ADULT EUSTACHIAN TUBE

FEATURES	INFANT	ADULT
1.DIAMETER OF THE ORIFICE	4-5MM	8-9MM
2.LENGTH	13-18MM AT BIRTH	36MM
3.DIRECTION	AT BIRTH 10° WITH THE HORIZONTAL.ITS MORE HORIZONTAL	FORMS AN ANGLE OF 45° WITH THE HORIZONTAL
4.TUBAL CARTILAGE	FLACCID,RETROGRADE REFLUX OF NASOPHARYNGEAL SECRETIONS CAN OCCUR	COMPARITIVELY RIGID.REMAINS CLOSED AND PROTECTS MIDDLE EAR FROM REFLUX
5.BONY VS CARTILAGINOUS PART	BONY PART IS LONGER AND RELATIVELY WIDER	ANTERIOR TWO-THIRD IS CARTILAGINOUS AND POSTERIOR ONE THIRD IS BONY
6.OSTMANN’S PAD	LESS IN VOLUME	LARGE AND KEEPS THE TUBE CLOSED

 

MIDDLE EAR PRESSURE REGULATION: NEW APPROACH

In the new approach, the ET is not only seen as pressure regulator but also a bi-directional conduct for diffusing of gases. The pressure regulating system is constituted by the co operation and continual interplay between these components

Ø  Bi directional diffusion of gas across the middle ear,(involves liberation and absorption)

Ø  Bi directional passage of gas through the ET.(up and down)

Ø  Bi directional exchange of fluid across the capillary production and elimination.

The infra tympanic pressure fluctuates close to the ambient pressure in a dynamic equilibrium. If a disturbance occurs, all three components work together in restoring the pressure, if one of the component is in capacitated, the remaining two can take over. It is seen an increase in the middle ear pressure will occur if:

Ø  More gas is liberated than absorbed

Ø  More  gas passes up than down the tube

Ø  If more fluid is protected than eliminated

In this approach, less attention is paid to equalization of artificially applied pressure in experimental situation and more attention is paid to the protective closing action of the ET in a physiological environment.

The role of cell system:

 The cell system enhances the area to volume ratio and its physiological function facilitates a rapid exchange of fluid and gases dissolved in the blood. This is important for the physiological pressure regulation

The network of blood capillaries in the thin mucosal lining in the cell system is exposed to the prevailing intra vascular to intra tympanic pressure difference. Because the exchange of fluid across the capillary wall depends on a close balance between the hydrostatic and the colloid-osmotic pressure, negative intra tympanic pressure can change the balance .This results in the transudation of fluid through capillary wall to the extracellular space in the mucosal  membrane and in to the lumen of the mastoid air cells.

The hydrostatic blood pressure at the level of the ear also depends on the body on the body position. During rest and sleep when one is lying down, the hydro static pressure across the capillary wall increases, which would shift the balance and transudation of fluid i.e. during active hours of a day when a person is upright, the hydrostatic pressure is lower, which would facilitate re-absorption of fluid.

 

TEST FOR EUSTACHIAN TUBE

A functional and patent ET is necessary for ideal middle ear sound mechanics. A fully patent ET may not necessarily have perfect functioning as in the case with the patulous ET or with mucociliaryabnormalities.Testing of both Eustachian tube patency and function are therefore important.

(A)     PNEUMATIC OTOSCOPY

Permeatal examination of tympanic membrane assesses the patency and perhaps the function of the tube. A normal appearing tympanic membrane usually indicates a normal functioning ET, although this does not preclude the possibility of a patulous tube.

Otoscopic evidence of tympanic membrane retraction or fluid in the middle ear indicates ET dysfunction but cannot be used to differentiate between functional impairment and mechanical obstruction of the tube.Normal,TM mobility on pneumatic otoscopy indicates  good patency of the ET.

(B)     NASOPHARYNGOSCOPY

Nasopharyngoscopy by posterior rhinoscopic mirror examination or more accurately by fiber optic endoscope helps visualization of any mass that may be obstructing the pharyngeal end of the ET.

(C)     TYMPANOMETRY

Measuring the middle ear pressure with an electro acoustic impedance meter helps to assess ET function. High negative middle ear pressure (>100dapa) indicates ET dysfunction. High negative pressure may be seen in individuals with normal hearing. In the presence of tympanic membrane perforation the air passes into the middle ear resulting in a large canal volume on tympanometry.

(D)      IMAGING:  

With the recent development of advanced imaging technology, studies have been used to better define the anatomy and pathology of the ET.MRI has been used to visualize the ET and to assess its anatomy and pathology in patients with nasopharyngeal carcinoma.

CT has also been used to assess the tube in normal individuals, in patients with patulous Eustachian tube and in otitis media. It has also been used in studying ET clearance.

(E)      POLITZER TESTS:

In this air is forced up the ET by having the patient close off their soft palate by either saying ‘k-k-k’ or swallowing water; then blowing air into one nostril while the other one is pinched. This can be unpleasant and should be done with a light touch.

(F)      EUSTACHIAN TUBE CATHETERIZATION

Catheterization for Eustachian tube with a curved metal canula via the transnasal approach has been used for more than 100 years. It can be done with the help of a nasopharngoscope or Trans orally with a 90° telescope.

The catheter is passed along the floor of the nose until it touches the posterior wall of the nasopharynx.The catheter is  then rotated 90° medially and pulled forward until it impinges on the posterior free part of the nasal septum. The catheter is then rotated 180° laterally, so that its tip lies at the nasopharyngeal opening of the Eustachian tube. A politzer bag is attached to the outer end of the catheter and and an auscultation tube with 2 ear tips is used with one tip is used with one tip in the patient’s ear and the other in the examiner’s ear. Air is pushed into the catheter by means of a Politzer bag. The examiner hears the rush of the air as it passes through the catheter into the ET and then into the middle ear.

Successful transferring of applied positive pressure from the positive pressure from the proximal end of cannula into the middle ear suggests tubal patency. Normal blowing sounds mean a patent Eustachian tube and bubbling indicates middle ear fluid. Whistling suggest partial ET obstruction while absence of sounds indicates complete obstruction or failed catheterization.

TYMPANOMETRIC MEASURES

The most common Eustachian tube measures are derived from tympanometric measures .These techniques are based on the assumption that the pressure location of the typanometric peak approximates the resting pressure in the middle ear space. Initially a pretest baseline tympanogram is obtained .Next the subject is instructed to perform a maneur, such as swallowing ,that normally result in opening of ET .A post maneuver tympanogram is obtained to assess tubal opening .Shift of the peak pressures of the tympanogram typically are used as an index of the tubal function lack  of pressure shift indicate tubal function

 Three techniques are used as tests of ET function in intact tympanic membranes:

(1) Inflation –deflation test

(2) Tone bee procedure

(3) Valsalva’s procedure

(4)  Sniff test

1.     INFLATION –DEFLATION TEST

The Inflation –deflation procedure is a pressure swallow technique for assessment of ET function. The test sequence is as follows:

Ø  A pretest, a baseline tympanogram is obtained.

Ø  A high positive pressure (inflation pressure ) or negative ( deflation ) air pressure is introduced into the external ear canal and the patient is instructed to swallow several times.

Ø  A post test tympanogram is obtained after swallowing.

 Tubal opening is indicated by a shift in pressure location for the post tested tympanometric peak relative to the pre test peak .The shift in pressure typically is in the opposite direction from the applied ear canal pressure.

2.     TONE BEE

The tone bee procedure is designed to introduce a negative middle ear pressure via Eustachian tube .The test sequence is as follows:

(a) A pretest baseline tympanogram is obtained.

(b) The patient is instructed to pinch the nose and swallow.

(3) A protest tympanogram is obtained.

   A successive Tone bee maneuver will cause the tube to open and will evacuate air from the middle ear, resulting in a tympanometric peak that is shifted in a negative direction.

3.     VALSALVA

The Valsalva procedure is designed to introduce a positive middle ear pressure via ET .Steps a and care identical to those of the tone bee procedure .In step b however the patient is instructed to close off the nose and mouth and blow gently .A successive Valsalva maneuver will cause the tube to open and allow air to be forced into the middle ear, resulting in a post test tympanogram peak that is shifted in a positive direction.

4.      SNIFF TEST

The Sniff test is designed to determine if sniffing induces significant negative pressure in the middle ear .The procedure is as follows:

 a) The patient performs a Valsalva maneur

(b) The baseline tympanogram is recorded

(c) The patient is asked to close off one nostril with a finger and to take several sharp sniffs

(d) A second tympanogram is recorded

  If the test is positive, the second tympanogram will have a peak that shifted in the negative direction.

ET function can also be measured with inflation –deflation procedure on ears in which TM is not intact.

(a) Positive pressure is increased to 800-400 daPa and the patient is asked to swallow

(b) The negative pressure is decreased to -200 daPa and the person again is asked to swallow

   If the ET is normal pressure will return to 0daPa after several swallows. If the ET is not normal, one of the two patterns will occur

  For the first pattern ,with positive ear canal pressure ,a pressure called opening pressure ,will cause the ET to open and the ear canal pressure to return to a level closer to 0 daPa .Subsequent swallows may allow the pressure to approach ,but not reach,0 daPa.For the negative pressure ,swallows will not cause the ET to open to equalize the pressure .The recorded ear canal pressure will not change, it remains the same.

REFERENCES:

(1)HEARING: An introduction to psychological and physiological acoustics.2^ND edition .Stanley A. Gelfend.

(2) Acoustic Immittance Measures in clinical audiology: Terry Wiley, Cynthia G.Fowler.

(3) Hand book of clinical audiology: 5^TH edition, Jack Katz.

(4)Internet.

(5) George .A. Mathew, George kuruvilla, Anand job  (2006) .American journal of otolaryngology.28(2007) 91-97.Dynamic slow motion video endoscopy in Eustachian tube  assessment.

(6)Zemlin.