Department of Otorhinolaryngology, University Hospital, Basel, Switzerland
Even though the diagnosis of otosclerosis is confirmed definitively during surgery, preoperative diagnosis and determination of the indication for surgery are made based on audio-logical evaluation. Audiological tests should firmly establish a conductive component to hearing loss. The measurement of pure-tone air and bone conduction thresholds has limitations that prevent an accurate diagnosis based solely upon these test results. Such limitations include general variability of threshold measurements, individual variations of tests in the bone conduction mode, and complex interactions between changes of middle ear mechanics and threshold. Objective audiometric tests should be added because of these uncertainties. The presence of otoacoustic emissions effectively excludes a diagnosis of otosclerosis. Standard clinical immittance measurements are used to confirm (or exclude) otosclerosis, the typical pattern being a normally shaped tympanogram and absent stapedial reflexes. Multifrequency tympanometry adds little information. Aside from establishing a preoperative diagnosis, audiological evaluation provides quantification of hearing loss, upon which the indication for surgery is based. Moreover, it lays the foundation for evaluation of surgical success and outcome measures. For both of these aims, speech audiometric tests such as a speech reception threshold should be included in the preoperative audiological evaluation of patients with otosclerosis.
Copyright © 2007 S. Karger AG, Basel
Audiological evaluation is essential for stapes surgery in three ways. First, it points to the diagnosis of otosclerosis, even though it cannot establish the diagnosis. Only surgery and possibly imaging can do so. Second, it quantifies hearing loss and thereby helps in making a decision on whether surgery is warranted or not. Finally, it serves as the most important, certainly the best reproducible measure of outcome or success of surgery.
Some audiometric measures can be used for all three of these aims; some may help just for one or two. Tests should be selected in such a way that they provide the necessary information effectively and efficiently.
The audiometric diagnosis of otosclerosis is based largely on interpretation of the air-bone gaps that establish the presence of a conductive component to hearing loss. Other measures such as tuning fork tests or more sophisticated behavioral tests may be used along with objective audiometric measurements for this purpose.
Tuning fork tests are a simple means of determining the presence of a conductive hearing loss. They are based on the same physical principles as the audiometric measurements. As such, they are subject to the same limitations and uncertainties, which are discussed in more detail in a later section on pure-tone audiometry.
A multitude of tuning fork tests exists; most were developed before elec-troacoustic audiometry was available as a routine clinical procedure. The two tuning fork tests still routinely used today by most otologists are the Weber and Rinne tests, both of which are generally well known. Weber described his classical test in 1825 . The test relies on the finding that acoustic energy transmitted to the inner ear by bone rather than air conduction is greater in an ear with a conductive hearing loss. Thus, the Weber test alone detects one-sided or asymmetric conductive hearing loss. An alternative without the need of a tuning fork is the humming test , in which lateralization occurs when the patient is asked to hum loudly with the mouth closed.
The Rinne test is helpful when both ears have a conductive hearing loss . It can be interpreted in two ways. First, it can be based on a comparison between bone and air conduction thresholds. Normally, the air conduction threshold is reached about 15 s later than the bone conduction threshold. Alternatively, it can be based on a loudness comparison between the two conduction modes.
The usefulness of the tuning fork tests derives from their being simple, inexpensive and quick. In addition, it is important that the surgeon performs these tests personally. Experienced surgeons can gain quick and useful information not only about the validity of the audiogram or the hearing of the patient, but also about the personality of the patient such as being very clear or hesitant during these tests.
If an audiometer is calibrated correctly, then air and bone conduction thresholds measured in decibels hearing level should more or less be superimposed when there is no conductive component present. Because both thresholds will show a slightly skewed distribution with a peak by definition at 0 dB HL in a normal hearing population, the two thresholds should cross one another in individual measurements due to this statistical distribution. Calibration or the measurement procedure may be skewed if bone conduction thresholds are always found to be better than air conduction thresholds on audiograms.
Even though air and bone conduction thresholds are by convention the same decibel hearing level in ears without a conductive component, we should keep in mind that the energy delivered by a vibrator for obtaining bone conduction thresholds is orders of magnitude greater than that needed with headphones used to deliver stimuli for air conduction threshold measures. This fact leads to inherent limitations in threshold tests with bone conduction stimulation. These include a restricted upper level of output (typically 50-70 dB HL, depending upon the equipment and the frequency being tested) and an upper frequency limit of 4 kHz because vibrators generate air-conducted sound at higher frequencies, which may then be heard through air conduction.
The higher energy level of vibrators is also partially responsible for clearly greater variance of bone conduction threshold measurements, besides calibration procedures being less reliable than those for air conduction. Other reasons for the higher variability of bone conduction thresholds include more individual variability in transmission of the vibration to the skull, from the skull to the inner ear, and the critical need of accurate masking of the nontest ear. All of these difficulties can make the measurement of bone conduction threshold challenging.
One difficult test situation involves the hearing sensitivity of an unaffected ear. It is not unusual to have conductive hearing loss from otosclerosis on only one ear with normal hearing on the opposite side. Masking is essential to eliminate participation of the nontest ear, and for reasons just mentioned, measurements may not reach a level of validity that is required as the basis for recommending surgery.
Bone conduction thresholds can be elevated in ears with otosclerosis, particularly in the 2-kHz region. This typical finding is called the Carhart notch, and it reflects the impedance mismatch between the middle and inner ear created by the fixation of the stapes. The Carhart notch is a useful hint for the presence of otosclerosis, but it does not provide definitive proof.
Not every difference between thresholds measured using earphones or bone vibrators for the same ears is invariably due to conductive dysfunction. Figure 1 displays the audiogram of such a case. Unusually low bone conduction thresholds and an air-bone gap in the frequency range below 2 kHz can be noted. The diagnosis of otosclerosis is improbable with such an audiometric pattern. An alternative possibility for such a pattern is pressure equalization between the perilymphatic and cerebrospinal fluid spaces as can occur with a spontaneous dehiscence of the superior semicircular canal. Figure 2 illustrates
this finding for the ears of the patient with the corresponding audiogram in figure 1. Mikulec et al.  have recently described a series of patients with similar findings, many of whom had undergone stapes surgery with a presumed diagnosis of otosclerosis.
Fluid mechanics and the impedance of the system change with such a 'third' window of the inner ear leading to increased sensitivity of the so-called bone conduction response . Several hypotheses have been proposed to explain how such an increased sensitivity may occur. These are linked to more general hypotheses about how sound reaches the cochlear fluids in the bone conduction mode. These mechanisms remain controversial; they are complex and not entirely clear.
One explanation of an air-bone gap in such cases has been proposed by Sohmer et al. . They concluded that so-called 'bone' conduction is not only due to sound reaching the inner ear by bone vibration. They demonstrated that acoustic energy delivered by a vibrator might reach the inner ear through soft tissues and cerebrospinal fluid without bone actually vibrating. Thus, 'bone conduction' may be a misnomer. It cannot and should not be taken literally. The point is that the presence of an air-bone gap in threshold measurements is not reliable enough to use as the sole basis for making a decision about surgery. Conductive components should be verified by other means.
Simple and effective methods are available to confirm or exclude a conductive dysfunction. Immittance measurements have proven their clinical value in evaluating the status of the middle ear for decades. This method represents the standard, and it should be included in every audiological evaluation for the differential diagnosis of otosclerosis. The usual pattern in an ear with otosclerosis is a normally shaped tympanogram with absent stapedial muscle reflexes for both ipsilateral and contralateral stimulation. It is essential to include contralat-erally induced stapedial reflexes because they are more reliable and less prone to artifacts.
Multifrequency tympanometry provides information on the middle ear resonant frequency, which is higher in ears with otosclerosis due to the fixation of the ossicular chain . However, a large overlap of the resonance frequencies between normal and otosclerotic ears limits the diagnostic usefulness of this testing.
The measurement of otoacoustic emissions (OAEs) can also be helpful in establishing a conductive component to a hearing loss. OAEs are not present in ears with otosclerosis. Their presence effectively excludes not only such a diagnosis, but also a clinically important conductive middle ear component. OAE tests are useful because they are easy, quick and reliable. The measurement of transiently evoked OAEs may be preferable over distortion product OAEs (DPOAEs) because they are less prone to artifacts. When measuring DPOAEs, the increased stiffness of the ossicular chain in otosclerosis may lead to artifac-tual distortions at sound levels for which they do not normally occur. Moreover, clinical DPOAE systems cannot differentiate such artifactual distortion products from those biologically generated within the inner ear.
An essential part of the audiological assessment is to demonstrate the need for rehabilitative measures, and the need for surgery in the particular case of suspected otosclerosis. A small air-bone gap with little overall hearing loss will not be an indication for surgery, even if the diagnosis of otosclerosis can be established. Three major components are to be considered when recommending surgery for otosclerosis: the overall hearing loss, the amount of the air-bone gap, and the handicap experienced by the patient. While these three components are interrelated, they cannot be predicted from one another. They have to be assessed independently.
The overall effects of the hearing loss are usually inferred from the air conduction thresholds on the audiogram. However, we should keep in mind that any definition of hearing loss on this basis, or for that matter on any other audiometric basis alone, is arbitrary. Air conduction threshold measures provide only an indirect means of determining the hearing difficulties that patients might experience in their daily lives. The inclusion of speech audiometry, usually by measuring the speech reception threshold, will not add information about these effects. The advantage of including speech audiometry relates more to the outcome measures discussed below.
The amount of the air-bone gap can provide an indication of the hearing gain that can be achieved by surgery. However, the derivation of the air-bone gap is limited by the general lack of precision in measuring bone conduction thresholds, as described previously, and by the presence of a Carhart notch, which can change after surgery in an unpredictable way. Thus, the possible gain of hearing thresholds due to surgery may be assessed in more general and intuitive ways rather than by relying strictly upon numeric data.
As a rule, the same thing is generally true for the assessment of hearing handicap in patients with otosclerosis. Surgeons tend to assess the degree of auditory handicap intuitively by talking to their patients. The use of formal questionnaires does not seem to be in widespread use for the purpose of evaluating pre- and postsurgical effects of stapes surgery. Many validated questionnaires have been used in other rehabilitative areas of audiology, and their use adds significantly to audiometric measures. Therefore, assessing hearing handicap in patients with otosclerosis using such questionnaires would be beneficial not only in evaluating handicaps before surgery, but also outcome and surgical success.
The Committee on Hearing and Equilibrium of the American Academy of Otolaryngology has published formal recommendations for how to evaluate and report audiometric improvement after surgery for otosclerosis . These recommendations include air conduction threshold measurements from 0.5 to 8 kHz and bone conduction between 0.5 and 4 kHz, both including 3 kHz. It is needless to say that all of these measurements have to be done before and after surgery, including the repetition of the measurement of bone conduction. The postoperative air-bone gap must be determined from these direct measurements because bone conduction thresholds may increase or decrease as a result of the surgery.
Even though these guidelines do not include speech audiometry, there are good reasons to include measures of speech audiometry to evaluate outcome. For example, stapedectomy has been shown to produce better threshold gains for pure tones but worse speech audiometric gains when compared with small fenestration stapedotomy . The reason for such findings may be because of different mechanical properties of the two methods of stapes replacement, favoring the transmission of high frequencies and transient events for stapedo-tomy because of less mass and less elastic fixation.
Thus, speech audiometry, particularly a threshold for speech (i.e. speech reception threshold) is recommended as an overall audiometric measurement to evaluate surgical outcome after stapes surgery. Improvement of speech reception threshold has been shown to be a reliable measure of postoperative success . It performs as well or better than any averages of pure-tone thresholds.
Overall hearing handicap and quality of life can be assessed using questionnaires, either as a single measurement after surgery, or as a repeated measurement before and after surgery. A multitude of validated questionnaires exists for such purposes, but few results have been reported, indicating that such methods are used rarely.
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Prof. R. Probst HNO-Klinik Universitatsspital Basel CH-4031 Basel (Switzerland)
Tel. +41 61 265 4105, Fax +41 61 265 4029, E-Mail [email protected]
Arnold W, Häusler R (eds): Otosclerosis and Stapes Surgery. Adv Otorhinolaryngol. Basel, Karger, 2007, vol 65, pp 127-132
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