Audiological evaluation of the patient with otosclerosis

The audiological signature pattern of fenestral otosclerosis include the following:
  • Conductive hearing loss with Carhart’s notch at 2KHz.
  • Excellent speech discrimination scores ( Also seen in cochlear otosclerosis)
  • Low compliance on Impedance (immitance) audiometry ( less than 0.3cc) Seen in fenestral as well as cochlear otosclerosis.
  • Normal middle ear pressure.
  • Absence of middle ear reflexes. Also seen in Cochlear otosclerosis.
The audiological pattern in otosclerosis is unique. The patterns of abnormalities can be directly explained by alterations in the middle ear transfer function produced by stapes fixation (Harrell 2002)

Pure tone audiometry

This is the most fundamental test that should be performed in any patient presenting with impairment in hearing. However it should be remembered that it is a subjective test and the results can vary from laboratory to laboratory. Nonetheless it is the single most necessary test that needs to be performed.

Air conduction.

The most prominent audiologic characteristics of otosclerosis are elicited with the use of low frequency stimuli.(Hannley 1993). The primary acoustic consequence of otosclerosis in its early stages is the increase in the stiffness reactance component of the total middle ear impedance. This results in a reduction of transmission effectiveness for low frequencies as seen in elevated thresholds. Another effect is that the resonant frequency of the middle ear is elevated.(Beales 1981) In the early stages a gradually progressive low frequency conductive hearing loss is first seen. Initially patients may be unaware of such a hearing impairment until it crosses the 25dB range. The hearing loss may be confined to frequencies below 1000Hz. High frequencies are typically unaffected at this stage. This characteristic rising audiogram configuration has been referred to as the “stiffness tilt”.
As the footplate becomes completely fixed and the otosclerotic focus proliferates, a mass effect is added to the audiogram. The low frequency hearing loss doesn’t increase and appears to stabilize. However the hearing loss progresses in the high frequencies and there is a gradual widening of the air bone gap. The audiogram configuration now changes to a flat pattern from the upward sloping pattern that it had in the early stages. In the absence of cochlear involvement, the pure conductive hearing loss produced by the complete stapes fixation is limited to 60dB to 65dB with a maximum air bone gap across the frequency range.
In cochlear otosclerosis air conduction thresholds continue to worsen and the loss starts to become mixed or sensorineural, with the high frequencies becoming severely affected. The typical pattern of cochlear otosclerosis in the early stages is the “cookie bite”pattern where the greatest degree of hearing loss occurs in the mid-frequency hearing range and is characteristically a mixed hearing loss.(Hannley 1993).
Tinnitus is usually present in a large percentage of patients. If the tinnitus is severe it may interfere with the ability of the patients ability to respond reliably to pure tone testing. Usage of pulsed or warbled tones may help the patient to identify tinnitus from pure tone test stimuli when being tested.

Bone conduction

Whilst air conduction curves give an indication of the hearing thresholds and its configuration may give valuable clues to the diagnosis of otosclerosis in its early stages, bone conduction audiometry is of great value in the diagnosis of otosclerosis and in selection of patients for surgery. Bone conduction is especially useful in when testing patients suffering from otosclerosis. It reveals characteristics that are typical of otosclerosis and it also helps reveal the amount of cochlear reserve in each ear. This helps identify if stapedial or cochlear otosclerosis is present. This in turn helps the surgeon make a decision as to which ear he should operate and helps predict optimum postoperative results.
The “Carhart’s notch”is thought to be typical of otosclerosis(Carhart 1950, Carhart 1962). Carharts notch is characterized by elevation of bone conduction thresholds of approximately 5dB at 500Hz, 10dB at 1000Hz, 15dB at 2000Hz and 5dB at 4000Hz. It was previously thought that this was due to the inertial component of bone conduction caused by stapes fixation. However the contribution of the inertial component is maximal for frequencies below 800Hz and thus a greater loss for bone conducted low frequencies might be predicted. This however is not borne out by clinical observation. Another more plausible explanation is that fixation of the stapes disrupts normal ossicular resonance, which in humans is around 2000 Hz. Furthermore the normal compression mode of bone conduction is disturbed because of the relative perilymph immobility caused by stapes fixation(Tonndorf 1972).. Carhart’s notch is a mechanical artifact and is not a true representation of cochlear reserve. Evidence that the Carhart’s notch is an artifact is seen in overclosure following stapes surgery.
Cross checks on the validity of bone conduction thresholds include careful consideration of the masking levels. Sensorineural acuity level (SAL) test is used to resolve masking dilemmas.
It must be appreciated that the Carhart Notch occurs in any condition which reduces the inertial vibration of the stapes footplate during bone conduction stimulation. One such condition is otosclerosis. It would however be incorrect if it is assumed that this is the only condition which can cause Carhart’s notch. Carhart (1950)give four postulates to indicate that stapes fixation induces mechanical modifications in the bone conduction audiogram.
  • In the Bing test in clinical otosclerosis there is no shift in loudness when the meatus is occluded, or when pressure is varied, as is seen in normal hearing and in sensorineural hearing loss. It is probable that the Bing test is negative in otosclerosis and on other forms of deafness because the middle ear element of bone conduction is attenuated by the middle ear element of hearing loss.
  • It is unusual to find a patient with otosclerosis whose bone conduction thresholds are normal.
  • Surgery improves bone conduction thresholds and the Carhart notch disappears following surgery.
  • Animal experiments producing stapes fixation cause bone conduction thresholds to become poorer.
It was thought that there was an abrupt shift in bone conduction thresholds and that this may reach its full magnitude when stapes fixation has progressed to the stage where it causes a mild air conduction loss. Beales (1981) disagrees and has evidence of a slowly progressive increase in the size of Carhart’s notch. In chronic otitis media Carhart notch like effects may occur but they are much less prominent than that seen in otosclerosis. Not all high frequency bone conduction losses are artifacts. Cochlear otosclerosis is characterized by the presence of mixed or sensorineural hearing losses in which the air bone gap is minimal. If the air conduction, bone conduction levels are roughly parallel the elevated bone conduction thresholds probably represent a sensorineural hearing loss.
Keleman and Linthicum(1969) have discovered that the severity and configuration of the pure tone audiogram do not match frequency for frequency those areas of the cochlea. Sensorineural hearing losses are most commonly associated with basal turn involvement and are invariably present with endosteal layer involvement. Sensorineural hearing loss varies directly with hyalinization of the spiral ligament.

Clinical value of bone conduction audiometry

In otosclerosis thee is a characteristic bone conduction curve and this helps the clinician in distinguishing otosclerosis from other causes of conductive hearing losses. The clinician can determine with accuracy the degrees of sensorineural reserve by correcting the bone conduction audiogram for mechanical distortion due to stapes fixation. Or stated another way it is possible to more precisely determine a patients hearing loss due to secondary cochlear loss by making allowance for the Carhart notch when interpreting bone conduction thresholds.

False lateralization

This phenomenon occurs when performing a weber test and the sound is referred to the other ear. This error is common when bone conduction audiometry is performed without masking and under these circumstances the bone threshold in the worst affected ear may seem to be better than it really is, by approximately 10dB or more, as the patient is hearing the sound in the other ear.


In some cases the introduction of a masking noise creates serious interference with the response of the ear under test, even though the sound is not strong enough to produce cross masking in this ear. Such patients do poorly even though the noise is too weak to be producing true masking in the test ear. This effect is termed ‘central masking’.

Shadow response

In some patients where the hearing losses are so great then efforts to mask the contrlateral ear become ineffective. This situation arises because effective masking is reduced at each frequency by the amount of the air conduction loss in the masked ear at that frequency. To illustrate this point a masking noise which causes a 50dB threshold shift in a normal ear will produce only about 10dB of shift in an ear with a hearing loss of 40dB. This same noise will not cause any threshold shift in an ear where the air conduction loss exceeds 50dB. This same noise will not cause any threshold shift in an ear where the air conduction loss exceeds 50dB. There is also the possibility of cross masking when the noise is increased even more in order to try and mask the ear only to find that the masking noise has crossed the head and is now masking the test ear.


When conductive hearing losses are present it is important that masking levels are appropriate and adequate. The goal of masking is to raise the threshold of the non test ear so that the sensitivity of the test ear can be evaluated in isolation. Inadequate masking leads to participation of the non test ear resulting in incorrect levels of hearing estimates. Excessive masking levels lead to cross over of the masking to the test ear, leading to an underestimation of its cochlear reserve. Correct masking is difficult to achieve when moderate conductive losses exist. In these instances the use of insert earphones for masking and the confirmation of thresholds by SAL may be necessary.


In the early stages of otosclerosis a low frequency conductive hearing loss is found. As the severity of the disease progresses the hearing loss increases in severity and changes in its configuration. Carhart’s notch is a mechanical artifact of stapes fixation. Carhart’s notch is characteristic of otosclerosis and it disappears with successful closure of the air-bone gap. Bone conduction thresholds become progressively elevated in the high frequencies in cochlear otosclerosis. Adequate masking techniques are necessary when evaluating a patient with otosclerosis.

Impedance audiometry

Impedance audiometry has three components: tympanometry, static compliance and acoustic reflexes.


The tympanogram is a graphic representation of the change in acceptance of sound energy through the middle ear as a function of air pressure applied to a hermetically sealed ear canal. Or stated another way tympanometry is the measurement of the tympanic membrane compliance in response to variations of air pressure in the external auditory canal. By varying the pressure in the external auditory canal the point at which the tympanic membrane has maximum freedom to vibrate in response to a pure tone is when air pressure equals that in the middle ear. Various schema have been proposed for classifying tympanograms. The classification proposed by Jerger(1970) is one of the more popular classifications and is the one used by the authors. There are 5 types and are described as Type A, B, and C. type A is further divided into two subgroups Type As and Type Ad . The distinguishing feature of the entire A category is a clearly defined peak of the function, which occurs in the range of+ 100daPa. The tympanometric peak demonstrates the presence of air in the middle ear space. The location of the peak on the pressure axis serves as an indicator of middle ear air pressure relative to ambient air pressure and thus indirectly it indicates the position of the tympanic membrane. The height of the tympanogram- the base- peak compliance difference- reflects the mobility of the middle ear apparatus, including the tympanic membrane, the ossicles as well as the inherent stiffness of the air enclosed in the external ear canal and that of the air in the middle ear space. Middle ear aeration is not affected in otosclerosis. The middle ear pressure is zero or atmospheric. A clear peak that remains within the normal range of 100daPa, Type A category, characterizes the tympanograms of patients suffering from otosclerosis. Further subdivision is decided on the basis of the base- peak compliance difference. This value will depend upon several factors, one of which is the condition of the tympanic membrane. Thus a stiffened tympanic membrane caused by tympanosclerosis will result in a stiff (As) tympanogram. A flaccid tympanic membrane would result in a Type Ad deep configuration. In the case of stapedial otosclerosis the height of the tympanometric peak decreases.

Static compliance

The static compliance is calculated by subtracting the compliance at +200daPa from the total compliance when the pressure is equilibrated across the tympanic membrane and energy transfer is at a peak.
Static compliance = Peak compliance-compliance200daPa.
Normal static compliance values fall in the range of 0.3 to 1.6cc. Values lower than 0.3cc are considered abnormally low, indicative of increased stiffness in the conductive mechanism. If the compliance is greater than 0.6cm3 it is probable that the footplate of the stapes will be relatively thin and if the compliance is less than 0.2cm3 there is a possibility that the footplate will be fairly thick with the possibility that the footplate may even be obliterative. If the hearing loss is symmetrical this information may be helpful in selecting which ear should be operated upon.

Acoustic reflexes

The principle of acoustic reflexes is this: The middle ear muscles contract reflexively to a stimulus. The stiffness of the entire middle ear system is increased resulting in a corresponding decrease in transmission efficiency for a low frequency tone The earliest evidence of otosclerosis is often the appearance of a diphasic pattern (Bel, Causse and Michaux et al 1976). The diphasic reflex is characterized by brief increases in compliance that occur at the onset and at the termination of the stimulus when the probe is in the affected ear (Flottorp and Djupesland 1970). Bel at al (1976)account for this phenomenon by postulating that although the anterior footplate may be fixed to the oval window the elasticity of the involved footplate and crura allows the posterior portion of the footplate to move independent of the anterior portion of the footplate creating the onset compliance change. The elasticity of the footplate returns it to its resting position where it remains until the pull of the tendon of the stapedius is relaxed allowing another brief change in compliance at the offset of the signal.
As the stapes gets progressively fixed the ipsilateral and contrlateral acoustic reflexes are affected even though the stapes fixation may be unilateral. This causes the impedance matching function of the ear to be less effective and as a result of the inefficient impedance match, the signal is attenuated before it arrives at the cochlea for the first stage of the reflex arc.
In the early stages of the disease, acoustic reflex abnormalities may be apparent only when the probe is placed in the affected ear which produces a ‘vertical pattern’ in the convention created by Jerger and Jerger(1977). As the degree of hearing loss increases , contralateral reflexes become abnormal as well producing the ‘inverted L pattern’ in which only the ipsilateral reflex in the unaffected ear remains observable.

Nonacoustic reflexes

Nonacoustic reflexes may be produced by tactile stimulation and this may help in distinguishing malleus and stapes fixation. The nonacoustic reflex, is generally believed to be related to tensor tympani activity and is characterized by its appearance as a part of a generalized startle reflex, by rapid habituation to repeated stimuli and by changes in compliance opposite to those seen with stapedial contraction.. The tactile stimulation may take the form of directing a jet of air to the ipsilateral cornea or by lightly stroking the tragus. If the acoustic reflex is absent or abnormal but the nonacoustic reflex is normal the fixation site can be inferred to be at the stapes.(Hannley 1993) However if the acoustic and nonacoustic reflexes are absent or abnormal then malleus fixation or generalized ossicular fixation may be suspected.(Klockhoff and Anderson 1960).
Normal tympanograms, static compliance accompanied by acoustic reflex abnormalities such as diphasic reflex, elevated thresholds and absent reflexes in one or both ears is the typical pattern of otosclerosis in its early phases. As the stapes footplate gets more firmly fixed the tympanogram becomes more shallow, with corresponding decrease in static compliance.

Speech audiometry

Speech audiometry completes the audiologic battery of tests. Persons suffering from sensorineural hearing loss will complain that they are unable to comprehend speech in a noisy background. However those who suffer from a conductive hearing loss will hear better in a noisy background. This is a paradox and is known as the phenomenon ‘paracusis of Willis’. This occurs because most speakers raise the level of their voices in order to be heard by themselves and their listeners (Lombard effect) and in doing so cross the thresholds of those who have a conductive hearing loss. The listeners conductive hearing loss attenuates the background noise leaving the speakers voice more audible through an apparent improvement in the signal to noise ratio Spondee reception threshold or known as speech reception threshold (SRT) is the first test. In the presence of a conductive hearing loss the SRT coincides with the 3 frequency pure tone average (500Hz, 1000Hz and 2000Hz) within a range of 5dB. Speech discrimination score (SDS) is a measure of word identification accuracy. SDS as measured by phonetically balanced words falls within the normal range of 90% to 100% at suprathreshold levels. Speech discrimination scores fall when there is a sensorineural component to the hearing loss. Thus poor discrimination scores would indicate that the prognosis would unlikely be excellent following surgery. Such patients would likely benefit more from hearing aid amplification.

Otoacoustic emissions

Otoacoustic emissions were first described by Kemp (1978). One study has found that transient evoked otoacoustic emissions (TEOAEs) recorded from patients with otosclerosis have a very low amplitude and may not be observable above the recording noise floor in the presence of relatively minor amounts of conductive hearing loss. At this time otoacoustic emissions have limited value in otosclerotic lesions with conductive hearing loss because of extreme vulnerability to middle ear transmission inefficiencies and by a lack of specificity(Hannley 1993). However future research may likely help in early identification of cochlear otosclerosis.

Vestibular tests

Vestibular tests only become necessary in otosclerosis when the patient also presents with vertigo. Causse and Causse(1991) have describe 3 types of vestibular dysfunction that are associated with otosclerosis. They postulate that vertigo occurs in these patients because of the release of toxic enzymes. According to them type 1 is characterized by mild dysequilibrium without a rotatory component. Caloric tests are normal. In type 2 attacks of acute rotational vertigo are experienced and may be accompanied by tinnitus and fluctuating sensorineural hearing loss. Paradoxically the caloric tests may likely be normal in these patients. In the Type 3 category Menieres disease and cochlear otosclerosis coexist.


Audiometry is a necessary tool to confirm and document the presence of otosclerosis. Otosclerosis presents in a unique and typical way. Pure stapedial otosclerosis has its unique signature which can be recognized on pure tone and impedance audiometry. Accurate reliable audiometric tests, which can be replicated, help the clinician decide on the appropriate form of treatment for otosclerosis. Should surgery be decided upon as the form of treatment, then audiometric tests help the surgeon decide upon which ear should be operated and help predict the outcome of such a surgery.


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