Vol. 12 • Issue 6
• Page 34
When patients present with outer or middle ear pathology, it is not possible using headphones, insert earphones or sound field to determine the degree of hearing loss that is due to cochlear impairment versus conductive pathology. It is therefore necessary to use bone conduction (BC) testing in order to determine cochlear function. From that information, pure tone thresholds may be compared between test modalities and the degree of conductive hearing loss inferred.
Presentation of pure tone stimuli via bone conduction is performed via bone conduction oscillator placed most commonly on the mastoid process of the temporal bone or on the forehead at Cz. Bone conduction stimuli vibrate the temporal bone surrounding the membranous labyrinth, which houses the cochlea and therefore effectively stimulates the cochlea directly. By comparing hearing thresholds obtained via bone conduction to thresholds obtained via air conduction, the examiner may determine whether outer or middle ear pathology is (negatively) affecting transmission of sound to the cochlea through the auditory canal, tympanic membrane and ossicles of the middle ear.
Because the transmission of acoustic stimuli presented via bone conduction involves mechanical transmission (vibration of the cranial bones), they lose very little acoustic energy when traveling across the head. For this reason, the decrease in intensity between the test ear and the contralateral ear is negligible. This measurement is called interaural attenuation and is considered to be 0 dB for all common audiometric frequencies when presenting via bone conduction. This essentially means that bone conduction oscillators stimulate both cochleae equally and that patient responses will be the result of hearing the stimuli in the better ear. It therefore cannot be decisively known by the examiner which ear is responding to stimuli presented via [unmasked] bone conduction.
When bone conduction testing results in hearing thresholds that are more than 10 dB improved compared to AC thresholds, an air-bone gap is considered to exist. This means that one ear potentially has a condition that is attenuating sound presented to that ear. In order to determine accurate sensory hearing thresholds and define the amount of hearing lost to outer ear or middle ear pathology in a particular ear, it becomes necessary to mask. The process of masking involves presenting sound to the non-test (NT) ear in order to artificially decrease hearing sensitivity of that ear-essentially keep it busy. If done correctly, the examiner can ensure that only the cochlea in the test ear (TE) will respond and therefore obtain accurate measurement of cochlear function in that ear. There are also cases in which both ears may have air-bone gaps simultaneously. In this case, more advanced methods of masking are required and are discussed later in this document.
Procedures to obtain masked BC hearing threshold vary, however are based on the principles described in this paper and therefore share many similarities. Procedures also exist which are intended to ease the job of the examiner, however it is important that all considerations are taken into account. If factors affecting masking level or stimulus level are ignored, incorrect sensory/cochlear hearing thresholds may be obtained and patients may receive incorrect counseling, medical pathology of the ear may be missed or amplification may not be fit properly. An examiner who understands the following concepts should be able to appropriately and correctly obtain masked BC hearing thresholds routinely and easily.
Which Ear to Test
When asymmetrical AC thresholds exist and unmasked BC thresholds are consistent with AC thresholds of one ear, it is initially assumed that the better ear is responding. The ear with poorer AC thresholds is therefore considered the "test ear" and the contralateral ear is considered the "non-test" ear. For ease of visualizing which ear is which, examiners are encouraged to place the bone oscillator on the mastoid of the test ear. This also makes placement of a headphone on the non-test ear much easier.
The Amount of Masking to Be Used
Various formulas have been published that describe the amount of masking to present to the test ear, however any formula used by the examiner must account for the following factors:
• The level of the stimulus presented to the test ear;
• Occlusion effect created by placing a headphone on-or inserting earphone in-the non-test ear;
• Interpersonal differences that may cause masking to be slightly lesser than read on the audiometer dial (remember that not all ears are the same);
• Examiner and patient factors that may affect transmission of the BC stimulus (placement, thickness of skin, conductivity of the stimulus to the cochlea).
While the latter two items, and particularly the last, may result in minor differences, it is important to remember that they exist and may affect presentation of both the stimuli and the masking noise. Occlusion effect is not minor, however, and may result in BC stimuli lateralizing to the non-test ear if insufficient masking noise is provided. Occlusion effect results from additional energy resonating within the bony labyrinth surrounding the occluded ear.
The risk of increased conduction of BC stimuli to the non-test ear is particularly of concern when the headphone or insert earphone is present only in the non-test ear. While it is very inconvenient-and nearly impossible from a practical standpoint-to place headphones over both ears with the bone oscillator present, many clinicians keep insert earphones in both ears when obtaining BC hearing thresholds so as to minimize the effect of occlusion in the non-test ear. As many examiners are taught to use only one headphone/earphone, that method will be described here. If using binaural insert earphones, the occlusion effect is neutralized and may be omitted from calculation. Procedures are otherwise the same.
The basic formula for determining the amount of masking to use is: EM = ST + OE, where EM is "effective masking" level (the dB level of masking you actually want to use), ST is the level of the stimulus in dB HL and OE is the amount of occlusion effect for the AC presentation being used. Evidence has been presented that occlusion effect of a supra-aural headphone may as much as 28 dB at 250 Hz. Occlusion effect is less for higher frequencies and the following recommendations are made, based on recommendations from multiple authors, ranging from 30 dB1 to 15 dB2 at 250 Hz and descending for higher frequencies (narrow-band masking is assumed):
250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz 8000 Hz
20 dB 15 dB 10 dB 5 dB 0 dB 0 dB
If insert earphones are positioned correctly (outside edge of foam insert is flush with the aperture of the auditory canal), occlusion effect has been measured as less than that for supra-aural headphones.1,2,3 Again, values range between authors, however it is recommended that the following values be used in order to account for inter-subject differences and possible variability in placement of the foam insert:
250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz 8000 Hz
10 dB 10 dB 5dB 0dB 0dB 0dB
It is widely recommended that an additional "safety factor" of 10 dB be added to calculations.1,2,3 This factor, in essence, accounts for inter-subject differences and inter-examiner differences, as described above. The addition of that factor results in the following, final, formula: EM = ST + OE + 10 dB
Therefore, the following examples would result for initial masking levels (complete test method will be discussed in the following section):
• A patient with a 10 dB [unmasked] BC hearing threshold at 1000 Hz in the test ear and examiner using supra-aural headphones, initial masking in the non-test ear should be: 10 + 10 + 10 = 30 dB of masking
• A patient with a 15 dB unmasked BC hearing thresholds at 250 Hz in the TE and an examiner using supra-aural headphones. Initial masking in the NT ear should be: 15 + 20 + 10 = 45 dB of masking
• A patient with a 15 dB unmasked BC hearing threshold at 250 Hz in the TE and an examiner using insert earphones. Initial masking in the NT should be: 15 + 10 + 10 = 35 dB of masking
Examiners always should begin BC pure tone presentation at the threshold previously obtained. Due to central (cognitive) effects of presenting noise to the NT ear, it is common for threshold to shift 5 dB. Provided the safety factor of 10 dB of masking and inter-subject differences in occlusion effect, it is possible that a 10 dB, 15 dB, or even 20 dB shift in threshold may occur if there is a significant difference between ears (if sensory threshold of the test ear is poorer). Therefore, if examiners present at that level with masking applied, they should not be surprised if the patient does not respond. The first step in obtaining masked BC thresholds is to re-establish threshold. This is typically done in the same fashion as unmasked BC or AC testing, using the modified Hughson-Westlake procedure.4
During the process of obtaining a masked BC hearing threshold, the following two concepts are crucial and assumed by the examiner:
• If the patient responds, it is possible that either ear may be responding. Masking should be increased to see if the stimulus will masked (meaning the non-test ear responded).
• If the patient does not respond, sensory/cochlear hearing threshold in the test ear has not been reached and the stimulus has been masked in the non-test ear. The stimulus level should be increased to see if the patient can hear it.
As stated previously, threshold should always be re-established in order to obtain an accurate BC hearing threshold in the test ear. Masking and stimulus levels are then raised (independently) until further increases in masking result in the patient responding at the same stimulus level. This results in the following procedure:
1. Set the initial masking level for the non-test ear and present masking (ask the patient if he or she hears the noise, to verify it is present).
2. Present the BC stimulus at the unmasked level and follow the standard (modified Hughson-Westlake) method for audiometric testing in order to re-establish BC hearing threshold.
3. Raise the masking level 5 dB.
4. Present the BC stimulus again.
5. If the patient responds, raise the level of masking 5 dB.
6. If the patient does not respond, raise the level of the stimulus 5 dB.
7. Present the BC stimulus again.
8. Repeat steps 5-7 until the patient has responded to three successive presentations. This establishes masked BC hearing threshold for the test ear with a plateau that ensures the non-test ear has not responded.
Note in this procedure that masking is raised only when the patient responds. It is possible that the initial amount of masking may be more than necessary, particularly when using the safety factor. Procedures that continually raise masking levels simultaneously with stimulus levels may result in substantial amounts of masking noise in the non-test ear and discomfort for the patient. In this event, some patients may even be distracted from the stimulus and fail to respond appropriately when the stimulus is at or near true BC threshold. Instead, masking is raised only when the stimulus is heard. This is due to Concept A, above, which states that the patient may be hearing the stimulus in either ear. If masking is not raised further, the examiner cannot know which ear has responded. By following a procedure in which effective masking is calculated according to the formulas described previously in this article and establishing threshold with three successive increases in masking, the examiner can be reasonably certain that the non-test ear has been properly masked and true sensory threshold of the test ear has been obtained.
Masking with Bilateral Conductive Hearing Loss
It is less common that patients present with binaural outer or middle ear pathology, however audiologists must be prepared for this situation as well. In this case, an air bone gap exists for both ears and the audiologist cannot, after obtaining unmasked bone conduction thresholds, know which ear is responding to bone conduction stimuli. When presented with this situation, it is possible that both ears have conductive pathology (possibly causing varying degrees of hearing loss) or it is possible that one ear may have poorer sensory hearing with conductive pathology. It will be the task of the clinician to determine which scenario is the case. In the latter case, the ear with better cochlear function (which therefore responded during unmasked bone conduction testing) may have conductive or mixed pathology.
The initial dilemma for the audiologist is which ear to choose as the initial test ear. If it is known from intake, prior medical examination or physical examination by the audiologist that the patient has unilateral conductive pathology, using the contralateral ear as the initial test ear may save time and effort. Should masked bone conduction thresholds in the ear without suspected conductive hearing loss be similar to air conduction thresholds, no further testing is required, as unmasked bone conduction thresholds must reflect cochlear function of the ear known to have conductive pathology. For the sake of thoroughness, both ears may be tested to ensure that masked bone conduction thresholds in the ear with conductive pathology are the same as unmasked thresholds. This would be particularly important for any patient with suspected central processing difficulties who may be susceptible to central effects of masking.
If both ears have possible conductive pathology (including the instance in which it is not known which ear may have conductive pathology), then the examiner will need to choose an initial test ear somewhat arbitrarily. In such instances, although particularly in any case of possible successive masking procedures, using insert earphones binaurally provides additional benefit. This is due both to decreased time and effort necessary to switch earphones in and out of each ear and also due to a lesser amount of masking required to overcome the occlusion effect. The latter concern becomes important when sensory hearing thresholds are elevated to the moderate hearing range or higher, as there is a greater possibility of instrumentation limitations preventing use of necessary masking levels.
Once a test ear has been chosen, masked bone conduction testing proceeds as previously described, with one exception: effective masking levels must include an adjustment for [suspected] conductive hearing loss. As described above, the non-test ear may not have conductive pathology, however masking must be presented as if it does. Otherwise, insufficient masking levels may be presented to prevent the cochlea of the non-test ear from responding. In this case, the amount of suspected conductive hearing loss must be overcome by the masking noise. In this case, our previous formula (EM = ST + OE + 10 dB) becomes: EM = ST + OE + HLC + 10 dB, where HLC is the suspected conductive hearing loss-the difference between unmasked BC thresholds and AC thresholds in the non-test ear. As before, occlusion effect is omitted when using binaural insert earphones. Once the amount of masking is established, the test procedure continues as described above for establishing unilateral masked bone conduction thresholds.
Once masked bone conduction thresholds are obtained for the initial test ear, masking is introduced to that ear (masking now in the original test ear) and the same procedure is used to obtain masked bone conduction thresholds in the second ear. HLC is now known to be the difference between AC thresholds and masked BC thresholds in the ear just tested. If the initial ear does not have a conductive hearing loss (perhaps was found to have an asymmetrical sensory hearing loss), then HLC will be zero.
Although not common, the audiologist may observe that masked BC thresholds in the second ear are different than suspected when masking levels were previously calculated for that ear (remember, the second ear was the initial non-test ear). This case may present when the initial test ear has a conductive or mixed hearing loss and the second ear is found to have an [asymmetrical] sensory hearing loss. It is then possible that too much masking was presented and it will be necessary to return to the initial test ear and re-test using appropriate masking levels.
To the student or new audiologist, these scenarios may initially be confusing (particularly when attempting to keep track of what is going on in each ear). However, with practice and repetition, any audiologist will learn to methodically analyze the masking needs of any ear and correctly obtain both unmasked and masked bone conduction thresholds.
1. Kent, RD. (2004). The MIT Encyclopedia of Communication Disorders. Massachusetts Institute of Technology.
2. Goldstein et al. (1985). Clinical Masking: A Decision-Making Process. From Katz, J., Handbook of Clinical Audiology, Third Ed. Williams and Wilkins.
3. Silman, S. and Silverman, C. (1991). Auditory Diagnosis: Principles and Applications. Academic Press, Inc.
4. Carhart, R, and Jerger, J. (1959). Preferred Method for Clinical Determination of Pure Tone Thresholds. Journal of Speech and Hearing Disorders. 24:330-345.
5. Martin, F. and Clark, JG. (2003). Introduction to Audiology, Eight Ed. Pearson Education.
John A. Coverstone, AuD, is president and CEO of Sentient Healthcare, an audiology consulting company; he also is the clinical audiologist at Innsbruck Hearing & Balance Center in New Brighton, MN and the educational audiologist at the Minnesota State Academy for the Deaf in Faribault, MN.