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 Table of Contents  
CASE REPORT
Year : 2020  |  Volume : 26  |  Issue : 3  |  Page : 182-185

The clinical value of bilateral bone conduction testing in hearing diagnosis


Department of Audiology and Speech Pathology, School of Health Sciences, Universiti Sains Malaysia, Kelantan, Malaysia

Date of Submission27-May-2020
Date of Acceptance19-Jul-2020
Date of Web Publication22-Dec-2020

Correspondence Address:
Dr. Mohd Normani Zakaria
Department of Audiology and Speech Pathology, School of Health Sciences, Universiti Sains Malaysia, 16150 Kubang Kerian, Kelantan
Malaysia
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/indianjotol.INDIANJOTOL_106_20

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  Abstract 


Pure tone audiometry (PTA) is the standard clinical test for diagnosing the severity and the type of hearing loss. For hearing diagnosis, air conduction (AC) and bone conduction (BC) thresholds are determined and analyzed. Typically, the unmasked BC testing is conducted in one ear as the interaural attenuation (IAA) for BC is considered to be 0 dB. In this case report, we demonstrate the usefulness of performing bilateral BC testing to achieve an accurate hearing diagnosis. As found, the IAA for BC could be as high as 15 dB, and the unmasked BC testing should be performed on both ears to avoid misdiagnosis (particularly on the type of hearing loss). Even though PTA is the standard clinical test for hearing diagnosis, it is still imperative to ascertain its results through comparisons with other clinical test findings.

Keywords: Air conduction, bone conduction, interaural attenuation, mastoid, pure tone audiometry


How to cite this article:
Romli M, Wan Mohamad WN, Awang MA, Aw CL, Zakaria MN. The clinical value of bilateral bone conduction testing in hearing diagnosis. Indian J Otol 2020;26:182-5

How to cite this URL:
Romli M, Wan Mohamad WN, Awang MA, Aw CL, Zakaria MN. The clinical value of bilateral bone conduction testing in hearing diagnosis. Indian J Otol [serial online] 2020 [cited 2021 Apr 10];26:182-5. Available from: https://www.indianjotol.org/text.asp?2020/26/3/182/304276




  Introduction Top


When the hearing organs are compromised, hearing loss would occur. When the outer ear or the middle ear is affected, the affected individual is said to have conductive hearing loss (CHL). Sensorineural hearing loss (SNHL) occurs when the inner ear is compromised. If both CHL and SNHL are present, the condition is known as mixed hearing loss (MHL). Hearing loss is in fact common among children and adults, in which proper assessments and timely management are required to minimize the negative consequences of hearing loss.[1]

In clinical settings, comprehensive hearing assessments are typically performed by audiologists. By conducting audiological tests such as pure tone audiometry (PTA), tympanometry, acoustic reflex test, and others, the severity and the type of hearing loss can be conveniently documented. In the PTA testing, air conduction (AC) and bone conduction (BC) thresholds are measured and analyzed for hearing diagnosis.[1],[2] In particular, AC thresholds provide information on the severity of hearing loss, whereas BC thresholds are useful in determining the type of hearing loss.[2] For determining the AC thresholds, both ears are tested with either headphones or insert earphones. On the other hand, by using a bone vibrator typically placed on the mastoid bone, the BC thresholds are measured from either ear or both ears.[1],[2],[3],[4] Moreover, masking noise is typically presented to prevent the participation of the nontest ear since cross-hearing can occur during the PTA testing which may affect the test results.[1]

Since the interaural attenuation (IAA) for BC is generally considered to be 0 dB, it is common that the unmasked BC thresholds are determined from only one ear.[1] That is, the IAA of 0 dB means that similar BC thresholds would be obtained regardless of which ear is tested. In fact, many clinics have been using a triangle symbol (“Δ”) to denote BC threshold from either ear on the audiograms (rather than symbol “<” that represents right unmasked BC threshold and symbol “>” that denotes left unmasked BC threshold). Nevertheless, since the IAA for BC is not necessarily 0 dB (in fact, it can be as high as 15 dB),[3],[4] conducting BC testing on only one ear can be problematic. In this paper, we report a case to demonstrate the clinical value of performing bilateral BC testing in achieving an accurate hearing diagnosis.


  Case Report Top


This is the case of a 57-year-old male teacher who reported hearing difficulties in both ears for the past 6 months. Worse in the right side, he had difficulties to understand speech, particularly male voices and in the presence of background noises. No otalgia, otorrhea, aural fullness, tinnitus, or vertigo was reported. His general health was also good. No other significant findings reported in the history.

As revealed by an otoscopic examination, the tympanic membrane (TM) appeared intact with clear ear canal for both ears. However, unlike the right side, the left TM looked dull (with absent cone of light). The audiological test findings are shown in [Figure 1]. Using a 226-Hz tympanometric device (AT235H, Interacoustics, Denmark), a Type A tympanogram was obtained bilaterally, suggestive of normal middle ear function in both ears. The acoustic reflex threshold (ART) was found to be within the normal range (i.e., 70–100 dB) for each tested frequency in the right ipsilateral condition. In contrast, the ART was abnormal (i.e., elevated or absent) in the left ipsilateral, left contralateral, and right contralateral conditions for each tested frequency. The PTA testing was carried out using a two-channel audiometer (GSI 61, Grason-Stadler Inc., USA), and appropriate masking procedures were performed. Using TDH-39 headphones and B71 bone vibrator (placed on the mastoid area), the AC and BC thresholds were determined accordingly based on the standard protocol.[1] It is worth noting that for the BC testing, the unmasked BC thresholds were recorded from the right ear first (i.e., the worse ear) and followed by the left side (i.e., the better ear). As revealed in [Figure 1], PTA revealed a moderate-to-severe SNHL in the right ear. In the left ear, when considering the unmasked BC and BC masked thresholds in both ears, significant air-bone gaps (ABGs) were observed (at 250 Hz and 4 kHz) implying the presence of conductive element. That is, in terms of type of loss, the left ear was likely to have MHL affecting low and high frequencies (with mild-to-moderate degree of loss). Of note, for the left ear, the unmasked BC threshold at 4 kHz was reassessed with the ear canal occluded (using the patient's own index finger) to rule out the acoustic radiation phenomenon.[5] It was then found that a similar BC threshold was produced (i.e., 15 dB) [Figure 1]. All audiological tests were carried out by an experienced clinical audiologist in a dedicated soundproof room using calibrated instruments.
Figure 1: Pure tone audiometry, tympanometry, and acoustic reflex results of the patient

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In terms of the consistency between the test results, the PTA findings were indeed consistent with those of otoscopy and acoustic reflex. That is, for the left ear, the presence of ABGs was consistent with the appearance of dull TM and abnormal ARTs. On the other hand, the tympanometric results (that showed no conductive element in both ears) were not in line with the findings of other tests. Collectively, by considering all test findings, it was concluded that the PTA results were reliable. The patient was then referred to the otorhinolaryngology clinic for further management.

As depicted in [Figure 1], the IAA for BC (the difference between the right and left unmasked BC threshold) was 10 dB for 250 Hz and 15 dB for 4 kHz. In this regard, if the unmasked BC thresholds are only measured from the right ear (i.e., the worse ear), misdiagnosis would occur. That is, if the right and left unmasked BC thresholds are considered similar (because the IAA is assumed to be 0 dB), the left ear would be mistakenly diagnosed with SNHL, which contradicts other test results (in this situation, BC masking is not required as the ABGs ≤10 dB for both 250 Hz and 4 kHz).


  Discussion Top


In clinical settings, both AC and BC thresholds were carefully obtained to properly diagnose the severity and the type of hearing loss. Since the IAA for BC is generally considered to be 0 dB, many clinicians would perform BC testing on only one ear (typically the worse ear) and proceed with the subsequent masking procedures (as applicable). As stated earlier, even some clinics have been using the triangle symbol to denote unmasked BC thresholds implying the insignificant IAA in BC testing.

In this case report, it was demonstrated that the IAA for BC was not necessarily 0 dB, and conducting unmasked BC testing on only one ear would lead to the misdiagnosis on the type of hearing loss (as well as inconsistency with other test results). As revealed in [Figure 1], the left unmasked BC thresholds (particularly at 250 Hz and 4 kHz) were better by 10–15 dB than the right masked BC thresholds. These findings are in fact sensible as the IAA for BC (when tested with the bone vibrator placed on the mastoid area) would range from 0 dB to 15 dB.[3],[4] On the other hand, if the bone vibrator is placed on the forehead, 0 dB IAA is expected.[3],[4]

In this case report, the PTA results were considered valid and reliable for several reasons. First, the PTA findings were consistent with those of otoscopy and acoustic reflex, suggesting the presence of a conductive element in the left ear and SNHL in the right ear. Even though the tympanometry revealed contradictory findings (i.e., absence of conductive element in both ears), it is known that the sensitivity of tympanometry in detecting middle ear disorders is not 100%.[1],[6],[7] In this regard, it would not be able to identify some patients with confirmed middle ear pathologies.[6],[7] As such, the test battery approach is always recommended to achieve accurate clinical diagnoses.[1] Second, the ABGs at 250 Hz and 4 kHz were considered “genuine” implying the presence of a conductive element in the left ear. Measuring BC thresholds at 250 Hz can be controversial due to issues such as vibrotactile stimulation and harmonic distortions.[2],[8] In this regard, “false” ABGs would be produced if vibrotactile sensation is perceived (rather than hearing sensation) or if harmonic distortions are present.[2],[8] Nevertheless, for B71 transducer, the minimum intensity level that would produce the vibrotactile sensation was around 35 dB HL at 250 Hz frequency.[8] Moreover, at 250 Hz, the harmonic distortions would start to occur when the intensity level reached 20 dB HL.[8] In relation to our case, the left unmasked BC threshold at 250 Hz was 15 dB HL, which was below the vibrotactile level and unlikely to produce harmonic distortions. The “false” ABGs could also occur at 4 kHz frequency.[2],[9],[10] Several reasons have been proposed to explain this including the occurrence of acoustic radiation.[9],[10] During the BC testing, the signals produced by the BC transducer would radiate to the ear canal activating the AC pathway (in conjunction with the BC pathway). Consequently, lower than expected BC thresholds are obtained (producing the “false” ABGs), particularly at high frequencies.[9],[10] Occluding the ear canal would eliminate the effect of acoustic radiation.[5],[9] However, as shown in [Figure 1], the left unmasked BC threshold at 4 kHz was not notably lower than those at other frequencies. In fact, when the ear canal was occluded, the BC threshold at this frequency did not change (eradicating the influence of acoustic radiation). Of note, even though significant ABGs were observed at 250 Hz and 4 kHz, masking was not necessary as the right masked BC thresholds had shifted more than 15 dB eliminating the central masking effect.[1],[11]

For the left ear, the type of hearing loss was likely to be MHL. That is, apart from the presence of conductive element, the inner ear may also be compromised. As revealed in [Figure 1], even though the BC thresholds were all within the normal range, most of them were approaching 20 dB HL (the upper limit of normal range) implying that the inner ear may not be completely normal. Since the PTA thresholds are calibrated in dB HL (i.e., normalized to 0 dB HL when testing normal hearing individuals), those with well-functioning inner ears would have BC thresholds around 0 dB HL.[1] Furthermore, the AC threshold at 8 kHz exceeded 60 dB HL (which is the upper limit of CHL). This pattern also suggests the presence of sensorineural deficit in the left ear.[1]

From this case report, several important considerations are pointed out, which would be beneficial clinically. First, after completing the BC testing (for both masked and unmasked conditions) in one ear, it is advisable to do BC testing in the other ear as the IAA may not be necessarily 0 dB. As such, both unmasked and masked BC thresholds should be determined as necessary for this ear. An accurate hearing diagnosis will be achieved with the complete BC testing. Second, the BC testing conducted at 250 Hz can provide useful information, particularly on confirming the type of hearing loss. Even though the BC testing at 250 Hz can be controversial, we found the BC thresholds at 250 Hz to be reliable, at least in this case.


  Conclusions Top


Even though PTA is the standard clinical test for hearing diagnosis, it is still imperative to ascertain its results through comparisons with other clinical test findings. Since the IAA for BC is not necessarily 0 dB, the unmasked BC testing should be performed on both ears to avoid misdiagnosis (particularly on the type of hearing loss).

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient has given his consent for his images and other clinical information to be reported in the journal. The patient understands that the name and initials will not be published and due efforts will be made to conceal the identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

1.
Katz J, Marshall C, English K, Hood L, Tillery KL, editors. Handbook of Clinical Audiology. Philadelphia: Lippincott Williams & Wilkins; 2015.  Back to cited text no. 1
    
2.
Wan Mohamad WN, Romli M, Awang MA, Abdullah R, Lih AC, Zakaria MN. The presence of unusual bone conduction thresholds in pure tone audiometry. Indian J Otol 2020;26:54-7.  Back to cited text no. 2
    
3.
Studebaker GA. Clinical masking of air- and bone-conducted stimuli. J Speech Hear Disord 1964;29:23-35.  Back to cited text no. 3
    
4.
Studebaker GA. Clinical masking of the nontest ear. J Speech Hear Disord 1967;32:360-71.  Back to cited text no. 4
    
5.
Sahu P, Mahallik D. Acoustic radiation: Relation with frequency and its impact on threshold estimation. Indian J Otol 2019;25:53-8.  Back to cited text no. 5
  [Full text]  
6.
Finitzo T, Friel-Patti S, Chinn K, Brown O. Tympanometry and otoscopy prior to myringotomy: Issues in diagnosis of otitis media. Int J Pediatr Otorhinolaryngol 1992;24:101-10.  Back to cited text no. 6
    
7.
Rogers DJ, Boseley ME, Adams MT, Makowski RL, Hohman MH. Prospective comparison of handheld pneumatic otoscopy, binocular microscopy, and tympanometry in identifying middle ear effusions in children. Int J Pediatr Otorhinolaryngol 2010;74:1140-3.  Back to cited text no. 7
    
8.
Eichenauer A, Dillon H, Clinch B, Loi T. Effect of bone-conduction harmonic distortions on hearing thresholds. J Acoust Soc Am 2014;136:EL96-102.  Back to cited text no. 8
    
9.
Lightfoot GR, Hughes JB. Bone conduction errors at high frequencies: Implications for clinical and medico-legal practice. J Laryngol Otol 1993;107:305-8.  Back to cited text no. 9
    
10.
Margolis RH, Eikelboom RH, Johnson C, Ginter SM, Swanepoel de W, Moore BC. False air-bone gaps at 4 kHz in listeners with normal hearing and sensorineural hearing loss. Int J Audiol 2013;52:526-32.  Back to cited text no. 10
    
11.
Snyder JM. Central masking in normal listeners. Acta Otolaryngol 1973;75:419-24.  Back to cited text no. 11
    


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