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 Table of Contents  
CASE REPORT
Year : 2020  |  Volume : 26  |  Issue : 1  |  Page : 54-57

The presence of unusual bone conduction thresholds in pure tone audiometry


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

Date of Submission08-Sep-2019
Date of Decision04-Oct-2019
Date of Acceptance20-Oct-2019
Date of Web Publication19-Feb-2020

Correspondence Address:
Dr. Wan Najibah Wan Mohamad
Audiology and Speech Pathology Programme, School of Health Sciences, Universiti Sains Malaysia, Kubang Kerian, 16150, Kelantan
Malaysia
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/indianjotol.INDIANJOTOL_99_19

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  Abstract 


In pure tone audiometry (PTA), the widely used transducer for bone conduction (BC) testing is Radioear B71. Nevertheless, the BC thresholds obtained with this bone vibrator can be questionable and invalid due to its technical limitations. In this study, we report a case where “abnormally good” BC thresholds were noted at low frequencies, possibly due to the occurrence of harmonic distortions by this transducer. In clinical settings, due to the technical limitations of the BC vibrator, it is imperative for hearing health-care professionals to properly interpret the BC results in PTA. Ultimately, having newly designed transducers (with low harmonic distortions) in clinical settings is advantageous to obtain a more accurate hearing diagnosis.

Keywords: B71 transducer, bone conduction, harmonic distortions, pure tone audiometry


How to cite this article:
Wan Mohamad WN, Romli M, Awang MA, Lih AC, Abdullah R, Zakaria MN. The presence of unusual bone conduction thresholds in pure tone audiometry. Indian J Otol 2020;26:54-7

How to cite this URL:
Wan Mohamad WN, Romli M, Awang MA, Lih AC, Abdullah R, Zakaria MN. The presence of unusual bone conduction thresholds in pure tone audiometry. Indian J Otol [serial online] 2020 [cited 2020 Jul 14];26:54-7. Available from: http://www.indianjotol.org/text.asp?2020/26/1/54/278744




  Introduction Top


Reliable and valid hearing results are important for the successful treatment of patients with hearing loss. Pure tone audiometry (PTA) has been regarded as the gold standard test for hearing diagnosis. It measures the softest audible sound at least 50% of the time, known as hearing thresholds. Hearing thresholds consist of air and bone conduction (BC) thresholds. Air conduction (AC) thresholds are the softest audible acoustic signals that travel through outer, middle, and inner ears using headphones or insert earphones. On the other hand, BC thresholds are the audible acoustic signals that vibrate the skull to stimulate the inner ear (cochlea) using a bone vibrator. The difference between AC and BC thresholds is known as air-bone gap (ABG). The ABG is typically used to determine the type of hearing loss (i.e., conductive, sensorineural, or mixed hearing loss). As such, if the ABG is 15 dB or more, the patient is said to have a conductive element in which a specific medical treatment is required.[1] A false ABG can result in an inappropriate diagnosis and case management.

It is worth noting that the BC thresholds are influenced by many factors, including the bone vibrator placement, the contact area of the vibrator, and the type of transducer.[1],[2],[3] Radioear B71 bone vibrator is the most common transducer used in the audiology clinics worldwide. However, obtaining valid BC thresholds can be troublesome with this transducer (particularly at low frequencies) due to the occurrence of harmonic distortions and vibrotactile stimulation.[2] In this paper, we report the presence of questionable unmasked BC thresholds in PTA testing that might be noteworthy to readers, students, and clinical professionals in the fields of otology and audiology.


  Case Report Top


This is a case of a 66-year-old male presented with reduced hearing in both ears (worse in the left ear) starting about 3 years ago. He felt that his hearing gradually became worse. As such, he reported difficulties in understanding conversations in quiet and noisy environments. No history of otalgia, otorrhea, and other ear, nose, and throat problems has been reported by the patient. He was also keen to wear hearing aids if suitable. He was a lecturer in a public university and has no history of noise exposure. He was generally healthy, and no other significant medical history was reported.

All audiological tests were performed by a competent audiologist in a dedicated soundproof room using calibrated instruments in the university hospital. The otoscopic examination found him to have clear ear canals with intact tympanic membranes. Using a 226 Hz tympanometer (AT235H, Interacoustics, Denmark), Type A tympanogram was obtained bilaterally, suggestive of normal middle ear functions. The PTA testing was performed using a two-channel audiometer (GSI 61, Grason-Stadler Inc., USA), and appropriate masking procedures were applied. Using TDH-39 headphones, the AC thresholds were determined at octave frequencies of 250–8000 Hz. The BC testing was carried out at 250–4000 Hz frequencies with the bone vibrator (B71) placed on the mastoid. Based on the complete audiogram, he was found to have a mild-to-moderate sensorineural hearing loss in the right ear and a moderate-to-severe sensorineural hearing loss in the left ear [Figure 1]. This result was consistent with the otoscopic findings and tympanometric results (suggestive of the absence of conductive element in both ears).
Figure 1: Pure tone audiometric and tympanometric results of a patient

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Nevertheless, a closer look at the audiogram revealed the presence of “abnormally good” unmasked BC thresholds at 250 Hz and 500 Hz in the right ear (i.e., 20 dB HL) and significant ABGs were observed [Figure 1]. From this perspective, it is sensible to predict that at least one ear would have some conductive element (even after the application of BC masking). These results, however, can be questionable as the other (left) ear showed poorer unmasked thresholds (i.e., 40 dB HL at 500 Hz and vibrotactile response at 40 dB HL for 250 Hz), producing a large interaural attenuation (IAA) value (i.e., 20 dB).

Surprisingly, when all the necessary masking procedures had been performed, the BC thresholds were shifted notably, leaving the questionable unmasked right BC thresholds (e.g., 20 dB HL) at the aforementioned frequencies (the masked BC thresholds were 40 dB HL). To further check the validity of this unmasked threshold, we redetermined the BC threshold at 250 Hz but with the BC transducer placed on the forehead. Interestingly, the unmasked threshold was obtained at 40 dB HL. This threshold value was consistent with the right masked threshold and the unmasked left threshold when the BC vibrator was placed on the mastoid. Collectively, the presence of questionable unmasked BC thresholds at low frequencies in the right ear could be due to the unavoidable technical limitation, i.e., harmonic distortions produced by the B71 transducer.


  Discussion Top


Obtaining valid BC thresholds is specifically important to diagnose the type of hearing loss.[1] The case presented in this paper was of a particular interest to demonstrate the presence of questionable BC thresholds in a clinical setting, possibly due to the technical limitation of the Radioear B71 bone vibrator. It is worth noting that the tympanometer, audiometer, headphones, and bone vibrator had been calibrated accordingly and were functioning well during the testing.

In this case, since otoscopic and tympanometric findings were unremarkable for both ears, the presence of initial significant ABGs (unmasked results) at 250 Hz and 500 Hz attracted our attention. That is, based on these results, at least one ear should have some conductive element (contradictory to otoscopic and tympanometric outcomes). However, after the applications of masking, the PTA result was indeed consistent with those of otoscopy and tympanometry, in which the type of hearing loss was sensorineural [Figure 1].

The unmasked BC thresholds obtained for the right ear at low frequencies (i.e., 20 dB HL) were considered “abnormally good” and invalid for several reasons. First, the IAA (the difference between the unmasked right and left BC thresholds) at these frequencies was atypically large (i.e., 20 dB) [Figure 1]. As reported elsewhere, the IAA for BC was around 0 dB at low frequencies.[4],[5] This implies that the BC thresholds in either ear were invalid (likely to be the right ear). Second, when the bone vibrator was placed on the forehead, the unmasked right BC threshold became worse (i.e., 40 dB HL) but was comparable to the unmasked left BC threshold. Typically, the difference in BC threshold between mastoid and forehead placements should not exceed 15 dB.[6],[7] This again raised the concern regarding the validity of the unmasked right BC threshold (obtained with the mastoid placement). Finally, after the applications of masking, the right BC thresholds were shifted significantly, and the type of hearing loss was, in fact, sensorineural. In this regard, the unmasked results were clearly not the “true” thresholds, and no conductive element was found in both ears. In addition, central masking can occur during BC testing and contribute to shifted masked thresholds.[1],[8],[9] Nevertheless, the maximum threshold shift provided by the central masking was reported to be 10 dB at low frequencies[8],[9] which was unlikely for this case.

It is worthwhile to highlight the possible basis of the questionable unmasked BC thresholds demonstrated in this case. Based on the literature, the validity of BC thresholds can be compromised in the presence of harmonic distortions, vibrotactile response, and acoustic radiation.[1],[2],[3] Laboratory studies have found the widely used B71 transducer to have significant harmonic distortions, especially at low frequencies.[10],[11] In particular, at 250 Hz, the harmonic distortions started to occur when the intensity level was around 20 dB HL.[10] The magnitude of distortions increased rapidly with the increment of intensity level.[10] Accordingly, the presence of harmonic distortions during the BC testing would produce invalid BC thresholds (i.e., lower than expected). As such, the “good” unmasked right BC thresholds observed in this case was likely to be contributed by the presence of notable harmonic distortions. In addition, lower than expected BC thresholds can also be produced by the vibrotactile stimulation.[10] The vibrotactile response is likely to be elicited by the transducer's vibration during the BC testing, especially at high-intensity levels and at lower frequencies. Clinically, the vibrotactile perception should not be regarded as the true BC threshold because the response is recorded based on the feeling of vibration rather than the auditory sensation. The reported vibrotactile levels for B71 transducer were 35 and 55 dB for 250 and 500 Hz frequencies, respectively.[10] In relation to our case, the atypically good unmasked BC thresholds for the right ear (20 dB HL) were below the vibrotactile levels and were unlikely due to the vibrotactile stimulation.

Transducers with better characteristics have been developed and studied to overcome the technical limitations of the Radioear B71 bone vibrator.[2],[10],[11] Eichenauer et al. compared the performance of Radioear B71 and B81 transducers among normally-hearing adults.[10] Using the mastoid placement, the BC thresholds for each transducer were determined at frequencies of 250, 500, 1000, and 2000 Hz. It was then found that the B81 transducer had notably lower distortions than the B71 transducer. In particular, at 250 Hz, the harmonic distortion of B71 vibrator was 78% at 45 dB HL. At a similar frequency and intensity level, the harmonic distortion of B81 transducer was only 0.85%. A similar pattern was seen for 500 Hz frequency. That is, lower harmonic distortions were produced by the B81 transducer (1.55% at 70 dB HL). For the B71 vibrator, the harmonic distortion was 0.1% at 50 dB HL and increased to 14% at 70 dB HL. This study also revealed the audibility of distortions to be lower for the B81 than for the B71 transducer. Taken together, the B81 transducer had shown better performance and was capable of providing higher output than the B71 transducer without much concern on the distortions.[10] In line with this, Fröhlich et al. compared the performance of three types of BC transducers (Radioear B71, B81, and KH70) among the normally hearing and hearing-impaired adults.[2] They then found that the total harmonic distortion was the highest for the B71, followed by B81 and KH70, particularly at low frequencies. Based on this finding, the newly designed KH70 transducer seemed to be superior to the other transducers in BC testing.

It is worth noting that the BC thresholds can also be affected by acoustic radiation.[3],[12],[13] In this regard, lower BC thresholds are produced during the BC testing as the signal (from the bone vibrator) radiates to the external ear canal (EAC) through the AC pathway. Sensibly, blocking the EAC may help to reduce the effect of acoustic radiation.[3] Nevertheless, the lower unmasked BC thresholds in our case were noted at low frequencies, which was not due to this phenomenon. That is, the acoustic radiation occurs only for high-frequency signals (>2 kHz).[3],[12],[13]

In terms of case management, due to notable hearing loss and no medical contraindication (the absence of conductive element), the patient was then fitted with receiver-in-canal Signia Pure 312 3NX in both ears in a timely manner to improve his hearing and communication abilities. His progress will be monitored regularly in the subsequent follow-ups.


  Conclusions Top


In clinical settings, it is possible to obtain lower than expected BC results at low frequencies that are likely contributed by the technical limitations of a widely used Radioear B71 bone vibrator. As such, it is imperative for hearing health-care professionals to be aware of this issue, and PTA results should be sometimes interpreted with care. In addition, it is also important for clinicians to perform masking procedures accordingly to obtain valid BC thresholds so that appropriate case management can take place. If inconsistency occurs, conducting other diagnostic tests (e.g., multifrequency tympanometry, acoustic reflex, and otoacoustic emission) can be useful to achieve accurate hearing diagnoses. With regard to the BC testing, having the newly designed transducers (with low harmonic distortions and vibrotactile response) in clinical settings is advantageous to obtain a more accurate hearing diagnosis in PTA testing.

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 image and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published, and due efforts will be made to conceal their 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 and Wilkins; 2015.  Back to cited text no. 1
    
2.
Fröhlich L, Plontke SK, Rahne T. Influence of transducer types on bone conduction hearing thresholds. PLoS One 2018;13:e0195233.  Back to cited text no. 2
    
3.
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. 3
  [Full text]  
4.
Studebaker GA. Clinical masking of the nontest ear. J Speech Hear Disord 1967;32:360-71.  Back to cited text no. 4
    
5.
Studebaker GA. Clinical masking of air and bone-conducted stimuli. J Speech Hear Disord 1964;29:23-35.  Back to cited text no. 5
    
6.
Studebaker GA. Placement of vibrator in bone-conduction testing. J Speech Hear Res 1962;5:321-31.  Back to cited text no. 6
    
7.
Dirks DD, Malmquist GM. Comparison of frontal and mastoid bone-conduction thresholds in various conductive lesions. J Speech Hear Res 1969;12:725-46.  Back to cited text no. 7
    
8.
Snyder JM. Central masking in normal listeners. Acta Otolaryngol 1973;75:419-24.  Back to cited text no. 8
    
9.
Liden G, Nilsson G, Anderson H. Masking in clinical audiometry. Acta Otolaryngol 1959;50:125-36.  Back to cited text no. 9
    
10.
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. 10
    
11.
Jansson KJ, Håkansson B, Johannsen L, Tengstrand T. Electro-acoustic performance of the new bone vibrator radioear B81: A comparison with the conventional radioear B71. Int J Audiol 2015;54:334-40.  Back to cited text no. 11
    
12.
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. 12
    
13.
Tate Maltby M, Gaszczyk D. Is it necessary to occlude the ear in bone-conduction testing at 4kHz, in order to prevent air-borne radiation affecting the results? Int J Audiol 2015;54:918-23.  Back to cited text no. 13
    


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