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 Table of Contents  
CASE REPORT
Year : 2020  |  Volume : 26  |  Issue : 4  |  Page : 274-276

The evidence of noninner ear involvement in the presence of depressed bone conduction threshold at 2 kHz frequency


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

Date of Submission26-May-2020
Date of Decision14-Sep-2020
Date of Acceptance25-Sep-2020
Date of Web Publication23-Apr-2021

Correspondence Address:
Prof. 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_105_20

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  Abstract 


Pure-tone audiometry is a standard clinical test for hearing diagnosis. Air–bone gaps derived from the difference between air conduction (AC) and bone conduction (BC) thresholds are important indicators to distinguish between different types of hearing losses. Even though BC thresholds are typically unaffected in conductive hearing loss (CHL) cases, a notch or depression at 2 kHz frequency (i.e., Carhart notch) could occur, which could be mistakenly considered as the involvement of sensorineural hearing loss (SNHL). In this article, it was clearly demonstrated that the BC notch at 2 kHz (that could exceed the normal limit) was in fact related to CHL (not SNHL). That is, in this case, both AC and BC thresholds became “perfectly” normal after the treatment (and the notch at 2 kHz disappeared). Several useful points were highlighted from the present case report to guide clinicians, medical trainees, and respective students to avoid misdiagnosis in clinical practice.

Keywords: Air conduction, bone conduction, Carhart notch, pure-tone audiometry


How to cite this article:
Zakaria MN. The evidence of noninner ear involvement in the presence of depressed bone conduction threshold at 2 kHz frequency. Indian J Otol 2020;26:274-6

How to cite this URL:
Zakaria MN. The evidence of noninner ear involvement in the presence of depressed bone conduction threshold at 2 kHz frequency. Indian J Otol [serial online] 2020 [cited 2021 Jun 16];26:274-6. Available from: https://www.indianjotol.org/text.asp?2020/26/4/274/314334




  Introduction Top


Hearing loss is a prevalent condition among children and adults.[1] In clinical settings, pure-tone audiometry (PTA) is the standard test for hearing diagnosis typically performed by hearing health-care professionals such as audiologists. With PTA, the severity and type of hearing loss are determined based on air conduction (AC) and bone conduction (BC) thresholds.[1],[2]

Conductive hearing loss (CHL), sensorineural hearing loss (SNHL), and mixed hearing loss (MHL) are the typical types of hearing losses found among patients with ear disorders.[1],[2],[3] CHL is caused by disorders affecting the external and middle ears.[2] If the inner ear and/or the eighth nerve are compromised, SNHL would occur. MHL, on the other hand, is the combination of both CHL and MHL.[2] Getting an accurate hearing diagnosis is crucial to proceed with a specific treatment.[1],[2],[3]

In typical cases of CHL, BC thresholds are within the normal limit (i.e., 20 dB) and AC thresholds are abnormal, producing significant air–bone gaps (ABGs) (i.e., ≥15 dB).[1],[2] While for SNHL, both AC and BC thresholds exceed the normal limit with no significant ABGs. Nevertheless, in some CHL cases such as those due to otosclerosis (stapes fixation), the BC thresholds would drop, resulting in the ABGs to be closed at 2 kHz frequency.[4],[5],[6] Several theories have been proposed to describe this Carhart notch phenomenon.[4],[5],[6] Because BC information is believed to be mainly contributed by the inner ear, the absence of ABG (due to depressed BC threshold) could be mistakenly considered the involvement of SNHL or even MHL. In this case report, the noninner ear involvement in the presence of a depressed BC threshold at 2 kHz was further demonstrated.


  Case Report Top


We report the case of a 7-year-old Malay boy who came to the Audiology Clinic, University Hospital, for hearing assessments. He has been complaining of reduced hearing in both ears for the past 2 months. As such, his parents and siblings had to speak louder when having conversations with him. According to his parents, he watched television at a higher volume. No history of otalgia and otorrhea was reported. There were no other significant findings reported on history taking. Previous examinations by an otorhinolaryngology (ORL) specialist found him to have otitis media with effusion. As such, using a 512-Hz tuning fork, Rinne test was negative for both ears, and he reported that the sound was lateralized to the right side in Weber test.

The boy was healthy during his first visit (2 days after his visit to the ORL clinic). Otoscopic examination revealed a dull tympanic membrane in both ears. As shown in [Figure 1], tympanometry revealed a Type B tympanogram bilaterally (with normal ear canal volume), suggesting the presence of fluid in the middle ears. In the right ear, PTA revealed a mild CHL with notable ABGs (with hearing within the normal limit at 1, 4, and 8 kHz) [Figure 1]. In the left ear, even though AC thresholds were all within the normal limit, significant ABGs were noted, implying the presence of a conductive element. The notch at 2 kHz for BC threshold was clearly present for both ears [Figure 1]. It is worth noting that for the right ear, the BC threshold at 2 kHz was abnormal (producing no ABG), mimicking a SNHL pattern. If it was believed that the type of hearing loss was CHL (not SNHL or MHL), the PTA results were in fact consistent with tympanograms, as well as with the results from the tuning fork tests. Based on the ORL report, he was prescribed with medications to treat the middle ear condition.
Figure 1: Pure-tone audiometry and tympanometric results of the child before the treatment

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The boy presented again for hearing reassessments a month later. On history taking, he reported that his hearing was much better. Similarly, his parents also noted improvements in their child's hearing. The results of hearing assessments are shown in [Figure 2]. The otoscopic examination found his tympanic membrane to be intact (with a visualized cone of light) in both ears. The tympanometry showed a type As tympanogram bilaterally (suggestive of normal middle ear pressure with slight decreased compliance). The PTA revealed hearing within the normal limit bilaterally. As indicated, notable improvements were seen, in which both AC and BC thresholds became better after the treatment. For both ears, the notch at 2 kHz was now absent, implying that it was in fact related to CHL pathology (not SNHL element). For the right ear, the abnormal BC threshold at 2 kHz (i.e., 25 dB, before the treatment) became normal (i.e., 5 dB, after the treatment).
Figure 2: Pure-tone audiometry and tympanometric results of the child after the treatment

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  Discussion Top


Even though the BC thresholds are typically unaffected in the presence of CHL, the BC notch at 2 kHz could occur. This notch was first described by Carhart in his study among patients with otosclerosis.[4] Due to stapes fixation, the middle ear resonant frequency (around 2 kHz) was altered and the BC threshold at this frequency was depressed.[4] Since then, the occurrence of BC notch at 2 kHz has been inclusively “assumed” to be caused by the stapes fixation (as in otosclerosis).[4],[7]

In this case report, it was evident that the presence of BC notch at 2 kHz did not imply that the inner ear was compromised. That is, it was indeed related to the conductive element as both AC and BC thresholds became better after the treatment. It is well known that SNHL is a permanent condition and BC thresholds would not typically become better over time.[1] Even though “confusion” may arise as the BC threshold at 2 kHz was abnormal for the right ear, it was clear that no SNHL element was involved and the hearing became “perfectly” normal for both ears after the treatment. Collectively, in the presence of notable BC notch at 2 kHz, the child was diagnosed to have pure CHL (not SNHL or MHL) [Figure 1]. In terms of case management, the parents were advised to monitor the child's hearing from time to time and yearly hearing assessment was recommended.

Several useful points could be gathered from this case report. First, as mentioned earlier, it was evident that the abnormal BC threshold at 2 kHz was not indicative of compromised inner ear and/or neural function. Second, the BC notch at 2 kHz may occur in various CHL cases, not just otosclerosis. In this regard, any pathologies that would alter the middle ear resonant frequency would produce the 2 kHz notch.[8],[9] Finally, as shown in [Figure 1] (left ear), the presence of normal AC thresholds (≤20 dB) may not necessarily mean that the middle ear is healthy (i.e., no conductive element). As such, BC testing must still be carried out (even though AC thresholds are normal) to rule out any significant ABGs.


  Conclusions Top


In this case report, it has been clearly demonstrated that the BC notch at 2 kHz (that can exceed the normal limit) is related to CHL (not SNHL). This information would be useful to guide clinicians, medical trainees, and respective students to avoid misdiagnosis in clinical practice and training.

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form, the patient's guardian has given consent for his images and other clinical information to be reported in the journal. The patient's guardian understands that the child's name and initial 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 & 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.
Lih AC, Zakaria MN, Mohamad RA, Nor Rashid MF. Effects of ethnicity and gender on the middle ear function in Asian adults. Indian J Otol 2017;23:94-7.  Back to cited text no. 3
  [Full text]  
4.
Carhart R. Clinical application of bone conduction audiometry. Arch Otolaryngol 1950;51:798-808.  Back to cited text no. 4
    
5.
Tonndorf J. Bone conduction. Studies in experimental animals. Acta Otolaryngol 1966;Suppl 213:132.  Back to cited text no. 5
    
6.
Stenfelt S, Hato N, Goode RL. Factors contributing to bone conduction: The middle ear. J Acoust Soc Am 2002;111:947-59.  Back to cited text no. 6
    
7.
Laitakari K, Löppönen H. Carhart notch and electric bone-conduction audiometry. Scand Audiol 1994;23:139-41.  Back to cited text no. 7
    
8.
Shishegar M, Faramarzi A, Esmaili N, Heydari ST. Is Carhart notch an accurate predictor of otitis media with effusion? Int J Pediatr Otorhinolaryngol 2009;73:1799-802.  Back to cited text no. 8
    
9.
Yasan H. Predictive role of Carhart's notch in pre-operative assessment for middle-ear surgery. J Laryngol Otol 2007;121:219-21.  Back to cited text no. 9
    


    Figures

  [Figure 1], [Figure 2]



 

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