What is the False Air-Bone Gap?

Introductory
10 mins
Reading
01 May 2017

Description

Air-bone gaps are the difference in sensitivity threshold between air and bone conduction transducers. This measurement helps distinguish between conductive and sensorineural hearing losses, as well as their combinations. The specific pattern of thresholds can also aid in diagnosing certain causes of conductive or sensorineural losses, for example otosclerosis or noise-induced loss. However, while not unheard of, relatively few types of conductive loss would be expected to selectively produce air-bone gaps at 4 kHz1. Moreover, in many cases, such air-bone gaps are seen in the absence of any other evidence for conductive hearing loss such as positive symptoms, otoscopic examination and tympanometry, hence the phrase ‘false’.

There have been a number of investigations into the causes of this apparently false reading centred on the Radioear B71 bone vibrator, but it was not until relatively recently that a full explanation was put forward (Margolis et al, 2013). The findings from this study attributed the false air-bone gap at 4 kHz to an error in the calibration reference levels (RETFLs). Other theories such as airborne radiation from the bone transducer that enters the ear canal where it can be heard via the air-conduction route are less able to fully explain the phenomena. 


References and caveats
1Age related changes in middle ear function have been described in the literature which may affect the high frequencies in particular (e.g. Feeney and Sanford, 2004), as have partial ear canal collapse/occlusion, partial ossicular disarticulation and non-organic hearing loss (e.g. Mustain and Hasseltine, 1981).

Feeney, M.P. and Sanford, C.A. (2004) Age effects in the human middle ear: Wideband acoustical measures. Journal of the Acoustical Society of America 116 (6) pages 3546-3558

Margolis, R.H. et al (2014) False air-bone gaps at 4 kHz in listeners with normal hearing and sensorineural hearing loss. International Journal of Audiology 52 (8) pages 526-32

Mustain, W.D. and  Hasseltine, H.E. (1981) High frequency conductive hearing loss: a case presentation. The Laryngoscope 91, pages 599-603

Presenter

Michael Maslin
After working for several years as an audiologist in the UK, Michael completed his Ph.D. in 2010 at The University of Manchester. The topic was plasticity of the human binaural auditory system. He then completed a 3-year post-doctoral research program that built directly on the underpinning work carried out during his Ph.D. In 2015, Michael joined the Interacoustics Academy, offering training and education in audiological and vestibular diagnostics worldwide. Michael now works for the University of Canterbury in Christchurch, New Zealand, exploring his research interests which include electrophysiological measurement of the central auditory system, and the development of clinical protocols and clinical techniques applied in areas such as paediatric audiology and vestibular assessment and management.

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