Maximum Conductive Hearing Loss—Revisited

By Cynthia Collier

This article is a part of the May/June, Volume 35, Number 3, Audiology Today issue. 

Pure-tone behavioral audiometry is the most fundamental of the audiologist’s core competencies. The down-ten, up-five threshold search is simple enough and can lull us into a sense of infallibility in our test results. Things can go from very simple to rather complicated, however, when we encounter a significant conductive component to a patient’s hearing loss. Obtaining accurate thresholds for the more complex patients depends on a solid understanding of maximum conductive hearing loss (mCHL) and its relationship to interaural attenuation (IA). The idea that mCHL is limited to 60 dB is the most intractable fallacy regarding pure-tone audiometry among audiologists and otolaryngologists alike and can lead to misdiagnosed hearing loss, even to the point of making a purely conductive hearing loss appear purely sensorineural.

Evidently, the seminal assertion of the 60 dB mCHL is from Scott Reger in 1944:

It appears that an approximate 60-dB loss is the maximal air-conduction impairment (with accompanying normal bone conduction) to be anticipated with middle-ear defects. Therefore, if the air-conduction loss in a patient with apparently typical middle-ear pathology exceeds 60 dB, it is likely that inner-ear impairment is superimposed on the middle-ear lesion, the extent of which will be revealed by bone-conduction measurements (Reger, 1944).

Reger’s observations have since been repeatedly supported with the use of supra-aural headphones to obtain thresholds in patients with ear canal atresia, both anecdotally and in research (Zernotti et al, 2013). There are examples in the research, however, where an air-bone gap greater than 60 dB is reported even with supra-aural headphones. For example, Wolf et al (2015) presented a case study of a 12-year-old boy with bilateral type C ear canal atresia (total atresia) whose audiogram showed at least a 70 dB air-bone gap at 500 Hz.

It is the aim of this article to disabuse audiologists and otolaryngologists of the idea that a conductive hearing loss cannot be greater than 60 dB. It will be demonstrated that air-bone gaps are both transducer specific and frequency specific and can feasibly be upward of 105 dB.

Pure-tone behavioral audiometry is the most fundamental of the audiologist’s core competencies.

The Physics of Sound Transmission

Sound transmits through different substances with varying degrees of efficiency. Solids and liquids (for example, the skull and cochlear fluids, respectively) are very efficient conductors of sound. Bone is such a good conductor that there can be as little as 0 dB sound attenuation from one cochlea to the other. 

As Liden et al (1959) pointed out, “In bone-conduction measurements the skull acts as a conductor of the test tone rather than as a barrier to it. For all practical purposes one must therefore regard both cochleae as being equally excited.” It is because of this effective crossing of the signal for bone conduction (crossover) that we mask the non-test ear so much more readily when presenting to the test ear via bone conduction.

Conversely, air is quite an inefficient conductor of sound compared to other elastic media. The intensity of an air-conducted sound must be much greater than that of a bone-conducted sound to overcome the IA and stimulate the cochlea of the non-test ear via crossover. This crossing over of the air-conducted signal has been referred to as bone conduction by air conduction because bone is the actual conduit of the air-conducted sound (Chaiklin, 1967). 

The minimum intensities at which crossover occurs for pure-tone audiometry range from 40 to 60 dB with the use of supra-aural headphones and from 55 to 85 dB with the use of insert earphones, depending on the frequency of the tone (Killion et al, 1985; Sklare and Denenberg, 1987). 

Killion et al found the average IA at 500 Hz to be 98 dB using deeply inserted earplugs. They observed IA as great as 110 dB at 500 Hz and as great as 95 dB at 1,000 Hz (1985). Likewise, Sklare and Denenberg (1987) found IA to be as great as 105 dB at 500 Hz with properly inserted foam earplugs.

It has been demonstrated that the intensity level at which the skull is set into motion is inversely proportional to the surface area of the transducer, with the IA corresponding “roughly to the ratios of the areas under the … earphone systems and, consequently, to the ratios of the areas of head surface exposed to sound pressure” (Zwislocki, 1953). That is, the smaller the area of the skull that is vibrated, the higher the intensity level needed to stimulate the cochleae. This is a manifestation of the physical relationship between pressure and area: pressure = force/area. And sound pressure is, of course, simply the force produced by a vibrating object and propagated through an area of elastic medium (in our present case, air, bone, and cochlear fluids).