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Audiometry: masking.



Audiometry provides information about hearing function, not about diagnosis. Many different causes of hearing impairment produce similar audiometric au·di·om·e·ter  
n.
An instrument for measuring hearing activity for pure tones of normally audible frequencies. Also called sonometer.



au
 results. Nevertheless, we rely upon audiometric test results to help establish a diagnosis, so it is essential that they be accurate. One common source of error occurs when the audiometric stimulus is perceived in the contralateral contralateral /con·tra·lat·er·al/ (-lat´er-al) pertaining to, situated on, or affecting the opposite side.

con·tra·lat·er·al
adj.
 (nontest) ear, and patient responses are unknowingly recorded as if they had resulted from stimulation of the test ear. Audiologists avoid this error by expert use of masking.

Since patients with conductive or mixed hearing loss have a good chance of being successfully treated with surgery, it is critical to determine the degree of the conductive component in these cases. However, testing of patients with conductive or mixed hearing loss poses a challenge to the audiologist Audiologist
A person with a degree and/or certification in the areas of identification and measurement of hearing impairments and rehabilitation of those with hearing problems.
. For example, interaural attenuation Loss of signal power in a transmission.
Attenuation

The reduction in level of a transmitted quantity as a function of a parameter, usually distance. It is applied mainly to acoustic or electromagnetic waves and is expressed as the ratio of power densities.
 produces cross-hearing in the nontest ear via bone conduction (BC). Although interaural attenuation is frequency-dependent, hearing scientists and audiologists generally accept a level of 40 dB across all frequencies for air-conduction (AC) testing. Masking is applied to the nontest ear if the AC threshold of the test ear is greater than 40 dB above the BC threshold of the nontest ear. During BC testing, masking is applied to the nontest ear whenever there is an air-bone (A-B A-B Air-Britain (UK-based aviation historical society)
A-B Research Centre Applied Biocatalysis (Graz, Austria) 
) gap greater than 10 dB in the test ear. Masking should be applied whenever the difference between the BC of the two ears at any given frequency exceeds 5 dB.

Audiometers are calibrated so that a 10-dB masking noise will block a 10-dB pure-tone signal. Masking of the nontest ear is achieved by using "effective masking" (i.e., 40 dB greater than the nontest-ear threshold) and/or by reaching a plateau of 15 dB (in 5-dB increments), during which the test ear hears the pure tone at the same level. Effective masking is used primarily to save time. Masking without a plateau can lead to either undermasking or overmasking.

Undermasking leads to false A-B gaps and is usually the result of operator inexperience or a failure to follow masking rules. In the case illustrated here (figure), the left, low-frequency A-B gap occurred as a result of not using the plateau method, and it disappeared when proper masking was applied. In this situation, the results of the 256- and 512-Hz tuning fork tests, which lateralized to the right ear, were a telltale sign of a false A-B gap. Although uncommon, BC hypersensitivity hypersensitivity, heightened response in a body tissue to an antigen or foreign substance. The body normally responds to an antigen by producing specific antibodies against it. The antibodies impart immunity for any later exposure to that antigen.  (BC thresholds in the nontest ear less than 0 dB) can lead to undermasking if the true thresholds are not used to calculate effective masking levels. (We described BC hypersensitivity, which is usually present with superior canal dehiscence Superior canal dehiscence (SSCD) is a very rare medical condition, first described in 1998 by Lloyd Minor of Johns Hopkins University, causing hypersensitivity to sound in those affected. , in the August 2006 AUDITORY AND VESTIBULAR MEDICINE CLINIC. (1)) In such cases, it is recommended that true BC thresholds be used for effective masking.

[FIGURE OMITTED]

Overmasking is commonly encountered in patients with bilateral conductive hearing loss Conductive hearing loss
A type of medically treatable hearing loss in which the inner ear is usually normal, but there are specific problems in the middle or outer ears that prevent sound from getting to the inner ear in a normal way.
. In such patients, in order to provide 40-dB masking above the BC threshold in the nontest ear, the masking level may exceed interaural attenuation and produce an inadvertent threshold elevation by masking the test ear, as well. It is difficult to establish an accurate masking plateau in this patient population. This is referred to as a "masking dilemma," and it can be rectified by using insert earphones, which have higher interaural attenuation levels.

In summary, masking is critical during audiometry. Masking rules, effective masking, masking plateaus, and insert earphones should be used routinely to achieve proper masking and to avoid under-and overmasking problems.

Reference

(1.) Brookler KH, Hamid MA. The "normal" audiogram au·di·o·gram
n.
A graphic record of hearing ability for various sound frequencies.


Audiogram
A chart or graph of the results of a hearing test conducted with audiographic equipment.
. Ear Nose Throat J 2006;85:486.

Mohamed A. Hamid, MD, PhD; Kenneth H. Brookler, MD

From the Cleveland Hearing and Balance Center, Beachwood, Ohio (Dr. Hamid), and Neurotologic Associates, PC, New York City New York City: see New York, city.
New York City

City (pop., 2000: 8,008,278), southeastern New York, at the mouth of the Hudson River. The largest city in the U.S.
 (Dr. Brooklet brook·let  
n.
A small brook.

Noun 1. brooklet - a small brook
brook, creek - a natural stream of water smaller than a river (and often a tributary of a river); "the creek dried up every summer"
).
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Copyright 2006, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

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Title Annotation:AUDITORY AND VESTIBULAR MEDICINE CLINIC
Author:Brookler, Kenneth H.
Publication:Ear, Nose and Throat Journal
Date:Oct 1, 2006
Words:626
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