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Audiometry: masking.

Audiometry provides information about hearing function, not about diagnosis. Many different causes of hearing impairment produce similar audiometric 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 (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. For example, interaural attenuation 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) 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 (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, 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. 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. 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 (Dr. Brooklet).
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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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