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How to eliminate air-bone gaps audiometrically: use too much masking.

Abstract

The excessive, indiscriminate use of masking during measurements of pure-tone bone-conduction thresholds can reduce or eliminate air-bone gaps. This may result in an abnormal, audiometrically induced bone-conduction threshold shift and suggest to the otologist the need for auditory brainstem response testing and/or magnetic resonance imaging. A case is presented in which the inappropriate use of the masking plateau method resulted in a reduction of the air-bone gap in an ear with a mild conductive hearing loss. The audiometric Weber test should be used in these cases, and nonmasked bone thresholds should be used to determine the actual level of the cochlear reserve.

Introduction

Remarkable, even revolutionary, strides have been made in a variety of diagnostic procedures, such as testing for otoacoustic emissions, auditorybrainstem response, and auditory steady-state response. Newer techniques apparently allow us to distinguish between central auditory processing disorder and attention-deficit/hyperactivity disorder. (1) But in the quotidian practice of audiology, important decisions continue to be made on the basis of air- and bone-conduction relationships. The distinction between conductive, sensorineural, and mixed peripheral hearing losses continues to guide the selection of treatment options, such as middle ear surgery, amplification, or some combination thereof.

More than 50 years ago, I wrote in the introduction to my doctoral dissertation that the use of masking in performing bone-conduction threshold measurements was "the messiest problem in clinical audiology." (2) In 2008, it continues to be a messy problem. Masking can lead to a variety of audiometric misinterpretations and consequent errors in patient management.

In 1959, Jerger and Wertz also warned of the dangers inherent in the indiscriminate use of masking in bone-conduction audiometry. (3) They described the case of a patient with surgically confirmed otosclerosis whose masked bone levels yielded a false sensorineural hearing loss secondary to overmasking of a conductively impaired ear. They cautioned that "too little is known of the extent to which the mere presence of noise on one ear may yield a completely false picture of bone acuity on the other ear." They noted that contralateral masking noise must be used with extreme caution and with a full awareness of the limitations inherent in this technique.

Because interaural attenuation for bone-conduction stimuli is minimal or nonexistent, many audiologists believe that measurement of bone thresholds should always be performed with masking of the nontested ear. However, the bone-conduction vibrator stimulates both cochleas about equally; in other words, there is no interaural attenuation (IA) for bone conduction (IA = 0 dB). (4) The routine use of masking in the nontested ear results in an occlusion of that ear of unknown degree for any particular patient. Furthermore, the spurious elevation of the bone-conduction level will result in lateralization of the test tone to the nontested ear, which is counterproductive when one is attempting to test the cochlear function of each ear individually.

An increasingly popular and widely used procedure for bone-conduction masking is the plateau method described by Hood in 1960. (5) This method involves a gradual increase in masking in the nontested ear in an effort to reach a plateau or a range of masking levels in which there is no increase in the threshold of the ear under test. Most audiologists who use this technique consider that the attainment of the same response on three consecutive masked levels is a reflection of the "true" cochlear level for that ear at that frequency. The plateau method is considered a valid measurement, but it is time-consuming and may induce patient fatigue. (6) Furthermore, not all patients reach a plateau level, so the procedure may result in overmasking in some cases, such as those involving mild conductive hearing losses.

Can tympanometry replace bone-conduction measurements?

In an effort to resolve problems associated with overmasking, it has been suggested that tympanometry be used to eliminate the need for bone conduction and masking in some cases. If the tympanogram is completely normal, some suggest that it is unnecessary to test bone conduction. Jerger believes that with an air-bone gap, the entire immittance battery cannot be normal (J.F. Jerger, PhD; oral communication, March 24, 2008). This includes determination of the acoustic reflex levels. However, some people with normal tympanograms have air-bone gaps, while others with abnormal tympanograms have no air-bone gaps. Certainly, tympanometry should be a part of every audiologic evaluation, but it should not be viewed as a substitute for traditional bone-conduction measurements.

Wilber and Feldman reported that immittance measurements cannot determine the extent of the air-bone gap with any degree of accuracy, but they can point to the probability of medically pathologic conditions. (7)

Sensory acuity level

In 1960, Jerger and Tillman introduced a test for sensory acuity level (SAL) in an effort to resolve the continuing problems associated with the use of masking for bone-conduction measurement, particularly in cases of bilateral conductive pathology. (8) Their test was a modification of a procedure developed by Rainville. (9) For the SAL test, air-conduction thresholds are obtained with masking presented at a fixed level via the bone vibrator. These thresholds are then compared with the patient's unmasked air-conduction thresholds. The difference between the two sets of air-conduction thresholds is then compared with the average shift obtained under the same conditions for a group of normal-hearing listeners. However, this test was found not to be in agreement at low and middle frequencies with conventional bone-conduction measurements on persons with conductive pathology. The SAL test cannot be used with confidence as a substitute for conventional bone-conduction measurements. The SAL test fails to compensate for the occlusion effect that is present in normal-hearing persons but absent in those with conductive hearing loss. The occlusion effect varies from one subject to another.

The SAL test eventually lost its appeal among clinicians--including one of its developers. In fact, Tillman wrote as early as 1963 that the SAL test was inadequate as a substitute for conventional bone-conduction testing and that the two approaches yielded different estimates of sensorineural sensitivity in the low frequencies in otosclerotic patients. (10) By 2001, Gelfand reported that the SAL test was no longer being used routinely, although it did retain some value when standard bone-conduction measurements were equivocal. (11)

Audiometric Weber test

The classic Weber test is performed with tuning forks to differentiate conductive from sensorineural hearing loss. Markle et al suggested performing the Weber test with the bone vibrator of the audiometer in order to determine which ear(s) should be masked in bone audiometry. (12) This test should be performed at frequencies at which bone conduction will be tested. The vibrator is placed at the midline of the forehead, and the intensity of the tone is increased until a frontal threshold is reached. The intensity is then increased by 10 to 15 dB, and the patient is asked to identify the ear in which the sound is louder or to report whether the sound is equally loud in the two ears or not perceived in either ear. The bone conduction of the lateralized ear is tested first without masking. The bone conduction of the contralateral ear is then tested while masking is delivered to the lateralized ear.

Like the SAL test, the audiometric Weber test fell into general disuse over the years, and today it is used by only a few of us. Some patients are unable to localize the signal or are hesitant to report that it is louder in their poorer ear. Others give inconsistent responses, and yet others localize to one ear at some frequencies and to the other ear at other frequencies, which may reflect the presence of a conductive component at some frequencies.

It is interesting that in the fourth edition of his classic audiology text published in 1972, (13) Newby described the audiometric Weber in considerable detail, but in later editions, (14) he ignored it completely. One of the few recent audiology texts in which the Weber test was mentioned is the second edition of Gelfand's Essentials of Audiology; alas, Gelfand mentioned it only to disparage it, calling its usefulness "questionable." (15) He argued that to use the Weber test to determine which ear to mask yields results that "are not sufficiently accurate or reliable for this purpose." Gelfand pointed out that even the test's proponents, such as Studebaker, (16) admitted that it is best to disregard inconsistent Weber results. Other advocates of the Weber test who suggested that lateralization should be disregarded if the results appear to be improbable are Liden et al (17) and Naunton. (18) They expressed an unwillingness to rely on it exclusively in all instances.

[FIGURE 1 OMITTED]

Yet despite the reluctance to accept the Weber test, it might be premature to disregard it altogether. When it is used appropriately, it can prevent overmasking in cases of conductive hearing loss. Therefore, the skepticism should be reconsidered, especially in cases of mild to moderate conductive hearing loss.

The following case report provides an example of the misuse of the masking plateau. Proper administration of the plateau method in this case would have prevented a false drop in bone conduction.

Case report

The subject of this investigation was a 30-year-old woman who was majoring in speech-language pathology and audiology at New York University. The author was providing practice in auditory screening in a required practicum. As part of the case history, the student reported better hearing in her right ear than in her left ear. She said that the hearing loss on the left had been longstanding.

[FIGURE 2 OMITTED]

The student/patient underwent pure-tone audiometry performed by the author (figure 1). The results on the right fell entirely within normal limits; the pure-tone average (PTA) for the three central frequencies was 8 dB. There was a mild hearing loss on the left; the PTA was 28 dB. The audiometric Weber test was referred to the left ear at all frequencies tested except 2,000 Hz, where there was no lateralization. Unmasked bone conduction on the left showed a PTA of 5 dB and an average air-bone gap of 23 dB, which accounted for all of the mild hearing loss. The speech recognition thresholds were 5 and 25 dB on the right and left, respectively. The word recognition score was 100% on each side. The results of this evaluation were interpreted as a mild conductive hearing loss in the left ear.

The author referred the patient to the otologist who covered the student health service, and he in turn referred the patient to the medical center with which the university is affiliated for a complete audiologic evaluation (figure 2). Unmasked bone conduction on the left was not performed. Masking was performed (level or levels unknown), and the air-bone gap averaged 6 dB. The audiologist concluded that there was a mild conductive hearing loss at 250 and 4,000 Hz and a mild sensorineural hearing loss at 500 and 1,000 Hz. The hearing loss at 2,000 Hz appeared to be primarily sensorineural. Speech recognition thresholds were in agreement with pure-tone air-conduction thresholds, and word recognition scores were 100%.

[FIGURE 3 OMITTED]

The referring otologist was concerned about the asymmetric bone thresholds and believed that the asymmetry warranted magnetic resonance imaging with contrast. The author told him that the asymmetry was the product of an excessive and unnecessary use of masking, and the otologist agreed that the acoustic reflex results were consistent with a conductive, not a sensorineural, hearing loss.

The author then referred the patient to an audiology colleague (figure 3). She performed bone-conduction testing of the left ear with and without masking and concluded that there was a mild conductive hearing loss at the low frequencies, a borderline sensorineural hearing loss at 1,000 and 1,500 Hz, normal hearing at 2,000 Hz, and a conductive component at 4,000 Hz. Speech audiometry findings were consistent with the pure-tone findings. The audiologist suggested a possible diagnosis of early otosclerosis or a congenital ossicular malformation.

A referral to yet another otologist was made. This otologist questioned the audiogram shown in figure 2, and she performed a tuning-fork test, the result of which was consistent with a conductive hearing loss in the left ear. She suggested otosclerosis as the probable diagnosis and said that the patient might be helped by surgery.

References

(1.) Shinn JB, Baran JA, Moncrieff DW, Musiek FE. Differential attention effects on dichotic listening. J Am Acad Audiol 2005;16(4): 205-18.

(2.) Miller MH. Clinical Applications of Paired Masking Enclosures in Pure Tone Air and Bone Conduction Testing of Subjects Having a Differential in the Hearing Acuity of the Two Ears of 30 Decibels or More [dissertation]. New York: Columbia University; 1956.

(3.) Jerger J, Wertz M. The indiscriminate use of masking in bone-conduction audiometry. Arch Otolaryngol 1959;70:419-20.

(4.) Gelfand SA. Essentials of Audiology. 2nd ed. New York: Thieme Medical Publishers; 2001:293.

(5.) Hood JD. The principles and practice of bone conduction audiometry: A review of the present position. Laryngoscope 1960;70: 1211-28.

(6.) Katz J, Lezynski J. Clinical masking. In: Katz J, ed. Handbook of Clinical Audiology. 5th ed. Philadelphia: Lippincott Williams & Wilkins; 2002:136.

(7.) Wilber LA, Feldman AS. The middle ear measurement battery. In: Wilber LA, Feldman AS, eds. Acoustic Impedance and Admittance: The Measurement of Middle Ear Function. Baltimore: Williams & Wilkins; 1976:345-77.

(8.) Jerger J, Tillman T. A new method for the clinical determination of sensorineural acuity level (SAL). Arch Otolaryngol 1960;71: 948-55.

(9.) Rainville MJ. New methods of masking for the determination of bone conduction curves. Translations of the Beltone Institute for Hearing Research 1959;11:1-10.

(10.) Tillman TW. Clinical applicability of the SAL test. Arch Otolaryngol 1963;78:20-32.

(11.) Gelfand SA. Essentials of Audiology. 2nd ed. New York: Thieme Medical Publishers; 2001:165.

(12.) Markle DM, Fowler EP Jr., Moulonguet H. The audiometer Weber test as a means of determining the need for, and the type of, masking. Ann Otol Rhinol Laryngol 1952;61(3):888-900.

(13.) Newby HA. Testing the hearing function: Pure-tone audiometry. In: Newby HA. Audiology. 4th ed. Englewood Cliffs, N.J.: Prentice Hall; 1972:127-8.

(14.) Newby HA. Audiology. 6th ed. Englewood Cliffs, N.J.: Prentice Hall; 1992.

(15.) Gelfand SA. Essentials of Audiology. 2nd ed. New York: Thieme Medical Publishers; 2001:298.

(16.) Studebaker GA. Clinical masking of the nontest ear. J Speech Hear Disord 1967;32(4):360-71.

(17.) Liden G, Nilsson G, Anderson H. Masking in clinical audiometry. Acta Otolaryngol 1959;50(2):125-36.

(18.) Naunton RF. Clinical bone-conduction audiometry. The use of a frontally applied bone-conduction receiver and the importance of the occlusion effect in clinical bone-conduction audiometry. AMA Arch Otolaryngol 1957;66(3):281-98.

Maurice H. Miller, PhD

From the Department of Speech-Language Pathology and Audiology, Steinhardt School of Culture, Education, and Human Development of New York University, New York City.

Correspondence: Maurice H. Miller, PhD, Department of Speech-Language Pathology and Audiology, Steinhardt School of Culture, Education, and Human Development of New York University, 665 Broadway, 9th Floor, New York, NY 10012. Phone: (917) 686-6020; fax: (718) 793-4645; e-mail: mhml@nyu.edu
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Title Annotation:ORIGINAL ARTICLE
Author:Miller, Maurice H.
Publication:Ear, Nose and Throat Journal
Article Type:Case study
Geographic Code:1USA
Date:May 1, 2008
Words:2504
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