Semantic descriptions

Whether the audiometric results have been recorded as numbers or plotted on a graph, they are interpreted in the same way. Results must be looked at for each frequency in terms of (1) the amount of hearing loss by air conduction (the hearing level), (2) the amount of hearing loss by bone con-duction, and (3) the relationship between air-conduction and bone-conduction thresholds.

A wide variety of pure-tone configurations may be seen among patients and within various pathologies. While some configurations may be suggestive of a given pathology, it is not always possible to identify pathology by audiometric configuration alone. Some distinct gender effects have been noted in audiometric configuration with a wider variation in configuration and a greater preponderance of more steeply sloping audiograms reported for men (Ciletti & Flamme, 2009).

The effects of different hearing-loss configurations on the perceived clarity of speech may be heard on the Companion Website under “Simulated Hearing Loss”

in the Learning Supplements section.

The audiometric results depicted in Figure 4.13 illustrate a conductive hearing loss in both ears. There is approximately equal loss of sensitivity at each frequency by air conduction. Measurements obtained by bone conduction show normal hearing at all fre-quencies. Therefore, the air-conduction results show the loss of sensitivity (about 35 dB);

the bone-conduction results show the amount of sensory/neural impairment (none); and

10. The conductive component of a hearing loss can be determined by the ______.

10. air-bone gap

92 C H A P T E R 4 Pure-Tone Audiometry

the difference between the air- and bone-conduction thresholds, which is called the air-bone gap (ABG), shows the amount of conductive involvement (35 dB).

Figure 4.14 shows an audiogram similar to that in Figure 4.13; however, Figure 4.14 illus-trates a bilateral sensory/neural hearing loss. Again, the air-conduction results show the total amount of loss (35 dB), the bone-conduction results show the amount of sensory/neural impairment (35 dB), and the air-bone gap (0 dB) shows no conductive involvement at all.

FIGURE 4.13 Audiogram illustrating a conductive hearing loss in both ears. The three-frequency pure-tone averages are 35 dB HL in each ear. Bone-conduction thresholds were obtained from the forehead, first without masking and then with masking, and average 0 dB HL. There are air-bone gaps of about 35 dB (conductive component) in both ears. An asterisk is used to denote that nontest-ear masking was used.

Bone-conduction results obtained from the forehead with the left ear masked are assumed to come from the right cochlea, and bone-conduction results obtained from the forehead with the right ear masked are assumed to come from the left cochlea. No lateralization is seen on the Weber test at any frequency. The determination of need for and correct amounts of masking are covered in detail in Chapter 6.

FIGURE 4.14 Audiogram illustrating sensory/neural hearing loss in both ears. The air-conduction thresh-olds average 35 dB HL and the bone-conduction threshthresh-olds (obtained from the forehead), about the same (33 dB HL). Tones are heard in the midline at all frequencies on the Weber test.

The results shown in Figure 4.15 suggest a typical mixed hearing loss in both ears. In this case the total loss of sensitivity is much greater than in the previous two illustrations, as shown by the air-conduction thresholds (60 dB). Bone-conduction results show that there is some sensory/neural involvement (35 dB). The air-bone gap shows a 25 dB conductive component.

Sometimes high-frequency tones radiate from the bone-conduction vibrator when near-maximum hearing levels are presented by bone conduction. If the patient hears these signals by air conduction, the false impression of an air-bone gap may be made. Since this only occurs at 3000 and 4000 Hz, it is unlikely that a misdiagnosis of conductive hearing loss would be made.

This small inaccuracy can be ameliorated by testing bone conduction from the forehead since

94 C H A P T E R 4 Pure-Tone Audiometry

FIGURE 4.15 Audiogram illustrating a mixed hearing loss in both ears. The air-conduction thresholds average about 60 dB HL (total hearing loss), whereas the bone-conduction thresholds average 35 dB HL (sensory/neural component). Masking, as will be explained in Chapter 6, was required for all frequencies for bone conduction. There is an air-bone gap of about 25 dB (conduc-tive component). Tones on the Weber test do not lateralize.

Harkrider and Martin (1998) found less acoustic radiation in the high frequencies from the forehead than from the ipsilateral mastoid.

Interpretation of the air-bone relationships can be stated as a formula that reads as follows:

The conduction (AC) threshold is equal to the bone-conduction (BC) threshold plus the air-bone gap (ABG), which is determined at each audiometric frequency. With this formula, the four audiometric examples just cited would read

Formula: AC⫽ BC ⫹ ABG

Figure 4.10: 5 = 5 + 0 (normal)

Figure 4.13: 35 = 0 + 35 (conductive loss) Figure 4.14: 35 = 35 + 0 (sensory/neural loss) Figure 4.15: 60 = 35 + 25 (mixed loss)

Air-Bone Relationships

Figure 2.2 suggests that (1) hearing by bone conduction is the same as by air conduction in individuals with normal hearing and in patients with sensory/neural impairment (no air-bone gap) and (2) hearing by air conduction is poorer than by bone conduction in patients with conductive or mixed hearing losses (some degree of air-bone gap), but (3) hearing by bone conduction poorer than by air conduction should not occur because both routes ultimately measure the integrity of the sensory/neural structures. It has been shown that, although assumptions 1 and 2 are correct, 3 may be false. There are several rea-sons why bone-conduction thresholds may be slightly poorer than air-conduction thresholds, even when properly calibrated audiometers are used. Some of these arise from changes in the inertial and osseotympanic bone-conduction modes produced by abnormal conditions of the outer or middle ears. More than four decades ago Studebaker (1967) demonstrated that slight variations between air-conduction and bone-conduction thresholds are bound to occur based purely on normal statistical variability. Slight differences between air- and bone-conduction thresholds are to be anticipated (Barry, 1994) and are built into the ANSI (1992) standard for bone conduction. Because no diagnostic significance can be attached to air-conduction results being better than those obtained by bone air-conduction, the insecure clinician may be tempted to alter the bone-conduction results to conform to the usual expectations.

Such temptations should be resisted because (in addition to any ethical considerations) they simply propagate the myth that air-conduction thresholds can never be lower than bone-conduction thresholds.

Tactile Responses to Pure-Tone Stimuli

At times, when severe losses of hearing occur, it is not possible to know for certain whether responses obtained at the highest limits of the audiometer are auditory or tactile. Nober (1970) has shown that some patients feel the vibrations of the bone-conduction vibrator and respond when intense tones are presented, causing the examiner to believe the patient has heard them. In such cases a severe sensory/neural hearing loss may appear on the audiogram to be a mixed hearing loss, possibly resulting in unjustified surgery in an attempt to alleviate the conductive component. Martin and Wittich (1966) found that some children with severe hearing impairments often could not differentiate tactile from auditory sensations. Nober found that it is possible for patients to respond to tactile stimuli to both air- and bone-conducted tones, primarily in the low frequencies, when the levels are near the maximum out-puts of the audiometer. When audiograms show severe mixed hearing losses, the validity of the test should be questioned.

See Case Studies on the Companion Website for this book.

Dean and Martin (1997) describe a procedure for ascertaining whether a severe mixed hearing loss has a true conductive component or if the ABG is caused by tactile stimulation. It is based on the fact that bone-conduction auditory thresholds are higher on the forehead than the mastoid, but tactile thresholds are lower on the forehead than the mastoid, at least for the low frequencies. The procedure works as follows (assuming that original bone-conduction

96 C H A P T E R 4 Pure-Tone Audiometry

results were obtained from the forehead): Move the vibrator to the mastoid and retest at 500 Hz. If the threshold gets lower (better), the original response was auditory. If the threshold gets higher, the original response was tactile.

If the mastoid was the original test position and a tactile response is suspected, move the vibrator to the forehead and retest at 500 Hz. If the threshold appears to get lower, the original response was tactile. If the threshold gets higher, the original response was auditory.

Cross Hearing

When there are differences in hearing sensitivity between the two ears, there may be a risk that sounds that are presented to one ear (the test ear) may actually be heard in the opposite (nontest) ear. It is important for clinicians to be alert to these possibilities and to deal with them effectively. These matters are addressed in detail in the chapter on masking (Chapter 6).