Syntactic descriptions

The purpose of measuring hearing by bone conduction is to determine the patient’s sensory/neural sensitivity. The descriptions offered in Chapter 2 were very much oversimplified, as bone conduction is an extremely complex phenomenon. Actually, hearing by bone conduc-tion arises from the interacconduc-tions of at least three different phenomena.

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

When the skull is set into vibration, as by a bone-conduction vibrator or a tuning fork, the bones of the skull become distorted, resulting in distortion of the structures of hearing within the cochlea of the inner ear. This distortion activates certain cells and gives rise to electrochem-ical activity that is identelectrochem-ical to the activity created by an air-conduction signal. This is called distortional bone conduction.

While the skull is moving, the chain of tiny bones in the middle ear, owing to its inertia, lags behind so that the third bone, the stapes, moves in and out of an oval-shaped window into the cochlea. Thus, activity is generated within the cochlea as in air-conduction stimulation. This mode of inner-ear stimulation is appropriately called inertial bone conduction.

Simultaneously, oscillation of the skull causes vibration of the column of air in the outer-ear canal. Some of these sound waves pass out of the outer-ear, whereas others go further down the canal, vibrating the tympanic membrane and following the same sound route as air conduction.

This third mode is called osseotympanic bone conduction. Hearing by bone conduction results from an interaction of these three ways of stimulating the cochlea.

For many years the prominent bone behind the ear (the mastoid process) has been the place on the head from which bone-conduction measurements have been made. This was prob-ably chosen (1) because bone-conducted tones are loudest from the mastoid in normal-hearing persons and (2) because of each mastoid process’s proximity to the ear being tested. Probably the bone-conducted tone is loudest from behind the ear because the chain of middle ear bones is driven on a direct axis, taking maximum advantage of its hinged action. The no-tion that placing a vibrator behind the right ear results in stimulano-tion of only the right cochlea is false because vibration of the skull from any location results in approximately equal stimulation of both cochleas (Figure 4.11).

Years ago, Studebaker (1962) demonstrated that the forehead is in many ways superior to the mastoid process for measurement of clinical bone-conduction thresholds. Variations produced by

8. The three ways by which we hear through bone conduction are ____, ____, and _____.

FIGURE 4.11 Vibrations of the skull result in bone-conducted stimulation of both inner ears, whether the vibrator is placed on (A) the mastoid or (B) the forehead.

8. distortional, inertial, osseotympanic

vibrator-to-skull pressure, artifacts created by abnormalities of the sound-conducting mechanism of the middle ear, test-retest differences, and so on are all of smaller consequence when testing from the forehead than from the mastoid. The greater amount of acoustic energy generated in the outer ear canal when the mastoid is the test site, as compared to the forehead, is further evidence of the advantage of forehead testing (Fagleson & Martin, 1994).

In addition to the theoretical advantages of forehead bone-conduction testing, there are numbers of practical conveniences. The headband used to hold the bone-conduction vibrator is much easier to affix to the head than the mastoid headband. Also, eyeglasses need not be removed when the vibrator is placed on the forehead, which is the case for mastoid placement.

The main disadvantage of testing from the forehead is that about 10 dB greater intensity is required to stimulate normal thresholds, resulting in a decrease of the maximum level at which testing can be carried out. Although bone-conduction testing should sometimes be done from both the forehead and the mastoid, the forehead is recommended here for routine audiometry. Despite negative reports on mastoid test accuracy and other problems that go back many years (e.g., Barany, 1938), a national survey (Martin, Champlin, & Chambers, 1998) shows that the mastoid process continues to be the preferred bone-conduction vibrator site among audiologists. It is hoped that this situation will change.

When the mastoid is the place of measurement, a steel headband crosses the top of the head (Figure 4.12A). If testing is to be done from the forehead, the bone-conduction vibrator should be affixed to the centerline of the skull, just above the eyebrow line. A plastic strap encir-cles the head, holding the vibrator in place (Figure 4.12B). All interfering hair must be pushed out of the way, and the concave side of the vibrator should be placed against the skull.

9. The most reliable placement for the vibrator when testing by bone conduction is the ______.

9. forehead

FIGURE 4.12 Bone-conduction vibrator placement on (A) the mastoid process and (B) the forehead.

For a demonstration of the proper placement of bone-conduction vibrators, see the video titled “Bone-Conduction Receiver Placement” in the Clinical Demonstra-tions section of the Companion Website.

(A) (B)

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

TABLE 4.2 Occlusion Effect for Bone Conduction Produced When a Supra-Aural

Earphone-Cushion Arrangement ( TDH-39 earphone in MX/41AR cushion) Is Placed over the Ear during Bone-Conduction Tests

The amounts shown indicate central tendencies and the range of occlusion effects may be considerable (Elpern & Naunton, 1963).

Frequency (Hz) 250 500 1000 2000 4000

Occlusion effect (dB) 30 20 10 0 0

Procedure for Bone-Conduction Audiometry

The decision of which ear to test first in mastoid bone-conduction audiometry is unimportant.

As a matter of fact, as is illustrated in Figure 4.11, clinicians really cannot be certain which cochlea they are testing. The procedure for actual testing is identical to that for air conduction, although the range of frequencies to be tested and the maximum intensities emitted are more limited for bone conduction than for air conduction.

Bone-conduction thresholds may be recorded in identical fashion to those for air conduc-tion, using the appropriate spaces on the audiometric worksheet. All audiograms should con-tain a key that tells the reader the symbols used and what they represent. The symbol for forehead (or ear unspecified) bone conduction is a black V when no masking is used. When the nontest ear is masked for forehead bone-conduction testing, a red 0 is used as a symbol for the right ear and a blue for the left ear. If the mastoid is the test site, a red arrowhead pointing to the reader’s left (>) is used for the right ear and a blue arrowhead pointing to the reader’s right (>) for the left ear. If the printed page is viewed as the patient’s face looking at the reader, the symbols for right and left are logical. When masking is used in the nontest ear, the symbols are changed to square brackets,[ in red for the right ear and ] in blue for the left ear. Some audiol-ogists prefer to connect the bone-conduction symbols on the audiogram with a dashed red line for the right ear and a dashed blue line for the left ear. Others prefer not to connect the symbols

0

Both ears must be uncovered during routine bone-conduction audiometry. When normal ears and those with sensory/neural impairments are covered by earphones or occluded by other de-vices, there is an increase in the intensity of sound delivered by a bone-conduction vibrator to the cochlea, which occurs partly because of changes in osseotympanic bone conduction. This phe-nomenon, called the occlusion effect (OE), occurs at frequencies of 1000 Hz and below. The occlu-sion effect is rare in patients with conductive hearing losses (e.g., problems in the middle ear) because the increase in sound pressure created by the occlusion is attenuated by the hearing loss.

The occlusion effect (Table 4.2) explains the results on the Bing tuning-fork test described in Chapter 2. Research findings confirm previous notions that the occlusion effect is the result of the increase in sound-pressure level in the external ear canal when the outer ear is covered (Fagelson &

Martin, 1994; Martin & Fagelson, 1995). Dean and Martin (2000) compared supra-aural earphones to insert earphones with slight and deep insertion and found the occlusion effect to be markedly de-creased when the insert phone was deeply inserted, virtually eliminating it at 1000 Hz. This has pro-found effects when masking for bone conduction, as will be discussed in Chapter 6.

For a demonstration of a pure-tone bone-conduction test, see the video entitled

“Bone-Conduction Testing” in the Clinical Demonstrations section of the Companion Website.

at all. The latter is the preference in this book. At times, a patient may give no response at the maximum bone-conduction limits of the audiometer. When this occurs, the appropriate sym-bol should be placed under the test frequency where that line intersects the maximum testable level. An arrow pointing down indicates no response at that level. Figure 4.10 shows the record-ing of results and plottrecord-ing of an audiogram for a hypothetical normal-hearrecord-ing individual.