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"TRANSCRANIAL OR INTERNAL CROS FITTINGS:
Evaluation and Validation Protocol"
By Max S. Chartrand
Despite the hearing healthcare profession's growing
acceptance of transcranial fitting principles and other
concepts in aural rehabilitation that were once regarded
as exotic, there remains a lingering stigma attached to
the practice of "fitting a dead ear."
According to the still prevailing conventional wisdom,
hearing aid dispensers
should fit "dead," that is profoundly deaf ears only
with a contralaterally mounted microphone that crosses
over to the better ear for amplification through a
hearing instrument. The only apparent benefits of this
methodology, known as contralateral routing of signal (CROS),
are removal of the head-shadow effect and, possibly,
training of the good ear to differentiate between the
natural sound on the good-ear side and the electronic
signal emanating from the dead side, thereby effecting a
form of localization.
Recently, however, the conventional wisdom appears to be
making room for more effective fitting approaches in
these special dead-ear/normal-ear cases, provided that
the practitioner has a scientific rationale for the
fitting strategy.
The dispensing professional must meet two criteria to
justify fitting a dead ear:
(1) He must follow a repeatable evaluation and
validation protocol, and
(2) He must counsel the client to understand and accept
the benefits and limitations of aural rehabilitation.
Unless these two criteria are fulfilled the skeptics
will remain in their entrenched position, and, I might
add, rightly so.
FITTING "DEAD" EARS: A LITTLE HISTORY
Recent commentators have pointed to the little known yet
often effective practice of fitting profoundly deaf ears
with high-power, high-frequency instruments, taking an
internal tactile/auditory path-way to the contralateral
cochlea - which must be normal or near normal in
sensitivity.
These articles report on the success of the authors with
their own clients and on the experiences of other
practitioners whose offices and clinics have been the
birthplace of many discoveries that were not reported in
the literature until years later. Often what we are
scrambling to document has been in practice far longer
than is generally known.
To illustrate, let me go back about twenty years ago to
my first real attempt at an internal CROS fitting. The
client, Mary Alice Smith, had been plagued since young
adulthood with a dead left ear. As a result, she felt a
severe lack of localization, selectivity, and summation
ability. She was particularly disturbed by her inability
to accept and adapt to the loss of hearing in her left
ear. And no amount of advice from the several physicians
and clinicians with whom she consulted over the years
discouraged her from believing that she could obtain at
least some use out of her left ear.
So, here she was, sitting in my evaluation chair,
expressing an emphatic desire to "try" a hearing aid on
her dead ear. Her persistence and willingness to
experiment overcame my own initial hesitation. The
audiometric results confirmed what past investigators
had found: She had a very profound sensorineural loss
(100 dB PTA) in her left ear and very normal hearing (5
dB PTA) in her right ear.
Unsure of what my colleagues and licensing boards would
think of this "trial," I had Mrs. Smith sign a
disclaimer stipulating that she was being fitted with a
hearing aid for purposes "other than amplification." In
addition, she had recently visited an ear physician, who
had ruled out the possibility of medically treatable ear
pathology.
FINDING AN EVALUATION/VALIDATION PROTOCOL
While I was testing my client's pure-tone thresholds
with masking, it occurred to me that it might be
important also to apply selected air-conduction signals
to her left ear without masking to get a better "transcranial"
(or interaural attenuation) picture. Results showed
there was a difference of only 55 dB at 2000 Hz. In
essence, I had merely applied a pure-tone air-conduction
test to her better cochlea via her dead ear. (Principles
affecting her interaural attenuation and vibratory
perception will be discussed later in this article.)
During speech testing, I repeated the procedure used to
find the transcranial
pure-tone thresholds by applying speech reception
threshold (SRT), most comfort-able loudness JMCL), and
uncomfortable loudness (UCL) parameters to the dead ear
without masking. Her unmasked SRT was quite elevated (90
dB HL) and was effective only in the -12 dB slope
setting of the master hearing aid. In the flat speech
mode she complained of "fuzziness" and a sensation of
"being far
off." In the sharpened, more consonant sensitive -12 dB
mode, she actually
began to distinguish a speech reception threshold per
se.
Moreover, her unmasked MCL (95 dB HL) was only 5 dB
higher than her unmasked SRT. Surprisingly, her left-ear
unmasked UCL came out at 105 dB HL or 125 dB SPL. Her
true UCL, I felt, represented more of a physiologic
parameter than acoustic, for she described the sensation
as a discomforting "vibration" at the point of 110 dB
HL, involving her facial (tactile) nerves more than her
auditory nerves.
After establishing MCL for her better ear as a balance
factor, out of curiosity I amplified both ears to MCL,
the left ear receiving its unmasked MCL. The object was
to detect a possible bilateral effect.
"Do you hear me in both ears?" I asked.
A look of surprise crossed her face, and she said, "Yes,
I think I do!"
I instructed her to close her eyes and to point her
finger in the direction from which she heard my voice.
Then I leaned toward the right microphone and asked,
"Mrs. Smith, from which ear do you hear my voice?" She
pointed to her right.
Leaning to the left microphone, I asked, "Now, where do
you hear my voice?" She pointed to the left.
"Do you actually hear my voice in your left ear?" I
inquired. "It feels as if that is where it is coming
from, but it's difficult to be sure."
Then, I thought, the ultimate impossibility would be to
find a midline in the aggregate tactile/auditory
sensation by directing my voice at the center point
between the two microphones. "Now, from which direction
do you hear my voice, Mrs. Smith?" I asked. "Slightly to
the right" was her response. With the choice of
decreasing the signal to her right ear by 5 dB or
increasing it to her left by the same amount, I opted
for the decrease. In this way, I established a midline
with the left ear set on 95 dB HL with a -12 dB slope,
and the right ear set on 50 dB HL with a flat (speech)
response.
When I clicked tuning fork handles from side to side, my
client displayed an even more dramatic localization
ability than she had with speech. That is because with
speech, the diffraction (bending around the head) of the
lower frequencies in the vowel sounds tends to confuse
critical direction-ability.
As expected, Mrs. Smith was elated at the possibility of
having some sense of direction of sound and, hopefully,
better function in noise. At this point I emphasized the
limitations of such a fitting and pointed out that the
possible benefits had yet to be affirmed through actual
experience.
In this case, the prescribed instrument was a power BTE
(HFA FOG 54 dB) with
the HFA use-gain level set at 45 dB* and the tone
control set at a -1-2 dB slope. (There was no
appreciable gain below 800 Hz.) The AGC knee-point was
set at
90 dB SPL and the output set at approximately 125 dB SPL.
A filtered ear hook was used for better feedback
control.
The prescribed acoustic coupler consisted of a lucite
skeleton earmold with a pressure vent (0.010"), a long,
tapered canal, and No. 13 HW tubing.
POST-FITTING REPORTS
Post-fitting visits indicated better-than-expected
function in noise, a gradually emerging sense of
direction, and a slight reduction in. gain from the
initial level. Further modifications to the earmold
resulted in additional tapering of the canal without
changing the actual receiver distance from the tympanic
membrane.
In counseling, my client received a conservative
prognosis, which included being
told that she would not be actually hearing through her
dead ear and that she would have to learn gradually to
interpret the vibratory or tactile information provided
and to differentiate the CROS-aided-dead-ear signals
from the unaided good-ear signals to gain a sense of
direction.
Mrs. Smith reported better speech-to-noise reception in
the presence of such
competing office sounds as ringing telephones and the
operation of copy machines, typewriters, air
conditioners, and computers. Furthermore, she felt that
she had developed some ability to block out unwanted
noises while on the phone.
When this was not possible, she simply turned off her
hearing aid. A feeling of
balance and a sense of fitting into her environment were
other benefits she described during post-fitting visits.
Her final assessment was summed up this way: "You
couldn't take this hearing aid away from me for a
million dollars!"
Granted, not all my case histories are as dramatic as
Mrs. Smith's. However,
over the past nine years, these dead-ear fittings have
consistently produced results that justify a favorable
prognosis for the procedure when both the specialist and
the clients are well versed in its limitations and
benefits.
DISCUSSION: TRANSCRANIAL PRINCIPLES
The human hearing mechanism comprises a complex
collection of acoustic, vibratory, tactile, and
interacting neural impulses and responses. At least four
neural pathways are involved in the human hearing
system: VIIIth nerve (auditory), VIIth nerve (facial),
Xth nerve (vagus), and the Vth nerve (trigeminal).
Together, these neurologic avenues connect the human
brain the auditory environment outside.
Let us consider first the vibratory/tactile branches of
the VIIIth nerve. It is generally assumed that temporal
tactile sensation is a conductive phenomenon detected
only by the VIIIth (auditory] nerve via conduction
across the
skull. However, when one considers the role of the
afferent (sensory) and efferent
(motor) neurons of the tympanic plexus branch of the
facial nerve, the evidence
of its sensitivity and response to vibration at the
tympanic membrane exhibits a strictly tactile sequence.
Theoretically, all auditory input is tactile until it
reaches the cochlea, a primary example of which is the
acoustic reflex of the stapedius muscle via the VIIth
nerve while the tensor tympani muscle is enervated by a
branch of the Vth nerve, both simultaneously resulting
from a tactile/ auditory stimulus.
MacAllister has demonstrated the direct conductive
pathway to the cochlea via
the cartilagenous area of the ear canal, which shortens
the actual distance from
aperture to cochlea for low-to-mid frequencies. The
existence of this pathway
may explain the more efficient interaural attenuation,
of the lows over the highs,
another reason for reducing the lows when attempting to
address diffraction
and upward spread of masking. Furthermore, any
conductive shortcut from the ear canal to the cochlea
will also provide a similar reduction in distance to the
contralateral cochlea. Transcranial conduction values
vary significantly from individual to individual, and,
hence, careful evaluation of each subject is mandated.
Another consideration was pointed out by Levitt and
Voroba relative to the interaural attenuation changes of
the fused sound image. Those authors noted the
differences in time and intensity that are important
considerations in binaural fusion. Although the use of
one cochlea in a transcranial fitting hardly represents
the binaural fusion effect itself, it does help clients
learn to recognize direction and proximity, because two
separate routes of transmittal are used (i.e., through
the normal-hearing ear and by conduction from the
opposite air-conduction amplified ear).
The contribution of other neural pathways |Vth and Xth]
is less clear, but we
may assume they play a significant tactile-gathering
role. One such role would be the sensitivity of the
trigeminal nerve from the jaw to the skull, and
interestingly, its crossover at the tympanic membrane,
where air-conduction sound pressure is collected.
REHABILITATIVE CONCEPT
In past papers and lectures I have discussed the role of
the homunculus - not
the imaginary "little man" of folklore, but the actual
sensory organization center of the motor cortex that
distributes and redistributes sensory development
throughout the body according to need and function.
Take, for example; patients
with apraxia (loss or lessening of a body function
resulting from damage to one or
both hemispheres of the brain) who exhibit an amazing
ability to transfer important functions from the damaged
area to other parts of the brain.
Another example is the sufferer of aphasia (receptive or
expressive) who develops new channels of communication
in other portions of the brain. And deaf
individuals are known sometimes to develop an
extraordinary sense of touch, that enables them to
respond to sound that is received via sensory taction
from various parts of the body instead of through the
auditory cortex.
My point is that when an individual with a profoundly
deaf ear is empowered
to receive discrete vibratory information via an
amplification device, the amazing rehabilitative and
compensatory capability of the human mind will allow the
person to make proficient use of that information.
However, when that information is too broad in spectrum
or contains low frequencies (below 800 Hz), it is more
confusing than helpful in communicating.
Hence, low-frequency amplification should be avoided in
transcranial or internal CROS fittings.
EVALUATION PARAMETERS
The dispenser should keep in mind certain parameters in
assessing and recognizing possible candidates for
transcranial or internal CROS hearing instrument
fittings. The following is a basic checklist of those
parameters:
* The fitted (dead) ear must exhibit a loss profound
enough (90 dB or greater)
to benefit. Precluded auditory and/or medical conditions
include dyplacusis/hyper-recruitment of either ear, or
physiologic lesions. Also, chronic conditions such as
diabetes mellitus require special consideration when
there are possible neural, tissue, and vascular
abnormalities, all of which would require medical
examination before proceeding.
* CROS-aided thresholds of the dead ear should be no
greater than 70 dB HL to 75 dB HL
* Pure-tone air-conduction thresholds of the of the good
ear should be no greater than 30 dB at 1000 Hz, 2000 Hz,
and 4000 Hz.
* The CROS-aided MCL of the dead ear should be no higher
than 95 dB HL nor less than 5 dB below the UCL.
* The CROS-aided balance of the two ears via air
conduction should be satisfactory (i.e., the client
should have some sense of direction).
SUMMARY
Transcranial or internal CROS fittings, which bypass the
cumbersome two-piece
hardwire and FM CROS systems, appear to be a viable
alternative for many dead-
ear/normal-ear clients. By carefully observing the
parameters and principles set
forth above, the practitioner is compensating for the
hearing loss in the client's dead ear in three ways: (1)
by making more efficient use of the good ear without
hampering its auditory collection apparatus, (2) by
making more direct use of the dead ear, (3) by taking
advantage of the tactile facility of the dead ear.
Together, these methods may prove to be of significant
benefit to the client.
The success of a transcranial or internal CROS fitting
depends on the ability of the practitioner to make
precise evaluative measurements and to demonstrate the
results of correction. It requires as well motivation,
patience, and effort on the part of the client. However,
when these conditions are met, this "holistic" approach
to aural rehabilitation can open up a new frontier to
the hearing health- care professional in the treatment
of many formerly "unfittable" cases.
REFERENCES
1. Miller AL: An alternative approach to CROS and Br-CROS
hearing aids: An internal CROS. Audicibel 1989;38(1).
2. Chartrand M; Course on Transcranial or Internal CROS
Fittings. Livonia, Ml, National Institute for Hearing
Instruments Studies, 1989.
3. Fishbein, H: The best-kept secret: Fitting a dead
ear. Hear Jour WO, 43 |31.
4. Zemlin W: Speech and Hearing Science: Anatomy and
Physiology, 2nd edition. Englewood Cliffs, N1,
Prentice-Hall, Inc., 1981.
5. Newby HA: The Handicap of Hearing Impairment, in
Audiology, 5th edition. Bugle-wood Cliffs, N1,
Prentice-Hall, Inc., 1985
6. MacAllister MD: Minimum Contact Technology. Livonia,
MI, National Institute for Hearing Instruments Studies,
1990.
7. Larson-Donaldson LL: Masking; practical applications
of. In Masking Principles and Procedures. Livonia, MI,
National Institute for Hearing Instruments Studies,
1988.
8. Levitt H, Voroba B: Binaural hearing, in Libby ER
(ed): Binaural Hearing and Amplification, Vol. 1.
Chicago, Zenetron, 1980.
9. Ramsdell D: The psychology of the hard-of-hearing and
the deafened adult, in Hearing and Deafness, 4th
edition. Davis & Silverman, 1968 ch.19.
10. Chartrand M: Hearing Instrument
Counseling.-Practical Applications for Counseling the
Hearing Impaled. Livonia, MI, National Institute for
Hearing Instruments Studies,1990.
11. Jerger S, Jerger, J ); Auditory Disorders - Manual
for Clinical Evaluation. Boston, Little, Brown &
Co,1981. |
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