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Contains in-depth counseling instruction for solving
the most difficult fitting problem cases. Resolving
Problem Hearing Aid Fitting Cases
Note: To have your individual hearing aid fitting
problem addressed, simply send your question to
digicarenet@aol.com or fax to (719)676-6882.
Inquiries must include your name, address, and phone
number for verification and reply.
(Excerpted from the second edition of Hearing
Instrument Counseling/NIHIS)
COUNSELING CONCEPTS IN SOLVING PROBLEM CASES
By Max S. Chartrand, M.A.,
DigiCare Hearing Research & Rehabiliation
“The scenes change, but the story line stays much
the same: Lack of foresight brings the most
unexpected of consequences, even though we’ve been
there, done that,
a hundred times already. Each perplexing scene makes
us more determined to do better “next time”.
INTRODUCTION
Many hearing losses appear identical in all respects
except for amplification outcomes (Larsby & Arlinger,
1999)(Moore, 1985)(Feston & Plomp, 1983). Like the
irrepressible problem child, demanding inordinate
attention, some of these fittings have caused more
than a few sleepless nights for dispensing
professionals. "What now?" one asks, with
exasperation. Treated at the superficial level,
blame becomes spread around to all contingent
parties: the demanding user, the faceless factory,
and, finally, the catch-all “limitations of hearing
aid technology.” Far too many hearing aid fittings
fit this profile, and are even further aggravated by
well-intentioned, yet misdirected consumer yin for
satisfaction within an inordinate amount of time, as
in the spirit of mandated 30-day trials.
In this paper we'll discuss some of the essential
areas of consideration for resolving problem fitting
cases, the disregard of which can drive all, user,
specialist, factory, and researcher to utter
despair, and ultimately toward totally unnecessary
credit returns. The starting point in resolving any
problem case is that one must never assume there is
such as thing as a typical or easy hearing
instrument fitting. Every fitting has the potential
to become a problem one, especially when we ignore
the idiosyncratic dynamics that comes with
individuals who suffer from hearing loss. For, as
soon as outward appearances lull us into a false
sense of security, out pops the internal quirks and
idiosyncrasies of that which makes each individual
ear unique: cognitively, spatially,
psychoacoustically, neurophysically, and
epithelially (McSpaden, 1995)(Chartrand,
1996)(Oliviera, 1997)(Oliviera, Gilliom & Goldstein,
1992).
We will, therefore, begin with two sets of common
problem fitting cases, one set of problems that
present the start of the process; the other set that
seems to appear near the end of the process. Then,
to help unravel the immensity of variables that make
even the most straightforward fitting case into a
problem one, we will follow a definitive approach,
describing terms, concepts, and ideas as they
pertain to solving problem fitting cases.
Too often, dispensing professionals of all
disciplines find themselves embroiled in what may
seem like a slapstick episode of Amos and Andy. The
scenes change, but the story line stays much the
same: Lack of foresight brings the most unexpected
of consequences, though we’ve been there a hundred
times already. Each perplexing scene makes us more
determined to do better “next time”. We get the
feeling that we’ve “been there, done that”, when
there seems to be a near bottomless pit of new
there’s and that’s. To combat the surprises, and get
a better handle on the unwieldy matters of
dispensing, we devise a host of inventories and
questionnaires, such as Profile of Hearing Aid
Benefit (PHAB), Client Oriented Scale of Improvement
(COSI), Global Satisfaction Scale (GSA), et al. If
enough questions are asked, and if enough possible
pitfalls have been addressed, no more Amos and Andy,
we think. But, then not a week goes by that we do
not find another episode unfolding before our very
eyes.
Three psychosocial problem scenarios
1.“I hear alright. I just can’t understand some
words.”
Al Smith came for his test because his wife, his
daughter, even his good buddy across the street
ganged-up on him. “You need to do something about
your hearing, Al,” chanted the chorus of
well-wishers.
During the case history, Al was in denial for the
most part. He did agree that people mumbled, there’s
too much background noise at the senior center, and
his phone has connection problems (blame-shifting).
In his APHAB questionnaire he indicated these were
all things he hoped to see corrected if he bought
hearing aids.
2. “I don’t think she has much of a problem.”
Doris dutifully brought her normal hearing husband
to accompany her for her first hearing test, just as
the specialist insisted. While she filled out the
questionnaires and read the information the
receptionist gave her, her husband read a year-old
copy of Field and Stream.
It’s now time for the test. Everything seems to go
well during the case history. The husband had little
to contribute, but kept glancing at his watch and
impatiently shaking his crossed leg in muted
disinterest. “How much longer is this going to
take?” he seemed to be thinking.
Near the end of the evaluation, the specialist
indicated that Doris appeared to have a bilateral
55dB PTA sensorineural loss, with lows gradually
sloping downward into highs. She needed two hearing
aids.
Everything seemed fine until the specialist named
the price. To that, the husband sat up straight in
his chair, and said, “Now, I don’t think she hears
that bad. She seems to get by OK. It doesn’t bother
me.”
It seemed a miracle to the specialist, but somehow
he was able to complete the purchase agreement. New
hearing aids were ordered for delivery in two weeks
hence.
3. “Clear hearing guaranteed or your money back!”
Steven was a young man of 26 years. He wasn’t sure
which ad he saw it in, but he could have sworn it
was this particular specialist’s ad that he saw
promising “clear hearing guaranteed or your money
back.”
Steven had already tried three sets of hearing aids
on trial from three different offices for his
moderate to severe corner audiogram. All were
failures according to his family and friends. “Why’d
you spend $2,000 if you still can’t understand us?”
they taunted him. Certainly, none of those gave him
the desired “clear hearing”. This specialist’s ad
said just what he wanted to hear. He knew this time
around would be the charm.
Three fitting/post-fitting problem scenarios
1. “They’re too big!”
John gets ready to go to his specialist’s office to
take delivery of his new programmable CICs. He
remembers the embarrassment, his daughter sitting
there, when the specialist flashed his dirty ear
canals up on a bigger-than-life-size screen for all
to see. To make sure that didn’t happen again, he
took a cotton swab and gave both ears what he called
a good “reaming out”. John didn’t notice that almost
immediately the friction of the cotton swab had
evoked a marked lymphatic response. This cause his
ear canal to swell up to about 10% its normal
size---a state that would remain until several hours
later.
His hopes up, he hurries to the specialist’s office
to take delivery of his new micro wonders. The
specialist attempts to place the hearing aids in his
ears, but can only insert them halfway before Mr.
Smith jerks his head backwards. “That hurts!”, he
yelps, as he tries to cover his even greater
disappointment. The specialist breaks into a cold
sweat, and checks the instrument over very
carefully. After mumbling something about the
factory making them “too big”, he decides to make
new impressions. While they are setting up, he gets
on the phone with the shell lab at the factory, and
proceeds to tell them that he’s about to take his
business elsewhere if they don’t get this right back
to him pronto and, “They’d better fit right, this
time!”
The hearing aids are remade to the impressions that
were taken during the lymphatic swelling. Upon
second attempt at delivery, they encountered another
problem: loose fit and feedback. The specialist
quickly made another set of impressions, and sends
them to another factory. The original set get sent
back as credit returns to the first factory.
2. “Your voice is OK, but mine’s hollow”
George has a precipitous, mild to severe high
frequency loss. Upon insertion of the right hearing
aid, the specialist instructs, “Now, listen to my
voice as I turn up the volume control on your
hearing aid, and tell me when it’s just right.” As
the volume is increased, the specialist does what
she’s done a hundred times, she speaks in slow,
even, distinct phrases as she slowly increases the
volume control. Finally, George raises his hand,
indicating that the specialist’s voice is “just
right”.
The specialist then asks, “How does your own voice
sound?”
“My voice? Uh, my (pause) voice…sounds too loud,
like it’s coming back at me,” is the reply. Too much
output? Need more venting? No, the procedure for
setting volume controls was simply backwards. (See
previous section under recommended delivery
procedure).
3. “I still can’t understand without lipreading!”
Mary is severely hearing impaired, and someday will
likely be a cochlear implant. Her best-aided speech
discrim is 48% with her new hearing aids, much “too
good” to qualify for cochlear implant candidacy. But
life must go on in the meantime. She accepted her
new max-power hearing aids on the basis that if she
was not satisfied that she could get her money back.
The specialist agreed, neither party quite
understanding what the other considered was
“satisfactory”. Would it mean good speech discrim?
No more lipreading? Hearing again on the telephone?
True, Mary’s hearing only discrim score went from
40% to 48%, but she still had to resort to
speechreading, and was totally lost in noise. She
utilized no assistive devices, and felt that since
she had spent a good sum of money for the latest
digital power instruments that she should not have
to resort to the inconvenience of other apparatus
and coping strategies. The specialist reluctantly
agreed, and accepted the instruments back for a
refund.
What constitutes a problem fitting?
The above scenarios represent every specialist’s
Amos and Andy episodes. Seeing it on paper, the
problems seem to leap off of the page. But under the
emotional heat of the moment, we become as blind as
Willie Coyote who, for the millionth time, hears the
taunting “beep-beep” long after his prey has
blithely exited his clever trap. The problem is that
problem fitting cases are nearly always more than
one-dimensional. They do not lend themselves to
neatly outlined “how to” manuals. They’re certainly
too messy for refined, high-sounding educational
seminars. But we will approach the messiness, and
hopefully be ready for the sly Roadrunner before he
rounds the next bend.
Almighty pure-tone thresholds
There’s a marked tendency in the hearing aid field
to attempt to identify all information about a given
loss by way of pure-tone threshold measurements.
Indeed, when the specialist sends an order to the
factory, pre-established computer programs often
blindly regard audiometric threshold information as
inclusive of the necessary information required for
circuit/shell design parameters. Speech scores (SRT,
UCL, MCL, Discrim, and Master Hearing Aid readings)
with their more inherent lack of precision are too
often shunted aside and merely aside information.
When examined more closely, however, we obtain
valuable information when including air-bone
threshold differences, speech testing results and
other suprathreshold measures into the prescriptive
picture:
·Conductive components intermingled with cochlear
thresholds can affect the user’s functional dynamic
window, often widening the window with more
headroom.
·Speech reception threshold (SRT) scores can expose
speech to pure-tone calibration inconsistencies to
signal need for retest.
·Most comfortable listening level (MCL) scores can
expose higher or lower than expected use-gain.
·Uncomfortable listening level (UCL) scores can be
compared to other aspects, such as conductive
component involvement, or lower than expected
use-gain levels.
·Relationship of MCL to SRT and UCL tends to be an
accurate predictor of where the fastest loudness
growth occurs. Does loudness grow fastest just above
SRT (as we expect in straightforward sensorineural
cases) or just before UCL (as expected in the most
damaged cochleae)?
·PB-Max scores, compared to PB at MCL, can provide
valuable counseling information to explain best
aided condition outcomes.
·An MCL set to an external voice (such as the
examiner’s) may have no relationship to the user’s
own voice MCL, which calls into question how we
counsel the new user to set their volume controls to
achieve an acceptable use-gain level.
Rethinking remakes
To avoid another Amos and Andy episode, both the
specialist and the shell lab at the factory must
make various pertinent observations to make more
informed judgments and decisions before they
blindly, like soldier’s marching off a cliff, follow
the status quo. Some of those considerations that
need explored and discussed are:
·If the patient coughed (vagus reflex) during
impression taking, or an abnormally prominent red
reflex (thickening and dilation of the M) evoked
during otoscopy, the insertion of a hearing
instrument will likely involve similar, albeit more
subdued effects. Are such sensitive cases noted on
the order form? Does the shell lab know what to do
when such is noted?
·Hypersensitive epithelium at the bony isthmus,
coupled with normal or excessive mandibular
movement, can cause even a perfectly fitted shell or
mold to be rejected. Do one’s ear impressions allow
for mandibular-caused distortions with an open-mouth
impression? Will the lab observe dimensional
differences when exercising its judgment during the
remake process?
·Fungus, yeast, otomycosis, psoriasis, eczema,
and/or dermatitis are rampant in the ears of
diabetics, hypo/hyper-glycemics, frequent swimmers,
and those who over-clean their ear canals, all too
often causing early rejection and discomfort with
hearing aids. Will such conditions be noted,
properly treated, and resolved before remakes and
modifications are wasted?
·In cases of dermatitis (skin allergy) standard
earmold materials or plastics that are still
chemically or ionically active, or where caustic
build-up materials (such as so-called hypoallergenic
nail polish) have been used to resolve a feedback or
retention complaint, users may experience a
discomforting lymphatic (swelling) response, making
the earmold seem too tight, when in fact it may be a
perfect fit. Are we attempting to remake or modify
the instrument to a temporarily swelled ear? Will
the factory recognize that the new (smaller)
impression is a result of the swelled condition?*
·Cartilaginous distortions or sharp turns at the
first and/or second bends of the external meatus (EM),
and prolapsed or collapsed canal tissue often call
for excessive rounding of ear impressions to make
insertion and removal of hearing instruments
possible. Was more material than necessary taken off
in the manufacturing and/or modification process,
compromising the higher priority acoustic and
retention aspects of the fitting?
·Some patients experience non-acoustic occlusion,
regardless of how much venting is introduced into
the hearing instrument. Did we vent MORE than is
acoustically feasible, and still not resolve the
occlusion complaint?
·Occlusion of the ear canal can cause an internal
amplification of the user’s voice by as much as
20-25 dB. Do we utilize available measurement and
detection methodology when faced with seemingly
unresolvable own voice complaints?
·If a new impression is required for a remake, is it
made in the same manner, the same material, and with
the patient’s jaw in the same position, with the
expectation of a better result?
*A side-note to the above: Retakes on impressions
can also be affected by materials used, presentation
pressure, and shrinkage during shipment (in oil &
powder cases). In silicone impressions the biggest
problem tends to be too much insertion pressure.
Actually, to minimize and resolve problem fitting
cases, it is essential that the specialist provide
more information than traditional order forms ask
for, and certainly more than is required to pass
state licensure examinations. It is also imperative
that manufacturers and labs know what to do with
that information when it is supplied. And when the
specialist does not supply it, they have an
obligation to ask the needed questions instead of
blindly repeating the same remake/modification
process. Of course, this poses a challenge in
working within the inherent limitations of time and
efficiency in manufacturing. Perhaps customer
service and audiological staff can more efficiently
provide that communicative interface between
specialist and lab. But the fact still remains that
as we discover more about the dynamics and
physiology of the human ear, that we must consider
those findings more closely in manufacturing and
shell technology if we ever hope to significantly
reduce the high rates of credit returns and remakes
in the industry. This will, no doubt, constitute an
enormous effort of ongoing training for specialists
and earmold laboratory personnel. Deficiencies at
either end of the system will result in unnecessary
remakes, returns, and unresolved “mystery cases.”
Fitting problem dilemmas
The above considerations represent some of the
aspects that should have been determined at the
diagnostic stage. It is the author’s opinion that
these represent the crux of most problem fitting
cases. Now, we will go into prescription aspects,
which can affect or cause a problem fitting case.
The literature is replete with practical information
about various circuit technologies and appropriate
applications. It would not be appropriate here to
try to duplicate that information, as important as
it is. Therefore, we will remain as generic as
possible in terms of technology-related problem
areas:
1. A fitting where the electroacoustic parameters of
the hearing instrument do not meet the
psychoacoustic profile of the user. Examples:
Gain/output do not accommodate insertion loss of the
ear canal, or frequency response of the instrument
involves too many mechanical distortions of the
cochlea and/or central auditory system, or automatic
signal processing thwarts the human brain’s ability
to perform the desired signal processing functions.
A simile may be drawn with long-term use of some
pharmacological medications, which rob the body’s
immune system of its ability to resist disease on
its own. Replacing a function that the body (or
brain) can do better on its own should never be an
artifact or goal of correction of any kind.
2. A fitting/counseling situation in which physical
limitations have not been identified and discussed
with the patient and family. Examples: Presence of
overlay problems, related health conditions, need
for visual/tactile compensation, assistive devices,
and or impossible environmental and sociological
situations. In other cases, perhaps cosmetic issues
surrounding a hearing aid fitting have taken
precedence over the more important communicative
issues. Only counseling can give the patient the
tools and understanding to prioritize appropriately.
3. Incorrect or incomplete delivery and validation
procedures. Examples: Rushing through steps of
delivery, lack of third party involvement, lack of
proper validation and adjustment, failure to provide
appropriate wearing schedule, etc. The specialist
should have a written delivery procedure that is
followed religiously so that every patient has the
maximal opportunity to succeed with their new
hearing aid fitting. Wearing schedules should be
specific and followed up on, even in cases of
previous users.
4. Lack of post-fitting care. Examples: Little or no
post-fitting assessments/adjustments, little or no
follow-up each calendar quarter and no retest each
anniversary, or lack of availability of adequate
aftercare after initial fitting. One should plan to
provide long-term care to meet the needs of each
patient, and consider accessibility issues related
to that care.
5. Under-recognition of poor motivational factors.
Example: Unsympathetic third party, lack of priority
perspective in cost/benefit, misinformation by other
professionals. (Refer to the Appendix on Third Party
Psychology in the back of this text for a complete
treatment of this topic).
Notice that little is mentioned about reliability of
the hearing instruments. It is a given that certain
care and maintenance aspects are part and parcel to
hearing instrument fittings, and patients need to be
advised of such. Ultimately, the manufacturer is
responsible for electroacoustic performance, the
specialist is responsible for recommendation,
validation, counseling and fitting, and the patient
is responsible for working with and following the
counsel of the specialist, as well as utilizing
options to pick up where hearing aids may leave off.
We may well expect that almost any hearing loss can
become a "problem fitting" when, for whatever
reason, reciprocating responsibilities of each party
are not properly observed.
Something that has long concerned the author is the
propensity by some in this field to assume that
hearing impaired persons lack the capacity to
understand complex issues such as evoked neural
reflexes, loudness growth abnormalities, critical
bandwidth distortion, cochlear microphonics, and
central auditory function. Of course, it is
absolutely critical for anyone that counsels the
hearing impaired to have an intimate, constantly
expanding knowledge of these topics. But today’s
increasingly more sophisticated consumer demands to
know more than the simple, repetitious basics of how
a hearing aid helps a hearing loss. They expect, and
should be given, more detailed explanations and
rationales for what we do to help correct near
insurmountable shortcomings of the patient’s own
hearing system. In so doing, the patient will
understand and accept limitations better, and will
be more likely to fully cooperate with the
specialist. In this way, the specialist becomes as
much an educator of the hearing impaired as any
other role in professional practice.
Adjunct to the “natural” hearing system
When a hearing instrument is prescribed, it is
expedient to visualize the hearing instrument as
becoming an integral part of the natural hearing
system. Indeed, new perceptions must be developed,
practiced, and eventually accepted as the "natural"
state by the client (Gatehouse & Killion, 1993). The
instrument literally reshapes the acoustic
information as it is coupled to that which travels
through the venting of the earmold, the sum total of
which must be perceived by the brain as a complete
and inseparable auditory picture.
Furthermore, the coupling of the instrument to the
abnormal ear must be acceptable to the body's
neurological system as well. This may be
accomplished by utilizing the self-adaptive ability
of the human body which some researchers have
attributed to the homunculus portion of the brain,
“the little man in the brain", so to speak. The
homunculi are comprised of both somatosensory and
motor neurons at the cortex. The "little man" is
actually comprised of a neural map, which outlines
the body's sensory pathways (Hooper & Teresi, 1986).
For example, within the homunculi each body part or
organ is proportioned in size according to its use
and sensitivity. A right-handed person will have
developed more neural sensitivity in the right hand
than the left, thereby forming a "larger" neural
network for the right hand than for the left.
Likewise, the greater use of the index finger
produces a larger sensory outline than in, let's
say, the little finger which receives considerably
less use by comparison. The thumb, though important
in grasping and lifting, is also considerably less
important in the sensory outline.
An interesting example of the rehabilitative powers
of the homunculus is when the loss of a finger is
experienced during experiments on monkeys (Rubin,
1989). Rubin reports "that the brain region
originally devoted to the missing finger gradually
begins to serve the two neighboring fingers. Within
a few weeks the takeover is complete." This example
provides a description of what happens to amputees
as their remaining limbs gain increased sensitivity,
and, hence, amazingly they are able to compensate
for the missing part by performing tasks they could
never had performed with the surviving parts before.
This principle has been found to apply to brain
damage, blindness, and deafness, as well as in other
lost functions of the body.
Using this approach, therefore, we find an
explanation for why a person with a severe or
profound hearing loss is able to readily adapt to a
tight fitting earmold, sound pressure levels that
normally would evoke cellular discomfort, and the
ability to withstand epithelial invasion of large
prosthesis without causing the customary lymphatic
swelling. Compare this to the case of a
mild-to-moderate hearing loss patient, where even
the slightest distortion of the earmold causes
extreme discomfort and rejection. In fact, in the
latter case the degree of hearing was barely
discernable to the patient, while the former example
has known of their loss for many years, and had
accepted its presence (Chartrand, 1990).
The Role of the Vagus Nerve in the Ear
Another example of neurological function, which can
have a debilitative or rehabilitative effect on
hearing prosthesis is Cranial Nerve X, specifically
the Vagus Nerve (Crouch and McClintic, 1971). The
vagus is known, among other functions, to act in
nonlinear conduit fashion as a cough reflex between
the external meatus and the larynx via the posterior
and inferior portion of the ear canal. This
wandering minstrel (hence, the meaning of the name "vagus")
meanders around the same area that is often occupied
by hearing aid earmolds, but just below the surface
of a sensitive epidermis. Some individuals are more
sensitive than others are, because of the varied
position it occupies, and variations in tissue
health. Many cough upon insertion of the otoblock,
or even during otoscopy. Eyes may water during the
same procedures. These reflexes are considered
“normal”, but can have a deleterious effect upon a
hearing aid fitting.
From the ear canal a branch of Cranial X joins the
tympanic plexus which cross-transmits sensory
information to even more neural pathways. The "red
reflex" sometimes evoked during otoscopy----where
dilated vessels and capillaries on the tympanic
membrane can turn red enough to appear to be otitis
media in some individuals----may be attributed to
this nerve and its interconnections to other nerves
such as the trigeminal (Hawke, 1981). This is an
important consideration because of the peripheral
manifestations in the interconnections of neural
"crossovers". These all play an important role in
the patient's ability to "couple" or adapt
comfortably to an earmold, particularly when
attempting to overcome unpleasant low-frequency
conduction (often mistaken as acoustic occlusion) of
the user's own voice while wearing the hearing aid.
To alleviate this complaint in CIC fittings, many
manufacturers routinely taper the end of the ear
canal (Oliviera, 1997). In some cases, more tapering
is needed in the specialist’s lab.
Cartilaginous distortions
For the elderly this phenomenon may be further
aggravated by the onset of cartilaginous atrophy
which causes a high incidence of prolapsed and
collapsed ear canals (Schow and Randolph, 1979). The
prevalence of this problem requires attention by the
one making the ear impression, with a high
probability of dimensional distortions due to
pressure of the impression material. Often some
specialist-performed modification of the earmold
will be necessary to resolve these complaints of
non-acoustic occlusion and physical discomfort (Hickok
et al, 1993). Proper fitting earmolds often require
an open-mouth impression technique. In most cases,
it is advisable to use incisor spacers or bite
blocks while the impression material is still
setting up in the ear canal, thereby shaping the
earmold or shell to its more dynamic shape. This, in
turn, may pre-empt or alleviate some of the common
complaints that come with closed-mouth impressions (Danhauer
and Danhauer, 1997).
Sensorineural hearing losses with near-normal
thresholds in the lows (in men, primarily), often
exhibit an abnormal voice conduction effect upon
first wearing a new hearing instrument. The
complaint is most often described as, "Hearing my
own voice inside my head" Or “Sounds like I have a
cold”. Many specialists have supposed, as often
cited in the literature, that this complaint stems
from too much sound pressure build-up caused by
occlusion and that, therefore, more venting is
needed (Chartrand, 1989). But this is not always the
case. It is common knowledge in the profession that
lack of venting does cause a significant build-up of
low-frequency energy, especially from a male
patient's deeper voice, hence the occlusion
complaint. But when occlusion persists AFTER
venting, it is most likely a non-acoustic occlusion
complaint caused by earmold pressure on wall of the
external canal. For many years now some specialists
report that they resolve the non-acoustic occlusion
effect by taking pressure off of the ear canal,
either through tapering the end of the canal or by
"slit leaking". Since almost all hearing instruments
may have venting today---even a pressure vent in the
most powerful instruments---it is the feeling of
this author that overly tight earmolds, pressing
against the ear canal wall (and vagus nerve branch)
is what is causing most non-acoustic occlusion
complaints.
Furthermore, overly tight fitting earmolds can evoke
a “red reflex” or thickening of the tympanic
membrane that can create greater impedance (and less
compliance). This will increase the need for more
gain and output in the amplified signal to overcome
the added impedance. No matter how we how look at
it, an overly tight earmold defeats its purpose and
sets the stage for another problem fitting case. We
may congratulate ourselves in reducing feedback with
an overly tight earmold, the enjoyment of which will
only last until the cartilage in the stretches
further over a period of about two weeks. Then, we
are back where we started! Likewise, those same
complaints are almost always as easily alleviated by
removing pressure between the canal wall and the
earmold.
As a side note, the author has, on numerous
occasions, heard M. Duncan MacAllister, Ph.D.
describe his innovative earmold technology called
Minimum Contact Technology or MCT (MacAllister,
1989), which reportedly avoids contact with the
region of the canal that creates this conductive
effect. It is this author's experience, as it has
for many others, that "slit leaking" or "tapering"
of the earmold provides a similar though somewhat
lesser accommodation without pressure at the more
sensitive, thinner tissue at the bony isthmus.
However, the point is that when the patient's need
of low frequency amplification increases there will
also be a proportionate increase in neurological
adaptation as the instruments are worn over time.
While some users have reported feeling tension in
their throats (vagus), or even experiencing a sore
throat while simultaneously speaking and wearing
their instruments, the degree of discomfort appears
to be relative to the need of (or lack of) low
frequency amplification because of one’s biofeedback
mechanism which helps one to modulate his/her voice.
There have even been reports of nausea related to
the involvement of the vagus nerve while wearing
hearing aids (Chartrand, 1989). Though rare, the
specialist must always be on the lookout for these
exceptional manifestations, and not treat them as
impossibilities. The mark of the professional is to
expect any and all, even the unexplainable, as
possibilities that may need addressed or
investigated. Granted most users will adapt over
time to an over-sensitivity problem, if they can be
gotten past the 30-day trial. Possibly, not even
realizing its existence, there are still those, for
any number of reasons, who have difficulty adjusting
without appropriate modification of the earmold.
Counseling example: "At first you may hear your own
voice sound different, but as you wear your new
instruments, you will gradually adapt until your
voice sounds more natural."
The point cannot be stressed enough that when one
considers the high failure rate among precipitous
high frequency sensorineural cases who ultimately
refuse amplification, not necessarily because of
difficult-to-detect aural benefits, but because of
lack of satisfactory counseling on the part of the
specialist about the "occlusion" or discomfort
phenomenon. By virtue of the above consideration,
many precipitous losses unnecessarily have become
"problem cases". It is the author’s objective to see
the frequency of these cases diminish through better
research, education, and practical application.
Psychoacoustic Concerns in Hearing Aid Design
The violation of psychoacoustic considerations may
inadvertently cause an otherwise relatively simple
fitting to become a most complex affair. Several
areas of concern will be focused upon here relative
to their importance in preserving, as closely as
possible, a "natural systems" accommodation in a
hearing prosthesis. Dissecting the "psycho"
(psychological perception) and "acoustic" (sound
transmission) elements in the field of
psychoacoustics, we will find several considerations
which may be considered the "ideal conditions" that
should be kept in mind in the research and
development, and in laboratories of hearing
instrumentation and, finally, in the aural
rehabilitation practice of the specialist.
Hearing Instrument Fidelity.
Acoustic properties such as frequency, intensity,
and resonance are psychoacoustically perceived as
pitch, loudness, and timbre (Myklebust, 1964).
Indeed, the physical properties of periodicity, or
the time component of frequency, become
psychologically perceived as tone or pitch. Within
each band of sound the brain, then, perceives a
center frequency, which is identified as a discrete
tone, while the surrounding frequencies around the
center frequency provide only the colorizing or
timbre of the discrete tone. The range of human
hearing has variously been reported as from 16Hz to
16,000Hz or from 20Hz to 20,000Hz. Regardless of the
precise range, it can be safely assumed that the
ideal fidelity in human hearing perception is
closely associated with that range and its spatial
properties.
While the human mind is capable of detecting rhyme
and reason from complex bandwidths of sound, the
hearing aid itself does not inherently have this
interpretive ability. Hence, the hearing aid serves
primarily as a conveyor of the acoustic signal. In
hearing instruments sound is transferred from
acoustic to electrical and back to acoustic energy
in changed amplitude and spectra, all the while
ideally maintaining an exact replica of the
periodicity of the acoustic signal. If all other
factors were equal, the main problem then would
become one of bandwidth replication.
For instance, when a given signal, let's say a
symphony orchestra, presents a range of bandwidths
(distinguishable to human hearing) of 16Hz (in the
lowest bass fundamentals) all the way up to 20,000Hz
(in harmonics, overtones, and cluster tones), the
question arises: What happens when that range of
frequency is compacted into a hearing circuit range
of only 200Hz to 4,800Hz (as it is in many hearing
instruments)? Obviously, acoustic information
occurring below 200Hz and above 4,800Hz are
essentially discarded or spatially compressed.
Recently, researchers (Tange, Dreschler, and van der
Hulst, 1985) have begun tackling the ominous
implications of hearing loss in the higher
frequencies (i.e. 8KHz-20KHz). Furthermore,
audiometric equipment and related evaluation
protocol in testing in the upper extremes of the
human hearing range have been developed (Brummett et
al, 1979, Fasti et al, 1979). With the advent of
micro-miniature electroacoustic transducers capable
of reaching up to and beyond 20KHz, and new hybrid
circuits reaching to 13KHz-14KHz or beyond, wider
band or multiband applications have become more
available to the specialist in providing higher
fidelity to their hearing aid users.
In broadening bandwidth in hearing instruments,
there are two "ideal acoustic condition" factors to
be considered:
1) The spreading effect of the acoustic, or
decompressed spatial landscape, when one increases
the range of F1-F2 frequencies in the hearing aid
circuit.
2) The ability of harmonics to acoustically reach
downward in search of their lowest common
fundamental, which tends to strengthen the
fundamental frequencies (Chartrand, 1989). Together,
these phenomena produce what might be experienced
when comparing the fidelity of an expensive home
stereophonic CD system versus a small AM pocket
radio: the home stereo provides the closest fidelity
or trueness to a live concert, while the smaller
unit attempts, with great loss of fidelity and with
other distortions, to "squeeze" the same spectral
and temporal information into a smaller acoustic
landscape.
The first consideration deals with the need to
expand, or at least, preserve the F1/F2 range of
output frequencies relative to input. The second
condition presumes that higher frequency harmonics
are an important element in providing fundamental
information to the listener, even in cases where the
harmonics exceed the frequency range of the
listener. The essence of these two phenomena is that
improved signal-to-noise ratio, quieter electronic
circuitry, lowered input sensitivity for distance
hearing all represent a realistic replication
(fidelity) in hearing aid amplification. Therefore,
the electroacoustic researcher and the practitioner
of aural rehabilitation must ask themselves whether
or not the widest range of fidelity is being offered
a given patient, the thesis here being that the
circuit that limits the most frequencies may be the
least acceptable. Of course, an exception to this
would be in cases of aberrant cochlear distortion,
as in diplacusis.
Bandwidth/Spatial Separation in Frequency-limited
Impairments.
It is sometimes erroneously supposed that if the
user has no residual hearing range after, let's say,
1KHz, that all amplification after 1KHz is
superfluous, unnecessary, and, therefore, to be
filtered out or minimized where possible. However,
one must keep in mind that by eliminating higher
frequency overtones and harmonics that the
enhancement---and "reidentification" function in
harmonic energy----of the fundamental frequencies
would also be removed, thereby weakening the
delivered signal to the impaired hearing system.
Furthermore, the circuit of the hearing instrument
is being asked to enlarge upon the inadequacies of
the impaired ear, that is, to amplify within a
narrowed space those frequencies that occur in a
much larger acoustic environment. Doing so can limit
the acoustic information that reaches the auditory
cortex. Therefore, the widest possible range of
acoustic reproduction should nearly always be
considered the "ideal", and also the most compatible
with psychoacoustic factors in human hearing, with
few exceptions to the rule (Skinner, 1978).
Related to the above is the opinion by some that the
low frequencies are relatively unimportant in the
over-all goal of improved audibility of the impaired
ear. As evidenced in previous sections of this text,
and by other researchers (Eriksson-Mangold and
Erlandsson, 1984), amplification of non-verbal
information such as movement of other people around
them, warning signals, and, even the lower
frequencies of one's own voice may be supply
important auditory and attentional clues for the
listener. On the other hand, over-amplification may
degrade the over-all listening experience. Corner
audiograms, particularly, can be both beneficially
and adversely affected by the amount of
low-frequency information provided by the hearing
instrument. See Case History No. 6 for an example of
where the “transposition effect” of high frequency
distortion artifact in the lows provided years of
useful amplification until the patient could be
helped with a multichannel cochlear implant. Hence,
the goal of improved speech intelligibility and
preferred acoustic gain level can be, at times, at
opposing odds with one another (Cox, 1985),
requiring that the specialist and design engineers
at the manufacturing plant utilize multiband
use-gain prediction formulae, dividing the actual
threshold-based prescription into two or three
larger bandwidth components (Berger et al, 1988),
instead of the traditional monotonic or broad-band
approach used in LDL/SSPL90 correlation (Kamm et al,
1979).
Critical Bandwidth.
In many presbycusis cases there may be a dramatic
lessening in bandwidth sensitivity (Stabb,
1989)----i.e., 2KHz may sound like any frequency or
band between 1.5KHz to 2.5KHz to the hearing aid
user, or it may like a non-tonal distortion. This
can cause severe difficulty in understanding speech,
even when the offending pure-tone thresholds are
brought near normal limits. Hence, there may be no
predictable relationship between pure-tone threshold
and speech intelligibility, even in some of the mild
and moderate cases. Not all of the blame goes to
sensory deprivation factors, such as phonemic
regression. Some of it has to do with mechanical
limitations in aberrant movement of broken
stereocilia, lessened endolymphatic and
perilymphatic space due to stria vascularis and
otosclerosis of the cochlea. In some cases,
electrochemistry is not in proper balance. Whatever
the reason, the problem of critical bandwidth
distortion can be just as prevalent and troubling in
the amplification process as abnormalities in
loudness growth (Larsby & Arlinger, 1999)(Chartrand,
1999).
For instance, the center frequency differences
between the letters "s" and "sh" can be as great as
1-1.5KHz. If, in that mid-to-high frequency range,
the user's difference limens (DLs) or "just
noticeable difference" (JND) are too broad, the two
sounds may be confusing or indistinguishable to the
listener. Of even greater concern are the second and
third formats (2KHz-6KHz) which are so critical for
speech-in-noise (SiN) ability (Arlinger &Dryselius,
1990)(Abel et al, 1989)(Durrant and Lovrinic, 1984),
and to minimize the occurrence of backward and
forward masking ((Elliot, 1971)(Fastl,
1976)(Jesteadt, et al, 1983). It is imperative that
the response of the hearing aid circuit will assist
the user in "selecting" the most important critical
bandwidths to assist in speech formant selectivity.
Since unvoiced consonants, comprised of complex
bands without accompanying voiced vowel sounds (such
as "s", "th", "f", "sh", and "h"), make up the most
prominent components of phonetic definition in the
English language, it is important that certain
discrete "peaks" be provided in the amplified signal
for better speech understanding (Preves and Curran,
1985)(Chartrand, 1989).
Whereas, there is a common practice in hearing aid
manufacturing to "smooth out" the peaks to allow
greater acoustic gain before the onset of feedback,
caution is given so that this practice does not
become counter to achieving better speech
understanding in the ear that has experienced
critical bandwidth sensitivity loss. It is essential
that such cases be provided frequency responses that
contain the needed discrete peaks to make up for the
loss of sensitivity, and, hence, an improvement in
speech-in-noise (SiN) discrimination ability.
Generally, the ideal frequency regions for peaks in
amplification are 1.5Hz-2.5Hz for the primary peak,
3.5Hz-4.5Hz for the secondary peak, and 6KHz-6.5KHz
for the tertiary peak (referring to the third peak
of the hearing aid analysis printout). To achieve
this will require a total amplified bandwidth of up
to 8KHz on the F2. The reader must also keep in mind
that these peaks may not be evidenced via the
standard 2cc coupler, but can be measured in real
ear or probe mic format.
To say it another way: Within the human hearing
range there are approximately 25 bandwidths across
the spectrum of 20Hz-20,000Hz (Zwicker, 1957). These
bandwidths expand in width and, consequently,
decreasing distinguishability of the center
frequency as the bandwidths ascend the frequency
spectrum. Going the other spectral direction, we
find the bands narrowing in width, as the center
frequency becomes distinguishable to a DL of 2 or 3
Hz in the lowest band. Disregard of the bandwidth
sensitivity limitations of the impaired ear may only
serve to deter the new user in overcoming phonemic
regression, mild receptive aphasia, and other
possible central auditory lesions. A case in point
is when a hearing aid user, upon the first
post-fitting visit after obtaining a new hearing aid
fitting, complains:
"I hear better, but I still cannot understand any
better with the hearing aids than I can without
them."
Again, if the frequency response characteristics of
the instrument are so "smooth" in configuration so
as not to assist the listener in the "selectivity"
of certain critical bandwidths the usual 60-90 day
adjustment period may yield little improvement.
Moreover, these considerations are important in the
design and engineering of the hearing aid circuit,
as well as the need for counseling on the part of
the specialist. Programmable hearing instruments,
primarily because of the greater flexibility of
active filtering, has served to significantly
resolve much of this challenging and often
perplexing fitting problem.
A related observation: the author has found that
Verbo-tonal methodology as advocated by the American
Verbo-tonal Society (Asp, 1984) to be of great
benefit in establishing the primary, secondary, and
tertiary peaks in amplification based on the
development of speech communication. Their findings
are must reviewing for any manufacturer or
researcher who desires to design hearing aid
circuits that foster maximum speech intelligibility.
Acoustic Echo as a Result of Over-amplification
Another acoustic aberration relative to
amplification and hearing function is the
physiological recurrence of amplified vibrations,
largely from the round window of the middle ear,
which causes a manifestation often described as an
"echo" by the listener (Durrant and Lovrinic, 1985).
For lack of a better term the author calls this
phenomena the "acoustic echo". In some cases, it
might be better described as "cochlear distortion"
that is manifested only at high levels of
amplification. One mechanical model describes the
physical pathway of this phenomena as follows: The
output signal from the hearing aid receiver reaches
the tympanic membrane, gains momentum as it crosses
the ossicular chain to the oval window. Then, the
enhanced vibration is transmitted through the oval
window into the scala vestibuli, around the
helicotrema into the scala tympani, and then back
out the round window into the middle ear cavity to
interact with the input signal traveling through the
ossicular chain. Another model would describe echo
in the impaired ear as simply the result of the
stapedial reflex to dampen over loud input signals,
and hence, causing transform distortion on the
ossicular chain (Zemlin, 1981). Regardless the
mechanism, we know that this complaint cannot be
verified in the electroacoustic analyzer as it is an
artifact occurring solely in the listener’s ear.
When sound pressures from the hearing instrument
exceed what is physically possible for efficient
transfer through the mechanical structure of the
ear, a "hollow sound" or "echo" may be heard by the
user.
Dynamic Range Limitations.
This area is covered in more detail elsewhere in
this text, so we only mention it here to establish
its proper place in the possibilities of things that
can create a problem fitting case. A patient’s
dynamic range, or the individual's hearing system's
range of efficiency from threshold to the maximum
level of intensity where sound may be processed, is
both a psychological and physical phenomena. On the
psychological side are such factors as the patient’s
everyday ambient noise environment, psychosocial
settings, and personal preferences. On the physical
side of the equation are transform efficiency of the
middle ear, neurological limits and chemical (ionic)
exchange in the cochlea, and central auditory
status. It is sometimes necessary for the specialist
to decide which set of phenomena is presenting the
problem. Those decisions can only be based upon the
proximity of speech scores in from the audiometric
evaluation, and by loudness balance testing.
One such predictor is the proximity of MCL to UCL
scores, and their relationship to monosyllabic
discrimination ability. In describing this
relationship to help clinicians screen for cochlear
implant candidacy, the author developed a visual
model, the design of which was gleaned from
thousands of severe-to-profound patient case
histories. It was noticed, during compilation of
data, for instance, that the closer the MCL came to
the UCL that speech discrimination decreased by a
proportionate, though variable, degree. (See figure
7.1)
//figure here//
Figure 7.1 A visual model showing the progressive
relationship of MCL to UCL and its effect upon
monosyllabic word discrimination in patients who
progressed from moderate to severe hearing loss over
time (Chartrand, 1997).
Introducing peak-clipping into the scenario. Related
to the above, but from an electroacoustic viewpoint
of the hearing aid circuitry, another problem which
may hamper the specialist in attempting to
accommodate narrow dynamic range cases---by limiting
output without a corresponding reduction in
gain----is the introduction of "peak clipping", and
unacceptable distortion into the system. Class A
circuits are notorious for this phenomena. A better
solution is in the use of Class D circuitry and/or
adding input compression (Agnew, 1993). Dr. David B.
Hawkins (1993) commented to the author that,
although the raised "headroom" of the Class D
circuit would theoretically appear to introduce
unacceptable output levels in patients exhibiting
abnormally low discomfort levels, that user reports
attest otherwise. One speculation is that, since the
Class D approach is virtually distortion-free at
high gain levels that possibly a less pervasive
intrusion upon the outer hair cells (OHCs) of the
cochlea is experienced by these users. Others have
called for near-universal utilization of input
compression where the user's dynamic range is
substantially compressed (Fabry, 1993). For mild and
moderate hearing losses, K-Amp and Automatic Signal
Processing (ASP) type circuits, on the other hand,
reportedly introduce even better approaches to the
limited dynamic range (Killion, Stabb, and Preves,
1990) (Killion and Fikret-Pasa, 1993), such as
BILL/TILL circuits, without unwanted peak-clipping.
And, with multichannel wide range dynamic
compression (WRDC) programmable instruments gaining
in popularity today, the specialist is afforded even
more possible abnormal loudness growth solutions
while avoiding peak clipping distortion.
Distortion, peak clipping, and intrusion upon
narrowed dynamic ranges and the corresponding
applications would not be so important to our
discussion here except that it is evident from
experience that:
1) It is difficult for the patient to verbally
describe distortion, peak-clipping, and acoustic
echo.
2) It can be even more difficult for the specialist
to adjust downward the output of a hearing aid
without inadvertently sacrificing some other crucial
sinusoidal information.
Therefore, when the user reports problems such
"barrel effect", "hollow voice", "sounds harsh",
"not clear", rather than over-venting---which can
introduce resonant distortion and acoustic
feedback---or other deleterious shell modifications,
it may be more appropriate:
· Reduce gain and/or output slightly
· Adjust balance between bands of frequencies
· Restore natural ear canal resonance (from
insertion loss)
· Explore other circuit/technology applications
Shell Design and Modification
Next to an accurate hearing evaluation, the most
important fitting activity in dispensing is the
taking of a complete, undistorted, and perfect ear
impression. Not surprisingly, this is where many
"problem" fittings stem, and where there are more
variables involved than we can give justice to in
this section. Variations exist in a host of ways:
·Impression material mix
·Pre- and post-injected viscosity
·Insertion pressure
·Backflow consistency
·Jaw/mandible position
·Facial muscle stress
·Cartilaginous stiffness
·Setting/curing time
·Removal technique
·Mounting and packing for shipping
·Time/temperature exposure
·Handling at factory
·Shaping & coating at lab
There are so many variables that can ultimately
affect or change even the best impression to one
that is distorted, that we can only scrape the
surface here In deciding ergonomic relationships in
ear impression taking and earmold fabrication and
modification, there are several observations that
need to be considered:
1. Tension on the wall of the canal, which may be
created by an ill-fitting or overly tight shell, can
produce not only discomfort, but also resonance
changes, an increase in feed-back tendency, and
hampered neurophysical adaptation by the homunculus.
2. Canal length often determines the acoustic
integrity of the amplified signal that reaches the
TM. Hence, sound pressure is lost in the expanded
cavity of the external meatus when the canal of the
earmold is shortened. In addition, a short canal
with a high-gain instrument may introduce resonant
distortion, adding spurious transients to the signal
that finally reaches the tympanic membrane.
Conversely, the closer the end of the earmold to the
TM, the more preserved the acoustic information. But
there are limits: First, at some point near the TM,
the residual chamber becomes too small to allow
efficient transmission of signal, and secondly, the
tissue may be too sensitive (or too rigid for
dynamic movement!) at such depths in the canal as to
cause physical discomfort and rejection.
3. Venting change decisions should include the
inevitable effects upon frequency response, peak
shifts, and amplitude changes, not just the more
immediately evident relief of occlusion. Generally,
venting should be approached conservatively. More
venting can destroy the acoustic qualities of even
the best technology!
4. Physiological abnormalities such as allergic
reactions in the helix area of the ear, excessive
mandibular movement, absence of a normal concha or
tragus, and deformed canals are all important areas
of consideration in the fitting of hearing
instruments. These abnormalities or, rather,
idiosyncrasies can easily cause a “typical” case
turn into an atypical "problem case", if due
consideration is not given. Hence, any superficial
accommodation of these problems or modification of
the earmold venting, without due consideration of
the effects upon sound quality, is an abrogation of
professional responsibility in the fitting process.
Earmold Modification for Complaints of Discomfort
We must consider the highly subjective nature of the
user's reports of discomfort. Caution is the
watchword when responding to these reports, because
they may not be accurate. It is one of the skills of
the specialist to be able to elicit better
descriptions from hearing aid patients, prompting
with terminology and concepts so that they can
assist the specialist in resolving the complaint.
Otherwise, we risk unscientific modifications away
from the best acoustic delivery. Hence, one may add
more negatives than positives in the process of
letting the patient guide subsequent modifications.
Communication must be refined before information can
be entirely trusted. What may happen is that the
user begins to guide the hands of the specialist,
making dramatic changes, which may prove dubious at
best. There are certain "check points" along the way
that can help the specialist exercise damage
control, while at the same time accommodating user
complaints. For instance, issues of acclimatization
must be separated from issues of accommodation.
Below is an example of a dialogue that attempts to
accomplish just that:
User: "It feels too tight right here," as they point
deep within the ear canal. "It feels way too long,"
they continue.
Specialist: Removes instrument from out of the
user's ear, and notes if it is seated correctly, or
if it could be dragging loose tissue into the canal
because of (dry) friction. He/she inspects the
external meatus (at the bony isthmus in cases of
CICs) for redness. If no redness is observed, he/she
lubricates the canal of the instrument and
reinserts, noting ease of insertion and asks, “How
does that feel now?”
User: “That feels better, but still feels kind of
sensitive deep inside”.
Specialist: Removes instrument again, notes if there
is a sharp edge to the tip of the canal or extended
receiver tubing that could be causing the problem.
If not, he/she takes a fine Dremel cutter and tapers
the canal back no more than ¼” back from the end of
the canal. After buffing (battery inserted to assure
polarization), he/she lubricates the canal again and
reinserts for user evaluation. They are asked to
move their jaw, smile, and note if the comfort has
improved.
User: “That feels much better. It doesn’t feel so
long now.”
Keep in mind that the role of the specialist is to
interpret the complaint, and then to exercise
professional judgement in the action taken. Here are
a few practical tips:
1. No hearing instrument should be inserted into an
ear by the specialist without applying a light
coating of lubricant (an antiseptic/lubricant such
as Eargene, alcohol w/lanolin, etc.) on the canal
portion of the earmold or at the aperture of ear
canal. That was likely part of the original problem,
the ignorance of which would have prompted the
specialist to remove more material from the
instrument canal than necessary. The practice of
always using a lubricant when adapting new
instruments or shells to the ear prevents the tissue
of the canal from "gathering up" upon insertion,
which can cause the instrument to feel tighter and
longer than it actually is.
2. The true area of offense might just well be the
area opposite the are of verbal complaint. For
instance, the over-fit of the concha could create
tightness deep in the canal, a distorted helix could
create pressure in the bowl of the concha. The depth
problem might actually be one of distorted curvature
of the canal of the instrument, or a mandibular
movement problem. In other words, the problem may
not be at all the point of soreness, but elsewhere,
where there is an opposing pressure.
3. Is the discomfort long term? Did the ear become
sore after 6 hours of wear? If so, the modifications
should be very conservative, for grossly misfitting
shells will create discomfort within minutes, not
hours. Therefore, where it has taken long periods of
time to create discomfort, it is prudent to remove
only small amounts of material to disturb the
acoustic properties of the fitting as little as
possible.
4. Is the discomfort short term? Again, make sure
the problem is not one that a lubricant cannot solve
upon initial insertion. For if loose skin at the
aperture is "bunched up" into the canal entrance,
the result will be an uncomfortable, tight feeling
to the user, regardless of actual “fit”. If that is
not the case, then consider removing only material
that will not have an acoustical effect upon the
instrument. Venting and canal lengths are two areas
that we do not want to disturb at any cost.
Generally, the helix portion (in ITEs), certain
bends in the canal, and areas around the
tragus-antitragus may be removed without substantial
acoustic aberrations occurring.
5. If the problem is an ill-fitting canal, always
consider tapering before shortening. The effect on
comfort is almost identical when one tapers rather
that shortens the canal. (Take note of the foregoing
discussions on the vagus nerve and its possible role
in earmold comfort).
When the above considerations are made before making
modifications, fewer problems will develop that
require a return to the factory. Of course, there is
not a lot to be done to make up for a poor ear
impression, or one that has become distorted in
transit to the factory. But with close cooperation
of factory and specialist, even those occurrences
may be reduced to a negligible level.
The above sections are not meant, by any means, to
be a complete explanation of earmold/shell
modification. Instead it is our purpose to show some
of the questions that need to be asked before
consigning a savable fitting to the folly of failure
and disappointment. The problems are not usually
one-dimensional; neither can be the solutions. If
that were not enough with which to contend, we can
also add to the mix what might be called a battle of
perspectives and varying levels of motivation. On
the converse side of this problem will be found some
hearing aid users that have had to devise their own
methods of adjustment and compensation, because of
lack of knowledge, skill, or caring on the part of
the practitioner. A failure to closely monitor the
progress of a given fitting only serves to add fuel
to the fire of misconceptions about hearing aids and
the specialists in the field (Sorkin, 1993).
Communication, therefore, becomes the most critical
and necessary attributes of the hearing instrument
specialist and the factory. Regardless the level of
skill and knowledge acquired, the ability to
communicate and translate that communication into
effective action and counseling is most critical for
the resolution of hearing and hearing aid problems.
Using the Aural Rehabilitation Correction Model
Prognosis and percentages of expected ratios of
improvement from aural correction and compensatory
utilization represent rough quantification (shown in
pie-graphs) of the general relationship between
hearing instruments and other forms of compensation.
Non-hearing aid compensation methods are constituted
by ALDs, speechreading, and any other method needed
by the patient.
Expected ratios of corrected improvement (%) are
approximated from two bases:
1) The Estimated Aural Correction (EAC) resulting
from the use of hearing aids (i.e., anticipated
degree of auditory capability when applied to the
subject's residual hearing ability), and
2) The Need for Compensatory Utilization (NCU)
(i.e., visual, auditory and tactile compensation,
and the use of assistive listening devises (ALDs).
Although some substantial changes have been made to
the original Estimated Aural Correction EAC formula,
the principles remain the same. The suggested
"rule-of-thumb" formula for determining EAC consists
of averaging the percentages of “best aided”
monosyllabic speech discrimination in both quiet and
in a 10dB signal-to-noise ratio (SNR) environment.
While there is always a need to reflect narrowed
dynamic ranges in any estimation formula, the author
has found that the 10dB SNR monosyllabic discrim
score essentially accomplishes our objective in the
simplest way. Per the visual MCL to UCL model set
forth earlier we understand that speech
discrimination drops in relative proportion to the
reduction of space better MCL and UCL; even more so
in noise. And while speech discrimination and
speech-in-noise represent only one dimension in
desired amplification outcomes, it is generally
accepted that this dimension takes priority over all
others. For our purpose here is find an estimated
proportion in our expectation of outcomes between
hearing instruments and the other methods of
compensation.
The Need for Compensatory Utilization (NCU) can only
be interpreted qualitatively (100%-EAC=NCU) as a
percentage of normal auditory function. It also
reflects a rough equivalency of what is needed to
complete the aural rehabilitative profile of the
patient.
Example:
Best aided discrim in quiet = 78%
Best aided discrim in 10dBSNR = 44%
Average of quiet & 10dBSN = 61%
So that...
78%+44%=61% Estimated Aural Correction
2
The need for compensatory utilization (NCU) is
derived by simply subtracting the EAC from 100%, so
that:
100%-61% (EAC) = 39% NCU
Of course, because these are approximate figures for
counseling purposes only, it is best to round off to
the nearest 5% increment. Hence, by rounding off the
figures in the above example, one may estimate that
60% of the total correction will come by way of the
hearing aids, while the remaining 40% (to reach 100%
of correction) will need to be realized by way of
compensation means other than hearing aids.
Conclusion
It is incumbent upon the specialist to treat every
hearing instrument fitting as unique, for every
client is very much an individual. Taking the myriad
of environmental factors, their home environment,
workplace, recreational pastimes, and factoring in
social relationships, family, friends, and
co-workers, the keen observer encounters a complex,
sometimes contradictory mosaic of need and
expectations. And that's before one considers
psychological, emotional, and attitudinal
attributes, which can make each patient a challenge.
Therefore, when one approaches the ominous question
of "What is a problem fitting?", it is imperative to
realize that any fitting can become such. By
skillfully acting upon this assumption, the
specialist will be able to reclassify many
previously considered problem cases back into the
classification of manageable, and therefore,
successful fitting cases.
The purpose of this chapter was not to
over-complicate an already complicated process.
Indeed, there are those specialists that may carry
the minutest detail of scientific information to the
utter extreme, thereby obscuring the greater role of
counselor and communicator, confidence builder,
vision expander, attitude adjuster, and peacemaker.
On the other hand, lesser inclined specialists who
tend to oversimplify the finer points of
communicative disorders, and the continual need for
self-improvement and learning, may need to gain a
greater sensitivity toward the multitude of
variables manifest in hearing health care…and, of
course, the plethora of available solutions to meet
those variables.
When all considerations are taken, the ones most
outstanding are those that lift each and every
sufferer of hearing impairment into more productive
and effective living. That should be a consideration
of the first order for all professional hearing
instrument specialists.
References
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Agnew, Jerry, "Update on Circuit Classification",
Technology Summit 1993, Starkey Laboratories, Inc.,
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Arlinger, S. D., and Dryselius, H., “Speech
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resolution in normal and impaired hearing”, Acta
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Asp, Carl W., "The Verbo-Tonal Method for
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Chartrand, M.S., Hearing Aid Repair & Modification,
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Chartrand, M. S., “Soundfield Concept: Solving
Problems Cases”, in Advanced Audiometric Course I,
UNIMAXtm Education Series, Gainesville, TX. (1989).
Chartrand, M.S., “Working with the psychology of the
hearing impaired”, Hearing Instruments, Vol. 41, No.
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Chartrand, M.S., Ergonomic Prosthetics in Hearing
Aid Fittings, continuing education course, Livonia,
MI: National Institute for Hearing Instruments
Studies, (1996).
Chartrand, M. S. Unraveling the Mysteries, Exploding
the Myths, continuing education course, Livonia, MI:
National Institute for Hearing Instruments Studies,
(1999).
Chartrand, Max S., Success in Fitting Today’s CICs,
continuing education course, Livonia, MI: National
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