Download Guidelines for the Fitting, Verification and Evaluation of digital signal

Guidelines for the Fitting, Verification and Evaluation of digital signal processing hearing aids within
a Children’s Hearing Aid Service.
Introduction
Amplification offers the deaf child an opportunity to access the spoken word and environmental sounds so
that they can benefit from that experience. Each child will have an individual response to the amplification
but it is the role of an audiologist to provide a child with a hearing aid that offers effective amplification by
taking into consideration the type and degree of hearing loss, the child’s developmental stage and situation.
The three stages of fitting a hearing aid (fitting, verification and evaluation) are described as recommended
procedures. Audiologists involved in hearing aid fittings and reviews will have high levels of skill, training
and background knowledge of digital signal processing hearing aids and be capable of performing advanced
testing and interpretation of results. Audiologists should be alert to unexpected results, and be able to
critically appraise the data from each part of the fitting and verification procedure. It is essential that these
procedures are carried out by staff with the ability to recognise and correct anomalies in the measurements.
Fitting hearing aids to infants within the first months of life is a highly specialised task. Audiologists involved
in the fitting should be experienced and have an understanding of the specific issues affecting this age
group as information regarding frequency specific and ear specific hearing thresholds may be incomplete.
Selection of Physical Hearing Aid Characteristics
In selecting a particular aid the child’s physical characteristics, degree of hearing loss, flexibility of the aid to
cover a conductive episode, compatibility with an FM system, use of standard and advanced features etc will
need to be considered.
Behind the ear hearing aids will be appropriate for most children. Body worn devices may on rare occasions
be appropriate for specific management reasons. In-the-ear hearing aids are not recommended for infants
due to rapid growth of the outer ear, but may be a management option for older children. Digital signal
processing hearing aids rather than analogue are preferred for their processing flexibility and added
features.
Fitting

Prior to the first fitting appointment, programme the selected aid using the child’s audiological
details. Match the output of the aid to the prescriptive targets generated for input levels of 50dB
65dB and 80dBSPL using a speech input signal.

Noise reduction features should be disabled during programming.

Consideration of the child’s amplification requirements will inform decisions regarding the
application of various standard and advanced features.

Aids should be fitted to children using a published, threshold-based prescriptive procedure (DSL
i/o, NAL-NL1).

The prescriptive procedure available within the hearing aid or real ear measurement software must
be an authorised version of the procedure (some software uses modified formulae which should not
be considered equivalent unless authorised).

If an authorised prescriptive procedure is not available within the hearing aid and real ear
measurement software, then stand-alone software should be used to generate target values.

Aids should be fitted using Wide Dynamic Range Compression unless the hearing loss is severe to
profound, or unless there is a strong preference from an older child for a linear fitting. .

The prescriptive procedure and the settings used should be documented as part of the clinical
record.
Verification
The verification stage is essential as it takes into account the child’s individual acoustic parameters as ear
canal acoustics vary considerably between infants and differ from the acoustics of 2cc couplers. Real ear
measures, either real ear to coupler difference (RECD) or real ear aided response (gain or output) (REAR)
must be measured.
Real ear aided gain (REAG) is an in-situ measurement of the aid’s performance which is used if the child is
able to sit still and be quiet. Alternatively, the real-ear performance of the hearing aid can be predicted using
coupler measurements corrected with a measure of the real ear to coupler differences (RECD).
If a real ear to coupler measurement was measured prior to the fitting appointment then the fitting and
verification stages are combined.

If an RECD was not measured the performance of the aid is adjusted during the real ear aided
responses (real ear aided gain or real ear aided response) to match the prescriptive targets for
input levels of 50dB, 65dB and 80dBSPL for non linear fittings (quiet, mid and loud speech levels).

For linear fittings this should be at mid-speech around 65dBSPL.

Tolerances to the prescription procedure of +/- 5dB at frequencies of 250Hz, 500Hz, 1000Hz and
2000 Hz and of +/- 8dB at 3000Hz and 4000Hz should be achieved in all cases. In addition the
slope in each octave should be within +/-5dB/octave of the target.

When doing real ear measurements on children, real ear aided gain (REAG) should be used rather
than real ear insertion gain (REIG). A variety of test signals can be used to verify the gain of a
hearing aid and the limitations and uses of each should be recognised.

Real ear measurements on young children should be done at 0 degrees azimuth. For older children
able to face away from a loudspeaker during sound presentation an angle of 45 degrees should be
used.

It is recommended that the probe tube be placed at a depth of 21mm for under seven year olds and
27mm for older children (measured from the inter-tragal notch). The end of the probe tube should
be at least 5mm past the end of the earmould.

The SSPL90 measurement should only be assessed with a high level 90dB SPL pure tone sweep.
Evaluation
This stage of the process is to evaluate how effective the programmed hearing aid is for a child and to
measure the benefit of the amplification to the child. This is monitored over time. The clinical evaluation
assessments are combined with the assessment from education and parental observations to produce a
rounded view of the effectiveness of the amplification which will lead to informing decisions regarding the
management of the hearing loss.
The outcome measures used to evaluate the effectiveness of the amplification should be age appropriate
and considerate of the child’s communication level.
The following outcomes measured may be used;
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aided speech testing in noise and/or quiet conditions
soundfield aided testing
standardised questionnaires of listening ability as perceived by child, teacher and parent
Common Monitoring Protocol
communication development
compliance of wearing the aids
behavioural observation
These outcomes should be shared and considered by the hearing aid review team, (comprising parents,
teachers, audiologists, speech and language therapist) to inform decisions regarding the management
of the child’s hearing loss and to consider any adjustments to the programming of the aids.
Hearing Aid Reviews

Babies, infants and children with hearing aids should be offered regular hearing aid reviews. Such
reviews should be scheduled for one and a half hours or more and involve two experienced
audiologists. Parents should be able to be present throughout, and the support teacher of the deaf
should be invited to attend and /or to send a summary report on the child’s progress with reference
to the outcome measures described above.

Liaison with parents/school/teachers is essential.

Hearing aid reviews will need to be very frequent in the early stages of fitting when information is
only partial and the child and family are adjusting to the aid. They should never be less than
annual.

Hearing aid function should be checked at every hearing aid review, and details of electroacoustic
function and settings confirmed and reported to education services. Education services should be
sent details of any adjustments made to the programming of the aid along with frequency response
curves.

Verification to prescription targets using real ear measures should generally be performed at every
hearing aid review.

Evaluation of the amplification must be assessed using appropriate outcome measures.

Parents should be provided with written guidelines on hearing aid use and management. This will
include instructions on how to carry out daily listening checks, advice on simple ‘trouble-shooting’,
advice on how to arrange for new impressions to be taken, a care kit and the phone number of a
‘key’ or ‘link’ worker often but not always a teacher of deaf.
Other

Close links between health and education services and agreed systems for repairs, breakdowns
and loan aids are essential in a children’s hearing aid service.

Audiologists will be required to keep abreast of new developments and features offered by digital
signal processing aids as they come onto the market to ensure a high standard of care to hearing
impaired children.

It is essential that audiologists working in a children’s hearing aid service are able to communicate
effectively with deaf children and their families.
Revised Sept 2005