Hearing Aid Fitting for Children

Anatomy and Physiology

The peripheral auditory system consists of three main parts, the outer, middle, and inner ear. 

• The outer ear includes the pinna and external auditory meatus (the ear canal). The purpose of the outer ear is to collect external sound waves, which can be funneled and transmitted to the tympanic membrane via the ear canal.[8]
• The middle ear is comprised of the tympanic membrane (the eardrum) and three ossicles: the malleus, incus, and stapes. The air-filled middle ear space is connected to the nasopharynx via the Eustachian tube. This tube is responsible for equalizing the middle ear pressure to the external pressure in the atmosphere. Sound transmitted from the external auditory meatus causes the tympanic membrane to vibrate, which in turn moves the middle ear ossicles, the third of which, the stapes, is connected to the oval window of the cochlea.[9]
• The inner ear is made up of the cochlea and semicircular canals, which assist with the brain's perception of head movement and angular acceleration. The cochlea is a perilymph-filled structure with a 'snail shell-like' appearance. Its inner turns contain the Organ of Corti, which sits on the basilar membrane and houses rows of auditory receptor cells (hair cells), tonotopically arranged and tuned to particular sound frequencies. Movement of the stapes at the oval window causes activation of the hair cells through the movement of the perilymph and tectorial shifts at the basilar membrane. This activation converts the physical properties of the sound into electrical signals, which can then travel across the vestibulocochlear nerve to the brainstem and auditory cortex.[10]

Bone Conduction vs. Air Conduction Pathways

The air conduction pathway of sounds utilizes the outer, middle, and inner ear structures, as discussed above. The bone conduction pathway, however, differs in that sound vibrations, if transferred through the skull, can stimulate the oval window of the cochlea, which is housed in the petrous portion of the temporal bone.[11] With regard to hearing aid usage, this knowledge is important when determining whether a bone- or air-conduction hearing device is most appropriate for the patient. In cases where the outer ear, middle ear, or both are compromised, such as children with middle ear effusion or microtia/atresia, bone conduction devices may offer a better solution.

Indications

Children under 18 years of age identified as having aidable hearing loss should be offered hearing aid devices. The following factors are taken into account when making decisions on whether a child may be suitable or unsuitable for hearing aids:

• Level of hearing loss: due to the acoustic nature of the sound delivery, hearing aid outcomes can be directly related to the amount of inner and outer hair cell function in the child's cochlea.
  • Severe to profound hearing losses, which are attributed to poor hair cell function, may not be well supported with hearing aids, as the amplification is often not sufficient to allow access to speech phonemes at a normal conversational level.[12] In these cases, electrical stimulation of the basilar membrane via a cochlear implant may allow better outcomes.[13]
  • There is no universal consensus in the literature as to the benefits of hearing aids for children with very mild, unilateral, or isolated hearing loss. Insufficient evidence that such interventions can improve speech and language delay in this population means that clinicians often prescribe hearing aids on a case-by-case basis.[14]
• Nature of hearing loss: in cases of temporary fluctuating conductive hearing loss, such as those secondary to middle ear effusion (glue ear), bone conduction hearing aids may provide more suitable aiding for the child to avoid over-amplification and the need for regular fine-tuning.[15]
• Anatomy of the ear: children with malformations of the outer or middle ear, which could prevent an air-conduction acoustic signal from reaching the inner ear, may have better outcomes with bone conduction devices. Malformed pinnae may need assistance in supporting a behind-the-ear hearing aid which can usually be achieved with retention equipment such as toupee tape. Malformations can include:
  • Microtia
  • Atresia
  • Absent or incomplete middle ear ossicles
  • Absence or malformed middle ear space
• The motivation of parents/guardians or older child: while not an absolute factor in the decision to offer hearing aids, the motivation of the parents/guardians or the cooperation and motivation of an older child should be well considered during the process. If hearing loss is not yet or well accepted by the older child or child's caregiver(s), devices may be fitted and subsequently under-utilized.[16] Supplying families with high-quality verbal and written information and employing expert counseling techniques will ensure they are confident with device management and benefit, thus encouraging good uptake and usage of the hearing aids.
• Completeness/reliability of the audiogram: the whole of a hearing aid's prescription is the sum of its audiogram. In situations where a child's audiogram is incomplete or suspected to be unreliable, a hearing aid may over- or under-amplify. In these cases, more time should be taken to assess the child, either behaviourally or through objective measures such as auditory evoked response testing, to ensure their true thresholds are obtained. If predicted audiograms are used as a last resort, much consideration is required to ensure a conservative yet robust approach and should be approached only by expert and experienced healthcare professionals.[17]
• Communication options: some families, particularly those who use manual sign languages at home or are considered culturally Deaf, may feel that hearing aids are not the preferred direction for their child. In these cases, a substantial amount of counseling should be done to ensure that families give a fully informed withdrawal of consent, particularly in children with pre-lingual deafness. An interprofessional team should work with the family so that they are well-supported in their decision.[18]

Contraindications

There are no absolute contraindications to the pediatric hearing aid fitting in a child with hearing that can benefit from hearing aids as determined from audiometry. Relative contraindications consist primarily of patient and family acceptance and usage. If the patient and/or family are not interested in utilizing a hearing aid, one should not be prescribed. Such feelings can have many causes, including the social stigma of hearing aid use, desires of the family (particularly if they are members of the Deaf community), or cognitive/motor inability to manipulate or insert the devices, among others.

Equipment

A considerable amount of appropriately calibrated and maintained equipment is required during the hearing aid fitting process. They are:

• Computer with installed hearing aid fitting and verification software
• Hearing aid programming cables or wireless link device
• Hearing aid(s) selected for features such as power, size, color, connectivity to additional devices, FM compatibility, tamper-proofing
• Couplings such as earmould or slim tubes for behind-the-ear devices
• If real-ear measurements are used: a test box and/or hearing aid verification equipment, real-ear measurement tubing, real-ear-to-coupler-difference tube

Additionally, the following may be helpful to the clinician:

• Demo hearing aid to visually orientate the family
• Written instructions and information
• Hearing aid maintenance pack, which often includes: tubing puffer, battery checker or charger (for rechargeable hearing aids), moisture-removing tablets, cleaning tools, antiseptic wipes, stetoclip, hearing aid case
• Retention devices such as toupee tape, hearing aid clothing clips, loop retainers
• Speech testing materials
• FM 'shoes'/'socks'/'boots' and ear-level receivers
• Medical tape to help hold the real-ear-measurement tube in place

Personnel

Hearing aid fittings to pediatric populations should be performed by a suitably experienced healthcare professional, often an audiologist with additional training in pediatrics. Although the technique for fitting is similar to that in adults, additional considerations are required to ensure precision and appropriate post-fitting evaluation, particularly in pre-lingual children and those unable to provide subjective feedback. A multi-disciplinary team including audiologists, physicians, early intervention specialists/teachers of the deaf, and speech and language therapists should be familiar to the child and family by the time of fitting.

Technique or Treatment

Air conduction hearing aid fittings to pediatric populations should be performed to evidence-based local, national, and international guidelines. Organizations such as the American Speech-language Hearing Association (ASHA) and British Society of Audiology (BSA) provide such detailed standards, which should be consulted alongside the following techniques below.

1. Preparation

• The child's audiogram should be entered into the fitting software to ensure that the correct hearing aid prescription is calculated for the patient. Additional information, such as gender and date of birth, should also be detailed as it may affect the prescription calculations of the gain/output target.
• Prescription choice: the appropriate prescription method should be chosen according to departmental protocol.

DSL vs. NAL? Desired Sensation Level method (DSL) was developed specifically for infants and young children to deliver a scientific and systematic amplification formula that would provide adequate speech audibility according to speech sensation level data from pediatric populations.[19] Its newest edition, DSL v5, reflects conventional digital processing to maximize audibility while maintaining comfortable loudness for its user, accounting for different gain-frequency target recommendations for varying speech input levels. Different algorithms for conductive hearing loss and binaural hearing aid fittings exist.[20] 

The fitting rationale from National Acoustic Laboratories (NAL), most recently NAL-NL2, aims similarly to increase speech audibility and overall loudness comfort but with a focus on speech tonality, localization, and naturalness of the sound.[21] Both formulae have been validated with similar speech intelligibility outcomes observed in children despite differences in gain-frequency output and, therefore, loudness estimates, although there is a lack of empirical evidence in real-world environments.[22]

• The chosen hearing aid should be connected to the software. Acoustic coupling (i.e., earmould), tubing, and venting should be selected as this will also affect the prescription of the target. The hearing aid should now have set the appropriate gain levels for the child.
• If hearing aid validation is being completed, real-ear measurement software and hardware should be prepared before the patient arrives.

2. Counseling of the Family/Older Child and Orientation of the Device

• Excellent counseling is crucial to encourage successful uptake and routine usage of a hearing aid device. Care should be taken to ensure that information on the following is available written and verbally for all families but tailored to the needs of the child:
• Benefits of the hearing aid in different listening situations
• Realistic expectations
• Optimal usage time
• Manipulation of the hearing aid controls
• Maintenance and cleaning of the hearing aid, including any additional equipment issued, such as a stetoclip, tubing puffer, battery charger, etc.
• Correct insertion
• Retention of the device
• How families can encourage good usage of the hearing aid
• Additional connectivity and listening device equipment such as FM systems, personal listening devices, connecting to a tablet or mobile phone, loop/T systems

3. Validation of the Hearing Aid

• For children who cannot subjectively report on the sound and loudness of the hearing aid, thorough validation is critical.[23] It can be achieved by performing real-ear measurements on the child during the fitting process.

REMs vs. RECDs Real-ear-to-coupler difference measurements (RECDs) can be performed with minimal cooperation of the child, making it the best validation choice for hearing aid fittings on children under four years of age or for those who are unable to sit for extended periods, such as children with ADHD, autism, global developmental delay, etc.[24] 

It works by measuring the SPL of a broadband signal in a coupler and subtracting it from the SPL of the same signal in the ear canal of the child, thus deriving a difference value that can be added to the prescription formula. It enables precise fine-tuning verification of the hearing aid in the coupler of a test box while considering the acoustic properties of the patient's ear canal. The RECD depends on audiometric data measured via an insert transducer to ensure accurate conversion of the thresholds from their dBHL (decibel hearing level) measure to dB SPL (decibel sound pressure level) measure.

Real-ear measurements (REMs) similarly help the professional but provide a 'real-time' verification method of the hearing aid at the patient's ear level. This method, however, is reliant on specific conditions, such as a quiet room with minimal sound interference and a still and obliging child for around ten to fifteen minutes. Both methods allow professionals to inspect the hearing aid's match-to-target while considering the different effects of the ear canal volume on the sound intensity of the incoming signal. This is of particular importance where small children and infants are concerned, as failure to do so can result in over-amplification; due to the intensity of sound increasing when delivered to smaller ear canal volumes.

• RECDs
  • First, measure the SPL of the broadband signal in the coupler by connecting the RECD tube to the coupler and running the signal. The display of this data will vary by the software manufacturer, but you should be able to visualize the intensity of the signal at different frequencies, often between 125 and 8000 Hz.
  • Following this, measure the SPL of the signal in the ear canal of the child. To do this, connect the RECD tube to either the earmould tubing or insert the tip and insert the REM tube into the ear canal of the child. The depth will vary according to the child's age and is often standardized in practice guidelines. The earmould or insert tip can then be placed into the ear canal, and the signal ran again. For children, the RECD will be positive (i.e., over 0 dB) if we assume that the child's ear canal is smaller than a standard 2 cc coupler (which reflects a normal adult ear canal volume of 2 ml). If the RECD value is negative, it may highlight a poorly fitting or inserted earmould or insert tip. Often, the software will give you an expected RECD value according to the child's age, which allows the professional to assess the accuracy of their measurement.
  • The RECD data can then be saved and will be added to the target calculation of the prescription formulae.
• REMs
  • The child should be sitting adjacent to the stimulus source with instructions to sit quietly and still during the duration of the measurement.REUR (real ear unaided response): the REM tubes should be placed into the ear canal at the age-appropriate depth for the child and the broadband stimulus run. The value displayed is the SPL of the stimulus in the unaided ear. Often, there will be a peak response around the 3000 to 4500 Hz region, which reflects the resonant frequency of the outer ear.
  • REOR (real ear occluded response): the earmould or slim tube should be carefully placed into the ear canal with the REM tube still in situ and undisturbed. When the broadband stimulus has run, the values should reflect the SPL of the signal with the ear now occluded. Depending on the nature of the acoustic coupling, these values demonstrate the level of occlusion achieved.
  • REAR (real ear-aided response): the hearing aid can now be unmuted, and the international speech test signal (ISTS) can be run to view the aided response values of the hearing aid in situ against the prescription target for different input levels.

4. Fine-tuning the Hearing Aid to Prescription

• The ISTS: the international speech test signal was developed to provide a standardized and repeatable speech stimulus for hearing aid measurements. It encompasses the natural properties of speech and uses phonemes from six of the most widely spoken languages. The signal replicates the international long-term average speech spectrum (ILTASS), which can be shown as an overlay on most verification software when the REAR is run.[25]
• For different input levels (mostly 50, 65, and 80 dB), the prescription target will appear, and the gain controls on the hearing aid software can be manipulated for the REAR to match.
• It is helpful during the fine-tuning process for all frequency compression and noise management features to be deactivated.
• Once the hearing aid gain matches prescription targets for the appropriate input levels, a maximum power output (MPO) can be run at 85 or 90 dB to ensure the hearing aid will not amplify over the uncomfortable loudness level of the child. This can be done through real-ear measurements or performed in the coupler.

5. Subjective Verification of the Hearing Aid Output

• All children, regardless of their ability to self-report, should be observed for loudness discomfort in the clinic. This can be achieved in different ways, such as presenting warble tones, clapping, or using noisemakers at high-intensity levels. Infants and non-verbal children can show eye-blinking where gain reaches uncomfortable levels. In these cases, the gain should be reduced at the discretion of the experienced clinician to increase comfort while avoiding under-amplification.
• For children who are communicative but cannot reliably verbalize their evaluation of the hearing aid sound, additional resources can be used to gauge satisfaction, such as pictorial loudness scales.

6. Speech Test Evaluation of the Hearing Aid Fitting

• Due to the critical objective of providing speech audibility, hearing aids should be evaluated post-fitting to quantify the child's aided access to speech.[23] The method in which speech testing is performed will always reflect the child's developmental age to ensure that the most information can be reliably obtained.
• Children under the age of 7 months: this can be performed using aided cortical evoked potentials (CAEPs), although the research in this area is currently developing.[26] Simple distraction techniques can be employed for certain populations but should be interpreted with caution.
• Children who can be tested via visual reinforcement audiometry (VRA): speech phonemes such as Ling-6 sounds can be played in the soundfield, and the aided child is assessed for speech audibility per visual reinforcement audiometry techniques. It is important to note that this evaluation method does not test intelligibility or discrimination but detection alone.
• Speech testing for older children: there is a wide range of available speech test materials that can be used to assess aided speech intelligibility, recognition, and discrimination and which are age-appropriate for the child. Care should be taken to observe phonemes consistently confused or poorly detected by the user. In these cases, additional fine tuning can be performed to increase the intensity of gain delivered at different frequencies to avoid overlapping the highest formant of a speech phoneme, which often results in consonant confusion.

Complications

Due to the non-invasive nature of hearing aids, their associated clinical complications are low. Hearing aids are often issued to patients with syndromic and anatomical features, which may cause higher complication rates in invasive interventions such as T-tube insertion and implantable devices like bone-anchored hearing aids.[27] Issues surrounding hearing aids in children may pertain more to their consistent usage post-fitting rather than within the logistics of their fitting in the clinic. 

Professionals are key in providing families with the appropriate pre-fitting counseling, device management skills, and post-fitting support to ensure that hearing aids are utilized optimally.[28] Data-logging records can be used in the weeks and months after fitting to quantify patient adherence and assess whether further fine-tuning, troubleshooting, or counseling is required. 

Clinical Significance

Digital hearing aids and the implementation of newborn hearing screening programs worldwide have been instrumental in reducing the long-term effects of deafness in children.[29][30] When performed by qualified professionals in adherence with local and national protocols, hearing aids show improved language, educational and psychosocial outcomes.

Enhancing Healthcare Team Outcomes

Interprofessional and multidisciplinary teams around children who use hearing aids are critical for them to achieve optimal benefits from hearing aids. A team approach includes the audiology professionals responsible for assessing and fitting the child, early interventionists/teachers of the deaf who work closely at home and in schools with the child to provide tailored (re)habilitation programs, and speech and language therapists, among others who will support the child in using their aided speech access to develop speech and language in line with their normal hearing peers.In addition to supporting the child, working collaboratively with parents/guardians and families will promote smooth uptake of the devices and encourage good engagement with services.