Conductive/mixed/sensorineural hearing loss
Bone conduction hearing devices:
Percutaneous
Transcutaneous (active/passive)
Active middle ear implants:
Semi-implantable
Totally implantable
Severe-to-profound sensorineural hearing loss
Cochlear implant
Auditory brainstem implant
15.2 Bone Conduction Hearing Devices
15.2.1 Physiology of Hearing through Bone Conduction
Conventional hearing by air conduction (AC) requires sound collection into the ear canal, where the sound produces vibrations of the tympanic membrane. The mechanical vibrations are, in turn, transmitted across the middle ear by the ossicular chain, producing sound pressure changes within the cochlea. These sound pressure changes move the basilar membrane and excite the sensory cells within the organ of Corti. Bone conduction (BC) sound transmission involves multiple pathways, which ultimately results in similar changes in the cochlea. 2
BC hearing devices (BCHDs) should be considered when there is good cochlea function but failure to gain benefit from appropriately fitted acoustic hearing devices, or when these devices cannot be fitted, for example, for anatomical reasons such as microtia or atresia of the external ear canal.
There are currently two broad categories of BCHDs:
Percutaneous: these involve penetration of the skin by a titanium abutment, which is anchored to the skull by an osseointegrated implant. An audio processor (AP) is fitted to the abutment as an ear level or body-worn device. Bone-anchored hearing aids (BAHAs) are in this category.
Transcutaneous: by definition, the processors are not directly attached to the bone. Passive systems rely on implanted magnets which attract the external processor. This is termed as “skin drive.” Active systems involve the implantation of a transducer fixed to the bone, giving a “direct drive” to the cochlea. The external processor is again held in place by a magnet.
15.2.2 Clinical Indications for Bone Conduction Hearing Device
Congenital Conductive, Mixed, or Sensorineural Hearing Losses
Typically, these are children with congenital microtia and aural atresia (CAA) where reconstructive surgery is not feasible. Jahrsdoerfer et al 3 developed a grading scheme based on the preoperative temporal bone computed tomography (CT) scan and the appearance of the external ear in an effort to select those with the greatest chance of surgical success. They recognized that surgery for congenital aural atresia was difficult and unpredictable. Bouhabel et al concluded that BAHAs were a safe and efficient therapeutic option, with significantly better audiological outcomes when compared to unaided external auditory canal reconstruction for patients with CAA. 4
Acquired Conductive, Mixed, or Sensorineural Hearing Losses
Acquired indications would commonly include chronic otitis externa, some cases of persistent and recalcitrant otitis media with effusion, chronic suppurative otitis media, and the effects of trauma to the external ear.
Unilateral Hearing Loss
It is now recognized that even a mild unilateral loss in children should not be disregarded as it can have an adverse impact on development and on educational achievement.
Conventional amplification with hearing aids, and in some circumstances BCHD, may be offered under the supervision of an experienced pediatric audiologist. BCHD may be indicated in children following a trial period of a Softband (Oticon Medical; ▶ Fig. 15.1) with their active participation.
Fig. 15.1 Child wearing a Softband with a bone conduction hearing device. (Reproduced with permission from Oticon.)
Children with Special Needs
A number of children have congenital ear abnormalities as part of a syndrome or within the context of significant comorbidity (see ▶ 5). These children may have complex medical, social, and educational needs. Conventional aiding can be challenging, with problems around fitting and compliance. Children with Down’s syndrome are at particular risk for some degree of hearing impairment and many will benefit from early use of BCHDs.
15.2.3 Selection of Children
There are many issues to consider before performing surgery. Selection should be addressed on a case-by-case basis by a multidisciplinary team (MDT). It would be accepted practice in most European countries to consider surgery from a minimum of approximately 4 years of age, following trialing of children with Softbands ( ▶ Fig. 15.1) or BC headbands. In the United States and Canada, regulatory indications suggest this type of surgery for children 5 years and older. The main reason for this delay is to allow appropriate assessment and skull growth. Currently, there is no convincing evidence for earlier surgical intervention in children with such congenital hearing loss, 5 but amplification using more conventional aids is, of course, still used in the interim period.
Audiological Criteria
The audiological parameters for fitment must always be fulfilled as appropriate for the device selected. Selection is typically based on BC thresholds at 500 Hz, 1 kHz, 2 kHz, and 3 kHz. Some ear level devices ( ▶ Fig. 15.2) can be worn to levels equal to or better than 55 dB.
Body-worn processors such as the BAHA Cordelle (Cochlear; ▶ Fig. 15.3) increase the fitting range thresholds to ≤65 dB.
There are continued developments with these devices, so it is always advisable to look at up-to-date fitting data and shifting audiological selection criteria.
The preliminary assessment of audiological performance using a headband is very helpful. Transcutaneous devices tend to reflect what is obtained using the headbands, but allow up to 15 dB in BC, especially in the higher frequencies, when fitting the processor on a percutaneous abutment.
Looking into the benefit of bilateral implants, improvements in hearing thresholds, sound localization, and speech perception have been reported. 6 Binaural fitting is preferred for bilateral conductive loss when the BC thresholds do not vary by more than 10 dB in the higher frequencies of 3 and 4 kHz.
In cases of unilateral hearing loss (single-sided deafness [SSD]), the contralateral ear should have a BC average of ≤20 dB HL to reduce the head shadow effects. Reports show good improvement in hearing in noise and even with mild-to-moderate losses in the contralateral ear. 7, 8
Fig. 15.2 Main components of a percutaneous bone conduction hearing device. 1, Ear level processor; 2, abutment; 3, bone implant. (Reproduced with permission from Cochlear.)
Fig. 15.3 Cordelle body-worn bone conduction. 1, Body-worn unit housing the microphone; 2, lead; 3, coupling/transducer to abutment. (Reproduced with permission from Cochlear)
15.2.4 Percutaneous Devices
There are currently two devices: the Baha (Cochlear) and the Ponto bone-anchored hearing system (Oticon Medical/Neurelec). Both are semi-implantable, with a titanium osseointegrated fixation into the skull and a skin-penetrating abutment to facilitate attachment for the sound processor aid ( ▶ Fig. 15.4).
Baha has been commercially available from 1984 and Oticon devices since 2009. Both systems can be trialed with the processor worn on a latex-free Softband ( ▶ Fig. 15.1) or headband. 9 This allows early amplification for children considered too young for surgical fitment of the implant/abutment. Softbands are also available for binaural fitting.
Fig. 15.4 Percutaneous abutment with direct sound transmission to the cochlea. 1, External processor; 2, abutment and implant. (Reproduced with permission from Cochlear.)
Surgery
There are two main goals of surgery:
To optimize osseointegration.
To prepare the implant site to minimize the occurrence of soft-tissue reactions. Recently, there has been a significant change from previous tissue reduction procedures to a simpler nontissue reduction using a “punch” technique with or without a minimal incision parallel or extension of the “punch” site ( ▶ Fig. 15.5). Measuring skin thickness aids selection of the use of the most appropriate length of abutment.
Two-stage surgery has been recommended for children up to approximately 10 years of age, with 3 or more months between stages to allow for osseointegration. 10 Some centers advocate insertion of a second or “sleeper” fixation. However, with newer designs in abutment technology, single-stage surgery is now commonly performed. Early results indicate higher stability and faster osseointegration with newer implants both in adults and children. 11 With reduction in integration time, there is earlier loading with a processor.
Fig. 15.5 Abutment in position using minimal approach.
Outcomes
The audiological outcomes should reflect the preoperative assessments with a slight improvement due to the direct coupling, and a lack of attenuation due to the skin, which can account for approximately 10 to 15 dB.
The main recognized drawbacks of the percutaneous abutment relate to varying degrees of soft-tissue reactions ( ▶ Table 15.2). These can occur in approximately a third of patients.
With improved implant design, using a curved abutment and tight connections between the implant components, such reactions are becoming less common. 13, 14 Similarly, hydroxyapatite coating of the abutment to allow soft-tissue integration and reduced pocket formation around the skin penetrating abutment show promising results in preclinical studies. 15
Loss of implants due to failure of integration and trauma are more common in the pediatric population as compared with adults. In children, the figures vary depending on the age at initial implantation and the group of medical conditions involved. Kraai et al 16 reported that obesity and adverse socioeconomic factors appeared to contribute to a higher risk for complications. Frequent follow-up and meticulous care of the implant site may minimize complications.
Grade | Description |
0 | No irritation and slightly red, < 1 mm from the implant: epithelial debris removed if present |
1 | Red 1 mm or more from the implant: temporary local treatment indicated |
2 | Red and moist: no granulation tissue present |
3 | Red and moist with granulation tissue, skin overgrowth, or scar formation: local treatment indicated |
4 | Extensive soft-tissue reaction: could require implant removal |
15.2.5 Transcutaneous Devices
Transcutaneous devices can be classified into those with either passive or active internal transducer systems. Passive systems rely on the processor sending vibrations through the skin to an internal magnet system, which, in turn, keeps the processor in place. This has been termed skin drive. Conversely, active devices induce the surgically implanted system, which is fixed directly to the bone, to vibrate. The external processor does not produce any movements.
Two passive systems are available for use with children: Sophono (Medtronic PLC) and the more recently introduced Baha Attract (Cochlear). Such systems are known as “skin drive.”
The Bonebridge is an active semi-implantable internal device produced by MED-EL (Vibrant Bonebridge).
Transcutaneous “Passive” Systems
Sophono
The Sophono Alpha 2 MPO bone-anchored hearing system comprises a surgically implanted internal plate that houses two magnets hermetically sealed in a titanium case. This internal component is attached to the mastoid bone behind the ear. It is completely passive and placed under the skin in a simple single-stage procedure. The external digital sound processor houses a bone oscillator and uses a metal disc and spacer (a “shim”) to magnetically couple to the internal component and deliver auditory stimulation through the closed skin. Siegert 17 has reported on 100 patients whom he had implanted. The additional benefits of such a system are reduced risks of injury or inflammation and less of the psychological problems associated with the more prominent percutaneous abutments. 18
Audiological Criteria
The Sophono is approved by the U.S. Food and Drug Administration and in Europe by Conformité Européenne (CE) Mark, for any type of conductive and mixed hearing loss with BC thresholds of ≤45 dB. The system is designed for patients 5 years of age and older. Criteria for SSD include patients with pure-tone average of ≤20 dB in the contralateral ear measured at 0.5, 1, 2, and 3 kHz.
Surgery
The principle involves fixing twin rare earth magnets encapsulated in titanium to the skull ( ▶ Fig. 15.6). To reduce attenuation, the skin flap should not be thicker than 4 mm. This is not usually a problem in children. The external Alpha auditory processor is held in position by twin magnets of similar geometry to those implanted.
Fig. 15.6 Sophono magnets secured in position.
Outcomes
The external processor is typically fitted at 4 weeks postoperatively, allowing the tissues to settle. To minimize any skin reaction or discomfort, close attention to the magnetic coupling needs to be taken into consideration. The audiological outcomes by the nature of the device are similar to levels gained when using the processor on a soft headband. Case series have shown improvement in hearing over unaided conditions. 19, 20
Baha 4 Attract
This is a BC implant system that uses magnet retention to connect the Baha sound processor with the osseointegrated Baha BI300 implant. The same implant is used for Cochlear’s percutaneous system. The BI300 implant and the implant magnet are placed entirely under the skin, providing a more cosmetically appealing design. The sound processor is attached with a single external sound processor magnet. By wearing a SoftWare pad, the force of the sound processor magnet allows the distribution of contact pressure evenly on the skin.
Audiological Criteria
The Baha 4 Attract System is approved for adults and children. In the United States and Canada, the system is not currently approved for children below the age of 5 years.
The audiological indications for the Baha Attract System are conductive, mild mixed hearing loss, and SSD with a pure-tone average of ≤20 dB in the contralateral ear. The fitting range is based on the performance of the selected sound processor. When evaluating candidates for the Baha Attract System, the outcomes are similar or better than using the same processor on a Softband. 21
Surgery
The surgical steps have the same principles of fitting as the BI300 implant, but with the principle of raising a flap under which will be positioned the internal magnet. Particular care is required to ensure perpendicular placement ( ▶ Fig. 15.7) of the implant so that the magnet when fitted does not contact bone. The tissue over the magnet should not exceed 6 mm. The final position should make appropriate allowances for skull curvature and proximity of the pinna ( ▶ Fig. 15.8).
Fig. 15.7 Surgical placement of a right Baha Attract. (Courtesy of Iain Bruce.)
Fig. 15.8 Baha Attract worn by a young boy. (Courtesy of Iain Bruce.)
Outcomes
Simulation of an Attract processor on patients with established percutaneous systems introduced an attenuation starting from approximately 5 dB at 1,000 Hz, increasing to 20 to 25 dB above 6,000 Hz. However, aided sound field threshold shows smaller differences, and aided speech understanding in quiet and in noise does not differ significantly between the two transmission paths. 22 Transcutaneous systems offer good improvement in pure-tone thresholds and speech reception thresholds. Also, early studies have lower complication rate compared to those featuring the percutaneous. 23
Transcutaneous “Active” Semi-Implantable System
Bonebridge
The Bonebridge (MED-EL) was clinically introduced within Europe in 2011. It is a semi-implantable system. The internal components are a receiving coil linked to the BC floating mass transducer (BC-FMT) that sends sound transmissions direct to the inner ear. The externally worn AP, held in place by magnetic attraction, transfers signals across the skin to the implant. The technology of the Bonebridge draws upon established technology developed for the Vibrant Soundbridge (VSB) system ( ▶ Fig. 15.9).
Fig. 15.9 Complete Bonebridge system with audio processor (SAMBA BB). 1, External audio processor; 2, internal receiver coil; 3, internal electronics (demodulator); 4, internal bone-conduction floating mass transducer. (Reproduced with permission from MED-EL.)
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