Conventional external hearing aids can be generally subdivided into air-conduction hearing aids and bone-conduction hearing aids. Air-conduction hearing aids require the use of ear molds, which may be problematic for individuals with chronic middle ear and ear canal infections, atresia of the external canal, or an ear canal that cannot accommodate an ear mold. In these cases, bone-conduction hearing aids may be an alternative option.
BONE-CONDUCTION HEARING AIDS
External bone-conduction hearing aids function by transmitting sound waves through the bone directly to the inner ear. The external devices must be closely applied to the temporal bone, with a steel spring over the top of the head, a headband, or with the use of a spring-loaded arm on a pair of spectacles. The purpose of this type of hearing aid is to bypass the middle ear and transmit energy directly to the cochlea. These devices may be associated with pressure headaches or soreness.
BONE-ANCHORED HEARING AIDS
In contrast to external hearing aids, bone-anchored hearing aids are surgically implanted hearing devices. They consist of an external sound processor and a titanium implant, which is attached to the temporal bone of the skull. Bone-anchored hearing aids transmit sound directly to the inner ear through the bone, thus bypassing the external auditory canal and middle ear. While the bone-anchored hearing aids were originally used for individuals with conductive hearing loss and mixed hearing loss, in 2002, the FDA approved their use in individuals with unilateral sensorineural deafness. In these cases, a bone-anchored device located near the deaf ear works as a transcranial contralateral routing of signal (CROS) to transmit sound to the contralateral functional cochlea via bone conduction. This application has been evaluated as an alternative to the traditional air-conduction CROS hearing aid. Bone-anchored hearing aids are based on osseointegration, the process through which living tissue integrates with titanium in the implant, allowing amplified and processed sound to be conducted via the skull bone directly to the cochlea. Individuals must have sufficient bone integrity and mass to undergo the implant procedure.
PARTIALLY IMPLANTABLE MAGNETIC BONE-CONDUCTION HEARING SYSTEMS
Partially implantable magnetic bone-conduction hearing systems are available as an alternative to bone conduction hearing systems connected percutaneously via an abutment. With this technique, acoustic transmission occurs transcutaneously via magnetic coupling of the external sound processor and the internally implanted device components. The bone-conduction hearing processor contains a magnet that adhere externally to magnets implanted in shallow bone beds with the bone-conduction hearing implant. Since the processor adheres magnetically to the implant, there is no need for a percutaneous abutment to physically connect the external and internal components. To facilitate greater transmission of acoustics between magnets, skin thickness may be reduced to 4 to 5 mm over the implant when it is surgically placed. The first partially implantable magnetic bone-conduction device, the Otomag Bone Conduction Hearing System, received 510(k) clearance from the FDA in 2011, followed by the BAHA Attract in 2013.
In a retrospective case series, Siegert (2011) reported on more than 100 individuals who had Otomag devices implanted over a 5-year period. While effectiveness outcomes related to hearing were not reported on, the author noted that except for temporary pressure marks in 4 percent of individuals, there were no other complications. Siegert concluded that the safety of the Otomag device system was significantly improved compared to conventional or percutaneous bone conduction hearing aids. Clinical and audiologic follow-up was ongoing, and results were simply the preliminary results (i.e., after fitting) of 12 individuals from the initial group of more than 100 individuals. The study is limited in its design, small sample size, short-term follow-up period, and lack of an appropriate comparative control group, which limits the validity of the study results.
Two prospective trials evaluating different transcutaneous systems were identified. Both trials were small (27 and 15 individuals, respectively), but both demonstrated improvements in hearing outcomes. Briggs et al (2015) reported on a prospective interventional evaluation of the percutaneous, partially implantable Baha Attract System among 27 adults with a conductive hearing loss (CHL) or mild mixed hearing loss in the ear to be implanted. The choice of sound processor was based on patient preference and hearing tests with various sound processors in conjunction with a Baha Softband prior to device implantation. All 27 patients enrolled received an implant. Sound processor fitting occurred 4 weeks postimplantation in all but 1 patient. At 9-month follow-up, pure-tone audiometry (mean of 500, 1000, 2000, and 4000 Hz) was significantly improved with the implant and sound processor compared with unaided hearing (18.4-dB hearing loss; SD=6.9 dB; p<0.001). Patients generally showed improvements in speech recognition in noise, although comparing results across test sites was difficult due to different languages and methodologies used for testing speech recognition at each site. Compared with the preoperative unaided state, scores on the Abbreviated Profile of Hearing Aid Benefit (APHAB) overall score (p=0.038) and reverberation (p=0.016) and background noise (p=0.035) subscales improved.
Denoyelle et al (2015) reported on a prospective trial of the Sophono device in children ages 5 to 18 years with uni- or bilateral congenital aural atresia with complete absence of the external auditory canal with pure CHL. The study included a within-subject comparison of hearing results with the Sophono devices to those obtained with the Baha Softband preoperatively. All 15 patients enrolled were implanted (median age, 97 months). At 6-month follow-up, mean aided air conduction (AC) pure-tone audiometry was 33.49 (mean gain, 35.53 dB), with a mean aided sound reception threshold of 38.2 (mean gain, 33.47 dB). The difference in AC pure-tone average (PTA) between the Baha Softband and the Sophono device was 0.6 dB (confidence interval upper limit, 4.42 dB), which met the study’s prespecified noninferiority margin. Adverse effects were generally mild, including skin erythema in 2 patients, which improved by using a weaker magnet, and brief episodes of pain or tingling in 3 patients.
MIDDLE EAR HEARING AIDS
Implantable middle ear hearing devices transmit acoustic energy directly to the mobile structures within the middle ear space through the use of a vibrational transducer, thereby bypassing the external auditory canal. Compared to traditional air conduction hearing aids, implantable middle ear hearing devices are said to eliminate outer ear occlusion (i.e., the feeling of having a plugged ear canal) and feedback problems. They have the potential to provide greater amplification at higher frequencies and improve speech perception in the presence of background noise. They are theorized to provide a more natural sound compared to conventional hearing aids. Implantable middle ear hearing devices are generally classified as semi-implantable or totally implantable.
SEMI-IMPLANTABLE MIDDLE EAR DEVICES
Semi-implantable middle ear devices contain both externally and internally worn parts. The external parts are placed behind the ear. The devices use the periodic attraction and repulsion of two magnetic fields (one electromagnetic and one static magnetic) to cause vibration of the ossicles. This ultimately transmits sound to the inner ear. FDA-approved in August 2000, the Vibrant Soundbridge System™ is an alternative to traditional hearing aids for adults with a moderate to severe sensorineural hearing loss (i.e., occurs from damage to the inner ear, the auditory nerve, or the brain). The Soundtec Direct Device Hearing System™ was approved in September 2001 for the same indications, although it was subsequently discontinued in 2004. The Maxum System™ was approved in October 2009 and represents an upgrade over the Soundtec Direct Device Hearing System™.
In a prospective study as part of a phase II clinical trial, Hough et al. (2002) evaluated the safety and effectiveness of the Soundtec Direct System (since discontinued and now marketed as the Maxum System). One hundred and three individuals from 10 sites across the U.S. with bilateral moderate to moderately severe sensorineural hearing impairments received therapeutic intervention, including semi-implantable device implantation. Outcome measurements were taken after a 10-week healing period and included functional gain, speech recognition in quiet and noise, articulation index scores, perceived aid benefit, sound quality judgment, individual satisfaction, and the presence of feedback and occlusion. Compared with an optimally fit hearing aid, there was a statistically significant average increase of 5.3 percent in speech discrimination. While the mean speech perception in noise test scores were improved, it was not statistically significant. The authors concluded that the Soundtec Direct System provided statistically significant reductions in feedback and occlusive effect, as well as statistically significant improvements in functional gain, articulation index scores, speech discrimination in quiet, perceived aid benefit, satisfaction, and device preference. The study is limited in its relatively small sample size and short-term follow-up period.
In a systematic review, Tysome et al. (2010) evaluated whether semi-implantable middle ear implants improved hearing as much as hearing aids. Of 644 studies, 17 met the inclusion criteria for adults with a sensorineural hearing loss, and outcome measurements compared middle-ear implants with conventional hearing aids. Middle-ear implant outcome analyses included residual hearing, complications, and comparison to conventional hearing aids in terms of function gain, speech perception in quiet and in noise, and validated patient-reported outcome measurement questionnaires. The authors reported that the quality of the studies were moderate to poor, with short follow-up periods. The authors stated that the evidence supports the use of middle-ear implants because, overall, the semi-implantable middle ear implants did not decrease residual hearing, resulted in a function gain in hearing comparable to conventional hearing aids, and may improve perception of speech in noise and sound quality. The authors recommended that additional follow-up studies were warranted, with long-term results comparing middle-ear implants with conventional hearing aids. The systematic review is limited by the heterogeneity and poor quality of the included studies.
The limited data suggest semi-implantable middle ear hearing aids may provide marginal improvement in hearing compared with conventional external acoustic hearing aids in individuals with sensorineural hearing loss. However, given the safety and effectiveness of external acoustic hearing aids and the increased risks inherent in a surgical procedure, the semi-implantable device must be associated with clinically significant improvement in various hearing parameters compared with external hearing aids. While safety concerns appear to be minimal, only a limited number of individuals have been included in the clinical trials, and few have completed more than 1 year of follow-up. Given the small number of individuals and the limited safety data, risks cannot be adequately evaluated and compared with the marginal improvement in hearing. Studies on individuals with conductive or mixed hearing loss and aural atresia, when external acoustic hearing aids are not an option, have also demonstrated hearing benefit with semi-implantable middle ear hearing aids. However, these studies are few and limited to small numbers of individuals. Therefore, conclusions on the safety and effectiveness of semi-implantable hearing aids in these individuals cannot be made, and further study with longer term follow-up is needed. Comparisons of semi-implantable devices with alternative hearing devices such as implantable bone conduction and bone-anchored hearing aids would also be useful to determine device appropriateness for individuals who are unable to use external air-conduction hearing aids. Due to the lack of adequate safety data in broader populations over a longer period of time, semi-implantable middle ear hearing aids are investigational for all indications.
In 2013, Butler et al published results of a systematic review of comparative studies evaluating partially and fully implantable (discussed in following section) middle ear hearing devices for sensorineural hearing loss. The review included 14 studies, none of which were randomized controlled trials, 13 of which evaluated a semi-implantable device, most often the Vibrant Soundbridge, with 1 study evaluating the Envoy fully implantable system. Outcomes reported across studies were heterogeneous. Among the 9 studies that reported on the primary outcome (functional hearing gain), 1 found that middle ear implants were statistically significantly better than hearing aids, 1 found that hearing aids were statistically significantly better than implants, and 6 studies found that middle ear implants were better than hearing aids, but without a clinically significant difference. The authors concluded that middle ear implants were at least as effective as hearing aids in improving hearing outcomes.
A systematic review by Kahue et al (2014) evaluated studies of 3 FDA-approved middle ear hearing aids, the Vibrant Soundbridge, the Maxum System, and the Envoy Esteem (discussed in the following section). Studies eligible for inclusion addressed purely sensorineural hearing loss, had at least 5 implanted ears, and reported comparative data between preoperative and postoperative audiometric performance. Seventeen studies (503 ears) were included, 3 of which evaluated the Soundtec System (now Maxum System, 190 ears), 5 of which evaluated the Envoy Esteem (102 ears), and 9 of which evaluated the Vibrant Soundbridge (211 ears). The 14 studies comparing preoperative unaided hearing and postoperative middle ear implant-assisted hearing demonstrated improvement in hearing thresholds (weight mean, 25.2 dB improvement; range, 15.6-48.2 dB). However, for the 12 studies that compared the best aided preoperative condition with the postoperative assisted performance, the functional gain was smaller (weighted mean, 8.1 dB improvement; range, -9.4 to 13 dB), and only 1 reported statistically significant improvements over optimally fitting hearing aids. Similarly, studies that compared the preoperative unaided condition and the postoperative middle ear implant-assisted hearing demonstrated improvements in speech recognition (weighted average, 44.8% improvement; range, 8.8%-64.0%), while speech recognition was similar for the middle ear implant-assisted condition and best aided preoperative condition. Ten studies reported on safety outcomes, including 5 studies that focused on partially implantable middle ear implants; in those studies 15 (11.4%) of 132 implants had a malfunction that led to explantation.
One series with long-term follow-up (mean, 7.5 years) focused on middle ear implants in patients who failed external hearing aids. Zwartenkot et al (2013) described outcomes for 33 patients with moderate-to severe sensorineural hearing loss who had severe chronic otitis externa and were implanted with the Vibrant Soundbridge system or the Otologics MET system, a middle ear implant system not available in the United States. Compared with baseline, at long-term follow-up, subjects had statistically significant improvements in total scores on the Abbreviated Profile of Hearing Aid Benefit (63.3 at baseline vs 55.6 at follow-up, p<0.05). Eighty-five percent of subjects reported wearing the device more than 4 hours a day.
In summary, the evidence for use of semi-implantable middle ear hearing aids includes the clinical trials that supported FDA approval of the Vibrant Soundbridge and the Soundtec devices, along with a large number of observational series. Most available studies address the Vibrant Soundbridge device. For the use of semi-implantable middle ear hearing aids in patients with sensorineural hearing loss, the body of evidence suggests that these devices may be associated with a modest improvement in functional gain compared with external hearing aids, with similar improvements in speech recognition scores. Case series reporting on alternative coupling methods for the Vibrant Soundbridge for patients with conductive or mixed hearing loss (off-label uses) also report improved hearing thresholds and word recognition. Although the devices appear to have a good safety profile in the short term, studies in larger series reporting on longer term durability, safety, and efficacy are needed to permit conclusions about the devices’ risks and benefits relative to external hearing aids.
TOTALLY IMPLANTABLE MIDDLE EAR DEVICES
Totally implantable middle ear devices are used to treat moderate to severe hearing loss caused by defective inner ear function. The Esteem® system (FDA approved in March 2010) includes three implantable components (sound processor, sensor, and driver), in addition to external testing and programming instruments. To implant the device, the Esteem® requires disruption of the normal middle ear structure by partially removing the incus. Unlike hearing aids, this device does not use a microphone or speaker.
In a prospective, non-randomized “controlled” (each individual was their own control) trial, Kraus et al. (2011) evaluated the safety and effectiveness of the Esteem® system in 57 individuals with bilateral, mild to severe sensorineural hearing loss, with discrimination greater than 40 percent. Outcome measurements included hearing-related outcomes and the number of adverse events. Speech reception thresholds (minimum intensity in dBs at which an individual can understand 50 percent of spoken words) improved from 41.2dB to 29.4dB with the Esteem®. There were 6 serious adverse device events (2 wound infections, with one requiring explantation), 1 delayed facial paralysis, and 3 revisions due to limited benefit. The authors concluded that 12-month follow-up demonstrated that hearing results with the device were statistically significantly superior to baseline best-fit hearing aids. The study is limited in its small sample size, short-term follow-up period, and lack of a separate comparative control group.
In a systematic review, Klein et al. (2012) examined the safety and effectiveness of fully implantable middle ear devices for the treatment of hearing loss. Thirty studies were selected for full review, of which 7 studies were on the Esteem® system (representing 105 individuals) and 13 were on the Carina Fully Implantable Hearing Device (representing 68 individuals). The Carina Fully Implantable Hearing Device is not yet FDA approved. Complication rates with the Esteem® system were higher than with the Carina device. With regard to the Esteem® system, the most common adverse events included chorda tympani nerve damage or taste disturbance (30 percent). Facial weakness was also reported in 8 percent of individuals and was permanent in 2 individuals. Device failure was common with the Carina device, mainly due to charging difficulties. For both devices, clinically significant improvements in function gain, speech reception, and speech recognition over the unaided condition were reported. In studies comparing the fully implantable middle ear devices to conventional hearing aids, findings were mixed. Although improvements in functional gain were similar to those of conventional hearing aids, speech recognition and quality of life were reportedly greater with the implants. The authors concluded that despite the limited evidence, fully implantable middle ear devices seem to offer a relatively safe and effective treatment option, particularly for individuals who are medically unable to wear conventional hearing aids. However, they noted that individuals should be informed of the relatively high risk of surgical complications involved with device implantation, particularly with the Esteem® device. The study is limited in the heterogeneity and low quality (e.g., small sample size, short-term follow-up) of its included studies.
In 2014, Pulcherio et al reported results of a systematic review of studies of 2 fully implantable middle ear hearing devices: the FDA-approved Esteem device and the Carina device, which does not have FDA approval. The review included 22 studies with a total of 244 patients, 134 implanted with the Esteem device and 110 with the Carina device. No randomized controlled trials were identified, and most studies were small, with the largest series including 57 subjects and 12 series including fewer than 10 subjects. All studies showed improvement of sound field threshold from unaided to aided conditions with the fully implantable device, but the magnitudes of the improvements varied.
A case series published since the Pulcherio and the Klein systematic reviews reported high rates of facial nerve palsies (10/34 subjects [29.4%]) after implantation of the Esteem device, which persisted to 3 months of follow-up in 6 (17.6%) of 34 subjects.
In summary, the evidence for use of fully implantable middle ear hearing aids includes the clinical trial that supported FDA approval of the Esteem device, along with systemic reviews and small observational series that report short-term results. These studies generally found improved hearing over unaided hearing, with modest improvements over hearing with best-fit aids. The available evidence for use of fully implantable middle ear hearing aids, however, is insufficient to demonstrate long term improvement in net health outcome. Concerns exist about adverse events with these devices. Therefore, fully implantable middle ear hearing aids are considered investigational.