Feb 2019 The Hearing Journal

Hearing aid technology has improved significantly over the past 50 years, and today's hearing aids are vastly more sophisticated than those two decades ago. However, only 20 percent of people who could benefit from hearing aids actually use them, and the number of people who own hearing aids but never use them ranges from five to 25 percent depending on the study you read.

auditory

Some patients become easily frustrated with traditional hearing aids. For example, many do not want to be seen wearing a traditional hearing aid and are concerned with the visible stigma of hearing loss; others become frustrated with device issues such as feedback and the need for manual adjustment. Some are displeased with the audio quality of traditional hearing aids, while others find hearing aids to be incompatible with their active lifestyle despite the availability of hearing aid models with notable features like extended wear, better feedback, and water resistance.

So what options are available to those who either can't benefit from traditional aids or simply won't use them? Fortunately, solutions abound.

ACTIVE MIDDLE EAR IMPLANTS

All active middle ear implants (AMEI) share a common feature—a battery that powers a sound processor that turns sound into an electrical signal. The electrical signal drives an implanted actuator that is directly attached to a middle ear structure. AMEIs can be divided into magnetic and piezoelectric crystal strategies. Magnetic devices have an external battery and a sound processor that drives a magnetic implant, which vibrates a middle ear structure. Their power requirements involve weekly battery changes and, consequently, are semi-implantable. Magnetic devices are not MRI-compatible. Alternatively, piezoelectric crystals deform or vibrate when exposed to an electrical current, but, if mechanically deformed, they also produce their own electric current. These crystals tend to be more energy-efficient than magnets, with batteries that can last for years, allowing for totally implantable strategies.

Because AMEIs deliver a signal directly to the middle ear structures, they avoid the sound distortion from the ear canal and speaker typically encountered when using conventional hearing aids. In addition, AMEIs avoid the occlusion effect by keeping the processor out of the ear canal or up against the tympanic membrane. The audiological indications for an AMEI are moderate-to-severe sensorineural hearing loss (SNHL). Thus, patients who are not satisfied with their hearing aids for various reasons may be a candidate for an AMEI. AMEIs can also eliminate the issue of skin irritation in patients who have chronic otitis externa or sensitivities to ear canal moulds. Lifestyle considerations are a significant factor driving interest in completely implantable devices.

In the United States, there are three U.S. FDA-approved AMEIs. The Ototonix Maxum® is a magnetic device with an advanced sound processor worn in the ear canal that drives a magnetic implant placed on the ossicular chain. Surgery can be performed in an outpatient setting under local anesthesia. The MED-EL Vibrant Soundbridge (VSB) is also a magnetic device, but the processor is worn behind the ear on the scalp and is magnetically coupled with the surgically implanted internal device. In the United States, VSB is in the final phases of FDA investigational device studies for mixed and conductive hearing loss indications, but other countries have approved it for indications beyond sensorineural hearing loss. Envoy Esteem, an MRI-compatible piezoelectric device, is the only completely implantable AMEI available in the United States. 

COCHLEAR IMPLANTS

Cochlear implants (CIs) have become a standard means of rehabilitating SNHL. Initially, CIs were approved for adults with bilateral profound SNHL. However, CI candidacy has expanded significantly over time, and implantation is now possible in all age groups and in patients with residual acoustic hearing, pushing the boundaries of electrical stimulation of the auditory system.

The drive to expand CI candidacy has led to a variety of changes in implant design. As it has become apparent that minimising trauma during electrode insertion can better preserve residual low-frequency acoustic hearing, electrode arrays have become smaller and more flexible to facilitate atraumatic insertion into the cochlea. Moreover, devices intended specifically for acoustic plus electric stimulation (i.e., cochlear hybrid) have been designed with a shortened electrode array, theoretically optimising the probability of meaningful hearing preservation. In essence, short electrode arrays leave the apical regions of the cochlea undisturbed to be stimulated acoustically while electrically stimulating the basal region of the cochlea responsible for high-frequency sound. The FDA criteria for a hybrid device permit candidates to have a greater degree of residual acoustic hearing, and though some degree of postoperative hearing loss is expected after implantation, durable preservation of acoustic hearing is possible. Furthermore, quality of life and satisfaction with these devices have been shown to improve significantly.

Many non-traditional CI candidates can benefit from implantation with standard-length electrodes. Fundamentally, CIs provide a rehabilitative option for patients who do not derive sufficient benefit from hearing aids. Though historical candidacy only accounted for patients with complete or near-complete hearing loss in both ears, contemporary patients with asymmetric hearing loss may represent a population whose hearing can improve with binaural, bimodal hearing (i.e., hearing aid in one ear, CI in the other ear). In other words, the presence of a “better” hearing ear should not necessarily disqualify a patient from implant candidacy. Other non-traditional candidates who have potential for benefit are those with longstanding SNHL. Though historical evidence would also suggest that extended periods of auditory deprivation are detrimental to performance with a CI, implants have more recently been shown to potentially benefit pre- and post-lingually deafened patients in this setting. Thus, it would appear that a patient history of prolonged auditory deprivation should not lead CI teams to summarily dismiss candidacy.

CI FOR SINGLE-SIDED DEAFNESS

Until recently, researchers, surgeons, and audiologists opined that patients with single-sided deafness (SSD) could not benefit from a CI since the discrepancy between acoustic and electric input would be disruptive to auditory processing. The first reports of CI for SSD approximately 10 years ago demonstrated tinnitus reduction, restoration of the head shadow effect, and significant subjective benefits. Several years later, Arndt, et al., demonstrated that patients with SSD showed improved outcomes for speech perception with a CI compared with CROS aids and bone conduction implants (BCI). Similarly, Zeitler, et al., reported that children and adults with CI for SSD showed not only significant improvements in speech recognition even in challenging listening conditions but also improved localisation accuracy with a CI compared with BCIs and CROS aids. Interestingly, some patients were able to localise with an accuracy close to that of normal-hearing listeners. Importantly, these studies showed no detrimental effect of the CI on the normal-hearing ear.

Two large studies evaluating the benefits of CI for SSD have been published in the United States. Hansen, et al., reported on 29 subjects, and although there was some variability in objective outcomes, the mean word and sentence recognition scores improved significantly. Most subjects demonstrated significant benefit in sound localisation accuracy, with scores improving with increasing duration of CI use. These outcomes were corroborated by Sladen, et al., who reported on children and adults with CI for SSD, again showing significant improvements in both word and sentence recognition scores in all patients.

Much of the literature reporting on the benefits of CI for SSD has examined adults and children with acquired, post-lingual deafness. There is a lack of information on the benefit of CI for congenital SSD in children. Studies have demonstrated that children with congenital bilateral deafness derive greater objective outcomes following CI if surgery is performed before 12 months of age. To date, it is not known if the same critical period exists in children with congenital SSD or if the one functional, normal-hearing ear is able to sustain bilateral cortical maturation and prolong the critical period either temporarily or indefinitely. Sharma and colleagues reported that despite undergoing CI for SSD beyond the critical period of auditory cortical development, some children showed reversal of cross-modal plasticity even after many years of unilateral deafness.

The advancement of technologies and results of ongoing studies will afford options for clinicians to share with those patients who, in the past, had difficulty finding effective, satisfactory solutions.

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