Operative techniques in Otolaryngology: Hearing restoration options

The pathophysiology of presbycusis is complex and is a result of an intricate interaction between genetic factors and environmental exposures. Based on the anatomy of hearing, there are several mechanisms by which hearing loss can develop. ARHL is typically a pathology of the inner ear (Figure 1) including the cochlea, which consists of the scala tympani, scala vestibule, and scala media (Figure 2). Common etiologies include but are not limited to cochlear degeneration, hair cell loss, and stria vascularis atrophy. Sensory presbycusis is typically associated with loss of hair cells in the basal aspect of the cochlea and results in a predominant high-frequency hearing deficit.^, This is the most common type of presbycusis seen in the elderly population. Another mechanism by which hearing loss occurs is via atrophy of the stria vascularis, known as strial presbycusis. These patients typically present with flat pure-tone thresholds. Neural presbycusis is characterized by reduced speech perception and is due to loss of auditory nerve fibers. A fourth type of presbycusis, known as cochlear presbycusis, is a result of damage or change in the microstructures of the cochlea. While different types of presbycusis exist, the majority of presbycusis seen in the elderly is likely due to a mixture of these individual subtypes and the extent of disease is dependent on the degree of damage to the different structures involve in hearing.

Restoring hearing loss at this point in time is limited to conductive etiologies (eg, stapedectomy for otosclerosis). Age-related hearing loss, which is sensorineural in nature, cannot be restored, but multifaceted treatment and rehabilitation options are available. One of the most pivotal factors in managing hearing loss in the elderly is the use of amplification devices referred to as hearing aids. Although hearing aids do not require surgical intervention, they are addressed here because candidacy for operative treatments for presbycusis, such as cochlear implants, is conditioned on a trial of hearing aids. Specifically, a patient is considered a CI candidate if no discernable communication benefit from a hearing aid is demonstrated.

A hearing aid functions as an amplifier of sound and is comprised of multiple parts; a microphone, amplifier, battery, receiver, and control buttons. The microphone receives sound waves and transforms them into electrical signals which are increased in intensity by the amplifier, and converted back to sound waves by the receiver. The functionality of hearing aids can range widely. They can contain manual or automatic adjustment. Advanced hearing aids have been developed over the last decade based on digital signal processing which allows for frequency specific amplification, increased noise cancellation, precise directionality, and decreased feedback.^, Advancements in wireless integration have allowed for improved special perception and hearing in noisy environments due to bilateral directionality, and the ability to connect to smartphones, televisions, and music streaming devices.^, These improvements in hearing aid technology provide a wide range of options to better individualize therapy. Hearing aids can be categorized into those which are implantable, and those which are not. There are many different options for hearing aids, and understanding their differences is crucial to providing the best care for patients suffering from hearing loss.

While hearing aids made to fit within the ear canal are fantastic for sound quality, and for eliminating the stigma behind hearing aids, they tend to pose problems for the elderly who may have manual dexterity issues. Another hearing aid, known as The Lyric (Phonak, Zurich, Switzerland), attempts to solve this issue. The Lyric was developed as a completely in the canal hearing aid that is placed about 3-4 mm from the tympanic membrane. What differentiates this product is the decreased need for manual manipulation by the patient. This hearing aid is an extended-wear hearing aid and is the first of its kind as it allows patients to keep the hearing aid in place for 3-4 months after insertion before it requires management.^, However, this device is not without its faults. Firstly, the device is relatively expensive. Furthermore, due to its location in the ear canal long term, the device is susceptible to damage due to build-up of ear wax. Despite these possible concerns, The Lyric remains an effective hearing aid and when applicable, can help to minimize many of the issues associated with other hearing aid styles.

The middle ear implant (MEI) is another option used in patients with moderate to severe hearing loss. These implants are typically used in patients who have tried other hearing aids but experienced irritation or discomfort due to the feeling of having a hearing aid or the feedback often associated with hearing aids. These are useful in patients whose hearing loss may include a conductive component, as well. The middle ear implant functions by stimulating the ossicular chain directly. A retrospective case review done by Shohet et al evaluated hearing and safety in patients with sensorineural hearing loss who received a full middle ear implant for treatment. They found that, not only did the implant provide more hearing benefit with minimal side effects, many subjects also had resolution of their tinnitus following implantation. A study performed by Ter Haar et al evaluated the effects of middle ear implants on ARHL in dogs. This study found that hearing improved with the implant and the procedure did not negatively affect residual hearing in these dogs. These middle ear implants are very effective and have been shown to significantly improve hearing loss in the elderly (>60 years of age) population. These implants can be divided into those which are fully or partially implantable (Figure 2), with the fully implanted devices having the advantage of constant use and ability to wet the ear without any issues. Furthermore, this implant offers a unique advantage of minimal handling needed by the patient as the battery only needs to be changed approximately every 5 years.^,, Of note are 2 specific fully implantable devices; the Envoy Esteem and the Carina. The Envoy Esteem is typically reserved for those with moderate to severe hearing loss (40-90 dB). The system contains no external component, and operates using piezoelectric transducers with a sensor on the malleus and a driver on the stapes. The batteries in the device have a lifetime of about 5-9 years. No separate microphone is required for the Envoy as signal occurs via the eardrum.^, In order to route the leads, the procedure requires a mastoidectomy and posterior tympanotomy.^, In order for a patient to qualify for this device, a CT scan must show adequate space within the mastoid to allow transducer placement, and a healthy ossicular chain. The Carina is another fully-implantable device, also utilized for moderate to severe sensorineural hearing loss.^, It has been approved for use in Europe and is currently undergoing trials in the USA for approval by the FDA. The device includes a subcutaneous microphone, digital processor, and a connector to the transducer placed on the incus. The procedure for implantation requires a limited mastoidectomy, antrostomy, and atticotomy. With this device, even if there is damage to ossicular structures, or abnormal middle ear anatomy, tympanoplasty can allow for implantation if the round window is intact. The battery requires daily recharge, but is expected not to need replacement for about 20 years. On occasion, patients have reported issues with feedback when using this device. A study performed by Lefebvre et al showed that feedback decreased significantly overtime and was typically associated with microphone location, the implantation on the posterior inferior mastoid line leading to less feedback. Another category of implantable devices is the semi-implantable, namely, the Maxum and the Vibrant Soundbridge. These devices consist of a receiver below the skin and another portion attached to the ossicles themselves, and an external processor. Sound waves are transmitted from the processor to the receiver, which relays the sound to the middle ear and causes vibration of the ossicles. The Maxum is a device reserved for severe hearing loss, typically greater than 75 dB. This device contains an implantable titanium magnet which is placed at the incudostapedial joint. The external component of the device has an in-the-ear processor. Most notable about this device is that, unlike conventional hearing aids, this processor is electromagnetic rather than electroacoustic, transferring electromagnetic energy across the tympanic membrane and displacing the ossicular chain. This procedure requires a tympanotomy, and very precise placement of the processor with no bending of the canal for optimal results. The Vibrant Soundbridge is another semi-implantable device used for moderate to severe hearing loss. The device contains an external audio processor and a surgically positioned subcutaneous implant. This implant is connected via a conductor link to a floating mass transducers titanium attachment, which is attached to the incus. This converts electrical signals to mechanical vibrations, or can directly stimulate the round window if the ossicular chain is absent.^, The procedure for implantation of this device requires mastoidectomy and posterior tympanotomy to allow routing of the conductor link. This device is often used in those who are unable to use conventional hearing aids due to chronic otitis externa or ear canal malformations. A 20-year follow-up study on this device found it to be safe with minimal complications, and particularly efficient at high frequencies allowing for better speech intelligibility in noisy environments. Implantable hearing devices provide a beneficial option for patients with severe hearing loss, and those with combined sensorineural and conductive losses. They have also been shown to have greater efficacy than conventional HA's in those with sloping high tone hearing loss. Studies comparing MEI's to conventional HA's show significantly increased word recognition and better gain in patients with MEI's.^{,  ,}  Ten-year follow-up studies on MEI's show significant increases in patient satisfaction when compared to conventional HA's. While this device has many advantages that can be beneficial especially for elderly patients, it is more invasive and requires surgical implantation within the ear. For this reason, it is typically reserved for patients who have not tolerated other hearing aids well. For patients who do not desire implantation, but are not satisfied with traditional hearing aid use, a new transtympanic hearing aid (THA) is in development. This device amplifies sound directly on the round window, and studies comparing THA to traditional HA's have shown significantly improved speech comprehension in those using the THA (Figure 3).

Bone anchored devices are a unique type of hearing device as they are typically indicated for patients who have a unilateral hearing loss with normal hearing in the contralateral ear.^, It can also be used in patients with severe conductive hearing loss. This device is commonly used in patients who have an acoustic neuroma resulting in a unilateral hearing loss. This device works through direct bone conduction to produce hearing. A titanium screw is typically inserted into the mastoid, where it must remain for 3 months to allow osteointegration and stabilization before the device is activated. Following this period, an external device which contains a microphone that detects vibration, is attached to the screw and transmits these vibrations to the mastoid bone via the screw.^, While the majority of bone-conducting devices are invasive and surgically placed, there are some that are not as invasive. An example is a headband that makes firm contact with the patients skull to allow for conduction.^, There have also been clinical trials of an adhesive bone conductive device that can simply be applied to the skin and are water resistant. However, due to the decreased interface between the device and the bone, the gain in noninvasive devices is less than in traditional bone anchored devices. These devices have shown success, though their use is limited to those with conductive, or unilateral sensorineural hearing loss.

Despite the advancements made in the aesthetics and the technology of hearing devices, there remains a significant stigma surrounding their use. It is estimated that approximately 20%-25% of adults with hearing loss do not use a hearing aid.^,, There are many factors that contribute to the lack of hearing aid use in those with hearing loss, and they are important to consider when treating a patient. These factors include high cost, fear of stigmatization, and lack of aesthetic appeal. Furthermore, as previously discussed, manual dexterity is often a limiting factor especially in the elderly population using hearing aids. It is important to evaluate the patients manual dexterity before recommending a specific hearing aid, as the small devices which require precise handling may be more difficult for elderly individuals.^,,, For a patient who may have difficulty with managing their device, it may be wise to offer them options that require less frequent battery changes for example. It is important to fully inform the patient of the expectations when using a hearing aid including complications, difficulties, and extent to which the patient must personally manage their hearing aid. The patient should undergo a full hearing assessment prior to fitting and should be counseled on his/her hearing loss, the difference between hearing impairment and understanding impairment, and how hearing aids may affect each of these. Furthermore, while the provider may have an opinion as to which hearing aid is best for a patient, it is important to illicit each individual patient's concerns and priorities when fitting for a hearing aid, and to cater the options to the patient's specific needs.

A topic of ongoing debate has been whether patients experience increased benefit from binaural rather than monaural hearing aids. With the cost of some hearing aids being so high, it is understandable that patients may prefer a single hearing aid over purchasing 2 devices. Some studies have shown that binaural hearing aids improve speech intelligibility, clarity, and understanding. Studies have also shown that, especially in patients with severe hearing loss, speech localization is considerably improved with binaural hearing aids.^{,  ,}  A large-scale, multicenter retrospective study performed by Boymans et al showed that there was no significant difference between bilateral and unilateral hearing aids in terms of device use and satisfaction. However, there were significant differences in detection, reverberation, and localization of sound with bilateral hearing aids having an advantage. Based on the available literature, the main benefits seen in using bilateral vs. unilateral hearing aids is seen in those with severe hearing loss. A study by Cox et al explored the possible factors associated with a patient's preference towards using a single or 2 hearing aids. They found that patients who preferred a single hearing aid typically reported comfort and quality as the main factors. Those who prefer bilateral hearing aids reported increased balance restoration, clarity, and comfort as the main factors influencing their decision. In a patient-centered approach, bilateral hearing aids are not always the best. It is important to elicit patient preferences, especially in patients with less than severe hearing loss who may receive enough benefit from a single hearing aid.