Implantable Hearing Devices

Anatomy and Physiology

The most common form of auditory rehabilitation in patients with sensorineural hearing loss and select patients with conductive hearing loss is with hearing aids. With advancements in technology over time, hearing aids have become more effective, comfortable, and conspicuous; however, some limitations persist, which limit their effectiveness for certain subgroups of patients. Some limitations include medical conditions comprising of a chronic draining ear, anatomic limitations, insufficient amplification, acoustic feedback, occlusion effect, and discomfort.[2][3] 

In addition, patients with severe to profound hearing loss receive limited benefits from traditional hearing aids; however technological advancements in implantable hearing devices provide viable treatment options that may significantly improve patient quality of life. However, these newer devices are more expensive than traditional aids and carry the risks of surgery. Therefore, clinicians must carefully select the most appropriate hearing devices for individual patients.

Bone-conduction hearing prostheses stimulate the cochlea by vibrations of the skull, thus bypassing the external and middle ear. These may be considered for treatment of hearing loss in the setting of a chronically draining hearing, rehabilitation of conductive or mixed hearing loss that is not amenable to traditional hearing aids, for patients that are not surgical candidates and are unable to tolerate conventional hearing aids, anatomic limitations such as atresia or postoperative canal changes, as well as single-sided deafness.[4] 

Surgical and non-surgical systems exist for bone conduction. Non-surgical systems generally include steady pressure to the mastoid cortex via an elastic band or eyeglasses. Compared to surgical implanted bone-conduction systems, these systems are limited by soft tissue attenuation and chronic pressure from the device. Thus, these devices are most commonly used to provide bone-anchored aid before surgical implantation, especially in young children with conductive or mixed hearing loss or in patients who are not surgical candidates with conductive or mixed hearing loss. In surgical bone-conduction hearing devices, an osseointegrated screw is placed into the skull coupled with the external device via either a transcutaneous abutment or a magnet. 

Newer to the medical scene, middle ear implants have been developed which couple external sound to an implanted device within the middle ear. This device transmits mechanical energy from the middle ear to the inner ear fluids via the oval or round windows. These devices are alternatives to traditional hearing aids, traditional middle ear reconstruction, and bone-anchored devices. Partially implanted (devices with an external component) and completely implanted (devices with no external component) are available. Some advantages of middle ear implants over traditional hearing aids include results of direct vibration of the ossicular chain, which results in an increased functional gain and improved audiometric outcomes.[5] 

Middle ear implants may be chosen in lieu of bone-conducting systems in patients with an air-bone gap of less than 30dB with mild to moderately severe hearing loss to avoid skin-related complications of abutment or magnetic systems or due to aesthetic concerns. Although middle ear implanted devices currently have widespread application worldwide, their use in the United States is limited due to economic and insurance constraints. 

William House first introduced Cochlear implants in 1963 to treat severe to profound sensorineural hearing loss. The initial device was a single-channel device that, when implanted, resulted in patients with previous severe to profound hearing loss to perceive sound. Advances in technology and surgical technique have continued to evolve, improving patient outcomes, reducing complications, and expanding indications. These devices consist of an external component with a speech processor and transmitter and an internal component with a receiver/stimulator, which is coupled to the electrode array and ground electrode. These implants are generally placed below the subperiosteum in the postauricular area.

A cortical mastoidectomy with a facial recess approach is performed, and the electrode ray is carefully advanced into the scala vestibuli of the cochlea, most commonly via a round window or extended round window cochleostomy. The most common indication for cochlear implantation is severe to profound sensorineural hearing loss. Please reference the dedicated StatPearls chapter on Cochlear Implants for more information regarding cochlear implants. 

In addition to surgical fitness and otologic surgical risks, limitations of all surgical devices include the cost of devices and the requirement of anesthesia (most often general anesthesia) to place or replace the devices. In addition, some implants are not MRI compatible, and manufacturing guidelines regarding MRI compatibility must be considered with the patient's needs.[6]

Osseointegrated Bone-Conduction Prosthesis

The concept of osseointegration was pioneered by PI Branemark, which described the formation of a natural bond between titanium and bone.[7] Initially, this technology was primarily utilized in the field of dental implants but was eventually applied to other facia prosthetics, including the use for bone-anchored hearing conduction.[8][9] 

These devices became available in Europe in the early 1980s and were FDA approved for use in patients five years or older in the United States in 1996. Since that time, the indications of bone-anchored hearing devices have continued to expand. The results of bone conduction implanted devices are effective with a pure tone audiometry (PTA) in free field 18.3 +/- 1.3 dB with an estimated gain of 38 +/- 5.5 dB.[10]

Indications

Percutaneous and Transcutaneous

Current indications for bone-anchored hearing devices include conductive and mixed hearing loss and single-sided deafness. 

For patients with conductive or mixed hearing loss, the devices are approved for patients five years and older with unilateral or bilateral conductive or mixed hearing loss and who are unable or receive suboptimal results from standard hearing aids. The common etiologies resulting in this situation include middle ear disease, congenital canal atresia, cholesteatoma, chronic otitis media, and post-surgical changes. In these circumstances, the bone-anchored hearing device bypasses the problematic external or middle ear.

Previously, patients with single-sided deafness (SSD) either adapted to unilateral hearing or were fitted with contralateral routing of signal (CROS) device, which is a hearing aid that routes sound from the deaf ear to the contralateral better hearing ear. However, studies have revealed that the handicap associated with SSD was previously underestimated. Bone anchored hearing devices have been shown to benefit sound localization and speech in noise in patients with SSD. As a result, indications for bone-anchored hearing devices have expanded to include single-sided deafness from etiologies such as genetic disorders, inner ear malformations, ototoxic treatment, trauma, degenerative neurologic disease, sudden sensorineural hearing loss, and vestibular schwannoma.[11]

Contraindications for bone-anchored hearing devices include scalp dermatologic problems due to the propensity for skin infection or breakdown related to the device, patients with bony anatomy not amenable to osseointegration, and the need for ongoing MRI evaluation, especially of the unilateral temporal lobe. Although improvements have been made in device MRI compatibility, specific device manufacture guidelines should be referenced and discussed before surgery in patients in whom ongoing MRIs are planned. 

Middle Ear Implantable Hearing Prosthesis

Multiple devices are available for middle ear implantation, and most are indicated for adults and children five years and older with conductive or mixed hearing loss. Bone conduction thresholds must be stable with mild to moderately severe loss at frequencies between 500 and 4000 Hz. These devices are also indicated for patients with mild to severe sensorineural hearing loss between 500 and 6000 Hz with stable thresholds.

Malformations of the inner ear are not contraindications as the implants may often be couples to the stapes or round window and bypass these abnormalities. Contraindications include retrocochlear/central hearing impairment, active middle ear infection, and comorbid medical conditions that preclude safe general anesthesia. Manufacturing guidelines regarding MRI compatibility should be considered pre-operatively. In many cases, a pre-operative CT scan is performed for pre-operative planning.

Technique or Treatment

Percutaneous Implants

The location of the implant is first mapped along the temporal line between 5.5 and 6 cm posterior to the external auditory canal. Many surgeons mark the periosteum at this point transcutaneously with methylene blue before making a skin incision. However, these measurements may be made (or confirmed) after the incision is made. 

The skin thickness at the anticipated implant location is then evaluated so that the appropriate length abutment may be chosen. When placing a transcutaneous device, it is crucial to ensure scalp thickness lies within manufacture guidelines to ensure the proper function of the transcutaneous magnet. Once an accurate measurement of the skin and subcutaneous tissues at the site are obtained, an abutment 2 to 3 mm longer than the measured skin and soft tissue thickness is chosen. Of note, it is important to perform this measurement before infiltration with local anesthesia to obtain an accurate measurement. 

The skin incision is usually designed approximately 5 mm anterior or posterior to the implant site and approximately 2 cm in length. Local anesthesia is then infiltrated according to surgeon preference. An incision is made through the skin and subcutaneous tissues to the periosteum. A subcutaneous flap is created by gently elevating the subcutaneous tissues from the periosteum until the previously marked implant site is located. Correct placement of this site is confirmed. The periosteum at the implant site is then incised and removed. 

During any osseointegration procedure, adequate irrigation is imperative during drilling to avoid thermal injury, which can adversely affect adequate osseointegration. A guide drill is then used to create an opening to a depth of 3 mm. The site is then inspected to ensure no soft tissue or dura is encountered. Once this is confirmed, the bone is drilled to a depth of 4 mm. A bony well for the implant is then created with a countersink drill.  

If the soft tissue thickness was measured to be more than 10 mm, thinning of the soft tissue over the implant site is performed. The abutment is mounted on the implant. The drill is decreased to a rate of thirty to forty rpm, and the implant is placed again with adequate irrigation to avoid thermal injury to the bone. The implant is tightened to 25 newtons. The soft tissue is then draped over the implant, and a 5 mm skin punch is performed to exteriorize the abutment. The incision is then closed according to the surgeon's preference. A pre-manufactured cap is usually attached to the abutment when it is left in place for one week. 

Transcutaneous Implants

The surgical procedure for transcutaneous implants is similar to that of the percutaneous systems, except no percutaneous punch is made. The bony area immediately surrounding the implant location must be flat or drilled until it is flat to facilitate the transcutaneous device.  

Percutaneous and Transcutaneous Outcomes

Percutaneous and transcutaneous bone-anchored systems have been shown to provide audiological benefits compared to unaided conditions. Additional studies have shown that percutaneous and transcutaneous implants provide audiologically equivalent conductive hearing rehabilitation, while percutaneous devices provide a minor advantage when a sensorineural hearing loss component is present.[12] 

Middle Ear Implantable Hearing Prosthesis

Operative Technique

Although the technique varies among devices, the key steps between most devices are consistent. The implant location is identified according to manufacture guidelines but often posterior pinna at the level of the zygomatic root. The skin over the expected implantation site is measured using the method described above. A postauricular incision is performed, the periosteum is incised, and a subperiosteal pocket is created.

A cortical mastoidectomy is then performed, followed by a facial recess approach. Of note, the placement of middle ear implanted devices often require larger facial recess exposure than cochlear implant procedures. Thus, close attention to the annulus and chorda-tympani nerve is imperative to avoid damage to these structures. The wound bed is then irrigated copiously to remove all blood and bone dust. Adequate exposure is then confirmed with a measuring device or dummy prosthesis according to the specific implant and location.

The implant is then secured to the target location according to the specific device and anatomy. It is important to avoid contact of the device with the cochlear promontory, tympanic membrane, or pyramidal eminence. After adequate device placement is achieved, the periosteum is approximate with suture, and the postauricular incision is closed according to surgeon preference. 

Outcomes

Klieb et al. conducted a systematic review of active middle ear implants. This showed improvement in sound localization and speech in noise, but current data regarding the audiological outcomes and complications associated with these devices remains limited.[13]

Complications

Complications of Transcutaneous and Percutaneous Implants 

Intraoperative complications of bone-anchored hearing device surgery are rare but include damage to nearby structures, including the dura and sigmoid sinus, with associated risk for CSF leak and bleeding, respectively. 

Complications in the postoperative period are most commonly associated with soft tissue-related issues. With technological improvements and innovations in operative technique, including smaller incisions and limited tissue dissection, the incidence of these complications has decreased. Soft tissue complications include infection, excess granulation tissue, and excess scar formation.

These complications are more common in children, as well as adults with comorbidities, including smoking, diabetes mellitus, and obesity. Bone-related complications occur less frequently, and are the most common failure is lack of osseointegration. Technical fractures, as well as patient factors including trauma, chronic soft tissue infections, and inadequate bony recipient site, account for the majority of bone-related issues. Osteomyelitis is an exceedingly rare complication of bone-anchored hearing surgery.   

Complications of Middle Ear Implantable Hearing Prostheses  

Risks include those associated with traditional mastoid surgery and facial recess approach, including hearing impairment, changes in taste, facial nerve weakness, spinal fluid leak, and vertigo. Due to the exposure necessary through the facial recess, there is an increased risk to the chorda-tympani and tympanic annulus compared to cochlear implant approaches.

Clinical Significance

Approximately one-half billion people currently suffer from disabling hearing loss, and it is also the fourth leading contributor to disability worldwide.[14] 

Hearing loss significantly impacts patients' lives in many aspects, including education barriers, workplace barriers, and communication barriers that can lead to social isolation and depression. Recently, research has also revealed that hearing loss is an independent risk factor for developing cognitive decline, including dementia.[15] 

Hearing rehabilitation for many patients may be achieved by traditional hearing aids; however, limitations exist in some patients. Thus, implantable hearing devices are alternative treatments in patients with hearing loss who do not obtain adequate benefits from traditional hearing aids. Implantable hearing devices provide treatment options for patients who previously received little benefit from hearing aids.

Enhancing Healthcare Team Outcomes

Enhancing healthcare outcomes for patients who receive implantable hearing devices is multifaceted and involves all healthcare team members. Physicians, audiologists, nurses, pharmacists, and other health professionals must consider implications on healthcare outcomes when treating patients with hearing loss since failure to adequately treat patients with hearing loss results in significant detriment to the patient and cost to society.

Multidisciplinary care for patients receiving implantable hearing devices is critical in the pre-operative and post-operative settings. Pre-operatively establishing appropriate goals and expectations is of vital importance. Post-operatively, the ongoing role of the audiologist is crucial for device programming, troubleshooting, and training the patient and families.[14] [Level 5]