From Hearing Aid to Auditory Brainstem Implant
With over 700 million people worldwide anticipated to have a hearing-related disorder by 2050, there are many solutions for hearing rehabilitation to consider.
Luis Lassaletta, MD, Javier Gavilán, MD, Mario Zernotti, MD,
and Erika Celis-Aguilar, MD
Hearing loss affects 5% of the world population, according to the World Health Organization (WHO). This means that over 430 million people need an accurate diagnosis and hearing rehabilitation. By 2050 over 700 million people could develop a hearing-related disorder. These facts urge us to rereview and offer new and exciting options available for hearing rehabilitation.
In a patient with sensorineural hearing loss (SNHL), the first option should always be a hearing aid. Hearing aids have advanced quickly to meet the requirements of the modern era and have developed technology such as Bluetooth and more amicable receptors that facilitate and make more appealing their use. The different hearing aids available are behind the ear (BTE), receiver in canal (RIC), in the ear (ITE) intracanal, and the contralateral routing of signal (CROS) system. The BTE is a more potent device but with less suitable cosmetic appearance, and the RIC system is of silicone material with no occlusion effect for mild-to-moderate SNHL. ITE is a personalized hearing aid, customized from the pinna and external ear canal of the patient.
Note: In the United States, the sale of over-the-counter (OTC) hearing aids became effective in October 2022, with OTC hearing aids now available in traditional retail and drug stores. An FAQ for patients related to OTC hearing aids in the U.S. is available on ENThealth.org.
Cochlear implants (CI) are hearing devices that are surgically implanted; these are indicated when a patient has a bilateral severe-to-profound sensorineural hearing loss and no longer benefits from a hearing aid. The CI has two components, one external and another internal. The external component has a microphone and speech processor that converts the sound to a digital signal, which is transmitted transcutaneously to the internal component: the receiver-stimulator. Finally, the signal arrives to the electrode that directly stimulates the cochlea, which in turn gives the sensation of hearing.
It is important to point out that bilateral CIs should always be the goal in the case of congenital SNHL. Bilateral CIs in postlocutive adults are controversial but possible in certain cases.
There are many new indications for CI such as single-sided deafness, which has been recently approved by the Food and Drug Administration (FDA). This topic is still subject to controversy. On the other hand, asymmetric hearing loss is based on the cochlear implantation of the worst ear and the indication of a hearing aid in the best ear. This is a very interesting approach since you can combine two different forms of hearing rehabilitation and obtain, in this way, the best hearing rehabilitation possible.
Furthermore, there is also electroacoustic stimulation. This is indicated in patients with preserved hearing in the low frequencies but severe-to-profound sensorineural hearing loss in the high frequencies. In these patients, the CI could stimulate the high frequencies and the hearing aid stimulates the low frequencies. The most important step in the CI surgical approach should be the atraumatic technique that allows the electrode to be inserted without damage to the residual hearing. This is another example of the combination of different hearing rehabilitation resources available today.
There are other novel indications for CI such as vestibular schwannoma, which has been described previously, as long as cochlear nerve fibers are preserved during surgery. This indication is also valid in observation therapy or radiation treatment.
A malformed inner ear is no longer a contraindication for CI. After a careful review of the case, the surgeon can plan the cochlear implantation with the different types of electrodes that are available today, like the compressed type. Meningitis and cochlear ossification are the only urgent indications for CI, due to the ossification that can compromised the insertion of the electrodes.
Bone Conduction Devices
Bone conduction devices are anchored to cranial bone and can transmit the sound directly to the cochlea, bypassing the external and middle ear. The main indications for these devices are conductive or mixed hearing loss; unfortunately, the bone threshold limitation described is 45-65 dB. Also, patients not able to tolerate a hearing aid for medical reasons can find in these devices a solution for their hearing loss.
There are two main types of bone conduction devices available today: the percutaneous and the transcutaneous. The percutaneous device has the external component positioned in all the layers of the skin while osseointegration of the implant occurs in the temporal bone. Examples of percutaneous are Baha Connect and Ponto. The transcutaneous, instead, preserves an intact skin and transmits the signal through two magnets, one in each side. Examples of transcutaneous are Baha Attract, Osia 2, and Bonebridge. Results with both devices are expected to close a 30 dB gap. Nonetheless, this is dependent on the previous bone threshold of the patient.
Also, there are new nonsurgical bone conduction devices that could also benefit our patients—the future of bone conduction devices is evolving.
Middle Ear Implants
Middle ear implants are medical devices that transform and amplify the sound energy to a vibration stimulation. This energy is applied directly to the ossicles or to the round window.
According to the type of energy they produce, they can be classified as follows:
- Piezoelectric: This has a piezoelectric sensor and drive that generates a vibration to the ossicular chain. Example: Esteem.
- Electromagnetic: An electromagnetic field stimulates a magnet that vibrates. It can be located at the ossicles or round window. Example: MAXUM.
- Electromechanical: Similar to electromagnetic, the floating mass transducer typically vibrates at the ossicles or round window. Example: Vibrant Soundbridge.
The main indications for these devices are mixed or conductive hearing loss. For these patients, the direct stimulation to the round window or the use of a stapes coupler are necessary. Patients with 60 dB in bone threshold in mid-to-high frequencies, with an air bone-gap of 30-40 dB, would benefit from these devices.
In patients with SNHL, the stimulation of the ossicles, particularly the long and the short processes of the incus, are recommended. The main indication for these patients is moderate-to-severe high-frequency hearing loss. As explained previously, this mechanism bypasses the external ear and tympanic membrane, directly stimulating the ossicles. It is important to remember that patients with SNHL need to be thoroughly evaluated and should be initially offered a hearing aid, and only in the case of contraindication a middle ear implant should be considered.
Auditory Brainstem Implants
Auditory brainstem implants directly stimulate the cochlear nucleus. They are surgically implanted at the lateral recess of the fourth ventricle. Their main indications are patients with no cochlear nerve, cochlear aplasia, or cochlear ossification. The surgery implies a neurosurgical access, through a retrosigmoid or translabyrinthine approach. This device offers improvement if implanted in children. In most cases, children perceived sound and some conversational cues.
There are myriad options in the hearing rehabilitation of patients with hearing loss. The decision depends on the type of hearing loss (sensorineural versus mixed or conductive), clinical history of the patient, the possibility to adapt a hearing aid, and patient preference. New research and scientific data are providing novel information on the best options available for our patients, and we shall remain alert to the latest developments in order to choose the best hearing device.
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