Hearing Implants

Bilateral (left and right) not just unilateral (either left or right) vestibular schwannoma (VS) tumors, acoustic neuromas (AN) is the hallmark tumor growth for individuals with the condition neurofibromatosis type 2 (NF2). These tumors affect ninety-five percent (95%) of individuals with germline NF2. While tumors may grow elsewhere before VS, these lesions are often the first noticed for NF2 diagnosis even if other issues occur first. ^{[Ardern-Holmes, 2017]} The growth rate varies from person to person, but tumor growth rate has been proven dependent on an individual's NF2 mutation type; other factors may also apply. ^{[Hexter, 2015]}

Hearing loss that is the result of the condition VS is a result of damage to the cochlear nerve (acoustic nerve), the connection of the nerve between the Cochlea and the brainstem. Traditional hearing aids that amplify sound might offer help while the nerve starts to become damaged. As more damage to the nerve occurs, a Hearing Aid, which only amplifies sound, will become ineffective. The next step is a cochlear implant (CI) and thanks to medical advancements, many people with NF2 do well with hearing from a CI for many years. If the cochlear nerve breaks completely; something that would be confirmed or rejected as a result of auditory brainstem response (ABR) test, sometimes also called brainstem evoked response audiometry (BERA), only an auditory brainstem implant (ABI) will offer any sound.

1. Cochlear Devices

CI - Cochlear Implant

A Cochlear implant (CI) will work if; CN8 (the vestibulocochlear nerve), the vestibular nerves, the cochlear nerve and the cochlea itself are functioning.

Drug-based tumor treatments like bevacizumab (avastin™) have begun to allow for an increased period of effectiveness with a CI.

ABI - Auditory Brainstem Implant

When most people suffer from hearing loss, sound amplification is all that is needed to regain the missing sound. The Cochlear Implant was designed to direct sound to the Cochlea when an eardrum is ruptured, or a bone in the ear or other internal parts of the Middle Ear is broken. But if damage occurs to the or the cochlear nerve or other locations on the vestibulocochlear nerve before it splits to become the vestibular nerve (balance nerve) and cochlear Nerves, an auditory brainstem implant (ABI) is the only option to regain hearing. ABIs direct sound straight to the brainstem, bypassing the ear components completely.

The brain will need time to adjust and adapt or relearn the new form of hearing that an ABI provides. Sound quality improves over the course of the first year but can change after that as a result of other tumor growth.

The ABI's quality of sound is different from natural hearing. But with an ABI speech reading (lip-reading) is often easier. The effectiveness of an ABI combined with lip-reading varies based on an individual's ability to lip-read and working electrodes. With an ABI, it often becomes easier to understand speech over time.

Processor - Nucleus 6 (N6)

The Current Sound Processor model for cochlear implants (CI) and auditory brainstem implants (ABI) is the Nucleus 6 (N6). When cochlear releases a new processor, it does not always work for individuals with an ABI. The N6 is the newest processor for both.

Differences of ABI from Typical CI

Assuming an understanding of a basic typical CI, these are the differences between the ABI systems:

1. Internal receiver: The Internal Receiver components are the same as a CI, except for the shape and function of the sensor, and the location the sensors are placed in the head. The ABI sensor is then placed at the brainstem where CN8 is meant to leave the brainstem, and with a CI the sensor is placed inside the Cochlear.
2. Receiver magnet removal: During surgery for the implant, individuals who have health issues like NF2 and require MRIs have the internal receiver magnet removed for safety to allow for MRI scans. Removal of the internal magnet results in the requirement of a retainer disc to hold the ABI processor in a fixed location on the head.

A surgeon may not know this when implanting these devices, but it is possible to remove the magnet later. Requirements of MRI brain scans after implantation is only common for NF2. Radiology departments for MRIs will not allow an individual to have a scan unless they see documentation from a surgeon confirming the magnet was removed during implant.

3. Magnet: People with CI's can have the magnet removed it is standard to have it inside the head. It is why facilities assume to refuse an MRI to patients with hearing implants.
4. Side Effects: Possible Side Effects of some Electrodes: Each ABI electrode tested can result in stimulating parts of the brain that could be dangerous.
5. Poor Pitch Reception: The quality of final sound of an ABI is not at the same level as CI's.
6. Competing Tinnitus: During tests, sounds might compete with Subjective/Objective/GET Tinnitus.
7. Switching on and Tune Ups: An ABI tuning results in longer session times with an Audiologist than a typical CI sessions.

Things to know about Switching on and Mapping ABI's:

1. Activation is possible six weeks after an ABI is implanted and takes two days.
2. Twenty-One Electrodes = Twenty-One Channels of Sound
3. Channel Deactivation: Due to the brain's response to a new method of sound side effects of some electrodes not all of the electrodes are likely to work and which that do may change over time.
4. ECG: A heart monitor for safety due to potential problematic electrodes is worn during switch on.
5. Mapping:
   • Individual electrode test
   • Tones of working electrodes are set into a range of high to low pitch.
   • The range for each electrode - The volume level of each electrode is gradually increased or decreased as needed.
   • Pitch comparison
   • Testing\live
6. During live test:
   • Side Effects: Side effects to electrodes can show up. (twitching/spasms)
   • Volume: Overall or individual electrode volume levels might need to be reduced or raised.
   • Speech Readers/Lip Reading: Good speech readers might think the volume is good, but it is too low.

CI and ABI Important Information

Medical Treatment Warnings

In their information packets, Cochlear issued a warning certain medical treatments done too close to the implant can break the implant. These treatments include:

• Electrosurgery Radiosurgery - Gamma Knife, CyberKnife, or Proton Therapy
• Diathermy
• Neurostimulation
• Electroconvulsive Therapy

Additional warnings on damage include the following:

• Electrostatic Discharge (ESD) - Static Electricity ^{[Journal of Educational Audiology, 2002]}

ESD is the sudden discharge of static electricity. Electronic devices, including cochlear implants (both the internal and external devices), are susceptible to damage from ESD. No sound from implant could be from; a programming issue, processor damage, or even receiver failure. ^{[Journal of Educational Audiology, 2002]}

• Causes of ESD ^{[Cochlear, 2017] [Journal of Educational Audiology, 2002]}
• Low Humidity - Hot or cold dry conditions
• Removing clothes
• Playing on plastic slides
• Walking across a carpet
• Handling polyethylene bags
• Pouring polyurethane foam into a box
• Latex balloon coming in contact with hair
• CRT's - Computer monitors and TVs
• Radio Frequency Interference - There is interference with Cochlear Implants and RF (Radio Frequency) Technology ^{[Cochlear, 2017] [Journal of Educational Audiology, 2002]}
• Mobile phones
• Electronic Article Surveillance (EAS) Systems
• Metal detection systems (Such as security checks at airports.)

2. Baha

A Baha (bone-anchored hearing aid), is a solution for people who have a unilateral hearing loss single sided deafness (SSD). It allows for hearing introduced on both sides of the head to be heard, by transmitting the incoming sound of a deaf ear to the hearing ear.

On the side of the deaf ear, a small hole is drilled into the skull that the processor can be attached to. When attached the sound picked up by the processor is echoed through the skull bone to the hearing ear. A Baha might be only a temporary hearing solution for a better hearing for a few years depending on changes in tumors.

A Baha is a titanium piece surgically attached to the skull. The processor is a removable piece that can be attached or disconnected as needed.

Baha piece in skull - MRI Safe

Titanium is not a metal that is an issue or dangerous in MRI scans. Removal of the outer device, the processor, is necessary before an MRI as well as any parts containing a magnet.

3. Nerve Damage and Hearing Loss

The Vestibulocochlear Nerve affects both the Vestibular Nerve and the Cochlear Nerve. The vestibular nerve affects balance, while the cochlear nerve affects hearing. When this happens, a CI will not replace hearing loss but leaves ABI as an option for sound. Learn more about the vestibulocochlear nerve damage.

4. Sources

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