What Does Neurofibromatosis Type II Have to do With Hearing Loss?

Most types of hearing loss are due to something happening between the outer ear and cochlea. Sensorineural hearing loss, conductive hearing loss, and mixed hearing loss are all examples of a hearing loss that is due to something happening in or before the cochlea. However, there are also types of hearing loss due to something happening between the cochlea and the brain. These are called retrocochlear hearing losses, and one of the most common is associated with a genetic condition called Neurofibromatosis Type II, or NFII.

An Auditory Brainstem Implant is a type of hearing implant used to help treat hearing loss due to NF II.

Retrocochlear Hearing Loss

“Retrocochlear” is a word which means behind the cochlea, and therefore retrocochlear hearing loss  that is caused by something happening behind the cochlea. Specifically, this refers to organs like the auditory nerve and the brain itself. Here’s a graphic showing where the auditory nerve is located:

The auditory brainstem convey electrical signals from the cochlea to the brain.

You can see here that the auditory nerve is highlighted. It’s the auditory nerve that helps carry electrical signals from the cochlea towards the brain, where they are interpreted as sound, and it just happens that the auditory nerve is what’s often affected by Neurofibromatosis Type II.

Neurofibromatosis Type II

NF II is a genetic condition that usually causes non-cancerous tumors to appear on the auditory nerve. It affects approximately 1 in 33,000 people (0.003%) depending on the part of the world REF. Because these tumors, sometimes called schwannomas or acoustic neuromas, appear on the auditory nerve they can have a significant influence on hearing.

It affects different people differently. Some may be born with numerous schwannomas already on the brainstem, while others may develop them later in life. Regardless of when it appears, NF II typically causes tinnitus, hearing loss, and balance problems that can develop over time.

Although there is no cure for NF II, it can be treated. A common treatment involves surgically removing the tumors on the auditory nerve, and this where the auditory brainstem implant comes in.

Auditory Brainstem Implant

An auditory brainstem implant, or ABI, is a type of hearing implant used by individuals with NF II. It’s sort of like a cochlear implant, but instead of an electrode array inserted into the cochlea it has an electrode array that is attached to the auditory nerve. The auditory nerve is a part of the brainstem and is located between the auditory nerve and the brain, and therefore an ABI completely bypasses the cochlea.

Here’s a picture of what an ABI electrode array looks like:

The electrode array of an Auditory Brainstem Implant

Each of the twelve silver circles is a single electrode contact and each contact is responsible for stimulating one channel of sound, similar to each of the twelve channels from a CI.

An ABI’s electrode array is attached to the auditory nerve during the same surgery in which the tumors on the auditory nerve are removed. Because it’s more complex than just inserting an electrode array into the cochlea, the surgery takes approximately 12 hours depending on the number of tumors.

During surgery the surgeon can use tools to determine when he or she has placed the electrode array in the right position. This is important because the auditory brainstem doesn’t follow the tonotopic principle in the same way that the cochlea does, and therefore the surgeon needs to check to see what frequency of sound each electrode stimulates.