An auditory brainstem implant (ABI) is a surgically implanted electronic device that provides a sense of sound to a person who is profoundly deaf, due to retrocochlear hearing impairment (due to illness or injury damaging the

cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus (cochlea), modiolus. A core component of the cochlea is the organ of Cort ...

or auditory nerve, and so precluding the use of a

cochlear implant A cochlear implant (CI) is a surgically implanted Neuroprosthetics, neuroprosthesis that provides a person who has moderate-to-profound sensorineural hearing loss with sound perception. With the help of therapy, cochlear implants may allow for imp ...

). In Europe, ABIs have been used in children and adults, and in patients with

neurofibromatosis type II Neurofibromatosis type II (also known as MISME syndrome – multiple inherited schwannomas, meningiomas, and ependymomas) is a genetic condition that may be inherited or may arise spontaneously, and causes benign tumors of the brain, spinal cord, ...

History

The auditory brainstem implant was first developed in 1979 by William F. House, a neuro-otologist associated with the House Ear Institute, for patients with

neurofibromatosis type 2 Neurofibromatosis type II (also known as MISME syndrome – multiple inherited schwannomas, meningiomas, and ependymomas) is a genetic condition that may be inherited or may arise spontaneously, and causes benign tumors of the brain, spinal cord, ...

(NF2). House's original ABI consisted of two ball electrodes that were implanted near the surface of the

cochlear nucleus The cochlear nucleus (CN) or cochlear nuclear complex comprises two cranial nerve nuclei in the human brainstem, the ventral cochlear nucleus (VCN) and the dorsal cochlear nucleus (DCN). The ventral cochlear nucleus is unlayered whereas the dor ...

on the

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is conti ...

. In 1997, Robert Behr at the University of Wurzburg,

Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...

, performed an ABI implantation using a 12- electrode array implant with an audio processor based on the MED-EL C40+ cochlear implant. The first pediatric ABI implantation was performed by Vittorio Colletti from

Verona Verona ( ; ; or ) is a city on the Adige, River Adige in Veneto, Italy, with 255,131 inhabitants. It is one of the seven provincial capitals of the region, and is the largest city Comune, municipality in the region and in Northeast Italy, nor ...

Italy Italy, officially the Italian Republic, is a country in Southern Europe, Southern and Western Europe, Western Europe. It consists of Italian Peninsula, a peninsula that extends into the Mediterranean Sea, with the Alps on its northern land b ...

, in 1999. In contrast to cochlear implants, ABI implantation is relatively rare. By 2010, there were only 500 patients worldwide who had undergone implantation.

Parts

An ABI system consists of an internal part (the implant) and an external part (the audio processor or sound processor). It is similar in design and function to a cochlear implant. The external audio processor is worn on or behind the

ear In vertebrates, an ear is the organ that enables hearing and (in mammals) body balance using the vestibular system. In humans, the ear is described as having three parts: the outer ear, the middle ear and the inner ear. The outer ear co ...

. It contains at least one

microphone A microphone, colloquially called a mic (), or mike, is a transducer that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and publi ...

, which picks up sound signals from the environment. The audio processor converts these signals into digital signals and sends them to the coil. The coil transmits the signals through the

skin Skin is the layer of usually soft, flexible outer tissue covering the body of a vertebrate animal, with three main functions: protection, regulation, and sensation. Other animal coverings, such as the arthropod exoskeleton, have different ...

to the implant below. The internal implant sends the signals to the electrode array. The design of the electrode array is the key difference between a cochlear implant and an ABI. Whereas the electrode array for a CI is wire-shaped and is inserted into the

, the electrode array of an ABI is paddle-shaped and is placed on the cochlear nucleus of the brainstem. By stimulating the brainstem, the ABI sends the sound signals to the brain, allowing the patient to perceive sound.

Indications

Neurofibromatosis Type 2

Until 2018, ABI was only indicated for patients with Neurofibromatosis Type 2 (NF2). NF2 is a genetic disorder that is characterised by the development of non-cancerous tumours along the nervous system. These

vestibular schwannoma A vestibular schwannoma (VS), also called acoustic neuroma, is a benign tumor that develops on the vestibulocochlear nerve that passes from the inner ear to the brain. The tumor originates when Schwann cells that form the insulating myelin sheath ...

s (also known as acoustic neuromas) often form on the auditory nerve, and surgical removal of these NF2 tumours can damage the auditory nerve and limiting the patient's ability to hear. NF2 generally presents in adolescence or young adulthood, so candidacy was previously limited to patients aged 15 years or older, with NF2 and bilateral non-functioning auditory nerves.

Other indications

In Europe and other countries, ABI is CE-marked and approved for patients 12 months and older who cannot benefit from a cochlear implant due to non-functional auditory nerves. This includes both

congenital A birth defect is an abnormal condition that is present at childbirth, birth, regardless of its cause. Birth defects may result in disability, disabilities that may be physical disability, physical, intellectual disability, intellectual, or dev ...

and accrued etiologies, including: * Auditory nerve aplasia * Auditory nerve

hypoplasia Hypoplasia (; adjective form ''hypoplastic'') is underdevelopment or incomplete development of a tissue or organ.Head trauma A head injury is any injury that results in trauma to the skull or brain. The terms ''traumatic brain injury'' and ''head injury'' are often used interchangeably in the medical literature. Because head injuries cover such a broad scope of inj ...

* Non-NF2 tumours * Severe cochlear

ossification Ossification (also called osteogenesis or bone mineralization) in bone remodeling is the process of laying down new bone material by cells named osteoblasts. It is synonymous with bone tissue formation. There are two processes resulting in t ...

The US

FDA The United States Food and Drug Administration (FDA or US FDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food ...

approved clinical trials of ABIs for children in 2013. A handful of medical centres, including

New York University New York University (NYU) is a private university, private research university in New York City, New York, United States. Chartered in 1831 by the New York State Legislature, NYU was founded in 1832 by Albert Gallatin as a Nondenominational ...

, are undergoing feasibility studies in the pediatric population.

Surgery

ABI implantation requires a

craniotomy A craniotomy is a surgery, surgical operation in which a bone flap is temporarily removed from the Human skull, skull to access the Human brain, brain. Craniotomies are often critical operations, performed on patients who are suffering from brain ...

and is therefore much more complex than CI surgery. It is normally performed by both a

neurosurgeon Neurosurgery or neurological surgery, known in common parlance as brain surgery, is the medical specialty that focuses on the surgical treatment or rehabilitation of disorders which affect any portion of the nervous system including the brain, ...

and an ENT surgeon together, who insert the electrode array through the

fourth ventricle The fourth ventricle is one of the four connected fluid-filled cavities within the human brain. These cavities, known collectively as the ventricular system, consist of the left and right lateral ventricles, the third ventricle, and the fourth ...

onto the surface of the cochlear nucleus. For patients with NF2, the surgeon will spend a significant amount of time removing the acoustic neuroma tumours before inserting the implant. Depending upon the surgical approach, this may involve sacrificing the auditory nerve, thus rendering the patient deaf. Patients with NF2, who undergo both tumour removal and implantation in the same surgery, generally experience a longer post-op stay than patients without NF2.

Outcomes

Speech perception outcomes with an ABI are generally poorer than those reported in cochlear implant multichannel CI users. Most patients are able to detect the presence of environmental sounds and speech. Speech understanding gradually improves during the first three years after activation, and most patients experience better speech understanding using a combination of lip-reading and the ABI, as opposed to lip-reading alone. However, most patients are unable to understand speech using only their ABI. There are two reasons that could explain the difference in outcomes between cochlear implants and ABIs. Firstly, non-auditory side-effects, such as

vertigo Vertigo is a condition in which a person has the sensation that they are moving, or that objects around them are moving, when they are not. Often it feels like a spinning or swaying movement. It may be associated with nausea, vomiting, perspira ...

, limit the overall number of electrodes that can deliver useful frequency information. Electrodes found to cause one of these side-effects are deactivated, resulting in fewer signals reaching the brain. In addition, the brainstem is unable to offer the same tonotopic range as the cochlea. With a cochlear implant, the electrodes positioned in the basal end of the cochlea elicit a higher pitch sensation than those positioned in the apical end. In contrast, the tonotopic map within the cochlear nucleus runs parallel and obliquely through the nucleus and the ABI positioned on the surface does not stimulate neural structures in such a clear, tonotopically ordered way. This makes it harder to achieve optimal results during fitting. Patients without NF2 tend to experience better speech outcomes with an ABI than those with NF2. A study by Colletti found that a significant number of patients without NF2 were able to understand speech with an ABI, including effortless telephone use. It is believed that the tumours caused by the NF2 damage specialised cells in the cochlear nucleus important for speech perception. There is some evidence to suggest that ABI can help to reduce the effect of tinnitus and improve quality of life. Better language outcomes are also expected with younger children implanted before the age of 2. Because of the wide range of possible outcomes, it is crucial that patients and/or their parents are counselled effectively about what they can realistically expect from an ABI. Parents are advised about additional communication modalities available, such as the use of sign language, as the ultimate goal is to facilitate language with the child.