Definition and Overview
An auditory brainstem implant, often known by its acronym ABI, is an electronic device that enables a profoundly deaf person to hear sounds. This device is surgically implanted into the head, directly into the brain where normal auditory mechanisms should be found.
Who Should Undergo and Expected Results
An ABI procedure is often recommended for adults, aged 18 or older, who have been diagnosed with neurofibromatosis type II (or NF2). This is an inherited disease that involves the development of benign brain tumours that affect the transmission of sensory information from the inner ear to the brain.
Other eligibility criteria for an ABI procedure include:
- Profound hearing loss in both ears
- Absence of hypoplasia or aplasia on the cochlea or cochlear nerve
Meanwhile, the procedure may not be suitable for patients with:
- A small or absent auditory nerve
- Scarring in the cochlea, often caused by infections such as meningitis
- Damage to the cochlea, auditory nerves, or brainstems due to skull fracture
- Anomaly or pathology in the brainstem or brain cortex
- Tumours in the cranial nerve and brainstem
- Medical and psychological contraindications
This type of implant is often recommended for patients who have suffered from illness or injury that caused significant damage to the auditory nerve or cochlea. However, it is not the first type of auditory prosthesis recommended for patients, as it involves brain surgery. To date, there are only around 1,500 individuals who have received this type of implant, mainly because of the very invasive nature of the implanting procedure.
An ABI device has two basic parts: the external part, known as the processor, and an internal part, which is implanted into the patient’s brain. The processor is worn on the ear and is fitted with a microphone-like device that transforms sound waves into electrical signals.
The internal part, on the other hand, is responsible for picking up the electronic signals from the processor and conveying them to the brainstem. The internal receiver is placed right below the skin and is connected to an implanted array that is directly positioned within the brainstem.
An ABI is capable of bypassing both the patient’s hearing nerve and the cochlea, bringing the electronic signals directly into the brainstem. This process provides the patients with a sense of sound, especially if they have suffered from a condition or an injury that significantly damaged the hearing nerve.
An auditory brainstem implant is ideal for patients who have lost any natural ability for hearing. Patients who have learned lip-reading often have the best chances for this device to work.
However, it is important to note that there will be complex neurosurgery involved in placing an ABI into a patient’s head. The results are widely varied, too, as the surgeon will need vast experience in ABI procedures to properly position the device into the brainstem to deliver optimal results. In some cases, patients who have undergone the procedure have failed to hear anything while some patients get a narrow range of hearing, which means that there is little differentiation in the sounds heard. The patient might need some time to adjust to the sounds, transformed into electrical signals, for proper hearing.
How is the procedure performed?
The procedure to implant an ABI device into the ear and brainstem is performed under general anaesthesia. The surgeon makes a small incision behind and above the ear where the processor part of the device will be placed. An electrode, which is directly connected to the brainstem’s auditory nucleus, is then inserted in the inner ear.
In the event that the patient is suffering from NF2, acoustic tumours will be removed through a translabyrinthine approach. Once the device has been properly implanted, the small incision will be closed up.
The entire procedure usually takes at least an hour to perform because of the complex structures involved.
The patient is allowed to recover from the procedure in a process that can take up to a month before the ABI device is turned on and tuned by a qualified medical professional.
Possible Risks and Complications
Risks and complications of an ABI procedure are rare, but a small percentage of patients can experience the following:
- Infection of the brain, such as meningitis
- Cerebrospinal fluid leaks
- Facial nerve weakness
Otto, SR; et al. (2002). "The multichannel auditory brainstem implant update: Performance in 61 patients". Journal of Neurosurgery 96: 1063–1071.
Colletti, L.; Shannon, R.; Colletti, V. (Oct 2012). "Auditory brainstem implants for neurofibromatosis type 2.". Curr Opin Otolaryngol Head Neck Surg 20 (5): 353–7.