An auditory brainstem implant (ABI) is a medical device providing sound sensation to individuals with profound hearing loss. It offers a pathway to sound perception for those whose inner ear or auditory nerve is either missing or not functioning. An ABI works by bypassing damaged parts of the auditory system and directly stimulating the brainstem, interpreting these signals as sound.
What is an Auditory Brainstem Implant?
An auditory brainstem implant is an electronic device with external and internal components. The external part, worn behind the ear, includes a microphone and a sound processor. The internal component, implanted, consists of a receiver-stimulator and an electrode array. Unlike traditional hearing aids that amplify sound or cochlear implants that stimulate the auditory nerve, the ABI’s electrode array is positioned directly on the cochlear nucleus in the brainstem. This direct stimulation of the brain’s hearing pathways offers an alternative for those unable to benefit from other hearing technologies.
Who Benefits from an Auditory Brainstem Implant?
The auditory brainstem implant is for individuals with profound hearing loss who cannot use cochlear implants due to a damaged or absent auditory nerve or cochlea. A primary group benefiting are patients with Neurofibromatosis Type 2 (NF2), a genetic disorder causing auditory nerve tumors. Surgical removal of these tumors can lead to bilateral profound hearing loss, making an ABI a viable option.
Beyond NF2, other conditions can lead to ABI candidacy, such as congenital malformations where the auditory nerve is absent or underdeveloped, severe trauma, or complete ossification of the cochlea, which prevents cochlear implant placement. The evaluation process for an ABI is comprehensive, involving a multidisciplinary team of neurotologists, neurosurgeons, audiologists, and speech therapists. This assessment ensures the patient meets specific medical and anatomical criteria.
How the Auditory Brainstem Implant Works
The ABI system begins when the external sound processor, worn behind the ear, captures environmental sounds. Its microphone picks up sound waves and converts them into electrical signals. These signals are then transformed into a coded digital format within the speech processor.
The coded signals are transmitted wirelessly from the external processor to the internal receiver-stimulator. From the receiver-stimulator, these signals travel to the electrode array. The paddle-shaped electrode array is placed on the cochlear nucleus complex, the brainstem’s hearing portion. Electrodes on the array then directly stimulate brainstem neurons, which the brain interprets as sound. This sequence happens rapidly, allowing the ABI user to perceive sounds as they occur.
The Surgical Procedure and Activation
ABI implantation involves surgery under general anesthesia, lasting three to six hours. It requires collaboration between a neurotologist and neurosurgeon. An incision, behind the ear, accesses the brainstem. Surgeons then place the electrode array directly onto the brainstem’s cochlear nucleus.
During the procedure, intraoperative monitoring of brainstem auditory evoked potentials is used to confirm optimal electrode array placement and ensure it does not stimulate other nerves. Once secured, the internal receiver-stimulator is anchored into the skull. Patients remain hospitalized for two to four days post-surgery, with longer stays possible if tumor removal was also performed. Device “activation,” when it is first turned on and programmed, occurs around four to six weeks after surgery, allowing for initial healing.
Expected Outcomes and Rehabilitation
Outcomes for ABI users vary, but most gain some sound awareness. While an ABI does not restore normal hearing, it significantly improves sound identification and lip-reading. Younger children implanted after shorter periods of profound deafness show better word understanding than adults with conditions like NF2 who experienced longer deafness.
Post-implantation auditory rehabilitation and therapy are important for maximizing ABI benefit. This process helps the brain adapt to and interpret new electrical signals as sound. Consistent therapy, involving regular programming adjustments and auditory training, helps users recognize environmental sounds, improve speech rhythm and intonation following, and enhance speech perception with lip-reading. While speech understanding without visual cues can be challenging, the ABI provides a foundation for improved communication and interaction.