5The ABI Device: A Cochlear Implant With a Different Electrode

FSame engine, different electrode

An auditory brainstem implant shares most of its architecture with a cochlear implant. There is an external behind-the-ear sound processor with a microphone, a transcutaneous coil that transmits power and data across the skin, and an internal receiver-stimulator that decodes the signal and delivers electrical pulses. The speech-coding strategies are borrowed from the cochlear-implant world as well. What changes is the business end: instead of a long, thin array designed to wind through the spiral of the cochlea, the ABI carries a small flat paddle of electrodes designed to rest against the surface of the cochlear nucleus in the brainstem.

This shared lineage is deliberate. Because cochlear-implant electronics were already mature and reliable, ABI development largely consisted of designing a new electrode and adapting the surgery, while reusing the proven stimulator and processor. As a result, an ABI recipient wears and uses external equipment that looks much like any cochlear-implant user’s.[2012][2002]

TThe paddle array versus the spiral array

The intracochlear array of a cochlear implant exploits the tonotopic map of the cochlea: electrodes spaced along its length sit at different frequency places, so position encodes pitch in an orderly, well-understood way. The ABI paddle cannot rely on the same precision. It is a flat silicone carrier, often backed by a Dacron mesh that encourages tissue ingrowth and helps anchor it, bearing a grid of platinum surface contacts. Contemporary surface arrays carry on the order of twelve to twenty-one active electrodes depending on the manufacturer.

Because the cochlear nucleus is not directly visualised at surgery and its tonotopic sheets lie partly below the surface, the relationship between a given electrode and the pitch it evokes is far less predictable than in the cochlea. The contact spacing must be matched to a structure only about one square centimetre in visible area, and a meaningful fraction of contacts may ultimately have to be switched off because they evoke non-auditory side effects rather than clean sound. This anatomical mismatch, rather than the electronics, is the central engineering limitation of the ABI.[2002][2012]

CThe penetrating array idea (PABI)

Surface electrodes stimulate the cochlear nucleus from the outside, and the neurons most relevant to clear hearing lie deeper than the contacts can selectively reach. To address this, researchers built a penetrating auditory brainstem implant, or PABI, that added a small number of microelectrodes on thin shanks designed to enter the nucleus and sit closer to the target neurons, alongside a conventional surface array.

The engineering rationale was sound: the penetrating contacts did achieve lower stimulation thresholds, a wider range of evoked pitches, and more selective stimulation than surface electrodes. Crucially, however, these gains did not translate into better speech understanding in the trial, and the penetrating approach was not adopted as standard practice. The PABI remains an instructive example of a device that improved the physical metrics without improving the clinical outcome, and surface-electrode ABIs remain the norm.[2008]

CWho makes ABIs

The ABI market mirrors the cochlear-implant industry. Cochlear Corporation produces a multichannel ABI built on its Nucleus platform, carrying a paddle of platinum surface electrodes on a mesh backing; this device has the longest track record and is the one cleared for use in the United States. MED-EL offers an ABI system used widely in Europe and beyond, with its own surface array and processor and a separate small placing electrode used during surgery to find the best location before the definitive array is laid down. Advanced Bionics produced a Clarion-based ABI that was used in Europe but was subsequently discontinued.

A practical consequence of NF2 candidacy shapes the hardware: because these patients need lifelong surveillance MRI for other tumours, the internal magnet that normally retains the external coil is often replaced by a non-magnetic spacer, and the coil is held by an adhesive retainer instead. The same MRI and cautery precautions that apply to cochlear implants apply to ABIs.[2002][2012]