Although cochlear implants (CIs) are highly successful at restoring functional hearing, some people have no remaining auditory nerve and cannot benefit from a CI. These patients have lost their auditory nerve (VIIIn) from a variety of causes, most commonly neurofibromatosis type 2 (NF2). NF2, a genetic defect on chromosome 22, causes tumors originating in the Schwann cells that insulate the auditory nerve where it exits the internal auditory meatus. When the tumors are removed the auditory nerve is usually cut and no connection exists between the still-functioning cochlea and the brain. Other causes of VIII nerve loss are temporal bone fracture, congenital aplasia of the cochlea and/or nerve, and severe ossification from congenital or post-meningitic growth.

The auditory brainstem implant (ABI) is similar in design and function to a CI, except that the electrode is placed on the first auditory relay station in the brainstem, the cochlear nucleus (CN). The ABI electrode array is a small (8 x 3 mm) paddle that contains 21 small electrode contacts. Unlike the cochlea, which has a single linear tonotopic organization from base to apex, the CN has several tonotopic maps that are at different angles to one another within the nucleus. The CN also has multiple cell types that are specialized to extract different types of information from the VIII nerve input. The ABI electrode array is placed along the surface of the CN and each electrode likely activates a variety of neuron types, possibly with different characteristic frequencies. 

Auditory Performance  

NF2 patients with an ABI receive sound sensation on most electrodes that produce distinct pitch sensations (Otto et al., 2002). Electrodes that produce only non-auditory sensations (mostly tingling sensations along the ipsilateral side of the body) are turned off and not included in the implant program. Patients with the ABI can detect and discriminate sounds based on their temporal and amplitude properties. This auditory information improves their face-to-face communication by about 30 percentage points by supplementing speechreading. However, most NF2 ABI recipients cannot identify words or sentences with only the sound from the ABI—a few patients have more than 20% word understanding.

When ABIs are provided to patients who have lost their auditory nerve from causes other than NF2 (non-tumor or NT patients), the outcomes of the ABI are dramatically different. Colletti and colleagues in Verona, Italy, have demonstrated that about 50% of NT ABI patients have sentence understanding of more than 50% with only the sound from the ABI without speechreading (Colletti & Shannon, 2005). Several of these patients achieved speech understanding equivalent to the best performance of CI patients. These results demonstrate that electrical stimulation of the human brainstem (CN) can provide excellent speech understanding for some patients. It also suggests that the cause of poor speech recognition in NF2 ABI patients may be related to NF2. However, recent results in Europe have demonstrated speech recognition levels similar to those of CI patients in NF2 ABI patients—a level previously not obtained by hundreds of patients with ABIs around the world (Behr et al., 2007). Several NF2 ABI patients recently have shown excellent speech recognition, even in noisy listening conditions as poor as +3 dB signal-to-noise ratio (Skarzynsky et al., 2009).

ABIs in Children  

In Europe, children with congenital malformations of the cochlea and/or the absence of an auditory nerve have received an ABI (Colletti et al., 2001; Senaroglu et al., 2009), and some of these children are making progress in auditory and speech development that is comparable to that of similar-age children with CIs (Eisenberg et al., 2008). In addition, children demonstrated improvements in cognitive development (Colletti & Zoccante, 2008).

Research is currently underway to investigate the causes of this large difference in performance across patients with ABIs. What is now clear is that excellent speech understanding is possible with an ABI. Poorer performance with an ABI may be caused by damage to the CN, either by the tumor or its removal, or by the trauma that caused the loss of the VIII nerve. Further research will help to define the patient populations that are most likely to receive the greatest benefit from an ABI.