At left, an ITE hearing aid has been altered, allowing for a tubing connection to the Nucleus speech processor. The Nucleus Freedom speech processor also has integrated hearing aid circuitry though it has not yet been used clinically. The Med-El DUET (at right), an investigational device in U.S. clinical trials, integrates a hearing aid and speech processor.

One of the newest applications of implantable hearing technology now in clinical trials combines electric and acoustic stimulation (EAS) into a hybrid device designed for individuals with binaural low-frequency hearing and severe-to-profound high-frequency hearing loss.

Research has shown that these individuals may not gain significant benefit from traditional amplification (e.g., Hogan & Turner, 1998; Ching et al., 1998; Turner & Cummings, 1999). One possibility for this lack of amplification benefit is the presence of cochlear dead regions, which are more common with thresholds above 70 dB HL (Vinay & Moore, 2007). Another possibility is that the high levels required to make high-frequency speech audible may, in fact, degrade the signal.

Although these individuals may not obtain much benefit from high-frequency amplification, their relatively good low-frequency hearing may disqualify them from conventional cochlear implant candidacy. As a result, individuals with good low-frequency hearing and severe-to-profound high-frequency hearing loss can experience significant difficulty in everyday communication, particularly in noisy backgrounds, where low-frequency information alone is not sufficient to allow high levels of speech understanding.

The hybrid devices use a shortened cochlear implant electrode array that is inserted just 10 mm-20 mm into the cochlea (versus 20 mm-30 mm for a conventional implant). A successful surgical outcome allows for monaural electric stimulation of the basal cochlea for high-frequency information without damaging apical cochlear structures that transmit low-frequency acoustic information (e.g., Gantz et al., 2005, 2006; Gstöettner et al., 2005; Kiefer et al., 2005; Leutje et al., 2007; Skarzynski et al., 2006). This combination allows for the integration of electric and acoustic perception in the same ear.

The audiologic candidacy criteria for EAS or hybrid technology vary somewhat across manufacturers. Low-frequency thresholds generally can range from 20 dB HL up to 60 dB HL through 750 Hz, and thresholds at 1000 Hz and above must generally exceed 60 to 70 dB HL. Preoperative speech perception criteria require that aided CNC monosyllabic word recognition in the ear to be implanted cannot exceed 50%-60% correct.

Following surgery, the average low-frequency hearing loss ranges from 10 to 20 dB, depending on the electrode array and the nature of the surgical technique (Gantz et al., 2005, 2006; Gstöettner et al., 2005; Kiefer et al., 2005; Leutje et al., 2007; Skarzynski et al., 2006). Some individuals, however, have a more significant loss of low-frequency acoustic hearing in the implanted ear ranging from >20 dB to total loss of hearing. It is interesting to note that for individuals with measurable post-implant hearing, there is no correlation reported between the degree of low-frequency hearing preservation and postoperative performance on measures of speech recognition.

EAS patients wear an in-the-ear (ITE) hearing aid in the implanted ear in combination with an external ear-level or body-worn speech processor or an integrated hearing aid/speech processor on the implanted side, as shown in Figure 1 above. These patients wear a hearing aid on the non-implanted side as well, allowing for binaural acoustic hearing. Thus, EAS patients are essentially utilizing three separate auditory prostheses, using both ears.

Options for Hearing Preservation

Hearing preservation with a cochlear implant is also possible with a conventional long electrode array. It had been assumed that any residual hearing in the implanted ear would be sacrificed due to surgical trauma; however, in some instances, this is no longer the case. Increasingly skilled surgeons employing soft surgical techniques—which may include a smaller cochleostomy or round window insertion and more careful electrode insertion—with thinner electrode arrays and/or perimodiolar electrodes (which also may allow for a relatively atraumatic cochlear insertion) have all helped contribute to hearing preservation with standard cochlear implants.

In the past, cochlear implant patients typically had little or no measurable hearing preoperatively, but today many cochlear implant candidates have significant residual hearing, making hearing preservation possible. Balkany et al. (2006) reported measurable acoustic hearing in 28 standard, perimodiolar cochlear implant recipients, demonstrating a mean change of 15 dB in the pure-tone average (250, 500, and 1000 Hz) and resulting in a mean postoperative pure-tone average of 114 dB HL. At Mayo Clinic in Rochester, six implant recipients (four adults and two children) were implanted with a standard long-electrode device, but in retrospect were generally found to meet the audiologic criteria for an EAS or hybrid device (Gifford et al., 2007). These six individuals demonstrated hearing preservation in the implanted ear that rivals short-electrode EAS or hybrid recipients (see Figure 2 at right [PDF]).

Yet another variation of EAS is the standard cochlear implant recipient who combines electric hearing with contralateral aided acoustic hearing, commonly referred to as bimodal listening. Though most would not group bimodal listeners into the category of hearing preservation, they are certainly combining the electric and acoustic perception, albeit across ears.

Speech Perception Performance

How might we expect cochlear implant recipients—including short-electrode, long-electrode, and bimodal listeners—to perform on measures of speech perception? Figure 3 (above right) [PDF] compares the speech perception performance for monosyllabic CNC word recognition for these various groups of cochlear implant listeners. Although the data were collected in different studies, the subjects had similar degrees of pre- and post-implant hearing loss, configurations, etiologies, and duration of deafness. Cochlear implant recipients without acoustic hearing (n=162) served as the control group for average performance with a standard long-electrode cochlear implant—without hearing preservation—as shown in the green bar (Gifford et al., in review). The mean word recognition scores for 36 bimodal listeners using a standard cochlear implant in one ear and a hearing aid in the other are shown in the yellow bar (Gifford et al., in review). The orange bars show average speech perception results in two studies with EAS recipients in their combined condition (short-electrode cochlear implant + binaural amplification), including one study with 13 recipients as reported by Leutje et al. (2007), and another study with 11 recipients as reported by Gantz et al. (2005). The blue bar shows the average speech perception performance of three standard long-electrode recipients with hearing preservation in the implanted ear (Gifford et al., 2007).

As shown in Figure 3 [PDF], average EAS recipients are performing as well as conventional, long-electrode cochlear implant recipients. Bimodal hearing with a conventional long electrode implant, however, tends to yield higher levels of performance than conventional unilateral cochlear implant users as well as hybrid patients. The average performance for the final listening condition includes the small group of long-electrode patients with hearing preservation in the implanted ear. Average performance for this group is higher than all other groups, but given the small sample, it is not possible to draw any firm conclusions. The bottom line is that monosyllabic word recognition performance for hybrid recipients is comparable to standard unilateral implant recipients; however, hybrid patients have two acoustic-hearing ears, a condition that may provide real-world binaural benefit that standard clinical test measures do not measure.

EAS or hybrid technology with a short-electrode implant offers a viable treatment option for individuals who do not receive benefit from hearing aids and may not qualify for a standard cochlear implant due to significant residual low-frequency hearing. Given that EAS/hybrid technology is still in the clinical trial phase, its availability is somewhat limited. It will likely be at least two years before the U.S. Food and Drug Administration grants approval for widespread market availability. Research efforts are underway to determine the appropriate length of the array, number of intracochlear electrodes, mapping strategy, spectral range for amplification, and preoperative criteria (audiologic and speech perception) to provide a balance between maximizing postoperative performance with minimal loss of low-frequency hearing.

Some professionals might argue that hearing preservation in the implanted ear may not be necessary to achieve high levels of speech perception performance, and current data for bimodal patients may tend to support this conclusion. Yet, we do not yet fully understand the potential positive implications for preserving binaural acoustic hearing.