3. Surgical intervention

The physiological/structural condition of the cochlea may affect electrical stimulation. A full battery of objective measures available to the surgical team conducted pre- and intraoperatively helps guide preoperative planning and postoperative device programming [7]. Aspects to consider are the size of the cochlea [8], the type of electrode design (straight or perimodiolar) and its potential insertion depth, as well as whether the insertion was solely into the scala tympani or dislocated into the scala vestibuli. In our study, scala dislocation reduced scores by 12–25 points at the 1-month evaluation interval [1]. Thus, selection of the implant device, in cooperation with the patient wishes, anatomical considerations and

#### Prognostics Factors of Cochlear Implant in Adults: How Can We Improve Poorer Performers? DOI: http://dx.doi.org/10.5772/intechopen.89577

surgical intervention each play a role in performance outcomes and account for 8–13% of the variance in performance scores at 1 year.

Preoperatively, it is essential to choose the appropriate electrode type and to target an insertion depth of one cochlear turn (i.e., 360°) as proposed by [1]. This aim is also supported by [9], who indicated a negative correlation between word scores and electrode insertion depth measures. The study by Lazard et al. [6] also found poorer outcomes for the most deeply inserted electrodes. These results need to be tempered against the potential of having larger frequency-place mismatches for shallower electrode insertion depths as discussed in the following section.

Any information that contributes to the first activation and mapping for listening programs is useful. The insertion depth provides a reference for better accessing appropriate frequency allocations relative to cochlear tonotopic organization [8]. Electrode design also plays a role not only because of its insertion characteristics, straight or curved, but also because of the spacing between contact electrodes.

Our studies have shown that an insertion depth of 300–360° yielded optimal performance. Moderate shifts in frequency-to-place may easily be accommodated by the listener, but larger shifts >1.5 octave may affect auditory performance, and adaptation may take longer [10]. Electrode placement can be detected by routine intraoperative X-ray. Shifts were approximately one octave for Nucleus Implants with 360° insertion depth, with shifts still <1.5 octaves for 300°, for the default frequency allocation table. For other devices, the shifts appeared greater for the same insertion depths due to the specific default frequency-to-electrode allocation used in the device. Thus, these devices may work most effectively with greater insertion depths or, alternatively, with the use of customized frequency allocation tables that can be adjusted in the specific programming software.

Avoiding a frequency-place shift of greater than 1.5 octaves will probably produce the best result for a given insertion depth. However, further optimization may be achieved by limiting insertion depth at surgery or deactivating the most apical electrodes (e.g., [11]). If electrode arrays are found to be inserted greater than one turn, we may consider deactivating the most apical electrode contacts to simulate the ideal insertion depth. This is consistent with the work of [8] whose temporal bone studies found correlations between specific insertion depth angles and tonotopic frequency locations. Deeper insertion, greater than 360°, was associated with frequencies lower than 900 Hz; however, one needs to consider that the spatial density of spiral ganglion cells increases considerably past this point, such that cross-turn stimulation can easily occur. As mentioned, depending on the device type, if the active insertion depth is limited to 360°, then it may be necessary to modify the frequency-to-electrode allocation through programming to avoid excessive frequency-place shifts.

#### 3.1 Intraoperative tests

After the electrode has successfully been placed into the cochlea, monitoring its position is accomplished through intraoperative X-ray [7]. The neural activity of device-activated electrical stimulation is evaluated with neural response telemetry (NRT), which replicates electrically evoked compound action potentials (ECAP). The NRT responses provide an objective measure of the integrity of auditory nerve function when stimulated through a CI [12, 13]. It can be administered intra- and postoperatively; a thorough description of the method is described by [14], and the newer application of auto-NRT is described by [15]. Intraoperatively, the focus is on gaining details relating to whether the device is operational and whether the responses per electrode indicate that electrodes are within the scala tympani and

2.1 Main factors influencing performance

\* p<0.05, \*\* p<0.01, \*\*\* p<0.001.

Advances in Rehabilitation of Hearing Loss

of sentence recognition.

\*\* p<0.01, \*\*\* p<0.001.

sentence recognition.

Table 1.

Table 2.

helps estimate potential outcomes.

3. Surgical intervention

144

A thorough patient history is needed to gain details of etiology and duration of hearing loss. Our studies indicate that 6–12% of the total variance for speech understanding in quiet is related to the duration of deafness and approximately 30% is related to the etiology [1]. For instance, congenital HL produces significantly poorer scores in the short term and chronic otitis media in the long term [1, 4]. Certain diseases may produce greater damage to the cochlea resulting in poorer signal transmission after implantation such as bony tissue growth induced by meningitis or trauma. Speech signals may be distorted more than expected by poor neural representation of speech features due to anatomical distortions from diseases that affected the hearing [5]. The challenge is that characteristics of even a known etiology may not be clear.

Surgical factors explaining significant variance (\*) at 1-month post-activation with respect to outcomes of

Etiology 0.34\*\*\* 0.25\*\* Duration of deafness per year 0.06\* 0.08\*\* Total in percent 40% 33%

Proportion of electrodes in the scala media 0.14\*\* 0.13\*\* Insertion length per degree 0.09\*\*\* 0.08\*\*\* Total in percent 23% 21%

Patient history factors explaining significant variance (\*) at 1-month post-activation with respect to outcomes

In quiet In noise (10 dB SNR)

In quiet In noise (10 dB SNR)

Details concerning the duration of deafness may be elusive; for instance, defining the specific onset of significant hearing loss may be difficult to determine and impacted by hearing aid use (i.e., how much was one or two hearing aids actually used (e.g., [6]), was the loss progressive, how rapid did the loss develop, and so forth). The impact of unanswered questions may be seen in later performance, especially in cases of unexpected poor performance. Applying the predictive model

The physiological/structural condition of the cochlea may affect electrical stimulation. A full battery of objective measures available to the surgical team conducted pre- and intraoperatively helps guide preoperative planning and postoperative device programming [7]. Aspects to consider are the size of the cochlea [8], the type of electrode design (straight or perimodiolar) and its potential insertion depth, as well as whether the insertion was solely into the scala tympani or

dislocated into the scala vestibuli. In our study, scala dislocation reduced scores by 12–25 points at the 1-month evaluation interval [1]. Thus, selection of the implant device, in cooperation with the patient wishes, anatomical considerations and

close enough to activate auditory nerves. Those outside, mislocated into the scala vestibuli, may yield no NRT response [13].

In fact, the development of speech understanding with a CI does not follow a linear function with time. High sentence recognition scores can be obtained at only 1 day after activation, and the first 2 weeks are as important as the next 6 months and the following 2–3 years. It is not fully understood why CI user's individual performance progress at different rates. In James et al. [1], they observed different patterns of growth in scores, both in quiet and in noise, from the first month, but always following a logarithmic growth curve, such that each additional increment in

Prognostics Factors of Cochlear Implant in Adults: How Can We Improve Poorer Performers?

Significant improvement will usually take place from activation to 1 month; thereafter increases continue but at a much slower pace. Increases in understanding will be about the same after 6 months of experience for sentences in quiet. Adapting to any new sensation requires time; an auditory signal presented through a CI will always first be perceived as very different. It is unclear why some new users immediately accept the new input and others reject it as sounding too foreign. In any case, we believe a month of exposure to the new signals is the minimum time to allow all patients for the initial accommodation to the input. Thus, all CI users are re-evaluated at 1 month. By the first month, there already is access to data logging to confirm speech processor program usage, the users are usually aware of which program they might prefer, and the speech recognition scores in quiet will have been tested. The outcome of sentence recognition testing and CI user reports may indicate a need for alternative device programming. Looking at Tables 1 and 2, approximately 40–50% of the variance is not explained by the patient-related and surgical factors. There are dynamics in play that may never be known such as the impact of certain disadvantages (insertion depth, dislocation, cochlear condition at surgery) and others. Alternative programs (differing mapping parameters) may also take into consideration speed of stimulation (refractory period) as demonstrated through different

performance took twice as long as the preceding increase.

DOI: http://dx.doi.org/10.5772/intechopen.89577

5. Optimizing maps and initial evaluations: 1-month follow-up

stimulation rates or spread of excitation via channel selectivity (perhaps

degraded speech signals delivered through a cochlear implant.

a better foundation for learning to overcome perceptual difficulties.

5.1 Initial performance evaluations

147

deactivating particular electrodes). These more advanced aspects of programing, however, are taken into consideration at every programming session, as indicated. Optimizing sound processor programs is the most direct way to compensate for the

The one aspect to be evaluated may be behavioral responses to changes in stimulation rate. Postoperative NRT testing may be indicated to assess neural recovery functions to gain information about beneficial stimulation rates. From their studies on the temporal characteristics of auditory nerve stimulation via CIs, [16] suggest that the programmed stimulation rate relates to the refractory period of the nerve. CI user performance may be addressed, in some cases, by reducing the stimulation rate. It is not possible to define when the so-called aging process begins, but it is clear that neural transmission times slow as one ages [17, 18]. Older CI users may be more susceptible to stimulation rate effects. Any means of enhancing auditory signals that occur in the presence of poor temporal processing will provide

During this test interval, it is possible to identify, with more clarity, the individuals who might be classified as potentially having poor performance. By definition, on average, approximately 50% of recipients will demonstrate "normal" performance, i.e., 70% or greater scores for sentence understanding in quiet. However, if
