1

Introduction

Continuing improvement in the outcome of cochlear implantation has led to some extension of the criteria for patient selection. As a consequence, more and more patients undergo monaural cochlear implantation while they still have some useful residual hearing in the nonimplanted ear. Nevertheless, many of these patients do not continue to use a hearing aid (HA) in the nonimplanted ear.

People who receive a unilateral cochlear implant (CI) generally perceive speech well in quiet but find speech perception in acoustically complex situations very challenging. This is in part because they do not have access to binaural hearing.

For bilaterally totally deaf patients, the only possibility of achieving binaural stimulation is through bilateral cochlear implantation. For candidates with some residual hearing in the nonimplanted ear, bilateral auditory input can be obtained by the use of a unilateral CI and a contralateral conventional HA. This condition is referred to as “bimodal hearing.”

The 2 types of different stimulation, acoustic via the HA in one ear and electrical via the CI in the other ear, can be successfully used together with significant benefit , especially in noise condition . Previous studies have demonstrated that the fitting of a contralateral HA can provide benefits in terms of speech perception, speech intelligibility, and localization over the unilateral CI-alone condition in both adult patients and children . In addition to binaural advantages, there is some evidence showing that an HA and a CI provide complementary information. Acoustic amplification, via conventional HA, provides adequate low-frequency information, whereas electrical stimulation by CI does not. This low-frequency information relating to voice pitch has been found to enhance the segregation of competing voices, thereby contributing to speech perception in noise condition . Furthermore, CI recipients have reported that using an HA in the nonimplanted ear contributes to a more natural sound to give the impression that sound is heard on both ears rather than exclusively through the implant; and some even reported that their own voice quality is improved, finally leading to a better communication .

Most of the studies on bimodal hearing results regard postlingually deafened adult patients or children . To our knowledge, results from prelingually deafened adult patients have not been specifically evaluated but only together with those of postlingual patients in heterogeneous samples .

The purpose of the present study was to evaluate benefits from bimodal stimulation in a group of prelingually deafened adult CI users. With this aim, a group of 10 adult experienced prelingually deafened CI users, with detectable residual hearing in the nonimplanted ear, was fitted with the same type of high-powered digital HA contralaterally to the implant side.

2

Materials and methods

The sample was composed of 10 prelingually deafened adult patients, 4 males and 6 females, with a mean age 28.4 years (range, 18–43 years), from a group of 24 prelingually deafened adult patients consecutively submitted to unilateral cochlear implantation at the ENT Unit of the University of Pisa, during the period July 1999 to July 2006. We selected the patients on the basis of the following parameters: the presence of an aidable residual hearing in the nonimplanted ear, good language development, constant use of HAs bilaterally before implantation, at least 1 year of implant use, stable CI Measurable Audible Percept (MAP) for the past 6 months, and ability to open-set auditory alone speech understanding with CI. All the patients gave their consent to participate to the study.

The characteristics of the patients, regarding the type of implant and speech processor used and the experience with CI and bimodal hearing, are summarized in Table 1 .

All the patients enrolled in the study were oral language users, with good language skills, had some detectable residual hearing useful for amplification in the nonimplanted ear, with a mean hearing threshold (0.25–0.5–1–2–4 KHz) in the nonimplanted ear of 95.2 dB HL (range, 88–106 dB HL) ( Table 2 ), and consistently used an HA in the nonimplanted ear before surgery. We also calculated the mean hearing threshold at the medium-high frequencies between 1 and 4 KHz, which was 103.7 dB HL (range, 86.6–116.6 dB HL). When measuring the hearing threshold both without HA and with HA in the nonimplanted ear, we assigned a value of 125 dB to any frequency threshold over the maximum output limit of the audiometer (105 dB for 0.25 KHz and 125 dB for 0.5 and 1 KHz). Any vibrotactile sensation was also excluded.

All the patients enrolled in the study were fitted with the Starkey PXP 675 (Minneapolis, Saint Paul, MN, USA) HA (digital) in the nonimplanted ear. The HAs were fitted according to individualized digital prescription algorithms. The Berger formula was used in 2 patients: one of them used a digital high-powered HA and the other an analogue programmable HA contralaterally to the implant before the study setting. The NAL-R formula was used in the remaining patients. The choice of the fitting algorithm was based on the hearing threshold level in the nonimplanted ear, on the type of amplification used before surgery, and the study setting (in the case of experienced contralateral HA users) and on patients’ preferences. The loudness was balanced between ears.

After the contralateral HA fitting, a pure tone audiometry in free field with the HA alone was executed. The mean pure tone threshold (0.25–0.5–1–2–4 KHz) in free field with the HA was 47.5 dB HL (range, 41–58 dB HL) ( Table 3 ).

Speech perception was assessed using a speech perception test in Italian language. The test was administered by the same speech therapist to all the patients to avoid bias. We evaluated the disyllabic words recognition score using lists of 20 disyllabic Italian words at a level of 65 dB . A babble noise was used as the masking noise at a signal-to-noise ratio of +10 dB.

The speech perception tests were performed in 5 conditions:

• 
  

  a. Quiet

  
  
• 
  

  b. Speech front and noise front

  
  
• 
  

  c. Speech front and noise back

  
  
• 
  

  d. Speech front and noise CI side

  
  
• 
  

  e. Speech front and noise HA side

For the first 3 conditions, the results between 3 different stimulation types were compared: CI alone, HA alone, and CI + HA (bimodal stimulation).

For the last 2 conditions, the CI-alone results were compared to the CI + HA (bimodal) results.

The speech perception tests were performed at the moment of the first bimodal fitting and again after 6 months of bimodal hearing.

To obtain information about the subjective benefits from bimodal stimulation in comparison to CI-alone condition, we administered a questionnaire at the time of the first bimodal hearing fitting and again after 6 months of bimodal experience. The questionnaire is modified from that recently proposed by Flynn and Schmidtke . The aspects investigated by the questionnaires are reported in Table 4 . A score was obtained by assigning to each response a 5-point scale score (from 0 to 4) so that the total score could range from 0 to 16, as reported in Table 4 .

2

Materials and methods

The sample was composed of 10 prelingually deafened adult patients, 4 males and 6 females, with a mean age 28.4 years (range, 18–43 years), from a group of 24 prelingually deafened adult patients consecutively submitted to unilateral cochlear implantation at the ENT Unit of the University of Pisa, during the period July 1999 to July 2006. We selected the patients on the basis of the following parameters: the presence of an aidable residual hearing in the nonimplanted ear, good language development, constant use of HAs bilaterally before implantation, at least 1 year of implant use, stable CI Measurable Audible Percept (MAP) for the past 6 months, and ability to open-set auditory alone speech understanding with CI. All the patients gave their consent to participate to the study.

The characteristics of the patients, regarding the type of implant and speech processor used and the experience with CI and bimodal hearing, are summarized in Table 1 .

Table 1

Main characteristics of the patients enrolled in the study

Patients	Sex	Age (y)	Etiology	HL diagnosis (y)	Progression of HL	Experienced bimodal hearing user (type of HA)	Ear implanted	Duration of CI use	CI	Speech processor	Strategy
1	Female	20	Toxoplasma	5	Yes	Yes (analogue programmable)	Left	1 y	Freedom Contour Advance	Freedom BTE	ACE
2	Female	34	Unknown	1 1/2	Yes	Yes (digital)	Left	2 y	Freedom Contour Advance	Freedom BTE	ACE
3	Male	18	Connexin 26	1	No	Yes (analogue programmable)	Right	4 y	Nucleus Contour Advance	ESPRIT 3G	ACE
4	Male	43	Rubella	6	Yes	Yes (analogue)	Right	4 y	Nucleus Contour Advance	ESPRIT 3G	ACE
5	Female	19	Unknown	2 1/2	Yes	No	Right	7 y	Nucleus 24	ESPRIT 3G	ACE
6	Female	32	Unknown	1	Yes	No	Right	2 y	Freedom Contour Advance	Freedom BTE	ACE
7	Male	33	Unknown	5	No	No	Right	7 y	Nucleus 24 Contour	SPRINT body worn	ACE
8	Female	30	Rubella	4	Yes	Yes (analogue)	Left	2	Freedom Contour Advance	Freedom BTE	ACE
9	Male	25	Connexin 26	2	No	No	Left	2	Freedom Contour Advance	Freedom BTE	ACE
10	Female	31	Unknown	2 1/2	No	No	Right	2	Freedom Contour Advance	Freedom BTE	ACE

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