Preliminary speech recognition results after cochlear implantation in patients with unilateral hearing loss: a case series
© Stelzig et al; licensee BioMed Central Ltd. 2011
Received: 9 August 2010
Accepted: 2 August 2011
Published: 2 August 2011
Cochlear implants known to provide support in individuals with bilateral hearing loss may also be of great benefit for individuals with unilateral hearing loss. This case report demonstrates the positive effects of cochlear implantation on speech understanding in noise conditions in patients with unilateral hearing loss and normal hearing on the contralateral side. To the best of our knowledge, the data presented here are from the first few cases to receive a cochlear implant for unilateral hearing loss.
Four Caucasian German men, two aged 48 and the others aged 51 and 57 years old, with post-lingual unilateral hearing loss and normal hearing on the contralateral side were implanted with a cochlear implant. All our patients were members of the German army. Before and after implantation, they were given a battery of speech tests in different hearing conditions to assess the effect of unilateral cochlear implantation on speech understanding in noise conditions. Test results showed that all patients benefited from unilateral cochlear implantation, particularly in terms of speech understanding in noise conditions.
Unilateral cochlear implantation might be a successful treatment method for patients with unilateral hearing loss not benefiting from alternative treatment options. The results of this case report open up the field of cochlear implantation for expanded criteria and new areas of research.
Many individuals with unilateral hearing loss (UHL) have genuine difficulties in understanding speech in noise conditions. Despite these impediments, the impact of a complete UHL is often minimized by the presence of (near) normal hearing (NH) on the contralateral side. However, different studies have shown that the normal hearing capabilities of these individuals do not compensate for their UHL . Lin et al.  addressed the auditory deficits of patients with UHL. They reported that monaural patients had the greatest difficulties when the sound or source of speech was localized on the hearing impaired side, presumably due to the reduced exploitation of binaural processes.
Although it is recognized that patients with UHL encounter problems in speech recognition in noise conditions or sound localization, only very few treatment methods are available to these patients . Currently, they are generally treated with contralateral routing of signals (CROS) hearing aids  or bone-anchored hearing aid (BAHA) implants . However, various studies have demonstrated a poor user satisfaction of CROS as well as only a minimal improvement in speech discrimination in noise conditions and none for sound localization with the BAHA implant [2, 5].
In contrast to these treatment options for UHL, bilateral HL is mainly treated by cochlear implantation. The effectiveness of bilateral cochlear implants (CIs) has been demonstrated in the last several years [6, 7]. Patients with bilateral CIs show better speech discrimination largely due to the exploitation of the head shadow effect (with the head obstructing noise sources from the receiving ear), the binaural summation effect (receiving redundant information at both ears) and the squelch effect (occurring due to temporal and spectral differences of spatially separated speech and noise sources) . According to Schleich et al. , bilateral CI users significantly benefit from head shadow, squelch and summation effects, substantially improving their performance of speech understanding in noise conditions. These results may also suggest that individuals with UHL could gain substantial benefit from CIs due to added binaural effects as shown in a tinnitus study by Vermeire et al. .
Patient S1 was a 48-year-old Caucasian German man with post-lingual UHL and NH on his contralateral ear. The etiology of his hearing loss was unknown, and his middle ear status was normal. He had a duration of deafness of 11 months before unilateral implantation with a CI. Patient S2 was a 51-year-old Caucasian German man also with post-lingual UHL and NH on his contralateral ear. His hearing loss was caused by acoustic trauma; his middle ear status was reported to be normal. Patient S2 had a duration of deafness of 45 months before unilateral CI implantation. Patient S3 was a 48-year-old Caucasian German man with post-lingual UHL and NH on his contralateral ear. He had lost his hearing due to a stapedectomy; he showed a normal middle ear status. Patient S3 was implanted unilaterally with a CI after a duration of deafness of 96 months. Patient S4 was a 57-year-old Caucasian German man with post-lingual UHL and NH on his contralateral ear. His hearing loss was due to a Borrelia infection. Patient S4 also showed a normal middle ear status and was implanted with a CI after a duration of hearing loss of 33 months.
Our patients did not wear hearing aids (HAs) before implantation as their hearing loss (HL) was too profound for HAs to provide sufficient acoustic amplification. Since all our patients were members of the German Army, which is obliged by law to provide the best possible compensation for any kind of disability, treatment costs were no issue. Our patients, who all had leading positions within the army, consistently reported a high level of distress often related to feelings that their job was at risk because of their HL and were thus highly motivated for CI treatment. All our patients were thoroughly counseled and signed informed consent before implantation. Authorization was provided by the Germany Army, as our patients were treated in an army hospital in Germany.
All our patients were implanted with the PULSARCI100 implant and a standard electrode of 31.5 mm in length (MED-EL, Innsbruck, Austria). Our patients also received the OPUS 2 speech processor including the FineStructure coding strategy. In all patients, first fitting of the CI was performed approximately four weeks after implantation and included the adjustment of electrical hearing thresholds, the most comfortable stimulation levels as well as frequency allocation and compression characteristics. Our patients received aural rehabilitation therapy for patients with CI in their hearing centers. Bilateral testing was performed once a monosyllable understanding in quiet conditions of 50% correctness was achieved.
Monosyllable test; CI-only*
Monosyllable test; acoustic-only and binaural*
HSM; acoustic-only and binaural*
OLSA; acoustic-only and binaural*
Dichotic listening test, CI-only, acoustic-only and binaural*
Audiogram of normal hearing ear
Audiogram of hearing impaired ear (acoustic-only)
Audiogram of implanted ear (CI-only)*
Specific testing decisions were taken considering the dichotic listening test, whose design deviated from the original test layout  by using loudspeakers instead of headphones. Thus, we could focus not only on the perception of two highly separated sounds but also on head shadow and binaural effects. The positive results of this test suggest that unilaterally implanted patients may benefit from a reduced head shadow, which in this case obstructed speech sources from the hearing ear, as well as from binaural effects known for NH individuals or bilateral CI users and the ability to integrate independent inputs on both sides.
The results of all speech tests demonstrate that patients benefit from binaural hearing when compared to the acoustic-only or CI-only conditions. However, due to the small number of patients tested, conclusions about the statistical significance cannot be drawn.
To obtain a tendency of the subjective perception, we used a visual analog scale (VAS), ranging from 0 (very low) to 10 (very high). Our patients stated a high level of CI acceptance, integration of CI hearing, increased ease of listening especially in noise situations and a regaining of acoustic orientation abilities. After our patients became accustomed to the CI sound, they also rated the quality of the sound signals generated by the CI to be good. No negative interference of NH when using the CI was reported. This can probably be attributed to advancing CI technology having developed modern coding strategies with high frequency resolution and temporal processing. Overall, the subjective ratings of the CI were more positive than results of the objective testing.
It should be emphasized, however, that the VAS was not validated, but shows a tendency of how the CI is perceived. It must furthermore be stated that the subjective results might possibly be influenced by psychological effects, such as our patients' high motivation and expectations towards CI implantation. It might thus be interesting for future studies to focus in greater detail on the subjective benefits of unilateral CIs in particular, as well as on possible psychological effects influencing the subjective perception.
This contrast between objective and subjective benefits might also be due to the fact that the speech tests are designed for bilateral HL and not for NH on the contralateral side. The results in the acoustic-only condition might be dominated by the NH ear, thus decreasing the measurable difference between acoustic-only and binaural test results. A further explanation might be that all our patients reported improved sound localization abilities, which, however, cannot be fully reflected in speech tests, even if two separately placed speech sources are used. It would be an attractive aspect for future unilateral CI studies to include localization tests and to investigate if subjective and objective results continue to show different levels of improvement. Furthermore, future studies should include a greater subject population and adapted speech tests focusing on unilateral hearing so that statistical significance can be demonstrated.
Our patients in the present study had profound UHL for which HAs would not have rendered sufficient acoustic amplification. Since all our patients were members of the German army, treatment costs were no issue; however, generally, cost effectiveness plays an important role in health care structures. Regardless, based on the results of studies by Bond et al. , for example, the decision of unilateral cochlear implantation should not be influenced or even restrained by cost-related arguments.
Our patients showed a small but important benefit from unilateral cochlear implantation in speech recognition in noise conditions. The subjective benefits suggest a high degree of integration of the artificial auditory input through the CI. Restored sound localization and a regained spatial awareness were also reported and might be of interest in future research.
Written informed consent was obtained from the patients for publication of this case report and any accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal.
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