Introduction

As a result of the technological progress of cochlear implants more and more patients with useful residual hearing became potential candidates for implants use1.

The indication of cochlear implants has extended to severe hearing losses in adults2, 3, 4 which has increased the number of subjects with good residual hearing in the preoperative audiometry.

It is believed that subjects with good residual hearing would benefit the best from cochlear implants1, 5. In a study with temporal bones of 13 patients who presented severe hearing losses, Incesulu & Nadol (1998)6 found a greater number of cells of the spiral ganglion in the ears with best residual hearing. It could implicate better performance of speech recognition with the cochlear implants placed in such ears.

Despite that, it is not known for sure if the introduction of electrodes could destroy the auditory structures involved in residual hearing. Boggess et al. (1989)7 showed that hearing conservation is possible in about 30% of the implanted subjects, but with significant reduction of the operated ear thresholds.

In a histopathologic study, Jackler et al. (1989)8 observed the formation of granulation tissue around the round window and inflammation of the scala tympani after the insertion of long electrodes in guinea pigs. Clark et al. (1988)9 showed that the introduction of these electrodes cause minimum lesions to the cochlear structures. In order to position the electrodes to stimulate neural elements as close as possible, they are inserted in the scala vestibular or scale tympani using posterior tympanotomy 10.

The conservation of residual hearing after cochlear implants can suggest that both surgery and presence of electrodes do not influence the perception of the operated ear. Currently, the concept of conservative surgery for cochlear implants11, 12 is very promising in order to prevent deterioration of residual hearing in patients. However, the idea that the insertion of electrodes is traumatic13 still persists, followed by the concept that it may cause remaining ganglion cell loss.

Therefore, the present study intended to check the conservation of residual hearing in patients users of multichannel cochlear implants comparing pre and post-operative audiometry of these subjects.

Material and Method

We have implanted 29 patients in the Department of Clinical Otorhinolaryngology, HCFMUSP, since 1999. There were 15 female and 14 male patients, ages ranging from 02 to 77 years. Among them, only one subject had complete absence of residual hearing in both ears (anacusia).

Both pre and post-operative audiometry were performed using audiometer model Madsen Midimate 622 with maximum output of 120dB for frequencies 500Hz to 4000Hz, 105 dB for frequencies 250Hz to 8000Hz and 115dB for 6000Hz. In addition to pure tone audiometry, tested from 250 to 8000Hz, we also conducted speech recognition thresholds (SRT), if possible, and speech detection thresholds (SDT). In children, audiometry was performed using the playful conditioning technique with or without visual reinforcement, in addition to behavioral responses. All patients underwent immitanciometry and electrophysiological tests (ABR - automated brainstem response and otoacoustic emissions), as part of the pre-operative assessment.

Out of the total patients, 25 were implanted with Nucleus 22, three received the cochlear implant Nucleus 24 and one, cochlear implant Combi 40 + device.

In order to compare pre and postoperative audiological results, we defined the pure tone thresholds means - MT in frequencies of 250Hz to 2000Hz and in high frequencies (4KHz to 8KHz). The non-operated ear was used as the control ear.



GRAPH 1. Pre and post-cochlear implant - pure tone threshold means


Results

Up to the moment, we have compared pre and post-operative results of 10 patients aged 12 to 73 years (mean age 56 years), being 4 female and 6 male patients. The mean use of the cochlear implant was 9.6 months (Table 1).

The mean threshold (MT) preoperatively for frequencies of 250Hz to 2KHz was 100.5dB for the implanted ear and 110.05dB for the control ear. Some patients (n=03) presented conserved hearing also in high frequencies, so the MT for this region (4KHz to 8KHz) was 97.5dB in both ears.

In the postoperative assessment, the mean thresholds (205Hz - 2000Hz) fell 4.72dB in the implanted ear and 0.20dB in the control ear. The mean of pure tone thresholds in high frequencies reduced 17.5dB in the implanted ear and 8.33dB in the contralateral ear (Tables 2 and 3).

Graph 1 shows the variation of hearing thresholds in both ears for the frequencies of 250Hz to 2kHz and 4kHz to 8kHz.

The conservation of hearing in the implanted ear (presence of response in at least two speech frequencies) was observed in 5 patients (50%), and two of them presented responses in four frequencies (250, 500, 1000 and 2000Hz). Out of the 10 studied patients, 5 (50%) remained with thresholds at 80dB and 100dB in one speech frequency in the implanted ear, 3 (20%) had thresholds above 100dB and 3 (30%) had absence of response in speech frequencies.





Discussion

It is believed that candidates to cochlear implants with greater residual hearing would have better performance. However, the choice of the ear to be implanted is based, in many cases, on the side with poor residual hearing in order to preserve the best side for amplification. Therefore, hearing conservation is clinically related to a functional benefit, that is, auditory skills would be expanded by the use of cochlear implants. Moreover, another commonly discussed issue is the use of hearing aids in the operated ear1.

In the studied group, auditory conservation occurred in 50% of the subjects when considering the criteria of mid frequency conservation (speech) and in 80%, if we consider also low frequencies. The mean of pure tone thresholds in speech frequencies (250 to 2000Hz) increased in 4.60dB in the implanted ear and in 0.08dB in the contralateral ear (control). In high frequencies (4kHz to 8kHz) this increase was 17.5dB in the operated ear and 8.33dB in the control ear.

Hodges et al. (1997)1 found increase of pure tone threshold of 12dB HL in the implanted ear and 4.4 dB HL in the contralateral ear; considering, however, the percentage of subjects with measurable preserved hearing they observed that 52% (21:40) of the subjects presented conservation of hearing.

In this study, we considered the frequency of 250Hz as speech frequency, because according to Redondo & Lopes Fº (1997)14, there is great importance of low frequencies for the recognition of words in Portuguese language. In addition, we decided to consider as hearing preservation the presence of thresholds of two or more frequencies between 250 and 2kHz in order to avoid vibration responses or sensation of pressure, which are common among subjects with profound loss.

The mechanisms of hearing loss caused by cochlear implants are still not clearly defined. As we observed, there was greater variation in high frequencies, which could suggest trauma in the basal region of the cochlea during the insertion of the electrodes12. Gstoettner et al. (1997)10 believed that when the insertion was limited to the first point of resistance the histopathological results of the apical cochlear region showed minute traumas. In another histopathological study after implantation of long electrodes, Jackler et al. (1989)8 observed degeneration of the basal region of Corti's organ in some guinea pigs. Boggess et al. (1989)7 believed that the hearing loss after the implant could be associated with loss of perilymph, loss of cochlear tissue or as effect of the mechanical vibration of the electrode or the electrical stimulation, rupture of the basilar membrane or damage to the spiral ligament (Nadol, 1997)13.

Some patients (n=04) improved their thresholds in some frequencies, even in the implanted ears. It could have happened owing to the great attention to sound and improvement of the hearing perception provided by the use of cochlear implants15.

After surgical intervention, changes of up to 10dB in thresholds are generally considered clinically non-significant (Hodges, 1997)1. They are a result of inherited limitations to the test (variations such as phone positioning, level of environmental noise and attention to the patient, for example). Thus, the reasons can both worsen or improve.

Therefore, we can consider that there was hearing conservation after the insertion of electrodes in the most important frequencies for speech intelligibility (250Hz - 2kHz). Kiefer et al. (1998)15 studied 17 adults and 18 children with pre-implant residual hearing and they observed partial conservation of adult hearing. Conversely, hearing in children was completely preserved. The hypothesis that the hearing deterioration in adults was due to progression of the loss etiology was excluded because the contralateral ear had unaltered results.

We did not analyze data such as age or duration of stimulation owing to the limited number of patients. However, we shall carry on with the study in order to have data about the remaining implanted patients from the Department and a longitudinal analysis of their performance.

It is important to point out that even those that did not have residual hearing in audiometry presented good perception in functional routine tests.

Conclusion

 Auditory conservation is possible with the use of multichannel cochlear implants.
 The variation of thresholds after insertion of electrodes was not clinically significant.
 The variation of thresholds is greater in high frequencies than in speech frequencies (250Hz - 2kHz).

References

1. Hodges AV, Schloffman J, Balkany T. Conservation of Residual Hearing with Cochlear Implantation. The American Journal of Otology 1997;18:179-183.
2. Mangabeira Albernaz PL. Implante coclear. Parte 2. RBM-ORL 1996; 3(2):119-122.
3. Costa Filho OA, Bevilacqua MC, Moret ALM. Critérios de seleção de crianças candidatas ao implante coclear do Hospital de Pesquisa e Reabilitação de Lesões Lábio - Palatais - USP Rev Bras Otorrinolaring 1996;62(4):306-313.
4. Bento RF, Sanchez TG, Brito Neto RV. Critérios de Indicação de Implante Coclear. @rquivos da FORL 1997a;1(2):66-67.
5. Shiomi Y, Naito Y, Honjo I, Fujiki N, Kaneko K, Takahashi H, Yamashita M, Kawano M. Cochlear Implant in Patients with Residual Hearing. Auris Nasus Larynx 1999 Oct;26(4):369-74.
6. Incesulu A & Nadol JB Jr. Correlation of Acoustic Threshold Measures and Spiral Ganglion Cell in Severe to Profound Sensorineural Hearing Loss: Implications for Cochlear Implantation. Ann Otol Rhinol Laryngol 1998 Nov;107(11Pt 1):906-11.
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9. Clark GM, Shepherd RK, Dowell RC. Histopathological Following Cochlear Implantation in a Patient. Acta Otolaryngol Suppl (Stockh) 1988;106:448.
10. Gstoettner W, Franz P, Hamzavi J, Plenk Jr H, Baumgartner W, Czerny C. Intracochlear position of cochlear implant electrodes. Acta Otolaryngol (Stockh) 1999;119:229-233.
11. Bento RF, Sanchez TG, Brito Neto RV. O Implante FMUSP 1. Técnica Cirúrgica. @rquivos da FORL 1997b;1(3):80-83.
12. Cohen HL. Cochlear implant soft surgery: Fact or fantasy. Otolaryngol Head Neck Surg 1997;117:214-216.
13. Nadol JB. Patterns of neural degeneration in the human cochlea and auditory nerve: implications for cochlear implantation. Otolaryngol Head Neck Surg 1997;117:220-228.
14. Redondo MC & Lopes Fº O. Testes Básicos de Avaliação Auditiva. In: Lopes Filho O. Tratado de Fonoaudiologia. 1ª edição. São Paulo: Ed. Roca; 1997.
15. Kiefer J, Von Ilberg C, Reimer B, Knecht R, Gall V, Diller G, Stürzebecher E, Pfennigdorff T, Spelsberg A. Results of Cochlear Implantation in Patients with severe to profound Hearing loss - implications for patient selection. Audiology 1998;37:383-395.

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1 Speech and Language Pathologist and Audiologist, Service of Speech and Language Pathology and Audiology, Clinical Division of Otorhinolaryngology, HCFMUSP, Ph.D. in Human Communication Disorders, UNIFESP/EPM.
2 Clinical Speech and Language Pathologist and Audiologist.
3 Clinical Speech and Language Pathologist and Audiologist. Master studies in Experimental Pathophysiology under course, FMUSP.
4 Assistant Physician, Division of Clinical Otorhinolaryngology, HCFMUSP. Ph.D. in Otorhinolaryngology, FMUSP.
5 Associated Professor, Discipline of Otorhinolaryngology, FMUSP.

Study conducted in the Division of Clinical Otorhinolaryngology, Hospital das Clinicas, Medical School, University of São Paulo.

Address correspondence to: Maria Valéria S. Goffi Gomez - Av. Dr. Enéas Carvalho de Aguiar 255 São Paulo - Clínica Otorrinolaringológica do HCFMUSP 6 º Andar - ICHC - Tel. (55 11) 3069.6288 - E-mail: goffigomez@uol.com.br

Article submitted on August 9, 2001. Article accepted on August 29, 2002