A COMPARISON BETWEEN NEURAL RESPONSE TELEMETRY VIA COCHLEOSTOMY OR THE ROUND WINDOW APPROACH IN COCHLEAR IMPLANTATION

Rogério Hamerschmidt,1 Luiz Henrique Schuch,2 Rodrigo Kopp Rezende,3 Gislaine Richter Minhoto Wiemes,4 Adriana Kosma Pires de Oliveira,5 Marcos Mocellin6

1Professor of the Department of Ophtalmology, Otorhinolaryngology of Federal University of Paraná, Brazil and Hospital IPO, Instituto Paranaense de Otorrinolaringologia; 2Physician of Federal University of Paraná, Brazil; 3Physician of Hospital IPO, Instituto Paranaense de Otorrinolaringologia, Curitiba, Paraná, Brazil; 4Physician of Hospital IPO, Instituto Paranaense de Otorrinolaringologia, Curitiba, Paraná, Brazil; 5Professor and Head of the Department of Ophtalmology, Otorhinolaryngology of Federal University of Paraná, Brazil

Introduction

Brazil is estimated to have about 347,000 deaf individuals, many of them with indications for a cochlear implant (CI). For patients with little cochlear reserve who cannot achieve good sound discrimination even with sound amplification, the CI is one option for their rehabilitation.1 The CI brings about an improvement in hearing quality and improvements in speech perception and production, rendering a permanent and ascending quality-of-life gain in many aspects – such as self-sufficiency and socialization.25 It is estimated that since the 1970s until today, there are 400 thousand implan ted patients.6

CI’s partially replace the cochlea by turning sound into electrical signals.7 The survival of enough neural structures in the cochlear nerve allows the transmission of electric stimuli to the cerebral cortex.

The surgical implantation procedure via the transmastoid approach has been well standardized. Cochleostomy was first described in the 1980s.8 There are two techniques to place CI’s: via a cochleostomy, in which the promontory is drilled to fixate the implant, or via the round window (RW). Less drilling is required in the RW technique, thus reducing trauma, loss of perilymph, and bone powder on the tympanic scale.9 Preservation of residual hearing has been viable and beneficial due to the combination of electrical and acoustic stimulation, but it requires non-traumatic insertion of the electrode to minimize damage to inner ear structures and enable lesser neural tissue degeneration.6

There are different ways to perform objective measurements on the auditory nerves of CI users from the electrical stimulation of the auditory system, such as auditory brainstem response (ABR), middle latency response, late potentials, and stapedial reflexes.1 Neural response telemetry (NRT) is a test used to measure electrically evoked compound action potentials (ECAP) during surgery or post-operatively in implanted patients. This is an important test used to accurately monitor external and internal hardware function, and assess cochlear stimulation through neural responses.10

This is a prospective cross-sectional study aimed at comparing neural response telemetry in the immediate post-operative care of 23 patients of both genders with CI placed through cochleostomy or the RW approach to verify whether the choice of implantation procedure produces differences in auditory nerve stimulation.

Materials and methods

This study was approved by the Medical Ethics Committee on Research with Human Subjects and given permit nº 004/2010. This study complied with the standards defined in Resolution 196/96 issued by the Ministry of Health.

Twenty-three patients, seven males and 16 females, were enrolled in this study. Six patients underwent implantation via cochleostomy and 17 via the RW approach. All patients were implanted the same device made by the Cochlear Corporation. The procedures were performed by the same surgeon.

The multi-channel CI’s used in this study have 22 electrodes placed on the cochlea. The electrodes are numbered from one to 22, 22 being the one placed more apically. These electrodes were grouped the following way: 1–7 high-frequency sounds, 8–15 mid -frequency sounds, 16–22 low-frequency sounds. This division was needed because during NRT, we could not always get neural responses on one same electrode without changing the assessment parameters, and thus we left it for the software program to randomly choose within the groups which electrodes would be analyzed. Electrodes were split by ranges into high-, mid-, and low-frequency groups for the purposes of statistical calculation, as not all electrodes were analyzed individually.

The surgical technique employed to place CI’s consists of the following steps: 1. General anesthesia for pediatric patients and local anesthesia plus sedation for adult patients; 2. Retro-auricular incision of about three cm; 3. Dissection of subcutaneous tissue and muscle plane; 4. Y-shaped periosteal flap; 5. Shift periosteum from skullcap at the site of implantation of the internal unit; 6. Mastoidectomy; 7. Posterior tympanotomy; 8. Perform cochleostomy in the anterior inferior area of the RW in cases where cochleostomy was used as the implantation approach; drill the upper lip of the RW and open it with a probe; 9. Insert electrode beam; 10. Perform neural response telemetry; and 11. Close the planes of muscle and skin tissue using vicryl 3–0.

All patients were discharged on the same day of surgery and had compressive dressings on for two days. Amoxicillin and clavulanic acid were administered for ten days. Implants were activated 30 days after surgery.

The Custom Sound AutoNRT measurement system comprises the following elements: 1. A computer with Windows Vista Home Basic, Intel® Pentium® Dual processor; 2. Software version Custom Sound EP 2,0 (2.0.4.7298) and 3,2 (3.2.3855); 3. Programming interface – POD; 4. Speech processor – Freedom sound processor and headset SPrint; 5. Freedom Implant (Contour Advance). The NRT software was developed by the Engineering Department at the Cochlear Corporation.11

A computer equipped with programming interface is used to stimulate specific electrodes inside the cochlea. A series of pulses of information bi-directional communication using encoded radio frequency is transmitted from the Freedom processor interface through an external antenna placed inside a sterile bag placed on the patient’s skin above the internal receiver-stimulator. The encoded radio-frequency signal controls the stimulation parameters used to evoke compound action potentials. The internal receiver-stimulator in the Freedom Contour CI is equipped with one amplifier and one analog -digital converter. These additional components allow the voltage recorded on a pair of intra-cochlear electrodes to be amplified, sampled, and transmitted back to the external antenna, and then to the programming interface. These voltages are analyzed and the resulting ECAP wave is shown on a screen and the data can be stored in a computer. The ECAP records show a negative peak (N1) at 0.2–0.4 ms after stimulus onset, followed by a positive peak (P2) at 0.5–0.7 ms after stimulus onset. Response amplitude is measured from N1 to P2 and ranges between 40–2000 μV. Response amplitude varies with current levels between individuals. The parameters used to measure the thresholds on AutoNRT are: search for thresholds starts at 170 CL, at standard intervals of 6 CL for stimulation levels, at a stimulation frequency of 250 Hz.

Results

The patients submitted to implantation through the RW approach were aged between four and 84 years, and had a mean age of 32 years and three months. Patients in the cochleostomy group were aged between four and 54 years, and had a mean age of 19 years.

The Mann-Whitney test was used to statistically treat the samples, as this test allows the comparison of two groups of independent samples of different size. No statistically significant differences were found between implantation procedures as patients were assessed for high-frequency sounds (electrodes 1 to 7) (Table 1).

No statistically significant differences were found between implantation procedures as patients were assessed for mid-frequency sounds (electrodes 8 to 15) (Table 2).

No statistically significant differences were found between implantation procedures as patients were assessed for low frequency sounds, as shown in Table 3.

Table 1. Mean current level values for high-frequency sounds in patients submitted to implantation via cochleostomy and the round window approach.

image

n: number of subjects; min-max: minimum and maximum values; SD: standard deviation; p: level of statistical significance. (Source: designed by Hammerschimdt R, Schuch LH.)

Table 2. Mean current level values for mid-frequency sounds in patients submitted to implantation via cochleostomy and the round window approach.

image

n: number of subjects; min-max: minimum and maximum values; SD: standard deviation; p: level of statistical significance. (Source: designed by Hammerschimdt R, Schuch LH.)

Table 3. Mean current level values for low frequency sounds in patients submitted to implantation via cochleostomy and the round window approach.

image

n: number of subjects; min-max: minimum and maximum values; SD: standard deviation; p: level of statistical significance. (Source: designed by Hammerschimdt R, Schuch LH.)

Discussion

The preservation of the structural tissues of deaf patients is not essential in CI proce dures. However, since the introduction of electrical and acoustic stimulation combined in patients with cochlear reserve, the preservation of structural tissue and hearing has become of paramount importance in implantation procedures.12 The loss of residual hearing is the outcome of a combination of factors, including the approach used in the cochleostomy procedure, the electrode neuronal stimuli, and the location of the cochleostomy.8 The advent of new electrodes and the increased emphasis given to residual hearing preservation have renewed the interest in using the RW as a portal to place electrodes.8 When compared to cochleostomy via the promontory, placement through the RW should significantly reduce the number of perforation events during electrode placement and thus the risk of trauma, loss of perilymph, and bone powder in the tympanic scale.8 Bumps around the edges of the RW may pose difficulties to implant placement and require drilling of the anterior inferior border8. Drilling in this area must be done care fully, given its proximity to the opening of the cochlear aqueduct.8

Each electrode in the CI has to be programmed so that proper levels of electrical stimulation are provided. The unit used to program electrodes has been arbitrarily chosen and named current level programming units (CL). An important factor concerning cochlear implants is the variation on the current levels needed to elicit hearing for each individual and stimulation channel. Consequently, electrical stimulation parameters must be adjusted on the speech processor for each individual to adapt to specific user needs. This is done through a pro cess called mapping.

A more direct way of measuring cochlear-nerve function is electrically evoked compound action potential (ECAP). ECAP reflects the synchronized triggering of co chlear nerve fibers and, in many ways, carried similarities with wave I on ABR, occurring under 0.5 ms after stimulus onset.11 Originally, these measurements could be done on human beings only during surgery or through CI’s using percutaneous stimulation.

Stapedial reflexes can be measured as a response to electrical stimulation of the cochlea through direct obser vation of stapedial muscle contractions during surgery or by measuring acoustic impedance on the ear contralateral to the implant. The thresholds of electrically evoked stape dial reflex may be used to estimate C levels; nonetheless, significant variability is present in measurements done intra or inter-subjects. Additionally, according to a number of authors these reflexes cannot be recorded in about 40% of the population.13,14

Therefore, NRT is a technique that allows ECAP to be measured directly in implanted patients du ring and after surgery with greater sensitivity, as it can be done in more than 80% of the assessed individuals. NRT is a valuable tool and can be used to confirm the integrity of the internal device, objectively determine whi ch electrodes can be included in a map, define the best stimulation frequencies and speech encoding strategies, estimate T levels to measure the current levels to induce hearing sensation, and estimate C levels of the maximum sensation intensity accepted by patients, a clinically im portant variable.15

No differences were observed on neural response telemetry between implants placed in the tympanic scale via cochleostomy or the RW. One cochleostomy patient had to be excluded from the analysis of mean values for electrodes 16 to 22 as no neural response was captured for low-frequency sounds.

Karatas et al.8 reported that electrodes placed using the RW approach provided better stimulation when compared to electrodes placed via cochleostomy through the promontory as electrically evoked stapedius reflex thresholds (ESRT) and duration of stimulation were analyzed. In summary, the best response was defined for the shortest response time.

Both CI placement approaches are well established in the literature, cochleostomy being the most frequently used approach today. The choice of surgical approach is based on the preferences and training of the surgeon. There are no significant differences between the two approaches in terms of time of surgery and risk of complications.

This paper presents preliminary results, and no analysis was done on neural stimulation to compare patients in different age ranges. The auditory nerves of children respond better to stimulation than the auditory nerves of older patients. In a future study the groups need to be randomized for their specific ages so that this variable is properly assessed for this criterion.

This paper can be used to support other studies with larger samples, specifically on what concerns the cochleostomy approach and the measurement of all electrodes in the immediate post-operative care period. It is also part of the efforts made to reach better results with sound stimulation and auditory rehabilitation of the countless patients affected by deafness.

Conclusion

The data shown in this preliminary study in dicate that there is no significant difference in the acquisition of action potentials from the distal portion of the auditory nerve through neural response tele metry in multichannel CI patients using the implant to elicit stimulation and record responses, whether implantation was done through cochleostomy of the RW approach. Both approaches provide for equal stimulation of the cochlear nerve, and surgeons are free to choose the procedure of their pre ference.

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Address for correspondence: Rogério Hamerschmidt, MD, PhD., Av. Presidente Getúlio Vargas 881 ap. 302-F Curitiba-PR-Brazil Bairro Rebouças CEP 80230–030. rogeriohamer@hotmail.com

Cholesteatoma and Ear Surgery – An Update, pp. 355–359

Edited by Haruo Takahashi

2013 © Kugler Publications, Amsterdam, The Netherlands