Otology & Neurotology 35:1385Y1393 Ó 2014, Otology & Neurotology, Inc.
Impedance, Neural Response Telemetry, and Speech Perception Outcomes After Reimplantation of Cochlear Implants in Children *†‡kCatherine S. Birman, †‡Halit Sanli, †‡§William P. R. Gibson, and *†Elizabeth J. Elliott *Discipline of Paediatrics and Child Health, Sydney Medical School, University of Sydney; ÞSydney Children’s Hospital Network (Children’s Hospital at Westmead); þThe Sydney Cochlear Implant Centre; §Emeritus Professor, Sydney Medical School, University of Sydney; and kDepartment of Linguistics, Faculty of Human Sciences, Macquarie University, Sydney, New South Wales, Australia
Objective: To compare mean impedance levels, neural response telemetry (NRT), and auditory perception after initial and explant-reimplant pediatric cochlear implants. Study Design: Retrospective case review. Setting: Tertiary referral hospital and cochlear implant program. Patients: Children 0 to 16 years inclusive who have undergone explant-reimplant of their cochlear implant. Intervention: Impedance levels, NRT, and speech perception performance. Main Outcome Measures: Impedance, NRT, and auditory perception at switch on, 3 months, 12 months, 3 years, and 5 years after initial cochlear implant and reimplantation. Results: The explant-reimplant group receiving Cochlear contour array had significantly ( p G 0.001) raised impedance at switch on, 3 months, 12 months, and 3 years, compared with
their initial implant. The explant-reimplant group receiving Cochlear straight array had marginally significant ( p = 0.045) raised impedance at switch on, 3 months, 12 months, and 3 and 5 years. Infection was associated with greater increases in impedance in the reimplant Contour group. NRT was increased in the explant-reimplant group but not significantly ( p = 0.06). Auditory perception returned to preexplant levels within 6 months in 61% of children. Conclusion: Impedance is higher after explant-reimplant and remains increased for years after explant-reimplant with Cochlear contour and to a lesser degree the straight array device. Key Words: Cochlear explantVElectrode impedanceVFibrosisV Language outcomesVNeural response telemetryVPediatricV Reimplantation. Otol Neurotol 35:1385Y1393, 2014.
Removal of a cochlear implant may be required for several reasons. For the child and their family, this is a stressful experience as the child must undergo repeated surgery, a period of nonuse, and a period of readjustment to their new device. There is also concern that complications such as increased intracochlear fibrosis or reduced performance may follow repeated surgery. Impedance is a measure of electrical resistance at the electrode. It depends on the design of the electrode, including materials used and size; and the surrounding tissues
and fluid through which the current exits (1). Impedance can increase because of the amount of cell cover (2) and fibrous tissue growth around the electrode array (2Y5). Repeated surgery can be associated with an increase in scar tissue formation. The aim of our study was to assess if a change in impedance level followed cochlear implant explant-reimplantation. Increases reflect increased inflammation or fibrotic tissue formed within the cochlea. Increased intracochlear scar tissue can adversely affect cochlear implant function, power consumption, and hearing preservation. We analyzed impedance results for both the Cochlear straight and contour arrays after the initial implant and explantation-reimplantation. We examined the changes in impedance for both devices over time and reasons for explantation. Retrospective review was undertaken to determine the time taken for children to adapt to the second implant and regain preexplantation auditory perception.
Address correspondence and reprint requests to Catherine S. Birman, M.B.B.S., FRACS, Suite 402, Macquarie University Clinic, 2 Technology Place, Macquarie University, NSW 2109; E-mail: [email protected] The authors have no conflict of interest and received no funding for this research. E. J. E. holds a Practitioner Fellowship from the National Health and Medical Research Council of Australia (No. 1021480).
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This information, which is not currently available, will be important for clinicians seeking informed consent from patients and their families to inform them of the consequences of explant-reimplantation and likely time to return to previous function. Knowledge of explant-reimplantation fibrosis will also guide the use of emerging treatments such as steroids. METHODS The Sydney Cochlear Implant Centre (SCIC) data base was searched for pediatric cochlear implant cases (aged 16 yr or less), who had undergone explantation of a cochlear implant. SCIC is a large pediatric and adult cochlear implant program, having performed more than 3,000 surgeries. Ethics approval was obtained from the Children’s Hospital at Westmead Research Ethics Committee. Impedance data were collected following both the initial cochlear implant and reimplant. Mean impedance levels (impedance of all working electrodes were added, divided by the number of working electrodes) were recorded for Common Ground mode, at switch on, 3 months, 1 year, 3 years, and 5 years postimplantation. Data were compared following receipt of the initial and second (explant-reimplant) cochlear implants. Neural response telemetry (NRT) results obtained during intraoperative testing at initial and reimplant surgery were collected and compared. Statistical analysis was performed using generalized estimating equation (GEE) models. We retrospectively reviewed medical records and auditory perception results to determine the time taken to regain previous auditory perception function after explant-reimplantation. Because of variations in the age of children and their duration of hearing after cochlear implantation, a number of auditory perception tests were used. These included It-Mais, Ling sound detection and identification, CNC words and phonemes, MJW words and phonemes, and BKB sentences. For some children in whom formal testing was not performed, we determined return to previous level of function from comments in the notes that the family or child felt that they had regained preexplantation speech understanding levels. The duration until return of function after reimplantation was grouped into 4: by 6 months, by 12 months, by 18 months, or longer.
RESULTS In the SCIC database, we identified 104 children (50 female and 54 male subjects) who had undergone cochlear implant explantation, representing approximately 12% of children with cochlear implants in our program. The 104 children identified underwent 123 operations, including 116 explantations. Table 1 shows the frequency of operations performed. Simultaneous explant-reimplant was the most common procedure, occurring in 75% of operations. Explant surgery occurred in 116 of the operations. The reason for explantation, frequency of occurrence, and time between initial implantation and explant are listed in Table 2. Device break down was the most common reason for explantation (67%; 78/116), followed by infection or extrusion of the device (10%; 12/116). For the 14 explantation-alone operations, 9 required the procedure because of infection or extrusion, 2 because
TABLE 1.
Operation procedure and incidence
Operation procedure Explant-Reimplant Explant alone Reimplant separate operation Explant- reimplant a second time Explant, contralateral ear implant Explant a second time Total operations
Incidence (percentage) 92 14 7 6 3 1
(75%) (11%) (6%) (5%) (2%) (1%) 123
of pain (not reimplanted), 1 because of misplacement, 1 because of facial and body tick, and 1 because of cholesteatoma. At a later date, 7 children (50%) of the explantationalone operations underwent cochlear reimplantation. Initial and Explant-Reimplant Impedance Impedance data were available for 29 children after both the initial and second (explant-reimplant) surgery. For the remaining cases, data were not available either because there were no initial impedance data (38 CI22), or software upgrades meant old data were inaccessible. Of the 29 children with impedance data, 26 were reimplanted with the same type of device, 18 straight array, and 8 contour array. Thus, 26 cases could be analyzed for changes in impedance after explant-reimplantation. Generalized estimating equation (GEE) models were used to examine the effects of implant number (first versus second), time since implant (0, 3, 12, 36, and 60 mo), type of implant (straight versus contour), and reason for explantation (infection, extrusion, and device breakdown) on impedance. The GEE models assumed a normal distribution of impedance levels and an exchangeable correlation structure between repeated measurements in the same subject. Because of the small number of extrusions (n = 1) and infections (n = 3), interactions between reasons for explantation and the other predictors were not explored. There was a significant interaction between the effects of device type, implant number, and time since implant on impedance ( p G 0.001), so separate models were fitted for each device type. Figure 1 shows the changes in impedance over time for individual patients according to the order of the implant (first or second) and type of device (contour or straight). For each device type, both first and second (reimplant), impedances are highest at switch on and decrease over time. Figure 2 shows the mean and 95% confidence interval for impedance measurements over time by both the order of implant and device type. Straight array impedances were relatively constant after 3 months, in contrast with data from the contour device, which shows that mean impedance continued to decrease over years. By 36 months after switch-on, initial device impedance levels were similar (È4.68 and 4.61 k6) for the contour and straight array devices, respectively. For the 8 children who received the Contour initial device, predicted mean (95% CI) impedance at switch on was 10.4 (9.3, 11.5) k6. Time since implant ( p G 0.001) was associated with a decrease in impedance of 3.7 (2.8, 4.7) k6, 4.7 (3.4, 6.0) k6, and 6.1 (5.1, 7.1) k6 at 3, 12,
Otology & Neurotology, Vol. 35, No. 8, 2014
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IMPEDANCE, NRT, AND SPEECH PERCEPTION OUTCOMES TABLE 2.
Cause of explantation, frequency and time from initial cochlear implantation to explant Number of Time from initial CI to explant operations explantation (average) (percentage) and comment
Cause Device Malfunction (non CI512) Device Malfunction (CI512) total Infection Infection and extrusion Extrusion Array moved Misplaced array Upgrade and skin thickness Neurological symptoms
Pain Upgrade Raised comfort levels Thick tissue Poor performance Cholesteatoma Total explant operations
74
7Y272 mo (72 mo)
4
5Y12 mos (8 mo)
78 (67%) 6 (5%) 5 (4%) 1 (1%) 5 (4%) 3 (3%) 3 (3%) 3 (3%)
3 (3%) 2 (2%) 2 (2%) 2 (2%) 2 (2%) 1 (1%) 116 (102%)*
0Y16 mo (8 mo) 2Y61 mo (26 mo) 9 mo 1Y92 mo (41 mo) 0Y3 mo (2 mo) 104Y159 mo (123 mo) 21Y84 mo (44 mo) 2 with hemifacial and upper body twitch, 1 with epileptic seizure 3Y134 mo (47 mo) All female aged 17Y18 years 190Y201 mo (196 mo) 104Y254 mo (179 mo) 123 mo (123 mo) 48Y92 mo (70 mo) 24 mo
Explant-reimplant impedance changes in pediatric population. *Rounding of percentages affected total percentage.
and 36 months, respectively ( p G 0.001 for the overall effect of time). For the second implant with a contour device mean (95% CI), impedance was 1.4 (0.6, 2.3) k6 higher after the second implant ( p G 0.001; Fig. 3). No data were available for contour devices at 60 months postimplantation. For the 18 participants with straight devices, the predicted mean (95% CI) impedance at switch-on was 6.3 (5.8, 6.7) k6 (Fig. 2). This decreased by 1.6 (1.1, 2.2) k6 after 3 months and 1.8 (1.3, 2.3), 1.8 (1.2, 2.4), and 1.8 (1.0, 2.6) at 12, 36, and 60 months, respectively. As indicated by the overlapping confidence intervals, the impedance did not continue to decrease beyond 3 months after switch on with the straight array device. For the straight array, the predicted mean (95% CI) impedance was 0.65 (0.02Y1.29) k6 higher after the second implant than the first ( p = 0.045; Fig. 3). Figure 4 shows mean impedance over time for each device type and reason for explantation. For the contour device, the predicted mean (95% CI) impedance was 2.3 (1.6, 3.0) k6 higher when explantation was due to infection compared with device breakdown. This was significant ( p G 0.001). However, the number of cases where devices were removed for reasons other than breakdown was small, and so the results should be interpreted with caution. Figure 4 shows for the straight array device, little change in impedance over time, and the reason for explantation did not affect impedance. The mean (95% CI) predicted
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impedance being only 0.02 (j0.61 to 0.66) k6 higher when the array extruded through the skin than when there was device breakdown and 0.24 (j0.51, 0.98) k6 higher when there was infection rather than device breakdown ( p = 0.6). NRT Initial and Explant-Reimplant Figure 5 shows the mean NRT measurements by channel in 9 individual children for whom data were available after both the first and second implant. Not all 22 electrodes were tested for each patient, at each surgery. Comparison of NRT at the first and second implant was made by fitting a line of best fit to the data points for each child (Fig. 6). A GEE model was fitted for channel and implant number (first or second). The NRT was 10.5 units (95% CI, j0.5 to 21.6) higher for the second implant than the first ( p = 0.06). Return to Previous Levels of Function Twenty-one of the 29 children with initial and second device impedance measurements had detailed clinical notes or formal auditory perception testing to allow estimation of time taken for return to previous speech perception levels. Of the 21 children, 13 (62%) had regained speech perception by 6 months, 6 (29%) by 1 year, and 1 (5%) by 18 months after reimplantation. One child did not feel he had regained preexplantation speech perception by 2 years after reimplantation and had stopped routinely wearing the cochlear implant. Of the 3 children in whom infection resulted in reimplantation, 2 were explanted too early (at only a couple of months postimplantation) to know their auditory perception levels, but the other child had regained levels achieved after the initial implant levels by 2 months after reimplantation. DISCUSSION Ours is the first study to report impedance changes after reimplantation with the same array type as used initially. This allowed impedance levels after the first and second implant to be compared. All our patients received Cochlear devices with either a straight or contour array. Our study showed for the straight array, impedance was highest at switch on, reduced by 3 months to be stable thereafter. The mean value at switch-on and subsequent values thereafter were significantly raised for the second compared with the first straight array implant. Impedance was significantly higher for the contour than straight device at switch-on and showed a different pattern from the straight array with continued reduction in impedance levels for the 3-year data period. The Clarion device has been reported to have high impedance levels at switch on and stay stable from 3 to 18 months (6). Early reduction in impedance is thought to be due to reduced fibrous tissue around the array and dispersal of a protein layer on the electrodes that disburses with electrical stimulation (7). Impedances were higher with the second device for both arrays but most significantly for the contour ( p G 0.001). Otology & Neurotology, Vol. 35, No. 8, 2014
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FIG. 1.
C. S. BIRMAN ET AL.
Impedance over time for individual patients (n = 29).
In the only other study to examine impedance after reimplantation, De Ceulaer et al. (8) reported on 5 reimplantation recipients who had not received intracochlear
FIG. 2.
steroids. Cochlear devices were used for reimplantation: 4 Contour and 1 straight array. The initial array was the LAURA flex. At 6 months after reimplantation with the
Mean and 95% confidence interval of impedance over time for each device type and implant (n = 26).
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IMPEDANCE, NRT, AND SPEECH PERCEPTION OUTCOMES
FIG. 3.
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Mean impedance over time, first and second devices for contour and straight arrays.
straight array device or 3 of the 4 Contour devices, patients had significantly higher impedance levels than following receipt of their initial device. This is consistent with our findings. Cochlear implant impedance is the measure of electrical resistance at the electrode. It depends on the electrode design (9) and materials used in the electrode, the structures of tissues and fluid surrounding the cochlear, and the location in the cochlea. Impedance can be increased by an increase in cell cover over the electrode (2), including fibrosis (3Y5,10,11), and is made worse when inflammatory cells are present (2). Thus, increasing impedance postoperatively can be used as a marker or proxy for increasing inflammation. Passing current through the electrodes removes protein layer buildup. Stimulation of the electrodes can also induce impedance changes by altering the amount of cell cover over the electrodes. Increased fibrosis can also reduce perilymph, which can affect the electrode interface (12). Reactive oxygen species are molecules that can cause damage to the inner ear with an inflammatory reaction. Levels are high in patients with profound hearing loss, about to undergo cochlear implantation (13). This preexisting cellular response to deafness may enhance the inflammatory reaction that occurs when the cochlear implant is inserted. Neuburger et al. (14) found that increases in impedance are often accompanied by clinical inflammatory precipitation, with exudate and labyrinthitis. They also report that in some cases without an inflammatory event, impedance was still high. All these patients had high stimulation rates and high comfort levels,
and for most of them, impedance was reduced on mapping changes. High impedance is of clinical concern because it can cause saturation (compliance) of electrodes and reduce the dynamic range of stimulation. Reduction in electrical impedance of cochlear implant electrodes is important as less energy is used, and this prolongs the cochlear battery life. Lower impedance with lower current allows for more focused stimulation of the neural elements in the cochlea, giving greater differentiation of sounds (1,15). Reducing intracochlear inflammation and fibrosis has become even more important with hearing preservation cochlear implants (5,16). Impedance measurements are conducted at the time of implantation to identify open circuits (very high impedance) or short circuits (very low impedance) (17,18), and this is one method used to determine electrode malfunction. Air bubbles present at the time of surgical insertion can cause transient high impedance, most commonly seen in the most proximal electrodes (18). Healon (hyaluronic acid), a common lubricant used to aid insertion of the array, has no effect on impedance (19). Steroid effect on impedance over time has been studied by a number of authors. De Ceulaer et al. (8), compared Cochlear Contour and Straight impedance levels in children over the first 12 months with and without intracochlear steroid injection. Initial impedance was maximal just before first mapping, and impedance then dropped for 2 to 3 months, as also seen in our study. At 12 months, the straight array group who received intracochlear steroid still had significantly reduced impedance levels compared Otology & Neurotology, Vol. 35, No. 8, 2014
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FIG. 4.
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Mean and 95% confidence interval of impedance over time for each device type, implant, and cause (n = 26).
with the nonsteroid (control) group, but this was not so for the contour devices, where steroids did not give lower impedance for 12 months.
Paasche et al. (1) found no difference in impedances in 4 adults groups who received the Cochlear Contour device (control, Iridium coated array, steroid, and Iridium
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IMPEDANCE, NRT, AND SPEECH PERCEPTION OUTCOMES
FIG. 5.
NRT measurements in individual patients at each implant.
plus steroid). He grouped these patients according to whether they had received steroids (11 patients) or not (15 patients) and followed them at 30 days postoperatively
FIG. 6.
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(1) and then between 3 and 4 years later (20). The group that received intracochlear steroids had significantly lower impedances in the first month but impedances then increased
Observed and predicted NRT at each implant and channel. Otology & Neurotology, Vol. 35, No. 8, 2014
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over time. The group that received no steroids had increased impedance for 2 weeks but levels then remained stable. At the 3-to 4-year follow-up, the group that received intracochlear steroids had lower but not significantly different impedances compared with the nonsteroid group. Early reduction in impedance by intracochlear steroids may be helpful in the hearing preservation cases, where hearing loss is seen a few months after surgery; however, intracochlear injection may cause pressure changes, affecting hearing preservation itself. Huang et al. (21) examined the relationship between impedance and intracochlear steroid injection in the short-term guinea pig model and the long-term cat model. Intracochlear steroids were beneficial in the short term (4Y5 wk) for the guinea pig (21), reducing impedance, although this did rise over time. Intracochlear steroids were not protective against a rise in impedance over the long term (4Y5 mo) in the cat. In our study, the 18 children reimplanted with the straight array had significant ( p = 0.045) increase in impedance levels compared with the initial implant from switch on to up to 5 years, although a limitation of this study is the small sample size, and for the straight array devices, the p value is only just significant. The Contour group had significantly higher impedances ( p G 0.001) after receipt of the second (reimplanted) than the first device from the time of switch on, to 3 years. The Contour device is larger than the straight device with greater surface area, and possible greater shearing trauma occurs with explantation and reimplantation, leading to greater fibrosis with the second device. In the contour group, impedance levels were significantly higher in cases with infection than without ( p G 0.001), but this was not so for the straight array. Sample numbers, however, are very small. The reason for the differential effect on impedance with infection is unclear but may relate to the greater tendency for fibrosis with the contour device. Reimplantation with the straight array or CI422 may reduce the likelihood of impedance increases in these explantation-reimplantation cases, and this required further evaluation. Steroids can decrease impedance levels and are used in hearing preservation techniques (1,8,16), although the duration of protection, areas of the cochlea protected, and best mode of delivery are still under investigation. The use of steroids use in reimplanted cases could be considered, as our study shows that reimplantation results in higher impedance, which lasts for years, reflecting greater fibrosis. In using steroids, the animal studies by Huang et al. (21), showing no sustained protection against raised impedance with intracochlear steroids, should also be considered. We are the first group to examine NRT after initial and reimplanted cochlear devices. Although the sample size is small, our data show a trend suggesting that higher stimulation current thresholds are required to elicit NRT in reimplant cases. Numbers are small and did not reach significance, and further study in this area is warranted. In our study, we found 61% of reimplanted children had regained auditory perception to preexplant levels by at least 6 months after reimplantation. Van der Marel et al. (22) found comparable speech perception outcomes within
weeks for their 15 children and adults reimplanted with the HiRes90K and HiFocus1J devices. Henson et al. (23) found more varied outcomes after reimplantation in 28 adults. In our study, only a few children were infected and explanted soon after surgery, with our very limited numbers, infection does not seem to be detrimental to regaining auditory perception. Our study used a variety of tests because of different pediatric ages and also in some used comments from the notes, to assess return of function. Routine speech perception testing postoperatively in all children reimplanted at 6, 12, 18, and 24 months would be helpful in providing more accurate measures. Although there were some limitations to our data, the new information provided by our study will be invaluable when counseling parents and their families regarding their expectation for their child regaining speech discrimination. In our study of children undergoing cochlear explantationreimplantation, the most common reason for explantation was device malfunction, occurring in 78 (67%) of 116 of cases. This represents only a small proportion of device malfunction, as most cases of device malfunction involve only a few electrodes and do not require explantation. Carlson et al. (24) reported a 9% incidence of one or more open or short electrodes, with an 11% in children, yet only 4 (1%) of 636 children and adults required revision surgery for deteriorating devices. Brown et al. (25) report 7.3% revision rate for children, with 78% of revisions because of device failure for children and adults. Most of our pediatric device failures occurred several years after initial surgery, and so follow-up mapping is useful in detecting these failures. Our higher revision rate is influenced by our center’s long running program of nearly 30 years. Infection was the next most common reason for explantation, and most infection-only cases were explanted within 7 months after initial surgery. Infection and extrusion occurred in 2 peaksV either within a few months of surgery or after 2 years. These results indicate that the surgical site needs to be watched, beyond a few months postoperatively. CONCLUSION Impedance levels in the cochlear implant array can be used as an indirect measure of inflammation and fibrotic changes associated with surgery. Explantationreimplantation of both straight and contour Cochlear devices results in raised impedance in the second device. The increase in impedance was only marginally significant for the straight array but very significant in the contour array over the long term. Auditory performance takes time to return after reimplantation, but the majority has regained their normal level by 6 months after surgery. Additional studies are required to examine the influence of steroids and other medicines on postoperative impedance and to establish the optimal dose, mode of delivery, and duration to reduce intracochlear fibrosis, particularly after explantationreimplantation. Acknowledgment: The authors thank Liz Barnes, biostatistician.
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IMPEDANCE, NRT, AND SPEECH PERCEPTION OUTCOMES REFERENCES 1. Paasche G, Bockel F, Tasche C, Lesinski-Schiedat A, Lenarz T. Changes of postoperative impedances in cochlear implant patients: the short-term effects of modified electrode surfaces and intracochlear corticosteroids. Otol Neurotol 2006;27:639Y47. 2. Newbold C, Richardson R, Huang CQ, Milojevic D, Cowan R, Shepherd R. An in vitro model for investigating impedance changes with cell growth and electrical stimulation: implications for cochlear implants. J Neural Eng 2004;1:218Y27. 3. Xu J, Shepherd RK, Millard RE, Clark GM. Chronic electrical stimulation of the auditory nerve at high stimulus rates: a physiological and histopathological study. Hear Res 1997;105:1Y29. 4. Tykocinski M, Duan Y, Tabor B, Cowan RS. Chronic electrical stimulation of the auditory nerve using high surface area (HiQ) platinum electrodes. Hear Res 2001;159:53Y68. 5. Choi C-H, Oghalai JS. Predicting the effect of post-implant cochlear fibrosis on residual hearing. Hear Res 2005;205:193Y200. 6. Henkin Y, Kaplan-Neeman R, Kronenberg J, Migirov L, Hildesheimer M, Muchnik C. A longitudinal study of electrical stimulation levels and electrode impedance in children using the Clarion cochlear implant. Acta Otolaryngol (Stockh) 2006;126:581Y6. 7. Tykocinski M, Cohen LT, Cowan RS. Measurement and analysis of access resistance and polarization impedance in cochlear implant recipients. Otol Neurotol 2005;26:948Y56. 8. De Ceulaer G, Johnson S, Yperman M, et al. Long-term evaluation of the effect of intracochlear steroid deposition on electrode impedance in cochlear implant patients. Otol Neurotol 2003;24: 769Y74. 9. Busby PA, Plant KL, Whitford LA. Electrode impedance in adults and children using the Nucleus 24 cochlear implant system. Cochlear Implants Int 2002;3:87Y103. 10. Clark GM, Shute SA, Shepherd RK, Carter TD. Cochlear implantation: osteoneogenesis, electrode-tissue impedance, and residual hearing. Ann Otol Rhinol Laryngol Suppl 1995;166:40Y2. 11. Durisin M, Krause C, Arnoldner C, et al. Electron microscopy changes of cochlear implant electrodes with permanently high impedances. Cochlear Implants Int 2011;12:228Y33. 12. Duan YY, Clark GM, Cowan RSC. A study of intra-cochlear electrodes and tissue interface by electrochemical impedance methods in vivo. Biomaterials 2004;25:3813Y28.
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13. Ciorba A, Gasparini P, Chicca M, Pinamonti S, Martini A. Reactive oxygen species in human inner ear perilymph. Acta Otolaryngol (Stockh) 2010;130:240Y6. 14. Neuburger J, Lenarz T, Lesinski-Schiedat A, Buchner A. Spontaneous increases in impedance following cochlear implantation: suspected causes and management. Int J Audiol 2009;48:233Y9. 15. Micco AG, Richter C-P. Tissue resistivities determine the current flow in the cochlea. Curr Opin Otolaryngol Head Neck Surg 2006; 14:352Y5. 16. Friedland D, Runge-Samuelson C. Soft cochlear implantation: rationale for the surgical approach. Trends Amplif 2009;13:124Y38. 17. Hughes ML, Brown CJ, Abbas PJ. Sensitivity and specificity of averaged electrode voltage measures in cochlear implant recipients. Ear Hear 2004;25:431Y46. 18. Schulman JH. Using impedance telemetry to diagnose cochlear electrode history, location, and functionality. Ann Otol Rhinol Laryngol Suppl 1995;166:85Y7. 19. Mens LH, Oostendorp TF, Hombergen GC, den Broek P. Electrical impedance of the cochlear implant lubricants hyaluronic acid, oxycellulose, and glycerin. Ann Otol Rhinol Laryngol 1997;106: 653Y6. 20. Paasche G, Tasche C, Stover T, Lesinski-Schiedat A, Lenarz T. The long-term effects of modified electrode surfaces and intracochlear corticosteroids on postoperative impedances in cochlear implant patients. Otol Neurotol 2009;30:592Y8. 21. Huang CQ, Tykocinski M, Stathopoulos D, Cowan R. Effects of steroids and lubricants on electrical impedance and tissue response following cochlear implantation. Cochlear Implants Int 2007;8: 123Y47. 22. van der Marel K, Briaire J, Verbist B, et al. Cochlear reimplantation with same device: surgical and audiologic results. Laryngoscope 2011;121:1517Y24. 23. Henson AM SW, Luxford WM, Mills DM. Cochlear Implant Performance After Reimplantation, A Multicenter Study. Am J Otol 1999;20:56Y64. 24. Carlson ML AD, Dabade TS, Gifford RH, et al. Prevalence and timing of individual cochlear implant electrode failures. Otol Neurotol 2010;31:893Y8. 25. Brown KD CS, Balkany TJ, Eshraghi AE, Telischi FF, Angeli SA. Incidence and Indications for Revision Cochlear Implant Surgery in Adults and Children. Laryngoscope 2009;119:152Y7.
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Impedance, Neural Response Telemetry, and Speech Perception Outcomes After Reimplantation of Cochlear Implants in Children *†‡kCatherine S. Birman, †‡Halit Sanli, †‡§William P. R. Gibson, and *†Elizabeth J. Elliott *Discipline of Paediatrics and Child Health, Sydney Medical School, University of Sydney; ÞSydney Children’s Hospital Network (Children’s Hospital at Westmead); þThe Sydney Cochlear Implant Centre; §Emeritus Professor, Sydney Medical School, University of Sydney; and kDepartment of Linguistics, Faculty of Human Sciences, Macquarie University, Sydney, New South Wales, Australia
Objective: To compare mean impedance levels, neural response telemetry (NRT), and auditory perception after initial and explant-reimplant pediatric cochlear implants. Study Design: Retrospective case review. Setting: Tertiary referral hospital and cochlear implant program. Patients: Children 0 to 16 years inclusive who have undergone explant-reimplant of their cochlear implant. Intervention: Impedance levels, NRT, and speech perception performance. Main Outcome Measures: Impedance, NRT, and auditory perception at switch on, 3 months, 12 months, 3 years, and 5 years after initial cochlear implant and reimplantation. Results: The explant-reimplant group receiving Cochlear contour array had significantly ( p G 0.001) raised impedance at switch on, 3 months, 12 months, and 3 years, compared with
their initial implant. The explant-reimplant group receiving Cochlear straight array had marginally significant ( p = 0.045) raised impedance at switch on, 3 months, 12 months, and 3 and 5 years. Infection was associated with greater increases in impedance in the reimplant Contour group. NRT was increased in the explant-reimplant group but not significantly ( p = 0.06). Auditory perception returned to preexplant levels within 6 months in 61% of children. Conclusion: Impedance is higher after explant-reimplant and remains increased for years after explant-reimplant with Cochlear contour and to a lesser degree the straight array device. Key Words: Cochlear explantVElectrode impedanceVFibrosisV Language outcomesVNeural response telemetryVPediatricV Reimplantation. Otol Neurotol 35:1385Y1393, 2014.
Removal of a cochlear implant may be required for several reasons. For the child and their family, this is a stressful experience as the child must undergo repeated surgery, a period of nonuse, and a period of readjustment to their new device. There is also concern that complications such as increased intracochlear fibrosis or reduced performance may follow repeated surgery. Impedance is a measure of electrical resistance at the electrode. It depends on the design of the electrode, including materials used and size; and the surrounding tissues
and fluid through which the current exits (1). Impedance can increase because of the amount of cell cover (2) and fibrous tissue growth around the electrode array (2Y5). Repeated surgery can be associated with an increase in scar tissue formation. The aim of our study was to assess if a change in impedance level followed cochlear implant explant-reimplantation. Increases reflect increased inflammation or fibrotic tissue formed within the cochlea. Increased intracochlear scar tissue can adversely affect cochlear implant function, power consumption, and hearing preservation. We analyzed impedance results for both the Cochlear straight and contour arrays after the initial implant and explantation-reimplantation. We examined the changes in impedance for both devices over time and reasons for explantation. Retrospective review was undertaken to determine the time taken for children to adapt to the second implant and regain preexplantation auditory perception.
Address correspondence and reprint requests to Catherine S. Birman, M.B.B.S., FRACS, Suite 402, Macquarie University Clinic, 2 Technology Place, Macquarie University, NSW 2109; E-mail: [email protected] The authors have no conflict of interest and received no funding for this research. E. J. E. holds a Practitioner Fellowship from the National Health and Medical Research Council of Australia (No. 1021480).
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This information, which is not currently available, will be important for clinicians seeking informed consent from patients and their families to inform them of the consequences of explant-reimplantation and likely time to return to previous function. Knowledge of explant-reimplantation fibrosis will also guide the use of emerging treatments such as steroids. METHODS The Sydney Cochlear Implant Centre (SCIC) data base was searched for pediatric cochlear implant cases (aged 16 yr or less), who had undergone explantation of a cochlear implant. SCIC is a large pediatric and adult cochlear implant program, having performed more than 3,000 surgeries. Ethics approval was obtained from the Children’s Hospital at Westmead Research Ethics Committee. Impedance data were collected following both the initial cochlear implant and reimplant. Mean impedance levels (impedance of all working electrodes were added, divided by the number of working electrodes) were recorded for Common Ground mode, at switch on, 3 months, 1 year, 3 years, and 5 years postimplantation. Data were compared following receipt of the initial and second (explant-reimplant) cochlear implants. Neural response telemetry (NRT) results obtained during intraoperative testing at initial and reimplant surgery were collected and compared. Statistical analysis was performed using generalized estimating equation (GEE) models. We retrospectively reviewed medical records and auditory perception results to determine the time taken to regain previous auditory perception function after explant-reimplantation. Because of variations in the age of children and their duration of hearing after cochlear implantation, a number of auditory perception tests were used. These included It-Mais, Ling sound detection and identification, CNC words and phonemes, MJW words and phonemes, and BKB sentences. For some children in whom formal testing was not performed, we determined return to previous level of function from comments in the notes that the family or child felt that they had regained preexplantation speech understanding levels. The duration until return of function after reimplantation was grouped into 4: by 6 months, by 12 months, by 18 months, or longer.
RESULTS In the SCIC database, we identified 104 children (50 female and 54 male subjects) who had undergone cochlear implant explantation, representing approximately 12% of children with cochlear implants in our program. The 104 children identified underwent 123 operations, including 116 explantations. Table 1 shows the frequency of operations performed. Simultaneous explant-reimplant was the most common procedure, occurring in 75% of operations. Explant surgery occurred in 116 of the operations. The reason for explantation, frequency of occurrence, and time between initial implantation and explant are listed in Table 2. Device break down was the most common reason for explantation (67%; 78/116), followed by infection or extrusion of the device (10%; 12/116). For the 14 explantation-alone operations, 9 required the procedure because of infection or extrusion, 2 because
TABLE 1.
Operation procedure and incidence
Operation procedure Explant-Reimplant Explant alone Reimplant separate operation Explant- reimplant a second time Explant, contralateral ear implant Explant a second time Total operations
Incidence (percentage) 92 14 7 6 3 1
(75%) (11%) (6%) (5%) (2%) (1%) 123
of pain (not reimplanted), 1 because of misplacement, 1 because of facial and body tick, and 1 because of cholesteatoma. At a later date, 7 children (50%) of the explantationalone operations underwent cochlear reimplantation. Initial and Explant-Reimplant Impedance Impedance data were available for 29 children after both the initial and second (explant-reimplant) surgery. For the remaining cases, data were not available either because there were no initial impedance data (38 CI22), or software upgrades meant old data were inaccessible. Of the 29 children with impedance data, 26 were reimplanted with the same type of device, 18 straight array, and 8 contour array. Thus, 26 cases could be analyzed for changes in impedance after explant-reimplantation. Generalized estimating equation (GEE) models were used to examine the effects of implant number (first versus second), time since implant (0, 3, 12, 36, and 60 mo), type of implant (straight versus contour), and reason for explantation (infection, extrusion, and device breakdown) on impedance. The GEE models assumed a normal distribution of impedance levels and an exchangeable correlation structure between repeated measurements in the same subject. Because of the small number of extrusions (n = 1) and infections (n = 3), interactions between reasons for explantation and the other predictors were not explored. There was a significant interaction between the effects of device type, implant number, and time since implant on impedance ( p G 0.001), so separate models were fitted for each device type. Figure 1 shows the changes in impedance over time for individual patients according to the order of the implant (first or second) and type of device (contour or straight). For each device type, both first and second (reimplant), impedances are highest at switch on and decrease over time. Figure 2 shows the mean and 95% confidence interval for impedance measurements over time by both the order of implant and device type. Straight array impedances were relatively constant after 3 months, in contrast with data from the contour device, which shows that mean impedance continued to decrease over years. By 36 months after switch-on, initial device impedance levels were similar (È4.68 and 4.61 k6) for the contour and straight array devices, respectively. For the 8 children who received the Contour initial device, predicted mean (95% CI) impedance at switch on was 10.4 (9.3, 11.5) k6. Time since implant ( p G 0.001) was associated with a decrease in impedance of 3.7 (2.8, 4.7) k6, 4.7 (3.4, 6.0) k6, and 6.1 (5.1, 7.1) k6 at 3, 12,
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IMPEDANCE, NRT, AND SPEECH PERCEPTION OUTCOMES TABLE 2.
Cause of explantation, frequency and time from initial cochlear implantation to explant Number of Time from initial CI to explant operations explantation (average) (percentage) and comment
Cause Device Malfunction (non CI512) Device Malfunction (CI512) total Infection Infection and extrusion Extrusion Array moved Misplaced array Upgrade and skin thickness Neurological symptoms
Pain Upgrade Raised comfort levels Thick tissue Poor performance Cholesteatoma Total explant operations
74
7Y272 mo (72 mo)
4
5Y12 mos (8 mo)
78 (67%) 6 (5%) 5 (4%) 1 (1%) 5 (4%) 3 (3%) 3 (3%) 3 (3%)
3 (3%) 2 (2%) 2 (2%) 2 (2%) 2 (2%) 1 (1%) 116 (102%)*
0Y16 mo (8 mo) 2Y61 mo (26 mo) 9 mo 1Y92 mo (41 mo) 0Y3 mo (2 mo) 104Y159 mo (123 mo) 21Y84 mo (44 mo) 2 with hemifacial and upper body twitch, 1 with epileptic seizure 3Y134 mo (47 mo) All female aged 17Y18 years 190Y201 mo (196 mo) 104Y254 mo (179 mo) 123 mo (123 mo) 48Y92 mo (70 mo) 24 mo
Explant-reimplant impedance changes in pediatric population. *Rounding of percentages affected total percentage.
and 36 months, respectively ( p G 0.001 for the overall effect of time). For the second implant with a contour device mean (95% CI), impedance was 1.4 (0.6, 2.3) k6 higher after the second implant ( p G 0.001; Fig. 3). No data were available for contour devices at 60 months postimplantation. For the 18 participants with straight devices, the predicted mean (95% CI) impedance at switch-on was 6.3 (5.8, 6.7) k6 (Fig. 2). This decreased by 1.6 (1.1, 2.2) k6 after 3 months and 1.8 (1.3, 2.3), 1.8 (1.2, 2.4), and 1.8 (1.0, 2.6) at 12, 36, and 60 months, respectively. As indicated by the overlapping confidence intervals, the impedance did not continue to decrease beyond 3 months after switch on with the straight array device. For the straight array, the predicted mean (95% CI) impedance was 0.65 (0.02Y1.29) k6 higher after the second implant than the first ( p = 0.045; Fig. 3). Figure 4 shows mean impedance over time for each device type and reason for explantation. For the contour device, the predicted mean (95% CI) impedance was 2.3 (1.6, 3.0) k6 higher when explantation was due to infection compared with device breakdown. This was significant ( p G 0.001). However, the number of cases where devices were removed for reasons other than breakdown was small, and so the results should be interpreted with caution. Figure 4 shows for the straight array device, little change in impedance over time, and the reason for explantation did not affect impedance. The mean (95% CI) predicted
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impedance being only 0.02 (j0.61 to 0.66) k6 higher when the array extruded through the skin than when there was device breakdown and 0.24 (j0.51, 0.98) k6 higher when there was infection rather than device breakdown ( p = 0.6). NRT Initial and Explant-Reimplant Figure 5 shows the mean NRT measurements by channel in 9 individual children for whom data were available after both the first and second implant. Not all 22 electrodes were tested for each patient, at each surgery. Comparison of NRT at the first and second implant was made by fitting a line of best fit to the data points for each child (Fig. 6). A GEE model was fitted for channel and implant number (first or second). The NRT was 10.5 units (95% CI, j0.5 to 21.6) higher for the second implant than the first ( p = 0.06). Return to Previous Levels of Function Twenty-one of the 29 children with initial and second device impedance measurements had detailed clinical notes or formal auditory perception testing to allow estimation of time taken for return to previous speech perception levels. Of the 21 children, 13 (62%) had regained speech perception by 6 months, 6 (29%) by 1 year, and 1 (5%) by 18 months after reimplantation. One child did not feel he had regained preexplantation speech perception by 2 years after reimplantation and had stopped routinely wearing the cochlear implant. Of the 3 children in whom infection resulted in reimplantation, 2 were explanted too early (at only a couple of months postimplantation) to know their auditory perception levels, but the other child had regained levels achieved after the initial implant levels by 2 months after reimplantation. DISCUSSION Ours is the first study to report impedance changes after reimplantation with the same array type as used initially. This allowed impedance levels after the first and second implant to be compared. All our patients received Cochlear devices with either a straight or contour array. Our study showed for the straight array, impedance was highest at switch on, reduced by 3 months to be stable thereafter. The mean value at switch-on and subsequent values thereafter were significantly raised for the second compared with the first straight array implant. Impedance was significantly higher for the contour than straight device at switch-on and showed a different pattern from the straight array with continued reduction in impedance levels for the 3-year data period. The Clarion device has been reported to have high impedance levels at switch on and stay stable from 3 to 18 months (6). Early reduction in impedance is thought to be due to reduced fibrous tissue around the array and dispersal of a protein layer on the electrodes that disburses with electrical stimulation (7). Impedances were higher with the second device for both arrays but most significantly for the contour ( p G 0.001). Otology & Neurotology, Vol. 35, No. 8, 2014
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FIG. 1.
C. S. BIRMAN ET AL.
Impedance over time for individual patients (n = 29).
In the only other study to examine impedance after reimplantation, De Ceulaer et al. (8) reported on 5 reimplantation recipients who had not received intracochlear
FIG. 2.
steroids. Cochlear devices were used for reimplantation: 4 Contour and 1 straight array. The initial array was the LAURA flex. At 6 months after reimplantation with the
Mean and 95% confidence interval of impedance over time for each device type and implant (n = 26).
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IMPEDANCE, NRT, AND SPEECH PERCEPTION OUTCOMES
FIG. 3.
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Mean impedance over time, first and second devices for contour and straight arrays.
straight array device or 3 of the 4 Contour devices, patients had significantly higher impedance levels than following receipt of their initial device. This is consistent with our findings. Cochlear implant impedance is the measure of electrical resistance at the electrode. It depends on the electrode design (9) and materials used in the electrode, the structures of tissues and fluid surrounding the cochlear, and the location in the cochlea. Impedance can be increased by an increase in cell cover over the electrode (2), including fibrosis (3Y5,10,11), and is made worse when inflammatory cells are present (2). Thus, increasing impedance postoperatively can be used as a marker or proxy for increasing inflammation. Passing current through the electrodes removes protein layer buildup. Stimulation of the electrodes can also induce impedance changes by altering the amount of cell cover over the electrodes. Increased fibrosis can also reduce perilymph, which can affect the electrode interface (12). Reactive oxygen species are molecules that can cause damage to the inner ear with an inflammatory reaction. Levels are high in patients with profound hearing loss, about to undergo cochlear implantation (13). This preexisting cellular response to deafness may enhance the inflammatory reaction that occurs when the cochlear implant is inserted. Neuburger et al. (14) found that increases in impedance are often accompanied by clinical inflammatory precipitation, with exudate and labyrinthitis. They also report that in some cases without an inflammatory event, impedance was still high. All these patients had high stimulation rates and high comfort levels,
and for most of them, impedance was reduced on mapping changes. High impedance is of clinical concern because it can cause saturation (compliance) of electrodes and reduce the dynamic range of stimulation. Reduction in electrical impedance of cochlear implant electrodes is important as less energy is used, and this prolongs the cochlear battery life. Lower impedance with lower current allows for more focused stimulation of the neural elements in the cochlea, giving greater differentiation of sounds (1,15). Reducing intracochlear inflammation and fibrosis has become even more important with hearing preservation cochlear implants (5,16). Impedance measurements are conducted at the time of implantation to identify open circuits (very high impedance) or short circuits (very low impedance) (17,18), and this is one method used to determine electrode malfunction. Air bubbles present at the time of surgical insertion can cause transient high impedance, most commonly seen in the most proximal electrodes (18). Healon (hyaluronic acid), a common lubricant used to aid insertion of the array, has no effect on impedance (19). Steroid effect on impedance over time has been studied by a number of authors. De Ceulaer et al. (8), compared Cochlear Contour and Straight impedance levels in children over the first 12 months with and without intracochlear steroid injection. Initial impedance was maximal just before first mapping, and impedance then dropped for 2 to 3 months, as also seen in our study. At 12 months, the straight array group who received intracochlear steroid still had significantly reduced impedance levels compared Otology & Neurotology, Vol. 35, No. 8, 2014
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FIG. 4.
C. S. BIRMAN ET AL.
Mean and 95% confidence interval of impedance over time for each device type, implant, and cause (n = 26).
with the nonsteroid (control) group, but this was not so for the contour devices, where steroids did not give lower impedance for 12 months.
Paasche et al. (1) found no difference in impedances in 4 adults groups who received the Cochlear Contour device (control, Iridium coated array, steroid, and Iridium
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IMPEDANCE, NRT, AND SPEECH PERCEPTION OUTCOMES
FIG. 5.
NRT measurements in individual patients at each implant.
plus steroid). He grouped these patients according to whether they had received steroids (11 patients) or not (15 patients) and followed them at 30 days postoperatively
FIG. 6.
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(1) and then between 3 and 4 years later (20). The group that received intracochlear steroids had significantly lower impedances in the first month but impedances then increased
Observed and predicted NRT at each implant and channel. Otology & Neurotology, Vol. 35, No. 8, 2014
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over time. The group that received no steroids had increased impedance for 2 weeks but levels then remained stable. At the 3-to 4-year follow-up, the group that received intracochlear steroids had lower but not significantly different impedances compared with the nonsteroid group. Early reduction in impedance by intracochlear steroids may be helpful in the hearing preservation cases, where hearing loss is seen a few months after surgery; however, intracochlear injection may cause pressure changes, affecting hearing preservation itself. Huang et al. (21) examined the relationship between impedance and intracochlear steroid injection in the short-term guinea pig model and the long-term cat model. Intracochlear steroids were beneficial in the short term (4Y5 wk) for the guinea pig (21), reducing impedance, although this did rise over time. Intracochlear steroids were not protective against a rise in impedance over the long term (4Y5 mo) in the cat. In our study, the 18 children reimplanted with the straight array had significant ( p = 0.045) increase in impedance levels compared with the initial implant from switch on to up to 5 years, although a limitation of this study is the small sample size, and for the straight array devices, the p value is only just significant. The Contour group had significantly higher impedances ( p G 0.001) after receipt of the second (reimplanted) than the first device from the time of switch on, to 3 years. The Contour device is larger than the straight device with greater surface area, and possible greater shearing trauma occurs with explantation and reimplantation, leading to greater fibrosis with the second device. In the contour group, impedance levels were significantly higher in cases with infection than without ( p G 0.001), but this was not so for the straight array. Sample numbers, however, are very small. The reason for the differential effect on impedance with infection is unclear but may relate to the greater tendency for fibrosis with the contour device. Reimplantation with the straight array or CI422 may reduce the likelihood of impedance increases in these explantation-reimplantation cases, and this required further evaluation. Steroids can decrease impedance levels and are used in hearing preservation techniques (1,8,16), although the duration of protection, areas of the cochlea protected, and best mode of delivery are still under investigation. The use of steroids use in reimplanted cases could be considered, as our study shows that reimplantation results in higher impedance, which lasts for years, reflecting greater fibrosis. In using steroids, the animal studies by Huang et al. (21), showing no sustained protection against raised impedance with intracochlear steroids, should also be considered. We are the first group to examine NRT after initial and reimplanted cochlear devices. Although the sample size is small, our data show a trend suggesting that higher stimulation current thresholds are required to elicit NRT in reimplant cases. Numbers are small and did not reach significance, and further study in this area is warranted. In our study, we found 61% of reimplanted children had regained auditory perception to preexplant levels by at least 6 months after reimplantation. Van der Marel et al. (22) found comparable speech perception outcomes within
weeks for their 15 children and adults reimplanted with the HiRes90K and HiFocus1J devices. Henson et al. (23) found more varied outcomes after reimplantation in 28 adults. In our study, only a few children were infected and explanted soon after surgery, with our very limited numbers, infection does not seem to be detrimental to regaining auditory perception. Our study used a variety of tests because of different pediatric ages and also in some used comments from the notes, to assess return of function. Routine speech perception testing postoperatively in all children reimplanted at 6, 12, 18, and 24 months would be helpful in providing more accurate measures. Although there were some limitations to our data, the new information provided by our study will be invaluable when counseling parents and their families regarding their expectation for their child regaining speech discrimination. In our study of children undergoing cochlear explantationreimplantation, the most common reason for explantation was device malfunction, occurring in 78 (67%) of 116 of cases. This represents only a small proportion of device malfunction, as most cases of device malfunction involve only a few electrodes and do not require explantation. Carlson et al. (24) reported a 9% incidence of one or more open or short electrodes, with an 11% in children, yet only 4 (1%) of 636 children and adults required revision surgery for deteriorating devices. Brown et al. (25) report 7.3% revision rate for children, with 78% of revisions because of device failure for children and adults. Most of our pediatric device failures occurred several years after initial surgery, and so follow-up mapping is useful in detecting these failures. Our higher revision rate is influenced by our center’s long running program of nearly 30 years. Infection was the next most common reason for explantation, and most infection-only cases were explanted within 7 months after initial surgery. Infection and extrusion occurred in 2 peaksV either within a few months of surgery or after 2 years. These results indicate that the surgical site needs to be watched, beyond a few months postoperatively. CONCLUSION Impedance levels in the cochlear implant array can be used as an indirect measure of inflammation and fibrotic changes associated with surgery. Explantationreimplantation of both straight and contour Cochlear devices results in raised impedance in the second device. The increase in impedance was only marginally significant for the straight array but very significant in the contour array over the long term. Auditory performance takes time to return after reimplantation, but the majority has regained their normal level by 6 months after surgery. Additional studies are required to examine the influence of steroids and other medicines on postoperative impedance and to establish the optimal dose, mode of delivery, and duration to reduce intracochlear fibrosis, particularly after explantationreimplantation. Acknowledgment: The authors thank Liz Barnes, biostatistician.
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