Large vestibular aqueduct syndrome: Impedance changes over time with different cochlear implant electrode arrays

Abstract

Objectives

The aim of this study was to assess if large vestibular aqueduct syndrome (LVAS), with the increase in perilymphatic pressure, affects impedance changes over time with different types of Cochlear^® implant electrode arrays Contour, Straight, and CI 422. To report speech perception outcomes for these cochlear implant recipients.

Methods

Retrospective case review of impedance levels and categories of auditory performance. Impedance data were collected at switch on, 1 month, 3, 6, 12, and 24 months after cochlear implantation and compared with control (non-LVAS cochlear implant recipient) data for each array type. Forty-seven patients with exclusive LVAS and no other vestibulocochlear abnormalities or other identifiable cause of deafness were eligible for inclusion in the study.

Results

In LVAS patients, there was a significant difference in impedance between the three types of device (P < 0.0001). Time since switch on was associated with a decrease in impedance for all three devices (P < 0.0001). The mean impedance reduced between switch on and 1 month and remained relatively constant thereafter. Sound variation with softening of sounds was seen in four CI 422 (Straight Research Array) recipients due to ongoing fluctuations in electrode compliance.

Discussion

For all three array types, there was no significant difference in the mean impedance between the LVAS patients and controls over the first 12 months. In keeping with previous studies cochlear implant recipients with LVAS hear very well through the cochlear implant.     

Electric compound action potentials (ECAPs) and impedances in an open and closed operative site during cochlear implantation

Introduction: In patients undergoing cochlear implantation, intraoperative measures of impedance and electrically evoked compound action potentials (ECAPs) are used to confirm device integrity and electrode array position. However, these electrophysiological parameters have been shown to decrease over time, with a small decrement observable as early as 24?h post implantation and becoming more apparent after 6 months. Whether the intraoperatively measured impedances and ECAPs recorded immediately after electrode insertion versus later in the operation or in an open versus closed operative site vary has not been documented. Such variation in measurement procedure may affect the ultimate operative outcome.

Patients and methods: Between February and October 2016, 38 patients received a cochlear implant (Cochlear^®), with half receiving a CI 522 device and the other half receiving a CI 512 device. These patients were distributed into three groups. In the first (group A; n?=?21), the impedance and threshold neural response telemetry (tNRT) measures were taken before (M1) and after cutaneous suture (M2), whereas in the second group (group B; n?=?11) they were taken twice in the open operative site, once at the time of electrode insertion (M1) and then again 10?min later (M2). The last group (group C; n?=?6) was measured only once after a 10?min waiting time before closing the operative site.

Results: tNRTs of both group A and B were significantly higher at M1 than measured at M2. The magnitude of change in tNRT did vary significantly by group (P?=?.027) with group A having a bigger decrease than group B. For impedances there was evidence for a significant difference in M2 between the three groups (P?=?.012), with group C having significantly higher values compared to group A and B.

Conclusion: Intraoperative tNRT measures change significantly over time, including within the first 10?min of implantation. One underlying etiology of this phenomenon for tNRTs seems to be the condition of the surgical site whereas changes of impedances can be best explained by the ‘electrochemical cleaning’ theory associated with the first stimulation of the electrode. However, for both impedances and tNRTs there also is an important impact of time as well as of acute perioperative changes in electrical conductivity.     

Potential insertion complications with cochlear implant electrodes

Objectives: The aim of this discussion paper and literature review was to estimate the incidence of a variety of complications associated with the surgical placement of cochlear implant (CI) electrode arrays and to discuss the implications and management of sub-optimal electrode placement.

Results: A review of the peer-reviewed literature suggests that the incidence of incomplete electrode insertion and kinking is more prevalent in straight arrays and not more than about 2% in CI recipients with normal cochlear anatomy/patency. Incidence of tip fold-over is greater with perimodiolar arrays but also occurs with straight arrays and is typically less than 5%. Conversely, electrode migration is more common with straight arrays, and high rates (up to 46%) have been reported in some studies. Scalar translocations have also been reported for both perimodiolar and straight arrays. Higher rates have been reported for stylet-based perimodiolar electrodes inserted via cochleostomy (up to 56%), but with much lower rates (<10%) with both sheath-based perimodiolar arrays and lateral wall arrays.

Electrode positioning complications represent a significant proportion of perioperative CI complications and compromise the level of benefit from the device. Careful surgical planning and appropriate pre- and intraoperative imaging can reduce the likelihood and impact of electrode positioning complications. There is also evidence that newer array designs are less prone to certain complications, particularly scalar translocation.

Conclusions: It is important that implanting surgeons are aware of the impact of sub-optimal electrode placement and the steps that can be taken to avoid, identify and manage such complications.     

Cochlear's unique electrode portfolio now and in the future

Abstract

Objectives

To review Cochlear's electrode portfolio and discuss the merits of current and future straight and perimodiolar electrode arrays. To present an update on implant reliability.

Methods

Performance and hearing preservation data from studies involving the Slim Straight (CI422), Hybrid L24 and Contour Advance electrode array were reviewed.

Results

While several studies in past found little difference in performance outcomes between subjects implanted with perimodiolar and straight arrays, recent studies demonstrated that proximity to the modiolus is correlated with better performance. Hearing threshold increase was lowest with the Hybrid L24, closely followed by the slim straight array and was largest with the Contour Advance array. The CI24RE receiver-stimulator used for the three arrays had a cumulative survival of 99% at eight years post implantation.

Conclusions

Combining the hearing preservation benefits of slim straight arrays with perimodiolar proximity is the design objective of Cochlear's next generation electrodes.     

Method to quantitatively assess electrode migration from medical images: Feasibility and application in patients with straight cochlear implant arrays

Objective: To propose a method for quantitative assessment of the migration of lateral-wall, straight electrode arrays after surgery based on postoperative Cone Beam Computed Tomography (CBCT) images and automated medical image analysis techniques.

Methods: A preliminary study is conducted on 19 implanted ears. For each implantation, two CBCT images are objectively analyzed. Electrode arrays are consistently projected into the same coordinate system in order to estimate precisely the migration of each electrode. Spatial configuration changes are characterized with the overall curvature of the electrode array.

Results: From the samples analyzed no significant electrode migration, extrusion or electrode curvature changes were found. Mean infinitesimal local migration reveals a tendency where apical electrodes tend to move away from the modiolus and basal electrodes away from the round window.

Conclusion: CBCT images demonstrate adequate resolution with limited artifacts to assess the electrode array position in vivo. Automated medical image analysis techniques and consistent coordinate system allow to quantitatively estimate migration and extrusion effect for lateral-wall, straight electrode array.     

Voluntary increase of acoustic middle ear impedances with simultaneous sound attenuation associated with mild hyperacusis (VIMH)

Background: Acoustic impedances, as measured with tympanometers, change with the pressure in the middle ears, or reflexively as a response to a range of stimuli. A few persons are able to influence their acoustic impedances voluntarily.

Aims/objectives: To describe this phenomenon and its clinical consequences.

Material and methods: Presentation of five case reports with measurements of the acoustic impedances, hearing thresholds, and otoacoustic emissions (TEOAE) in the relaxed state and while the change in acoustic impedance was invoked.

Results: All five subjects were able to voluntarily increase the acoustic impedance of their ears for short periods of time. While the change was active, TEOAE levels decreased by 0.8–9.7?dB and a mild transient conductive hearing loss in the low frequencies occurred. Four out of the five subjects reported a mild hyperacusis when the voluntary change was not invoked and showed unusual features in their acoustically evoked reflex.

Conclusions and significance: Subjects who can control their acoustic impedances voluntarily seem to have an increased prevalence of mild hyperacusis. Our results suggest interference of the voluntary control with the acoustic stapes reflex as a mechanism and we propose to name this phenomenon ‘Voluntary Impedance increase with Mild Hyperacusis’, or VIMH.     

Objective,audiological and quality of life measures with the CI532 slim modiolar electrode

Objectives: To report on electrode array measurements for the Nucleus^® CI532 Slim Modiolar Electrode device including: ECAP thresholds, electrode impedances, and psychophysical comfort levels, as well as speech perception results pre- and post-operatively and standardized evaluations of quality of life.

Methods: Forty-four subjects were implanted with the CI532. Electrically evoked compound action potential (ECAP) thresholds and impedances were measured using automatic Neural Response Telemetry intra-operatively, at activation and at six months post implant. Pre- and post-operative measures of words in quiet and sentences in noise were made in multiple languages. Quality of life was assessed using The Speech Spatial Qualities questionnaire (SSQ) and Glasgow Benefit Inventory (GBI).

Results: Intra-operative ECAP thresholds were recorded successfully from 90% of electrodes tested. ECAP thresholds varied across the array and a post-hoc Dunn’s test showed that median thresholds for electrodes E1–E13 were significantly greater than those for E17–E22 (all P?<?0.001). Impedances increased significantly between surgery and activation. Speech recognition scores for words in quiet and sentences in noise showed a significant improvement for the group at six months, when using the cochlear implant, compared with pre-operative performance (P?<?0.001). There was a significant increase compared to pre-operative ratings for all sections of the SSQ at six months post activation (P?<?0.001). The GBI gave scores significantly above zero for the ‘general’ subscale and total score.

Conclusion: Objective ECAP and impedance measures for this new electrode array were as expected and similar to results reported for other array types. Speech perception and quality of life improved significantly following implantation.

Trial registration: ClinicalTrials.gov identifier: NCT02392403.     

Impact of cochlear tonotopy on electrically evoked compound action potentials (ECAPs)

Background: A wide range of cochlear implant electrode designs exists. Lateral wall electrodes may be favored for their potential to preserve residual hearing by virtue of being thin and delicate; whereas perimodiolar electrodes may have advantages in case of profound hearing loss, due to electrode positioning in close proximity to the auditory nerve fibers.

Aim: The aim of this study was to investigate the impact of these two array designs on the interaction between electrodes and the auditory nerve in different tonotopic regions of the cochlea.

Patients and methods: A retrospective study of both adult and pediatric cochlear implant recipients (CI24RE/CI512 or CI422, Cochlear®) was undertaken. The differences of threshold Neural Response Telemetry (tNRT) acquired 12 months after surgery were analyzed with respect to the tonotopic location.

Results: The results of 168 implants showed that perimodiolar arrays had lowest thresholds in the basal region whereas straight arrays had lowest thresholds in the apex. Highest thresholds for both array types were encountered in the medial parts.

Conclusions and Significance: tNRTs differ depending on electrode type and location inside the cochlea. This should be considered pre implantation when choosing the electrode array type and post-implantation when mapping the CI program.     

Electrode impedance changes over time in MED El cochlear implant children recipients: Relation to stimulation levels and behavioral measures

Objective: Electrode impedance measures resistance encountered by electric current passing through wires, electrodes and biological tissue. This study was designed mainly to evaluate changes in electrode impedance values and psycho-electric parameters changes (i.e. threshold levels, comfortable levels, and dynamic range) in cochlear implant patients over time.

Methods: It was a prospective study encompassing 20 patients implanted by MED-EL device programd using behavioral programs. Electrical stimulation levels and electrode impedance values were examined at 0, 1, 3 and 6 months after the first fitting session.

Results: Electrode impedance values were reduced from the time of activation to the 6 months visit. Most comfortable levels increased and dynamic range widened until the 6 months visit. There was an inverse correlation between impedance values and most comfortable level as well as dynamic range, over time.

Conclusion: Frequent monitoring of electrode impedance (for device and electrodes problems) and electric stimulation levels (for better performance, mapping and habituation) during the first 6 months of implant use is recommended.     

Electrically evoked compound action potentials are different depending on the site of cochlear stimulation

One of the many parameters that can affect cochlear implant (CI) users’ performance is the site of presentation of electrical stimulation, from the CI, to the auditory nerve. Evoked compound action potential (ECAP) measurements are commonly used to verify nerve function by stimulating one electrode contact in the cochlea and recording the resulting action potentials on the other contacts of the electrode array.

The present study aimed to determine if the ECAP amplitude differs between the apical, middle, and basal region of the cochlea, if double peak potentials were more likely in the apex than the basal region of the cochlea, and if there were differences in the ECAP threshold and recovery function across the cochlea.

ECAP measurements were performed in the apical, middle, and basal region of the cochlea at fixed sites of stimulation with varying recording electrodes. One hundred and forty one adult subjects with severe to profound sensorineural hearing loss fitted with a Standard or FLEX^SOFT electrode were included in this study. ECAP responses were captured using MAESTRO System Software (MED-EL). The ECAP amplitude, threshold, and slope were determined using amplitude growth sequences. The 50% recovery rate was assessed using independent single sequences that have two stimulation pulses (a masker and a probe pulse) separated by a variable inter-pulse interval. For all recordings, ECAP peaks were annotated semi-automatically.

ECAP amplitudes were greater upon stimulation of the apical region compared to the basal region of the cochlea. ECAP slopes were steeper in the apical region compared to the basal region of the cochlea and ECAP thresholds were lower in the middle region compared to the basal region of the cochlea. The incidence of double peaks was greater upon stimulation of the apical region compared to the basal region of the cochlea.

This data indicates that the site and intensity of cochlear stimulation affect ECAP properties.     

Research software in cochlear duct length estimation,Greenwood frequency mapping and CI electrode array length simulation

Background and objectiveThe size of the cochlea varies a lot among the human population bringing the necessity for electrode arrays to be available in various lengths irrespective of the cochlear implant (CI) brand. This research software helps in the estimation of the patient''s cochlear duct length (CDL) which is then used for the simulation of the correct length electrode array matching the patient''s cochlear size and as well in getting the patient specific cochlear frequency map.MethodsVisual Studio Express 2012 for Windows Desktop is used in the architecture of this research software. The basal turn diameter of the cochlea (“A” value) needs to be measured from the pre-operative computed tomography (CT) image of the patient''s temporal bone. This “A” will be taken as the input for the CDL equations proposed by Alexiades et al for estimating the CDL along the basilar membrane for various insertion depths. Greenwood''s equation is then used in combination with the CDL for the full length of the cochlea in getting the patient specific frequency map.ResultsThe research software with the help of the “A” value as input, with few button clicks, gives the patient specific CDL for various insertion depths and the Greenwood''s frequency map. The users have the choice to select any electrode array of their choice and place it under the frequency map to see how good it fits to that particular patient''s cochlea. Also, given the possibility to drag and move the electrode array picture to mimic the post-operative actual electrode insertion depth.ConclusionsThis research software simplifies the overall process of CDL estimation and in getting the patient specific cochlear frequency map. The clinicians get the chance to simulate placing the various electrode array lengths in patient cochlea in identifying the best fit electrode. This could help in pushing the CI field into the concept of individualized CI electrode array solution that ultimately benefits the patients.     

Long-term electrode impedance changes and failure prevalence in cochlear implants

Objective: This study assessed the prevalence of electrode failures and electrode impedance measures in Nucleus cochlear implants around initial activation (an average of 16 days after surgery) and after 8 to 12 years of device use. Design: Retrospective data from the Melbourne Cochlear Implant Clinic was collated and analysed. Study sample: Included in this study were 232 adults, all of whom were implanted at the clinic between March 1998 and August 2005. Results: Overall 0.5% of electrodes failed over the entire test period, with 5.6% of devices showing one or more electrode failure. The majority of these failures were recorded by initial activation. The numbers of electrode failures have decreased over time with array type, such that no failures were recorded with the currently available Contour Advance array. Array type was shown to affect electrode impedance at both time points, with the Contour and Contour Advance arrays having significantly higher absolute values than the Banded array. However, the Banded array had significantly higher area-normalized impedances at initial and final measures than the Contour and Contour Advance array. Conclusions: A relatively low incidence of electrode failures were recorded for the Nucleus devices of these recipients. Electrode impedance dropped for all array types after 8 to 12 years of device use.     

The importance of intra-operative imaging during cochlear implant surgery

Abstract

Two unusual cases of cochlear implant (CI) surgery complication are reported: introduction of the electrode array into the superior semicircular canal with normal morphology and a growing amplitude of neural potential (neural response imaging, NRI) during intra-operative monitoring control. In the first case, a two-year-old patient affected by congenital sensorineural profound deafness was bilaterally implanted with two Clarion 90 k devices and intra-operative electrophysiological and radiological controls were performed. After introduction of the array in the right side NRI was performed and a neural potential was found only on two apical electrodes. Radiological intra-operative control with antero-posterior trans-orbital plain films was performed to assess the position of the electrodes inside the cochlea. Radiography showed the electrode array in the superior semicircular canal in the right ear. The electrode array was removed and reinserted correctly. In the second case, a 72-year-old man underwent left cochlear implantation for sensorineural profound deafness of unknown origin. Intra-operative electrophysiological testing (NRI) showed the presence of neural potential on three tested channels. In this case, as routinely employed since 2006, an intra-operative static fluoroscopy control was performed, this showed the electrode array in the superior semicircular canal. The electrode array was removed and reinserted correctly. In conclusion, intra-operative monitoring tests during CI surgery play different roles: measurement of impedances and NRI can evaluate the integrity of implant electrodes and the status of the electrode cochlea interface, but they cannot be the only way to confirm correct positioning of the array. In our opinion the intra-operative radiological check is helpful during CI surgery, especially when there is any doubt about correct electrode insertion. Copyright © 2009 John Wiley & Sons, Ltd.     

Protecting against electrode insertion trauma using dexamethasone

Objective: To compare the benefits of a dexamethasone-eluting array for hearing preservation and cochlear histopathology in low trauma (soft-surgery) and high trauma models of cochlear implant surgery.

Methods: Adult guinea pigs were implanted with an intra-cochlear array using two different surgical procedures: either a soft-surgery approach or following generation of electrode insertion trauma (high trauma). Two methods of dexamethasone delivery were evaluated: elution from an electrode array alone, and elution from a cochlear implant electrode array in combination with a pre-operative systemic injection. All electrode arrays were implanted for a period of 4 weeks. Outcome measures at 4 weeks post-implantation included auditory brainstem response (ABR) thresholds, histological analysis of spiral ganglion neuron density, fibrotic tissue, new bone growth, and cochlear damage.

Results: Animals exposed to high surgical trauma showed greater hearing loss than those in the low trauma model, irrespective of the presence of dexamethasone. Whilst the area of intra-cochlear fibrotic tissue growth post-implantation was also independent of dexamethasone administration, new bone growth was significantly reduced in its presence. Our high trauma model effectively obliterated the organ of Corti and significantly reduced spiral ganglion neuron densities in the lower basal turn. This trauma-induced reduction in spiral ganglion neuron survival decreased with the inclusion of a dexamethasone-eluting array. A pre-operative systemic injection of dexamethasone did not significantly improve any outcome measures beyond those provided with a dexamethasone-eluting array alone.

Conclusion: Dexamethasone-eluting intra-cochlear arrays may inhibit osteoneogenesis, and reduce spiral ganglion neuron loss following traumatic cochlear implantation.     

Flat-panel computed tomography versus multislice computed tomography to evaluate cochlear implant positioning

Abstract

Objective

To evaluate and compare image quality between flat-panel volumetric computed tomography (fpVCT) and multislice CT (msCT) in temporal bones with cochlear implants (CIs), and to evaluate fpVCT imaging for accuracy in determining CI electrode positioning.

Methods

Six cadaveric temporal bones were imaged prior to CI using fpVCT. Each bone was implanted with an electrode array and rescanned in order to create radial reformatted images through each electrode contact. Electrode–modiolar interval (EMI) distances were measured. The bones were fixed and cut in order to grossly evaluate for CI intrascalar positioning and insertional trauma.

Main outcome measure

To compare image quality between fpVCT and msCT in temporal bones with CI, and to evaluate the utility of fpVCT in post-implantation temporal bone analysis.

Results

The mean EMI distances did not differ significantly between fpVCT and msCT images, while the image quality was significantly better for fpVCT. Furthermore, information about intracochlear trauma and intrascalar electrode array positioning can be ascertained using this radiographic technique.

Conclusion

fpVCT and msCT do not differ significantly in the evaluation of EMI distances in implanted temporal bones, but the image quality is significantly better using fpVCT. Additionally, useful information regarding intracochlear trauma, electrode depth of insertion, and intrascalar positioning can be gained from fpVCT imaging. Given the ease of use, superior image quality, improved convenience, reduced levels of radiation, and agreement with histology, fpVCT is a valuable option for post-implantation temporal bone imaging.     

Cochlear implantation in patients with canal wall down mastoidectomy cavities

Abstract

Background: Safe cochlear implantation (CI) is challenging in patients with a canal wall down (CWD) mastoidectomy cavity.

Objectives: We reviewed the outcomes of CI and proposed surgical management principles according to the presentation status of CWD mastoidectomy cavity.

Material and methods: The cases of eight patients (nine ears) with CWD mastoidectomy cavity who underwent CI were retrospectively reviewed. The basis of the surgical decision, postoperative complications, and postimplant auditory performance were analysed.

Results: In seven patients (eight ears), implantation was performed in a single stage; in six ears, the external auditory canal (EAC) was oversewn. In two patients with ossification, the electrode array was inserted into the scala tympani by drilling of the basal turn or in the second turn of the cochlea through a drill-out procedure. Seven patients had a follow-up of 12–50 months and one patient was lost to follow-up. None of the followed-up seven patients suffered complications.

Conclusions: CI is safe and effective in patients with profound hearing loss after radical mastoidectomy. In patients with CWD mastoidectomy cavity, CI does not cause a higher rate of postoperative complications relative to standard CI procedures. The outcome is excellent and comparable to that in the general CI population.     

Impedance changes in chronically implanted and stimulated cochlear implant electrodes

Abstract

Objectives

Electrode impedance increases following implantation and undergoes transitory reduction with onset of electrical stimulation. The studies in this paper measured the changes in access resistance and polarization impedance in vivo before and following electrical stimulation, and recorded the time course of these changes.

Design

Impedance measures recorded in (a) four cats following 6 months of cochlear implant use, and (b) three cochlear implant recipients with 1.5–5 years cochlear implant experience.

Results

Both the experimental and clinical data exhibited a reduction in electrode impedance, 20 and 5% respectively, within 15–30 minutes of stimulation onset. The majority of these changes occurred through reduction in polarization impedance. Cessation of stimulation was followed by an equivalent rise in impedance measures within 6–12 hours.

Conclusions

Stimulus-induced reductions in impedance exhibit a rapid onset and are evident in both chronic in vivo models tested, even several years after implantation. Given the impedance changes were dominated by the polarization component, these findings suggest that the electrical stimulation altered the electrode surface rather than the bulk tissue and fluid in the cochlea.     

Cochlear function following implantation with a full electrode array

Abstract

Objectives: Clinicians traditionally advise patients that implantation with a conventional full cochlear implant array will lead to loss of any remaining hearing in the implanted ear. We sought to assess the extent to which ‘standard’ insertion of a full electrode array affects cochlear function.

Methods: Air conduction pure tone audiometry was performed as part of the routine 6-month postimplantation audiological assessment for 81 adult patients who preoperatively had identifiable pure tone thresholds. All patients were implanted with the nucleus CI24 device, with complete insertion of a straight array in 68 and a contour array in 13 patients. A separate cochleostomy was fashioned anterior to the round window using a 1 mm diamond burr, and the array inserted without lubrication. The operating surgeon was blinded to the inclusion of patients in this study at the time of surgery.

Results: Of 81 patients, 58 preserved some residual hearing at 6 months. The rate of preservation of hearing was higher in the low frequencies. At 500 Hz 77 patients had an identifiable preoperative pure tone threshold, still identifiable postoperatively in 48. At 4 kHz only 25 patients had an identifiable preoperative threshold, preserved in 9, and at 8 kHz 6 of 13 patients still had identifiable pure tone thresholds postoperatively.

There was no correlation between the depth of insertion and hearing preservation, both groups having a mean of 5 stiffening rings inserted in addition to the 22 active electrodes.

The mean deterioration in threshold in those who had residual hearing was 15 dB at 500 Hz, and in 29 patients the deterioration in the threshold was less than 15 dB.

Conclusion: Insertion of a full conventional electrode array is not as damaging to cochlear function as often assumed. Residual low-frequency hearing may be preserved in approximately two-thirds of patients 6 months postoperatively. This study supports our counselling patients that despite the delicate nature of the inner ear there is a significant chance that their air conduction hearing will not be lost.     

Electrical impedance measurements with the CI24M cochlear implant for a child with Mondini dysplasia

Electrical impedance measurements can give useful information about the status of individual electrodes of a cochlear implant. Impedances within the normal range (when measured in the common ground mode of stimulation) suggest that current flow occurs in the tissue and fluid of the cochlea. Low impedance measurements may suggest that particular electrodes are short circuiting, whereas high impedances might be due to a broken electrode wire or an electrode only in contact with air. In the case discussed, low impedance measurements were recorded intra-operatively from a child with Mondini dysplasia on electrodes 1-13 of a Nucleus CI24M device. Post-operatively the impedances had returned to the expected range and were comparable with other patients implanted with the CI24M device. Possible reasons for this are discussed. It was thought that impedances were low intra-operatively due to a larger-than-normal proportion of fluid surrounding the electrode array, rather than short circuits occurring along the array, as suggested by the dps7 software.