Comparison between Long Electrode Arrays vs. Short Electrode Arrays Insertion in Cochlear Implantation of IP II cases
By Mohamed Alaa, Clinical Engineer for Inner ear Implants and student of
Abstract
There are different types of hearing loss which have various reasons to occur as one of the reasons maybe cochlear malformations, which is going to be the focus of this research. Cases with problems in the inner ear having severe to profound hearing loss could benefit from cochlear implants.
Incomplete partition II is a condition where the cochlea consists of 1.5 turns; the apical and middle cochlea turns are undifferentiated and form a cystic apex. The vestibule is normal while the vestibular aqueduct is always enlarged. Developmental arrest occurs at the seventh week of gestation. Characterized by absent or deficient modiolus, only normal at level of basal turn.
This study aims to show the effect of cochlear implantation of long electrode arrays vs. short electrode arrays in cases of Incomplete partition type II, the rest of malformations will not be reviewed in detail and will only be referred to as appropriate.
The results also match other studies showing the effects of long electrode arrays in normal cochlea and its effect on speech perception when compared to short electrode arrays, indicating similar results between normal cochlea an incomplete partition type II.
INTRODUCTION
There are many types of cochlear malformations. However, since the focus of this research is on the effect of cochlear implantation of long electrode arrays vs. short electrode arrays in cases of Incomplete partition type II, the rest of malformations will not be reviewed in detail and will only be referred to as appropriate.
Hearing loss appears when one or more part of the ear is not functioning normally. It can affect one ear or both ears, depending on the defected part; the type of hearing loss is specified.
The four main types of hearing loss are: Conductive Hearing Loss, Sensorineural Hearing Loss, Mixed Hearing Loss and Neural Hearing Loss.
Also hearing loss is classified according to its severity or degree, starting from mild hearing loss and ending with profound hearing loss. An audiogram is a graph that displays the audible thresholds a person can hear at different frequencies. The list below (Figure 1) shows different hearing loss thresholds ranges when compared to normal hearing thresholds.
Figure 1. Hearing Loss Degree (How to Read an Audiogram and Determine Degrees of Hearing Loss) How to Read an Audiogram and Determine Degrees of Hearing Loss. (n.d.). Retrieved from: http://www.nationalhearingtest.org/wordpress/?p=786
Hearing loss causes
It can occur at different periods across lifetime, people are most vulnerable to hearing impairment during crucial periods in their life. Whether it is in the prenatal period, perinatal period, postnatal period or adulthood period across lifetime.
According to World Health Organization key facts, it states that around 430 million (5% of the world’s population) have hearing loss. It is expected that by 2050 over 700 million people will have disabling hearing loss.
Disabling hearing loss describes having more than 35 dB of hearing loss in the better ear in adults and more than 30 dB in the better ear in children. Almost 80% of these people live in poor countries. Hearing loss prevalence increases with age, especially with people aged 60 years or over.
Cochlear implant is an electronic device designed for people suffering from severe-to-profound sensorineural hearing loss. A cochlear implant bypasses the damaged hair cells in the inner ear and directly stimulate the neural cells inside the cochlea electrically, which will pass through the auditory nerve then transmitted to the brain where they are perceived as sound.
The external part called the “External Audio Processor” (Figure 3) detects the sounds and sends them to the internal part “Implant” (Figure 2), which is surgically placed just under the skin behind the ear.
The internal part or the Implant consists of a demodulator, coil, magnet and an electrode array inserted into the cochlea while the external part or the audio processor consists of a microphone, speech processor, transmitter coil and power source.
Figure 2. Cochlear Implant Figure 3. External Audio Processor
MED-EL SYNCHRONY 2 implant and SONNET 2 external audio processor.
Retrieved from: https://www.medel.com/hearing-solutions/cochlear-implants/synchrony2
Previously malformations of the inner ear were considered as a contraindication to cochlear implantation, but the progress in cochlear implants technology and surgical techniques have permitted implanting malformed cochleae. The developmental interruption during pregnancy at different stages causes different malformations, also inner ear malformations are mostly due to genetic etiology.
Normal cochlea
It is important to define the normal anatomy as seen on the CT scans before describing the characteristics of inner ear malformations. It is very important to determine the normal cochlear architecture and IP anomalies in the modiolus (Figure 5).
Incomplete Partition of the cochlea
It is a set of malformations in the cochlea accompanied by other defects in the inner ear. All types of incomplete partition have malformations affecting both the modiolus and the interscalar septa.
According to the radiological data (CT classification), we have three types of incomplete partition:
Figure 5. Incomplete partition types. Sennaroglu, “Cochlear implantation in inner ear Malformations— a review article,” Cochlear Implants International, vol. 11, no. 1, pp. 4–41, 2010.
IP type I.
The cochlea is known as ‘cystic cochleovestibular malformation’ as it has a cystic appearance (Sennaroglu and Saatci, 2002). The cochlea has an absent modiolus and interscalar septa. It comes with a dilated vestibule. There is also defect between the internal auditory canal and the cochlea, due to the cochlear defect and the modiolus absence.
Figure 6. Incomplete partition type I.
IP type II.
The cochlea has normal proximal basal turn of the cochlea and a normal modiolus basal coil, while it has a deficient modiolus and interscalar septum at the apical part, therefore, the apex of the cochlea looks like a cyst. The cochlea is usually accompanied with an enlarged vestibular aqueduct and a minimal vestibular dilation constituting the Mondini deformity triad. The cochlea’s external dimensions are like the normal cochlea.
Figure 7. Incomplete partition type II.
IP type III.
The cochlea has a totally absent modiolus and a present interscalar septum. It is reported as a genetically inherited chromosome X linked deafness. The cochlea lies laterally to the IAC. This gives the cochlea a characteristic appearance. The cochlea’s external dimensions are like the normal cochlea.
Figure 8. Incomplete partition type III
A prospective trial (Canfarotta et al., 2020), made over 4 years of follow-up, showed better speech recognition for patients implanted with a long lateral wall electrode array covering the whole duct length of the cochlea compared to subjects with a short array covering only the basal turn of the cochlea.
As long electrode arrays offer temporal information closer to tonotopic place than shorter electrode arrays, especially as the spiral ganglion extends in the apex from 630° to 720° angle of insertion depth.
This study (Dhanasingh, A., N Jolly, C., Rajan, G., & van de Heyning, P., 2020). showed better comprehension of spiral ganglion cell bodies’ number and distribution in the modiolus of a cochlea with normal hearing besides in a cochlea with hearing loss due to different malformations. It also showed that SGCBs are present inside the modiolar trunk extending to 630°–680° of angular depth, which almost ends at the helicotrema, the cochlea’s second turn.
Figure 9. Density of SGCBs in each segment as a percentage of the entire number of SGCBs (Y axis) vs. angular depth (X axis).
Figure 10. SGCB density of each Segment as a percentage of the entire number of SGCB (Y-axis) vs. angular insertion depth (xaxis). Adapted from Otte et al. (1978), Nadol et al. (1989), Pollak et al. (1987), Schmidt (1985), and Kawano et al. (1996) and redesigned by adding additional information. Organ of Corti is represented by the gray spline line with the Greenwood frequency distribution along with the angular depth measured from the RW. The SGCB distribution is shown by the prominent black spline line with the vertical dotted line differentiating the four segments. The SGCB density is given in brackets for each segment.
Cochlear Implant in IP II
IP type II. In these malformations, the basal part of the modiolus is normal and the apex is cystic. Normally, spiral ganglion cells are located in the basal part of the modiolus and there are no ganglion cells are found in the apex (Slattery and Luxford, 1995). In theory, it should be achievable to supply the stimulation to the inner ear by the cochlear implant in the same way as we stimulate the normal cochlea with a cochlear implant, since the IP II basal turn of the cochlea is normal. In these cases, any type of electrode can be used either pre-curved electrodes (modiolar hugging) or straight electrodes.
Aim of Work
This study was conducted to compare between two main approaches for cochlear implantation and their influence on speech recognition in cases with incomplete partition type II malformation, the long electrode arrays vs. the short electrode arrays insertion with respect to auditory nerve responses in the apical region of the cochlea and the speech perception of both groups.
Materials and Methods
This is a retrospective cohort study. A total of 31 patients were diagnosed with severe to profound sensorineural hearing loss with an incomplete partition type II deformity were implanted with MED-EL implants. Eighteen implanted with CONCERTO and 13 implanted with SONATA, where the only difference between CONCERTO and SONATA implants is the stimulator housing thickness with is 5.9mm in case of SONATA and 4.5mm in case of CONCERTO which only affects the surgical fixation and it doesn’t affect the tests. Their age ranges from 5 to 39 years with a mean age of 11 years. Of the thirty-one patients, 16 were women (51.7%) and 15 were men (48.3%). All MED-EL electrode arrays are lateral wall electrodes with 12 channels.
Subjects were divided into two groups according to the insertion depth of the electrode array inside the cochlea, in order to find out the influence of the full insertion of electrode arrays (Group A) and the incomplete insertion of electrodes (Group B) on the apical stimulation of the cochlea.
Group A (Fully inserted subjects)
A total of 17 subjects had fully inserted long electrode arrays into the cochlea. 14 subjects were implanted with FLEX28 electrode array which is 28 mm long and 3 subjects were implanted with Standard electrode array which is 31 mm long. They were tested for neural stimulation in the apical region of the cochlea with both electrophysiological tests and behavioral tests since the electrodes are completely covering the cochlea, thus achieve tonotopy. In physiology, tonotopy is the spatial arrangement of where sounds of different frequency are processed in the brain. Tonotopic organization refers to the systematic topographical arrangement of neurons as a function of their response to tones of different frequencies.
Group B (Incompletely inserted subjects)
A total of 14 subjects. 8 subjects were implanted with FORM24 electrode array which is 24 mm long, 1 subject was implanted with FORM19 electrode array which is 19 mm long, 4 subjects were implanted with FLEX28 electrode (not fully inserted as shown in Table 2) and 1 subject was implanted with Standard electrode (not fully inserted as shown in Table 2). They were tested with behavioral tests only since the electrodes are not completely covering the cochlea, therefore, they were not tested with electrophysiological tests for neural stimulation in the apical region of the cochlea.
Subjects were tested intra operatively and post operatively on several visits to check the device functionality and electrophysiological tests in order to test the ability to stimulate neurons via MED-EL software MAESTRO 9.0.3 using tools such as: IFT (Impedance and Field Telemetry) and ART (Auditory Nerve Response Telemetry) & audiometry test (Audiogram)
The resistance of the electrodes to the current stimulated. It depends on the liquids and tissues surrounding the intracochlear electrodes inside the inner ear. The lower impedance values, the less resistance; accordingly, better electrical conduction. In order to evaluate the integrity of the implant electronics and electrodes. IFT (Impedance and Field Telemetry) test is done before surgery (in-package), during/after surgery and at every fitting appointment.
The ECAP represents a synchronous response generated by a group of electrically stimulated auditory nerve fibers and is essentially the electrical version of Wave I of the auditory brainstem response (ABR). The ECAP is recorded as a negative peak (N1) at about 0.2-0.4 ms following stimulus onset, followed by a much smaller positive peak or plateau (P2) occurring at about 0.6-0.8 ms. The amplitude of the ECAP can be as large as 1-2 mV, which is roughly larger in magnitude than the electrically evoked ABR.
ECAP (Evoked Compound Action Potential) was measured for all subjects using Maestro 9.0.3 MED-EL software to investigate the accuracy of intraoperative electrophysiological studies in detecting the position of electrodes in cochlear implant surgery. Amplitude growth function inside “ART” task is used. ART has been recorded in the 12 channels by stimulating a channel and recording from a successive one. Amplitude levels are set to be 10 in order to increase the accuracy of THR (Thresholds) calculation. In MED-EL software you can manually adjust the P and N points to determine the ART thresholds. ART is measured intraoperatively where it is set to maximum stimulation levels since the subject is already under anesthesia and postoperatively where we asked the subjects to raise their hands in case, they felt any discomfort loudness to skip that level.
The concept of Amplitude growth function is to divide the maximum stimulation level introduced to each channel into several ascending levels. Theoretically the auditory response will increase as far as the stimulation is increased. If the measured signal is an artifact; it will be independent on the growth of the stimulation level, therefore by simply plotting the P and Nx points on all the levels. It will show inclined slope intersect with x-axis (current units) showing the threshold value for each electrode.
Figure 12. shows a screenshot from Maestro 9.0.3 MED-EL Software for the ART task using amplitude growth function. On the left side it shows the stimulation and recording electrodes used in this test. On the upper right side shows the ECAP response divided into several stimulation levels set before running the test. The lower part shows the amplitude growth function slope and ECAP threshold is the intersection point between the slope and the x-axis. Usually, the software automatically detects the P and N points but sometimes it may need manual adjustment to have accurate results.
Figure 12. Amplitude Growth Function in MED-EL Maestro 9.0.3
All subjects underwent audiological assessment starting from the 2nd visit to the audiology physician. These assessments include aided audiometry and speech audiometry. Speech recognition assessment was completed in a soundproof booth with the subject seated 1 meter away from the sound source.
Free-Field Aided Test
This free-field aided test is performed with the patient putting on his/her implant devices and sitting in a soundproof booth with the test signals delivered through speakers. An aided hearing threshold will assist the audiologist to further fine tune the hearing devices.
Speech Audiometry
Is a hearing test that uses speech stimulus to find out about a person’s ability to hear and understand speech. It helps in cross checking the pure-tone hearing thresholds, identifying functional hearing loss, selecting the most suitable hearing aid and assessing the suitability of various implant solutions. A speech score will then be calculated based on the number of words that are repeated correctly.
Arabic language was introduced live to the patient, no recorded material was used, SRT and WRS using Arabic spondees and mono syllabic word list of 25 words were used respectively.
Speech Reception Threshold (SRT) and Speech Discrimination Score (SDS)
In audiology, speech recognition ability is measured by the tests Speech Reception Threshold (SRT) and Speech Discrimination Score (SDS). The SRT corresponds to the softest sound intensity level at which an individual can recognize 50% of the common words given. Typically, SRT and SDS values are consistent with the average of the hearing thresholds obtained for the speech-related frequencies. The SDS evaluates speech discrimination using a list of monosyllables and bisyllables 40 dB above the SRT thresholds.
At 4 weeks after surgery, all subjects underwent the implant switch-on programming session by an audiology physician. Regular programming was done to the cases. All subjects were clinically tested over 4 visits, for impedance of the electrodes and ECAP (Evoked Compound Action Potential) before adjusting the subjects’ processors with the new fitting.
1st visit:
In the operation room, subjects were tested to check the impedance of the electrodes and the ECAP via Impedance Field Telemetry test and ART test which is set to maximum stimulation level since the subject is already under anesthesia.
2nd visit:
In the audiologist’s clinic, subjects were first tested for impedance and ECAP before adjusting a fitting map of the external processor according to the behavioral responses from the MCLs (Maximum Comfortable Levels). For the channels that the maximum ART stimulation level is more than the behavioral MCL levels, we asked the subjects to raise hands for any discomfort sounds to skip that level, but no patient felt any discomfort, so all the levels are recorded completely.
3rd visit:
In the audiologist’s clinic, subjects were first tested for impedance and ECAP before adjusting a fitting map of the external processor according to the behavioral responses from the MCLs. Subjects undergone aided audiological assessment, pure-tone testing, with their processors using the behavioral fitting uploaded during the previous visit.
4th visit:
In the audiologist’s clinic, after 1 year, subjects were first tested to check the impedance and ECAP before adjusting a fitting map of the external processor according to the behavioral responses from the MCLs (Maximum Comfortable Levels) if needed. Subjects undergone aided audiological assessment with their processors using the behavioral fitting uploaded during the previous visit including pure-tone and speech testing.
Most Comfortable Level (MCL)
The maximum comfortable level or the upper stimulation level. The MCL is measured in charge units (qu) and represents the highest electrical stimulation intensity which elicits a loud, but not uncomfortable auditory percept.
Results
The present study included all 31 subjects with age range from 5-39 with incomplete partition type II. Patients were asked to visit the audiologist after a year for speech testing.
Group A Table:
Subjects | Electrode | Electrodes inserted | Insertion depth (mm) | SDS Score | Channel 1 (cu) ECAP responses thresholds | Channel 2 (cu) ECAP responses thresholds |
S1 | FLEX28 | 12 | 28 | 60% | 458 | 422 |
S2 | FLEX28 | 12 | 28 | 80% | 336 | 310 |
S3 | FLEX28 | 12 | 28 | 76% | 402 | 536 |
S4 | FLEX28 | 12 | 28 | 68% | 410 | 473 |
S5 | FLEX28 | 12 | 28 | 60% | 512 | 578 |
S6 | FLEX28 | 12 | 28 | 56% | 420 | 553 |
S7 | FLEX28 | 12 | 28 | 72% | 329 | 364 |
S8 | FLEX28 | 12 | 28 | 56% | 433 | 472 |
S9 | FLEX28 | 12 | 28 | 72% | 340 | 291 |
S10 | FLEX28 | 12 | 28 | 68% | 294 | 274 |
S11 | FLEX28 | 12 | 28 | 64% | 327 | 367 |
S12 | FLEX28 | 12 | 28 | 56% | 495 | 428 |
S13 | FLEX28 | 12 | 28 | 48% | 382 | 461 |
S14 | FLEX28 | 12 | 28 | 64% | 290 | 311 |
S15 | Standard | 12 | 31.5 | 80% | 274 | 326 |
S16 | Standard | 12 | 31.5 | 76% | 305 | 288 |
S17 | Standard | 12 | 31.5 | 72% | 379 | 346 |
Subjects | Electrode | Electrodes inserted | Insertion depth (mm) | SDS
| Channel 1 (cu) ECAP responses thresholds | Channel 2 (cu) ECAP responses thresholds |
S1 | FORM24 | 12 | 24 | 56% | N/A | N/A |
S2 | FORM24 | 12 | 24 | 52% | N/A | N/A |
S3 | FORM24 | 12 | 24 | 60% | N/A | N/A |
S4 | FORM24 | 11 | 22 | 64% | N/A | N/A |
S5 | FORM24 | 10 | 20 | 60% | N/A | N/A |
S6 | FORM24 | 12 | 24 | 68% | N/A | N/A |
S7 | FORM24 | 11 | 22 | 56% | N/A | N/A |
S8 | FORM24 | 12 | 24 | 48% | N/A | N/A |
S9 | FORM19 | 12 | 19 | 52% | N/A | N/A |
S10 | FLEX28 | 10 | 23 | 52% | N/A | N/A |
S11 | FLEX28 | 10 | 23 | 60% | N/A | N/A |
S12 | FLEX28 | 9 | 21 | 48% | N/A | N/A |
S13 | FLEX28 | 10 | 23 | 56% | N/A | N/A |
S14 | Standard | 10 | 26 | 68% | N/A | N/A |
This graph shows the relation between the subjects on the x-axis and the Speech Discrimination Score (SDS) given after 1 year of wearing the device.
Group A: individuals implanted with long electrode arrays is in blue
Group B: individuals implanted with short electrode arrays is in orange.
We can see better speech discrimination score in group A whose subjects are implanted with longer electrode arrays than group B whose subjects are implanted with shorter electrode arrays.
Discussion
The objective of this research is to show whether long electrode arrays insertion has better results than short electrode arrays in cases of Incomplete partition type II being that it is better in normal cochlea. Thirty-one subjects with incomplete partition type II were divided into two groups A & B according to the insertion depth of the electrode arrays in their cochlea.
Matching with the results found in Canfarotta et al. (2020), the American Laryngological showed that recipients implanted with long electrode arrays (31.5-mm) experience better speech recognition than recipients implanted with short electrode arrays (24-mm), initially and with long-term listening experience.
First, when Auditory Nerve Response Telemetry (ART) was performed on both groups it was found that there were good apical responses in both groups, denoting that there are neural responses at the apical part stimulated by long electrode arrays like a normal cochlea.
Second, when comparing Speech Detection Scores between both groups it was found also that SDS scores were better in the group implanted with long electrode arrays than those implanted with short electrode arrays.
Following the steps normal hearing and tonotopic stimulation as the cochlea is space-specific since each place in the cochlea is responsible for certain frequencies, where the high frequencies lie in the basal turn and the low frequencies lie in the apical turn. Hence, long electrode arrays are better than short arrays when implanted in normal cochlea, therefore, achieving tonotopy and avoiding any place mismatch resulting in more natural hearing experience.
No complications were recorded or any discomfort for the cases of incomplete partition type II when implanted with long electrode arrays (31.5 mm)
Generally, results are motivating to do cochlear implants using long electrode arrays in incomplete partition type II cases.
In order to do cochlear implantation in candidates with incomplete partition type two, you can either implant long or short electrode arrays. With long electrode arrays, contacts inserted up to the apex showed an apical stimulation and good auditory nerve responses were recorded as well as better speech discrimination scores were recorded in group A with long electrode arrays when compared to group B with short electrode arrays.
There is neural survival in the apical region as ECAP responses were recorded in group A having long electrode arrays reaching up towards the apex. Relying on imitating natural hearing which proves that fully inserted electrodes lead to better results as mimicking natural hearing includes doing full insertion of long electrode arrays to cover the whole duct length of the cochlea, therefore, larger frequency range in order to achieve tonotopy and better pitch match.
Overall results are encouraging towards full insertion of electrode arrays in incomplete partition type II cases.
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