Long CI Electrodes: Hearing Outcomes
Very-long CI electrodes (28mm, 31 mm), elegantly flexible and minimally traumatic, are designed to be deeply inserted into the low-frequency areas of the upper cochlear turns. However, it is not yet clear whether this additional depth of insertion provides outcomes superior to standard-length electrodes (< 24 mm). This is important because reaching the upper turns comes at a potential cost.
Very-long electrodes have previously been associated with greater loss of residual hearing and balance (1) as well as a higher rate of incomplete insertion (18%) than standard-length electrodes (2). (CI Surgeons Blog 12/1/15)
Failure of complete insertion
Daniele De Seta and colleagues of a research consortium led by Isabelle Moniere of the Groupe Hospitalier Pitié-Salpêtrière in Paris, reported that in a 5-year, prospective, multi-center study, failure of full insertion of very-long electrodes occurred in 12 of 38 (32%) implanted ears (3).
De Seta et al also found that the size of the cochlea had no effect on failure of full insertion. “The size of the cochlea…was similar between the ears with a full insertion and the ears with a partial insertion.” “No significant difference in the size of the cochlea between ears with incomplete and complete insertions was found in our study (3).” This differs from prior studies suggesting that the high partial insertion with very-long electrodes is due to the variable size of the cochlea. Since the size of the cochlea did not appear to be the cause of incomplete insertion, other factors might be considered, including the additional length of the electrode.
Loss of residual hearing
Kisser et al of the University of Munich reported sub-total loss of residual hearing with 28 mm electrodes, concluding that the 28 mm electrode “does not allow for usable additional hearing at present (4).” The authors recognize technical difficulties that may somewhat limit their study.
Hearing not improved
De Setta shows that the depth of insertion of very-long electrodes was not associated with better hearing outcomes. In considering only ears with full electrode insertion, but variable angular depth of insertion (510° to 880°), the authors found that deeper insertion into the apical region does not correlate with better hearing. “If we consider the ears with full insertion of the electrode array, despite a large variation of the angular depth of insertion, no correlation was found between this variable and the hearing performance.” 3In other words, “No correlation was found between the speech perception scores and the angular depth of insertion, both in quiet and in noise….(3)” Nonetheless, it may be important to note that even the 510° insertions are deeper than most standard-electrode insertions.
In expert hands, incomplete insertion of very-long electrodes occurred in 18 - 32% of ears.
Deeper cochlear penetration with very-long electrodes added no hearing benefit in this study. (Angular depth is not the same as length. Depth depends on cochlear size and electrode trajectory).
The De Seta study includes 38 subject ears, a notable accomplishment for a 5-year study. The findings reported are statistically significant. Nonetheless, as the authors point out, it is possible that larger studies may have different findings. It should also be noted that previous studies have found benefits of very-long electrodes. (5,6)
1. Nordfalk K F, Rasmussen KH, Bunne, M et al. Insertion Depth in Cochlear Implantation and Outcome in Residual Hearing and Vestibular Function. Ear and Hear 2015. Epub ahead of Print -
2. Brito R, Alves T…Bento RF. Surgical complications in 550 consecutive cochlear implantations. Braz. J. Otorhinolaryngol. 78; 3: May/June 2012.
3. De Seta D, Nguyen Y, Bonnard D, Ferrary E, Godey B, Bakhos D, Mondain M, Deguine O, Sterkers O, Bernardeschi D, Mosnier I. The Role of Electrode Placement in Bilateral Simultaneously Cochlear-Implanted Adult Patients. Otolaryngol. Head Neck Surg. (05/2016) E-Published before print: 0194599816645774.
4. Kisser U, Wunsch J, Hempel JM, Adderson-Kisser C, Stelter K, Krause E, Muller J, Schrotzmair F. Residual hearing outcomes after cochlear implant surgery using Ultra-flexible 28-mm electrodes. Otol Neurotol, PAP 2016) doi: 10.1097/MAO.0000000000001089.
5. Roy AT, Penninger RT, Pearl MS, Wuerfel W, Jiradejvong P, Carver C, Buechner A, Limb CJ. Deeper Cochlear Implant Electrode Insertion Angle Improves Detection of Musical Sound Quality Deterioration Related to Bass Frequency Removal. Otol Neurotol. 2016 Feb;v37(2):146-51.
6. Hochmair I, Hochmair E, Nopp P, Waller M, Jolly C. Deep electrode insertion and sound coding in cochlear implants. Hear Res. 2015 Apr; 322:14-23.