Long Cochlear Implant Electrodes: Incomplete insertion, loss of residual hearing and balance
October 16, 2015
Preservation of residual hearing has been a goal of CI surgery since it was first reported in 1989 (1). It is an indication of good surgical technique and may result in enhanced speech perception in conventional (2) and hybrid devices (3). Very-long electrodes (> 28 mm) have been used successfully for over two decades, but continue to be debated due to anecdotal inferences of loss of residual hearing and vestibular function, as well as a high incidence of insertion failure.
A recent study by Nordfalk et al (4) of Oslo University provides new data to address these issues.
Loss of Residual Hearing
In this study, 35 adult subjects received very-long electrodes (31.5 mm and 28 mm.) In addition, four received more typical, 24 mm electrodes of similar design.
A statistically significant relationship between loss of residual hearing and very-long electrodes occurred at 250 Hz (p < .05) but not for the low frequency pure tone average (lf-PTA.) A possible ceiling effect is difficult to avoid due to greater pre-op hearing loss at 500 and 1000 Hz.
Loss of Vestibular Function
Electrode Length 24 mm 28 mm 31.5 mm
Vestibular Symptoms 0% 6% 12%
Lost VEMP 0% 36% 64%
The table suggests that very-long electrodes are associated with subjective and objective indications of vestibular system damage proportional to their length. No vestibular symptoms or VEMP abnormalities were seen with the standard-length electrodes
Failure to completely insert long electrodes
Nearly 1/5 (18%) of very-long electrodes could not be fully inserted by this group of experienced surgeons. These findings confirm anecdotal reports and clinical experience of a high rate of full-insertion failure with very-long electrodes. Insertion failure rates for standard-length electrodes is significantly lower, in the range of 3 to 4% (5).
Nordfalk and colleagues at the University of Oslo have shown that very-long CI electrodes (> 28 mm) are associated with a higher incidence of cochlear and vestibular dysfunction when compared to standard-length electrodes (< 24 mm). The authors also demonstrated that failure to completely insert this type of electrode occurred in 18% of recipients, even in highly competent and experienced hands.
1. Boggess WJ, Baker JE, Balkany TJ. Loss of residual hearing after cochlear implantation. Laryngoscope. 1989; 99:1002-5
2. Sheffield SW, Jahn K, Gifford RH. Preserved acoustic hearing in cochlear implantation improves speech perception. J Am Acad Audiol. 2015 Feb;26 (2):145-54.
3. Roland JT Jr, Gantz BJ, Waltzman SB, et al. United States multicenter clinical trial of the cochlear nucleus hybrid implant system. Laryngoscope. 2015 [Epub ahead of print]
4. 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.
5. Brito R, Alves T…Bento RF. Surgical complications in 550 consecutive cochlear implantations. Braz. J. Otorhinolaryngol. 78; 3: May/June 2012.