Posts in Rollover
CI Tip Fold-Over II: Intraoperative Electrophysiology and Imaging
 

Background

Intraoperative evaluation of CI electrodes has been available for many years, although many CI centers have found testing to be unnecessary in routine cases. Such judgments have been based on a low overall rate of positive findings, a relatively high rate of false-positives (presumably due to air in the cochlea), extended anesthesia time, non-reimbursable audiology time, cost, the rarity of out-of-the-box failures, exposure to radiation, etc. However, the advent of very delicate electrodes, which may be susceptible to tip foldover (ICIT Surgeon’s Blog 8.28.17) suggests the need to re-evaluate the advisability of routine electrode evaluation.

Intra-operative evaluations consist of electrophysiological tests and X-ray imaging. The former include electrically evoked compound action potential (ECAP), electrical impedance (EI), and spread of excitation (SOE). ECAP is a measure of neural responses. EI can identify open circuits (high impedance) and short circuits (low impedance). SOE represents the location of the electrical field around each electrode and overlap suggests tip foldover. These tests are evaluated using reverse telemetry. Intraoperative ECoG is evolving and a number of X-ray imaging studies (plain films, fluoroscopy, 3-D rotational X-ray, CT scan, etc.) are used in evaluation of tip foldover.

New Information

Page, Murphy, Kennett, Trinidade et al. of the University of Arkansas for Medical Sciences(1) recently reported that, based on ECAP and EI, a backup device was used in only 2 of 266 (0.8%) consecutive implants performed between 2010 and 2015. In one case, the backup device showed the same high impedances (open circuit) as the initial device but was left in place and worked normally post-op (probably due to resorption of air). The authors appropriately make a case against routine intraoperative electrophysiological testing of the CI electrodes used during the study period. However newer, more delicate perimodiolar electrode arrays were not evaluated and X-ray imaging was not routinely performed.

Zuniga, Rivas, Hedley-Williams, Gifford et al. of Vanderbilt University(2) reported that foldover was associated with perimodiolar electrodes (5 of 6 cases) but was uncommon (<2%) in electrodes used prior to mid-2015. Some of their important findings regarding foldover:

  1. SOE had limited predictive value

  2. foldover was not apparent to the surgeon during insertion

  3. foldover was not associated with aversive stimuli to the patient

  4. diagnosis was made by CT scan

  5. deactivation of overlapping electrodes resulted in improved hearing.

Garaycochea, Manrique-Huarte and Manrique(3) report failure of ECAP and EI to identify tip foldover with a newer perimodiolar array. Like Zuniga et al(2), diagnosis was made by X-ray imaging. It appears that intraoperative imaging has a higher predictive value for foldover and may be less expensive than electrophysiology (the cost of an audiologist’s time.)

Take Home

Tip foldover appears to be more common in perimodiolar electrodes. Although many CI surgeons agreed that intraoperative electrode evaluation may have been unnecessary using previous electrodes, it may be time to reconsider that decision when implanting very delicate perimodiolar electrodes. At this time, X-ray imaging appears to have higher predictive value in identifying rollover. Further study is called for to determine the actual rate of tip foldover in newer arrays.

 

References

1. Page JC, Murphy L, Kennett S, Trinidade A, Frank R, Cox M, Dornhoffer JL. The influence of intraoperative testing on surgical decision making during cochlear implantation. Otol Neurotol 2017 38(8): 1092-1096.

2. Zuniga GM, Rivas A, Hedley-Williams A, Gifford RH, Dwyer R, Dawant BM, et al. Tip foldover in cochlear implantation: Case series. Otol Neurotol 2017 38:199-206.

3. Garaycochea O, Manrique-Huarte R, Manrique M. Intra-operative radiological diagnosis of a tip roll-over electrode array displacement using fluoroscopy, when electrophysiological testing is normal: the importance of both techniques in cochlear implant surgery. Braz J Otorhinolaryngol. 2017.  http://dx.doi.org/10.1016/j.bjorl.2017.05.003

 
Preventing Electrode Tip Fold-Over
 

Background

Electrode array design has continuously evolved to reduce insertion trauma and improve hearing conservation. However, one downside of some delicate (smaller diameter, more flexible) electrodes has been a tendency for tip fold-over. Tip fold-over may occur during insertion when the electrode array tip impinges the modiolar wall (or other structure) and is temporarily held stationary while the more proximal electrode advances past it. The phenomenon has also been called “tripping” and may be more common in perimodiolar electrodes.(1,2) Tip fold-over may result in a variety of negative consequences, ranging from the need to program-out electrode contacts all the way to removal and replacement of the entire electrode array. Fold-over is also associated with cochlear insertion trauma.

New Information

Prof. Angel Ramos and colleagues at Las Palmas University have recently analyzed insertion techniques for a newer delicate perimodiolar electrode array using both a plastic model of the cochlea and human cadaver temporal bones.(1) Dynamic insertion was studied with fluoroscopy while final electrode position was studied with cone beam CT. The authors analyzed the outcomes of the manufacturer’s recommended insertion technique and the effects of three common errors that could be made in surgery:

1. Improper alignment (rotation) of the pre-curved electrode at the cochleostomy (should be toward the modiolus);

2. Over-insertion of the electrode ‘sheath’;

3. Pre-extrusion of the array from the sheath prior to proper positioning of the sheath.

Using the recommended technique, the authors report good results with no tip fold-overs. However, in each of the three technique-error conditions, tip fold-over was common: improper alignment--100%; over-insertion—60%; pre-extrusion—40%. Imaging also demonstrated dislocation of the electrode into Scala vestibuli in most bones with tip fold-over. It should be noted that, typical of anatomic studies, the number of insertions in this study are limited.

 

Take Home

As electrodes become more delicate, the potential for tip fold-over becomes more substantial. This may be especially true with perimodiolar electrodes. Ramos et al. demonstrate the need meticulously to follow good surgical techniques and to be aware of common errors.

It is important to remember that initial studies of new electrodes are performed by surgeons who were highly involved in the experimental development of the electrode over a period of months or years. These surgeons are highly aware of the characteristics of the new array as well as what can go wrong during insertion. Subsequent FDA pre-market approval/CE Marking studies are usually carried out by a small group of highly selected senior CI surgeons who receive more training on a new electrode than other practicing CI surgeons usually do. As a result, sometimes surgical issues are not apparent until the new electrodes are released for widespread use.

For these reasons it is important to obtain specific training on any new device prior to performing the procedure. Manufacturers will provide this on request in a number of ways. Reading the surgeon’s manual and having a rep standing by in the O.R. for a first case are helpful but may not be sufficient.

References

1. Ramos-Macias A, De Miguel AR, Falcon-Gonzalez JC. Mechanisms of electrode fold-over in cochlear implant surgery when using a flexible and slim perimodiolar electrode array. Acta Oto-Laryngol 2017. PAP DOI:10.1080/00016489.2016.127149.

2. Briggs RJ, Tykocinski M, Xu J, et al. Comparison of round window and cochleostomy approaches with a prototype hearing preservation electrode. Audiol Neurotol. 2006;11:42–48.