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.
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:
SOE had limited predictive value
foldover was not apparent to the surgeon during insertion
foldover was not associated with aversive stimuli to the patient
diagnosis was made by CT scan
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.)
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.
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