Packing of the Cochleostomy Site Affects Auditory Nerve Response Thresholds in Precurved Off-Stylet Cochlear Implants

Objectives: To determine whether packing of the cochleostomy site influences electrically evoked compound action potential (ECAP) thresholds. Design: Prospective nonrandomized control study. Setting: Tertiary referral pediatric hospital. Patients: Eleven consecutive children who received unilateral or bilateral cochlear implants (16 Nucleus 24RE Freedom precurved advanced off-stylet electrode arrays [Cochlear Corp., Lane Cove, New South Wales, Australia]) between June and September 2008. Main Outcome Measures: Electrically evoked compound action potential thresholds were evoked by stimulation of basal (electrode 3), mid (electrode 9), and apical (electrode 20) electrodes during cochlear implant surgery in the operating room. The first recording was completed immediately after insertion of the electrode array before cochleostomy packing, and the same measures were collected immediately after the packing. The time between recordings was 5 minutes. Results: Electrically evoked compound action potential thresholds evoked by the apical electrode did not significantly change from pre- to post-cochleostomy packing, but ECAP thresholds increased significantly after packing when evoked by the mid-array electrode (mean, +10 clinical units; p < 0.001) and decreased significantly when evoked by the basal electrode (mean, -5.6 clinical units; p = 0.023). These changes were unrelated to passive changes occurring over the first 5 minutes after insertion of the electrode array. Conclusion: Packing of the cochleostomy causes changes in ECAP thresholds evoked by mid and basal areas of the implant electrode array. These findings suggest that packing shifts the electrode array in the cochlea, increasing the distance from the modiolus in the mid array and decreasing this distance at the basal end. This study is concerned with a single electrode array from a single manufacturer and a single insertion method (precurved off-stylet electrode). These conclusions may be too broad to apply to all precurved electrode arrays.