The Effect of Electrode Placement on Cochlear Implant Function and Outcomes

Abstract

Cochlear implants have been an effective treatment for restoring profound sensorineural hearing loss to those who do not benefit from traditional hearing aids. Advances in surgical technique and electrode design allow for preservation of residual hearing. This allows cochlear implant candidacy criteria to expand to those with good low frequency hearing and severe high frequency hearing loss above 1000 Hz with poor speech discrimination. With a less traumatic surgical approach, low frequency hearing can be preserved resulting in combined low frequency auditory perception and mid- to high-frequency electric perception resulting in electro-acoustic stimulation (EAS). Despite the improvements in cochlear implantation, outcomes continue to vary significantly from one user to another. The variance in performance may potentially be due to the placement of the electrode within in the cochlea. This study focused on performance of patients compared to insertion depth, age, pitch perception and electrophysiologic measures. Patients with residual hearing were included and outcome measures were measured via speech perception tests. Radiographic imaging confirmed insertion depth, and the change in pure tone average was compared to this depth. Hearing preservation was further accomplished with two patients who presented with residual mid and high frequency hearing. Custom atraumatic electrodes were inserted, and hearing was preserved across all frequencies. These cases allowed for electric and acoustic pitch matching experiments to be conducted in the same ear providing information on where in the cochlear the implant is actually stimulating. Several pairs along the cochlea were run between electric and acoustic pitches at varying rates of stimulation. Place to pitch mismatch varied depending on the area within the cochlea. Lastly, objective measures were used in attempt to determine the variance in outcomes. Two main contributing factors govern implant performance, 1) the ability of the processor to effectively deliver the electrical signal to the ear, and 2) the patient's ability to process the information. Peripheral mechanisms were analyzed with the electric compound action potential and its amplitude growth function. The slope of the amplitude growth function was measured at the corresponding electrodes and compared to speech discrimination scores. Steeper slopes correlated with increased word understanding abilities. For further insight into the health of the cochlea, age effects were compared to hearing preservation. The pure tone averages were calculated before and after surgery. Pure tone averages following surgery elevated with increased age suggesting that the elderly may be at more risk for loss of residual hearing as compared to the general population.